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THE AM BULLETIN BOARD => Technical Forum => Topic started by: ssbothwell SWL on August 20, 2011, 02:44:49 PM



Title: Building my first oscillator
Post by: ssbothwell SWL on August 20, 2011, 02:44:49 PM
hi everyone. my first post here was about building simple linear amplifiers. my next goal is to learn how to make a decent oscillator that i can use in various hf projects in the future. i have a copy of the 2011 arrl handbook which has a really great chapter on oscillators which i have been reading but i think i need some advice from you guys.

i would like to know what you think would be a really useful type of oscillator for someone fairly new to radio. should i try to build a vfo, vfxo, or frequency synthesizer? colpitts, series tuned colpitts, hartley, butler crystal, some specific pll ic, something else? the arrl handbook has tons of designs and i'm sure theres countless more out in the field. what i am hoping for is something simple enough for me to be able to understand how it operates and which can operate on the whole hf spectrum. i dont mean i want one oscillator that can tune over 1.8-30mhz but rather a design that can be tweaked for various hf applications.

thanks for any suggestions!


Title: Re: Building my first oscillator
Post by: WU2D on August 20, 2011, 08:41:56 PM
This is s great way to learn! Over the summer of my junior year in high school while not working at the Chevron station, I basically built every HF oscillator type described in the Handbook and in a military manual that an Army tech gave me. I used old TV and radio parts. Transistors - tubes whatever, it did not matter, I built them all up on a pine board. My power supply was a 9V battery and for the tubes, an old radio transformer power supply with 150V and a 6.3V for the filament. You would be surprised what you can learn from a 2N2222 or a 6J5.

Mike WU2D


Title: Re: Building my first oscillator
Post by: W4AMV on August 21, 2011, 10:49:16 PM
If you are mathematically inclined, just a little bit, an excellent paper and book by Clarke and Hess, treats the subject of oscillators in an extraordinary way. Also, as I recall, you have LT Spice. Once again, I encourage you to try the oscillator examples included in the program. However, if you are anxious to melt solder, than the direct answer to your question for L-C oscillators, is the Hartley configuration. I suggest this configuration mainly because it is mechanically a bit easier to configure then the others. If you don't have a box of CAPS, you only need one coil suitably TAPPED to provide feedback and just ONE CAP to form the completed resonator (or TANK). Add your 2N2222 and you are off. However, what is more exciting, is build the same design in LT Spice. The agreement with bench measurements is quite incredible through several 100 MHz!


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 21, 2011, 11:20:20 PM
thanks for the advice both of you! today i tried to put together a colpitts oscillator. the example in the arrl handbook uses a fet so i searched around online and found a colpitt using a bjt here: http://www.electronics-tutorials.ws/oscillator/colpitts.html

the example circuits in the arrl handbook didnt have specific values for the tank circuit components so i was pretty surprised to see the inductor value being given on that website in millihenries. that seems huge! every inductor i have worked with has been in the microhenries range. i would imagine winding a coil in the mH range to take hundreds of turns on ferrite.

should i be try to wind my own or do you guys buy them when they have such high values? if you wind them, what type of core do you use and what size?


Title: Re: Building my first oscillator
Post by: w3jn on August 21, 2011, 11:51:17 PM
couple of tips on building a stable oscillator:

Avoid using toriods, unless you get the most stable mix.  They're hard to wind and not adjustable.  Better is ony of those old Miller slug-tuned ceramic forms.

Use the best quality capacitors you can in the tank circuit.  Silver micas or ceramic NP0s.  Temperature compensate with a 10 pF or so N750 to start, then adjust as necessary.  Use air-variable trimmers if possible instead of mica compressions.

Use the best quality variable cap you can.  One with ball bearings on each end is best.  Use one with a straight-line frequency plate design (google it if you don't know what that is).  Also use the smallest (value-wise) cap you can to hit your frequency range.  Don't use a 10-365 pF variable and then use padders if you're building a 3.5-4 MHz oscillator.  Use a 50 pF

Use a single point ground for all components - ie ground the coil (if applicable) any trimmers, and the active components to one spot.

Use a regulated PS with plenty of bypassing at RF.  Best bypass is a .001, .1, and 10 uF caps all in parallel.

Strive for mechanical rigidity.  A cast aluminum box, or thick plate, makes the best mounting.

Use the lightest coupling possible from the tank to the active device that sustains reliable oscillation.  This ensures the tank Q is at its highest, gives the lowest phase noise, and best stability.

Avoid using those clipper diodes in the base circuit that you see in many circuits.  If you're blowing FETs due to RF voltage on the gate, reduce the supply voltage and reduce the coupling to the tank.  The clipper diodes create phase noise and drift due to their capacitance.

The L/C ratio is important.  Use as low a L/C ratio as you can get away with.  Caps are more stable (physically and electronically) than coils. 

Don't even consider using an oscillator to drive something else without using a buffer circuit.

This is a really nice aide to designing the tank, if you're building a superhet.  You can use it just for the oscillator, just enter "0" for the IF frequency.  Ignore large padder values, just delete the padder capacitance unless you don't have an adjustable coil then use a varaible padder in parallel with other larger caps.

http://ironbark.bendigo.latrobe.edu.au/~rice/superhet.js/superhet.html



Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 22, 2011, 01:50:39 AM
wow thats a great list of tips.

would it be better to use a fixed or tunable inductor?

would these power inductors work well? http://www.delevan.com/seriesPDFs/4590.pdf
what about these rf choke ones? http://www.epcos.com/inf/30/db/ind_2008/b82144a.pdf
any reason i would need a high current rating?

i know that the inductor values are relative to what capacitors i use but is there a general range i should be looking for if i want to be oscillating in the range of 1.8-30MHz?

i think i better listen to W4AMV and try assembling something in lt spice before i start buying parts.


edit: oh interesting. the colpitts examples in ltspice use smaller inductors (100uH) and larger capacitors (500pF) compared to the electronics-tutorials.ws example i posted earlier. what difference does it make to have larger inductors versus larger capacitors?


edit2: w3jn, with regards to that superhet site you linked, what is the aerial circuit? and is the padder the unlabeled capacitor in the circuit?


Title: Re: Building my first oscillator
Post by: WU2D on August 22, 2011, 08:06:48 PM
SWL,

Keep it simple and use a couple of Radio Shack MPF-102 FETs to start. The first FET is the Colpitts (actually this variation of the Colpitts is called a Clapp oscillator). The second is a buffer. Your power supply should be regulated. A 6.8V zener or a 7806 or 7805 make a good supply for this oscillator. This oscillator will make a good VFO or HFO for a receiver.

The coil can be solid # 16 to #14 preferably enamel covered wire, wound on a 1 Inch Dia. piece of PVC pipe about 3 inches long, securely mounted to your breadboard. Lets say we want something in the 80M band.

With about 120 pf of capacitance (100 pf fixed silver mica cap and a 20 pf air variable on a vernier dial) we would need about 15 uH of inductance to hit 3.7 MHz.

This turns out (pun) to be about 15 turns. Once happy with the number of turns, cement them down with Epoxy. The FET source RFC can be about anything from 500 uH to 2.5 mH. In fact a 1K resistor may work.

Give it a try.

Mike M. WU2D


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 22, 2011, 08:24:54 PM
i just did a couple ltspice tests.

using that superhet generator website that w3jn posted i made this:
http://i.imgur.com/fpmUx.png

i have a 400pF air variable capacitor so i entered min and max tuning capacitor values of 10pf and 400pf. i arbitrarily enter 1.8 and 3.5Mhz as the low and high frequencies. i assumed that was what sets the tuning range of the oscillator but when i loaded the model up in ltspice i had a range of ~2mhz to ~10.5mhz when i adjusted C2 from 400pF to 10pF. i didn't include any of the other caps from the circuit because the website did not list any values and w3jn said to skip the pad capacitor.


i then tried to build the colpitts oscillator from here: http://www.electronics-tutorials.ws/oscillator/colpitts.html
here is my ltspice schematic: http://i.imgur.com/rFRDu.png
this one didn't work at all. :(

WU2D, which schematic are you referring to as a clapp oscillator?


Title: Re: Building my first oscillator
Post by: WA1GFZ on August 22, 2011, 09:07:57 PM
The All time best cap for an oscillator circuit is the tuning cap out of an old BC221 with the spring loaded worm gear drive.


Title: Re: Building my first oscillator
Post by: w3jn on August 22, 2011, 11:42:37 PM
The circuit you posted has no feedback and it's just an amplifier with a resonant input and no input signal.

In that superhet program, the "aerial" circuit would be the RF stage in a superhet receiver.  It's meant to calculate components to make sure the RF and oscillator circuits track each other.  In our case all we're interested in is the oscillator.  The padder is there to make sure the oscillator tank range tracks the RF tank range, at the correct variable cap settings.

Your 10 pF-400 pF cap is way too wide.  I'd throw a 100 pF cap in series with it to narrow its range a bit (yep, that's a padder).

Those RF chokes you pointed to are not made for oscillator service.  Probably too much parasitic capacitance and definitely not temp stable.

Best way is to wind your own.  A piece of PVC pipe might even make a decent form, at least to experiment with.  Don't close-wind (space your turns at least a wire diameter or two) and use enameled wire (PVC or other insulation isn't mechancially stable).    By winding your own you can also include the feedback tap for a Hartley circuit.



Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 23, 2011, 01:03:20 AM
oh adding a 100pF in parallel helped quite a bit. now when i adjust c2 from 10 to 400pF i get a frequency range of 1.6 to 3.5MHz. if i wanted to make the oscillator switchable between multiple hf bands should i have different tank circuits setup on a rotary switch or is there a better way?


as far as that other circuit.. isnt there feedback from the collector to the base via the tank circuit?
how is it different from this circuit: http://www.electronics-tutorials.ws/oscillator/osc13.gif  ??


edit: and what exactly is a buffer circuit? wikipedia says "A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another." is that the same thing as a matching network?

edlt2: WU2D, sorry i asked a silly question about what schematic you were referring to. i didn't see the attachement to your post. thanks!


Title: Re: Building my first oscillator
Post by: w3jn on August 23, 2011, 12:04:44 PM
A buffer is just a single stage amplifier.  Could have no gain (like an emitter follower) or have some gain.  The purpose is to isolate the oscillator from the load.  A variable load will mess with the oscillator stability.

This is the schmatic you posted that has no feedback http://i.imgur.com/fpmUx.png

You could switch the tank with a rotary switch, that's the way receivers have done it prior to the synthesized era, but switches will introduce instability.  Collins receivers using a PTO do not bandswitch the variable oscillator and hence are extremely stable.



Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 23, 2011, 05:07:43 PM
but that is the oscillator i made based on the superhet generator link you sent. also it actually seems to work whereas the colpitts with the bjt transistor does not work:

http://i.imgur.com/BEhrl.png
http://i.imgur.com/40Rgr.png


i was ultimately hoping to be able to use this oscillator (or whichever one i end up building) in qrp transmitters. are these designs stable enough for transmission?


Title: Re: Building my first oscillator
Post by: W4AMV on August 23, 2011, 05:39:01 PM
In the Bipolar NPN, select an actual device, do not use the generic NPN device. Try again.

In the FET ckt change the 500 ohm to 50 then to 5000 and run and check the frequency. You can use the time marker reading or use the FFT function. This is a good example of what the buffer fixes. What does the frequency do for those 3 cases and WHY? 


Title: Re: Building my first oscillator
Post by: W4AMV on August 23, 2011, 05:54:06 PM
oh, the reason the FET circuit does work, per last post, is you used a "real device" and there is intrinsic feedback present. In this case gate-to-source capacitance. Again, in the case of the bipolar, try a real device, not the ideal BIPOLAR model. And finally, remember these are all models (devices, etc...) and in the end you do NEED TO BUILD these items. These simulations are just approximations. As a rule... if it works in simulation, it may work on the bench. If it does not work in simulation, it may still work on the bench, however, it is key to understand where is the disconnect. Finally, don't trust the simulator, but do trust the trends they predict. All that said, it is incredible many times to see how well they do predict operation. If that is the case, pat yourself on the back, you have good models!


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 23, 2011, 10:03:19 PM
i see what you mean about simulations not always matching up with the actual components. i put together one of those fet oscillators using a mpf102 on my breadboard. i haven't made a coil yet so i used an 18uH axial inductor i had laying around. maybe its the reason this didn't work but the circuit was oscillating at about 11MHz 100mV Pk-Pk regardless of how i adjusted my air variable capacitor. the sine wave looked really messy and was not stable at all. if i moved my hands around near the circuit it i would see the sinewave move around on my oscilloscope.

i tried two different variable capacitors, one is 600pF and the other is 400pF. i had the capacitor connected to the circuit using alligator clips. i tried i removing the alligator clips and used short lengths of wire to attach the tuning capacitor to the breadboard. this caused the circuit to stop oscillating.  ???



Title: Re: Building my first oscillator
Post by: w3jn on August 23, 2011, 11:33:33 PM
Yuo've just discovered the miracle of parasitic capacitance and inductance  ;D

The clip lead is acting like an inductor and its capacitance to the rest of the circuit is acting like a higher-Q circuit than your molded inductor (see why you don't use those in an oscillator?).  Short, stiff wiring and a single point ground and/or a good ground plane is the order of the day for more or less predictable results.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 24, 2011, 12:29:27 AM
oh cool. so my oscillator is so bad it turned into a theramin  ;D

i put together a junky coil using #16 enamel wire and an empty spool. i got the circuit working to some very very modest degree. it is oscillating at about 6MHz but it jumps between 5.8 and 6.2 and the sinewave looks really bad. you can see it is slightly misshapen in this photo but in person you can see that it is really 'jittery.'

http://i.imgur.com/kxpWw.jpg

here is a photo of the circuit: http://i.imgur.com/1bbrQ.jpg

the circuit stops oscillating when i connect my air variable capacitor.  ???

edit: i didn't have any mylar capacitors and had to use ceramic discs. could that have something to do with whats happening?


Title: Re: Building my first oscillator
Post by: Opcom on August 24, 2011, 02:31:50 AM
I remember my first oscillator, a tuned plate tuned grid using a 2A3 and a 300V power supply. The result was my father coming into my room, asking me what I was doing, and letting me know he was not happy about the TV reception.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 24, 2011, 05:18:57 AM
that sounds crazy opcom! how much power was that thing putting out?

i managed to clean up the sinewave a little bit. i removed the source to ground capacitor and the signal looks like this:
http://i.imgur.com/vMUKs.jpg4

using the capacitors i had available i figured out what frequency ranges the oscillator can produce. i discovered that if the tank circuit has greater than 150pF of capacitance that the circuit stops oscillating. heres the frequency values in relation to tank circuit capacitance i was able to achieve:
151pF = no signal
146pF = 5.0MHz
136pF = 5.181MHz
115pF = 5.6MHz
100pF  = 6.05MHz
50pF = 8.08MHz
33pF = 9.39MHz
20pF = 11.0MHz
15pF = 12.12MHz
10pF = 13.1MHz

at 5MHz the sinewave looks really great but at any higher frequency it starts to get deformed.

i'm still not able to get any signal at all when i have either of my air tuned capacitors in the circuit. this is what they look like: http://i.imgur.com/RImea.jpg

is there some way i can troubleshoot them and make sure they aren't damaged? i did do continuity checks and nothing seems to be shorted in them.

i checked my coil using this website: http://www.k7mem.150m.com/Electronic_Notebook/inductors/coildsgn.html
and based on the number of turns and it's dimensions the coil is only about 4.7uH. i'm going to make a better coil as soon as i can find some scrap pvc. is there a particular inductance value i should aim for?


Title: Re: Building my first oscillator
Post by: KM1H on August 24, 2011, 09:51:42 AM
Quote
Avoid using toriods, unless you get the most stable mix.  They're hard to wind and not adjustable.  Better is ony of those old Miller slug-tuned ceramic forms.

The 7 Mix toroid is very stable even with a very low power tube oscillator. Just dont get the smallest you can find, a T68 or larger is better.

Those old cheap slug tuned coils were far from stable but maybe OK for just AM. OTOH a large diameter one from a scrapped old high end radio will be fine.

Carl


Title: Re: Building my first oscillator
Post by: steve_qix on August 24, 2011, 07:07:49 PM
This is what I use (I use completely homebrew oscillators for everything).  This is easy to build, and is fairly foolproof.

The neat thing about it, is that it uses a DIVIDER rather than a multiplier.  In this way, you can leave the oscillator running all of the time, and never hear anything bleeding through on the fundamental frequency.

Here is the oscillator-only schematic.
(http://www.classeradio.com/oscillator.jpg)

And, here is the complete VFO article with schematics, plans, pictures, etc.

http://www.classeradio.com/vfo.htm (http://www.classeradio.com/vfo.htm)


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 24, 2011, 10:43:45 PM
today i found a smaller air variable capacitor at the electronics scrap yard. a guy at the yard guess it was 75pF or so. i also built a new coil with 20turns on a 2inch plastic spool. heres a photo of the oscillator with both devices installed: http://i.imgur.com/s06Yz.jpg

heres what the schematic looks like. the tank cap is air variable but i dont know how to draw a variable cap symbol in ltspice.
http://i.imgur.com/1zJ0V.png

this circuit doesn't work in LTSpice without a capacitor from source to ground but in real life the sinewave looks better without one.

here are images of the waveform at the lowest and highest frequencies it is getting:
2.8MHz: http://i.imgur.com/y0m0i.jpg
7.5MHz: http://i.imgur.com/e4UI2.jpg

just for fun i tried putting both of my home-made coils in the circuit. this bumped up the frequency range to ~5MHz to ~13MHz.

so here are my questions:

1. what can i do to clean up the waveform? at lower frequencies the bottom of the wave is slightly flattened and at higher frequencies the top and bottom of the wave have lumps.

2. how should i tweak the tank circuit so that adjusting the tuning capacitor gives me a more fine grained control over one specific band?

3. on a related note, it seems that the higher the inductance of the tank circuit the lower the frequency range. is this correct? if so, is it possible to have a tank circuit with one inductor in it that is adjustable between all the hf ham bands or do i need different coils for each band?


Title: Re: Building my first oscillator
Post by: w3jn on August 24, 2011, 11:25:47 PM
Answers to your questions:

1.  Filter the output with a lowpass or bandpass filter.  If your waveform is distorted you're most likely overdriving the transistor.  Reduce the feedback and/or coupling from the tank to the transistor.

2.  Put a fairly large amount of FIXED capacitance in parallel with the coil.  If you have a 75 pF variable cap put 300 pF or so in parallel with the tank.  This reduces the influence of the variable capacitor on the tank.  Alternatively (or in combination with the above) put a capacitor in SERIES with your variable. 

3.  Yes.  And no, that's not practically possible for an oscillator.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 25, 2011, 02:58:14 AM
thanks w3jn.

i feel kind of dumb, i had been changing parts around a lot and things got a little confusing. i took the whole circuit apart and put it back together and now the waveform looks great. for whatever reason i can't have more than about 150pF in the tank circuit otherwise it stops oscillating, but i have 100pF in parallel with the variable capacitor (which i now think is closer to 50pF) and the full rotation of the cap is a 1MHz sweep.

is it normal for the voltage to increase or decrease depending on the frequency of the oscillator?

here is a photo of the waveform:
http://i.imgur.com/3FX3K.jpg
and here is the correct schematic for how i have it setup currently (inductor might be a little off from my real life coil but its close):
http://i.imgur.com/4X02J.png

this is really exciting. i guess the next step is to build a nicer coil that gets me onto a specific band and add a buffer circuit.


Title: Re: Building my first oscillator
Post by: W4AMV on August 25, 2011, 05:38:41 AM
Looks like you are doing very well with your results. The answers to your questions (all) are available in an excellent text by Clarke and Hess, Communication Circuits Analysis and Design. In chapter 8 he treats both the bipolar and FET oscillator which are unique in the way the devices operate TO LIMIT THE FEEDBACK in an oscillator. On the one hand as JN indicates it is important to control the level of positive feedback, on the other how do I limit the feedback. The resulting limiting analysis and the resulting distortion is presented in their book. In the end, you can actually calculate quite exactly the values of the L and C required in the resonator (tank) and the required C's or L's (Colpitts, Hartley) in the feedback network to control the output voltage and distortion of the oscillator. I would search for a re-print of their text. Although the math requires the calculus, the resulting tables and graphs in the text, reduce the process to simple algebra! As time permits, I'll see if I can put their work into a simple example which closes the gap between simulation and calculation.

By trying different C values and their placement (location) in your circuit you are in affect modifying the feedback. Look at the total L and C in the resonator and the feedback, be careful to see if the L's and C's are in series or parallel as you find their total values. Then the operating frequency is APPROX 1/(2*PI* ROOT(LC)). 

Play with that 500 ohm drain R value in LT Spice. You will note that as you vary it in simulation and in real life the frequency will change. Why? Therefore, you may be better off, AC shorting the drain. And remove signal voltage from the source with your buffer.

You can draw a variable C in LT spice by modification of their C using their drawing tool.

 


Title: Re: Building my first oscillator
Post by: w3jn on August 25, 2011, 11:35:14 AM
thanks w3jn.

i feel kind of dumb, i had been changing parts around a lot and things got a little confusing. i took the whole circuit apart and put it back together and now the waveform looks great. for whatever reason i can't have more than about 150pF in the tank circuit otherwise it stops oscillating, but i have 100pF in parallel with the variable capacitor (which i now think is closer to 50pF) and the full rotation of the cap is a 1MHz sweep.

is it normal for the voltage to increase or decrease depending on the frequency of the oscillator?


Yes.  That's because the Q of the tank and feedback isn't constant over the frequency range, most likely due to parasitic capacitance and inductance.

It's not oscillating at the lower frequency due to insufficient feedback or too low a Q at that frequency in the tank.  Perhaps your variable cap has vanes that are shorting when it's close to being closed as well.





Title: Re: Building my first oscillator
Post by: W4AMV on August 25, 2011, 11:52:45 AM
Yes, JN is on target. Other issue is the reactance of 150 pf or more is just to low at the frequency of interest and the gain of the oscillator ( so-called loop gain) is too low to sustain oscillation. If you calculate the frequency using the eqn. from prior post you get ~ 2.9 MHz. One point, you have no C across the gate-source, so the feedback (in the Colpitts configuration) is now established by the FET gate-source C. You really want to swamp that out. So you will need to add a C across gate-source and adjust the C source to ground to set the feedback. Furthermore, the source to ground reactance might be best set with a choke in combination with R2. Lower R2 now to increases the open loop gain, since the FET gm will increase with increased current. If you monitor the drain current as you tune the oscillator, you will note that the current will vary. You are seeing the oscillator actually balancing the loop gain, by controlling its gm.  :)


Title: Re: Building my first oscillator
Post by: W4AMV on August 25, 2011, 12:11:21 PM
Here are some notes from LT Spice that illustrate some of the prior comments.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 25, 2011, 06:49:49 PM
thanks for all the great suggestions. i added a 100pF cap between gate and source and things improved quite a bit. i am able to add way more capacitance to the tank circuit which enables me to dial in my frequency range much more precisely. this also seems to have stabilized the amplification quite a bit.

w4amv, the only rf choke i have is a radioshack 100uH one. would that work okay with R2? also, it should go in series right?
i'm going to monitor the drain curent while adjusting R2 later today. i'll report back what i discover.

does this buffer amplifier design look like what i would want to use for my oscillator? http://www.electronics-tutorials.com/amplifiers/buffer-amplifiers.htm


Title: Re: Building my first oscillator
Post by: W4AMV on August 25, 2011, 08:17:40 PM
The RFC value you mention should be fine. For your frequency, say 1-5 MHz, 100 uH has a reasonable reactance. The issue with large chokes eventually is their SRF, self resonant frequency, approach that and the osc. will stop. You can diddle the source R, up and down will change the output voltage ( as observed at the source) and the distortion. I'll look at the buffer, but at these frequencies a source follower should be fine, gate of follower tied to source of oscillator, lightly coupled, then you can add power amplification after that and a LPF. Will turn a blind eye for now to osc noise and a bunch of other complications. However, feel free to experiment. Taking the voltage from the resonator directly, again lightly coupled can be a far better arrangement. 


Title: Re: Building my first oscillator
Post by: W4AMV on August 25, 2011, 08:21:05 PM
Yes, the web site design is fine.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 25, 2011, 08:48:42 PM
i included the rfc and dropped r2 down to 500ohms. vrms output jumped up quite a bit and was pretty stable as i swept the variable capacitor but the circuit stopped oscillating at lower frequencies. i went through various resistors between 500 and 1000ohm and found that around 800ohms i still get the full frequency range and vrms is reasonably stable but not perfect.

i'll report back again once i manage to get a buffer amp built. probably later tonight or tomorrow.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 26, 2011, 12:06:21 AM
hmm, i dont have any zener diodes or 1mh rf chokes so i cant put this circuit together tonight.

what is the deal with chokes? can i use a little axial inductor like this: http://www.allelectronics.com/make-a-store/item/CC-1000/1000-UH-CHOKE-COIL/1.html

or do i want something else?


Title: Re: Building my first oscillator
Post by: W4AMV on August 26, 2011, 10:12:03 AM
a 1 mH choke is a bit large and the SRF could be an issue. Ideally try to choose a choke value whose reactance is 5x the value of the source-gnd feedback C at the center frequency of the oscillator. So if you use 100 pf for a 3 MHz unit, a 270 uH would be appropriate.

The deal with chokes is the same deal with C's and R's. They all have associated parasitic elements.  So when you buy an L you get a C and and an R at no extra charge :) The same is true for C and R.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 26, 2011, 03:37:44 PM
sorry i was referring to a choke for the buffer amplifier. it has no feedback and the schematic mentions a 1uH choke.



Title: Re: Building my first oscillator
Post by: W4AMV on August 26, 2011, 03:54:47 PM
1 uh ok, any thing in the range of 1-100 uH should be fine. 1mH to large, however, try it if it is your junk box. Otherwise, order what was specified 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 26, 2011, 04:14:37 PM
oops! sorry again! i keep making things unclear. the buffer amplifier calls for a 1mH choke on the source.

http://www.electronics-tutorials.com/amplifiers/buffer-amplifiers.htm

am i right to assume that the purpose of the choke in this buffer amp circuit is to prevent the ac signal from getting sent to ground?

i'm just gonna go to the surplus shop and get a bunch of different size chokes to experiment with.

in this buffer circuit what would be the symptoms of too much and too little inductance in the choke?


Title: Re: Building my first oscillator
Post by: W4AMV on August 26, 2011, 05:59:41 PM
No problem. The 1 mH is fine and any reasonably large reactance will work well. The goal of the source inductor is to raise the Z of the source to RF AC so the voltage gain of the follower is nearly unity. Ideal, looking back into the source terminal the Z is 1/gm and no less. If less the voltage gain will fall below unity.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 26, 2011, 09:36:00 PM
i just got some really nice scores from the scrap yard. i found a 2"x4" ceramic coil form, 1x1.5" ceramic coil form slug tuned, 3/4"x3.5" ceramic coil form, five air-variable capacitors (some the same size and some slightly larger than the one in the photos i have posted here), a mixed bag of silver mica caps, and 10x of 1mH tuning coils and 10x 2.2mH tuning coils. all that for $30.

heres the big coil form: http://i.imgur.com/Irz3C.jpg
the wiring on it is as i found it. i didn't think to get enamel wire in the proper gauge to fit the grooves in it so i am leaving it as is for now. i don't want to mess with recoiling it until i have wire that will fit it properly.


i just tried putting together the buffer amplifier and i'm not sure it is working properly. i used a MPF102 and i was not able to find a zener diode. the output directly from the oscillator is 8.8vrms with a perfect sinewave whereas the output from the buffer amp is 1.3vrms with the top of the waveform clipped off. heres a photo of the buffer amp output: http://i.imgur.com/artqK.jpg

any idea whats going on?


Title: Re: Building my first oscillator
Post by: w3jn on August 26, 2011, 11:39:17 PM
Too much coupling from the oscillator to the buffer.  Try a 10 pF or so capacitor between the two.

A zener isn't necessary if your power supply is regulated.  That coil you have is really nice.  It's built for a high power transmitter tank but the size and large wire will make a nice stable oscillator, if you mechanically mount it right.

One thing I noticed is you're using those white plugin proto boards.  Those aren't good for experimenting at RF - been there, done that.  A better way is to get some scrap PC board and engrave some islands with a moto tool, and just solder the components directly to the PC board.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 27, 2011, 12:32:24 AM
the coil has nice brackets on the bottom for mounting it. everything is really precarious right now but once i get the whole circuit operational i will make it mechanically stable.

i'm on the fence about what to do with this coil. with it as is i can't get below 2.5MHz. i want to rewrap it with more turns but at the same time it is in really nice shape the way it is. i'm thinking i might try to keep it as is and use something else to make a coil for lower frequency operation. given the sizes (2" diameter) and the number of turn on it currently (10) what frequency band would it work the best on?

well it turns out i had been messing with the buffer circuit to much and had some misplaced parts on my bread board. i rebuilt it and i am getting slightly better results: http://i.imgur.com/7ritk.jpg

i also put together the buffer circuit in ltspice: http://i.imgur.com/EvdMh.png
the blue wave is the gate of q1 and the green wave is the source of q2 (buffer output)

the ltspice results are waaay better than my real life results.


Title: Re: Building my first oscillator
Post by: w3jn on August 27, 2011, 03:11:51 AM
Get rid of the breadboard and use short leads on your components.  The parasitic capacitance and inductance is killing you.  That RF choke is way too big and it's lined up in parallel (physically) with your tank coil.  There will be coupling between the two.


Title: Re: Building my first oscillator
Post by: W4AMV on August 27, 2011, 03:54:36 AM
Agree with all of W3JN comments. Pickup some scrap copper pc card, FR4 is fine. If it is only single sided so much the better to reduce capacitance per each pad you cut. A dremel tool with a fine grinder bit will allow you to remove copper and form the tie points.

LT Spice of course is now giving you fantasy results. Welcome to the world of trying to achieve accurate simulation models.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 27, 2011, 09:08:36 AM
do you guys really think the breadboard is causing enough stray capacitance and induction to give me the results i am getting? without the buffer amp i get really nice and stable sinewaves.

i do have a bunch of single sided pc board. i usually cut little .25" squares out of it and glue it onto a larger piece which acts as the ground plane. would that be okay or would there be too much capacitance between the little pieces and the large piece they are glued onto?


Title: Re: Building my first oscillator
Post by: w3jn on August 27, 2011, 11:15:36 AM
Naw, the squares should be fine so long as you don't try above 15 MHz or so.  Easier though is to just cut islands and traces with a MotoTool. 

And yes, I think parasitics are part of your problem.  Did you reduce the coupling to the buffer amp?  Try a capacitive voltage divider on the input of the buffer amp.  10 pF between the oscilaltor and the amp input, and 200 pF or so to ground.  I think you have a combination of loading down the oscillator with overdriving the buffer amp.


Title: Re: Building my first oscillator
Post by: W4AMV on August 27, 2011, 07:33:44 PM
I agree with JN. From the voltage wave forms you posted, it appears that the gate -source of the FET buffer is driven to hard and you are seeing the gate-source diode junction clamping the waveform. Try a Cap voltage divider or simply use a small C to couple the source point of the osc to the gate of the buffer. You can get all the voltage gain you want from a third stage and low harmonic content with a LPF latter added. The main job of the buffer is to isolate the oscillator section from variations in the external load.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 28, 2011, 03:05:38 AM
hi guys,

i figured out the problem and now it is producing pretty much the same results as the ltspice model. i'm just gonna say the problem was really dumb and i am kind of embarrassed. it involves a misplaced jumper cable.

i didn't get a chance to put together a pc board version of the circuit today. but i'll be doing that possibly tomorrow.

other than mechanically securing the oscillator, what do you guys think i should try to do at this point? is it now safe to try hooking up a 2n3866 linear amp like the designs i worked through in my previous thread?

i want to make two of these oscillators so that i can try building a receiver and a qrp transmitter. i guess it makes sense to attempt a receiver first?


Title: Re: Building my first oscillator
Post by: W4AMV on August 28, 2011, 04:13:25 AM
I would do a Rx first. Also, I assume you are going to transfer this work to a nice stable setup, not use the patch board. In any case, if you do the Rx first, it will serve you as a test bed. The Rx will tell you how stable is this thing, how noisy is my osc, is their jitter or a steady drift, etc... Plus its fun to listen. Good Luck.

If you use a high level passive mixer, like a diode ring mixer, a 2N3866 works well. A 100 mW or +20 dBm will suffice and a low pass filter prior to mixer injection is useful. The 3866 will have 15-20 dB gain, run at 13 V and 50 mA. Designs are on the WEB, in the HDBK, etc... or roll your own since you have the tools. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 28, 2011, 03:17:19 PM
okay i'll definitely start with a receiver. it will be fun to pickup signals on a home brewed rig. i started reading the arrl handbook chapter on receivers. i'm going to start with a direct conversion design and then add an IF stage or two afterwards. http://i.imgur.com/5irON.png

just to be clear, in your last post you recommend i amplify the vfo to 100mW and run it through a low pass filter prior to mixing it?

regarding mixers:
all the arrl design examples use sbl1 mixers but they dont specify forward/reverse voltages, and current. i see bunch of variations on sbl1 available from digikey: http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1376383&k=sbl1
which of these would you recommend?

also, would this schottky diode work? http://www.allelectronics.com/make-a-store/item/STPS30L40CW/SCHOTTKY-DIODE-40V-30A/1.html
allelectronics is way more convient for me than ordering online so it would be great if i could use this part.


Title: Re: Building my first oscillator
Post by: W4AMV on August 28, 2011, 08:29:06 PM
These are SMALL SIGNAL diodes, tiny fellows. Hot carrier or Schottky diodes are desired, small signal Ge, the old 1n34a would work as well. Follow the design info in ARRL handbook to construct the trifilar xmfr needed and try to match the 4-diodes that form the ring mixer. It is not essential, but if you can, try to match the forward current against the applied V.


Title: Re: Building my first oscillator
Post by: W4AMV on August 28, 2011, 08:36:02 PM
Yikes, ok SBL1 is not the part number of a mixer diode, but the part number of a completed mixer module from Mini Circuits Labs. They are cheap to buy I believe in small quantities, or try E-bay. However, if you want the thrill of building your own, go for it!


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 28, 2011, 10:24:22 PM
i definitely want to experience the thrill of building my own mixer ;D

i'm gonna be super excited when i pick up my first station on an entirely home brewed receiver.

is the mixer you are talking about Figure 10.22 from chapter 10 (mixers, modulators and demodulators)?
thats the only DBM circuit i saw in the chapter. for whatever reason the book says that it uses a square-wave LO. is that gonna work with a sine-wave LO?

i believe this is the same circuit: http://www.qrp.pops.net/images/2008/2008-larger/big_dbm.gif


Title: Re: Building my first oscillator
Post by: W4AMV on August 29, 2011, 12:10:35 AM
That is the exact circuit and a very clear diagram. You do not need a square wave. However, it will function with a square wave. Please read the details of how the mixer works in the text. Extreme brief, the LARGE LO signal operates each leg of the diode ring as a commutative switch. Therefore, the LO CHOPs the RF signal at an LO rate. This SWITCH function of the LO on the RF signal gives rise to an IF signal.This process of MULTIPLICATION of the LO and the RF to produce the IF will occur even with a pair of SINE waves. Break out your TRIG book and check it out :) Good Luck.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 29, 2011, 08:38:30 PM
i've assembled the trifiliar wound transformers but i need to find some schotky diode.

in the mean time i tried putting together a 2n3866 linear amp stage for the amplifier in LTSpice. first i tried adding a linear amp exactly as i had learned to do in my previous thread about single stage bjt amps:
http://i.imgur.com/xxYwO.png
the two sinewaves displayed are the input and output from the 2n3866 stage. this didn't work at all and completely ruined my nice sinewave output from the buffer stage.

on a hunch i added an rf choke between emtter-ground resistor and base-ground resistor:
http://i.imgur.com/unV3n.png
this cleaned up the waveform massively.

is this circuit producing 480mW into a 50ohm load or am i misreading something? 14V Pk-Pk = 4.9VRMS. (4.9^2)/50 = 0.4802W


Title: Re: Building my first oscillator
Post by: W4AMV on August 29, 2011, 09:17:58 PM
Your 50 ohm resistor is not returned to ground directly per your schematic, bit via the 1 mH choke. So that calculation is not correct. Is this the arrangement you really wanted?


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 29, 2011, 09:23:09 PM
oh whoops. that was a mistake. i'm not trying to get 480mW i just wanted to see if i had built an amplifier in the right way. i was actually suprised at that output power. when i directly ground the 50ohm resistor i get 200mV Pk-Pk and the sinewave is a little pointy on the positive peaks.

my goal is to get 100mW output to feed into the mixer as you recommended.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 29, 2011, 11:13:15 PM
i did some adjustments and now i'm getting a much better waveform. its only 160mV Pk-Pk into a 50ohm load. i tried adding another transistor stage but things got really hairy.

http://i.imgur.com/L0McY.png

what kind of load should i be using if i plan to run this thing into a DBM?


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 02:01:24 AM
First, pleased check your DC power dissipation in the 3866. If my calc. are correct, you're going to fry that poor fellow. On the mixer Z, that is a complex question! However, a good estimate is,,, drum roll, 50 ohms.

On the power amp, and the 3866 stage, let me make a reading suggestion. You need to study the idea of impedance matching, load lines, Class-A amplifier, and transforming a series circuit to its parallel equivalent and visa versa. I would simplify the Spice item, drop the oscillator buffer elements and concentrate on the 3866 design. Replace all of that with just a sine wave source whose frequency and voltage level is in line with what you are measuring on the bench. The goal of the 3866 stage will be to deliver between 7-to-20 dBm of power across 50 ohms. Start there. Hint, the 3866 can supply you with 15-20 dB of power gain at HF. Your supply V is fine, perhaps 12 V is a nice selection since voltage regulators are available.

Also, please check your biasing of the 3866, a simple standard 4-R bias is fine, although the emitter R selection is critical as their are tricks for maintaining the gain and achieving the bias stability. One trick there is split the R into 2-pieces, AC bypass one and leave the remaining R not bypassed to improve AC linearity, i.e. lower distortion. 

As time permits, I'll try to put a design flow for the 3866 class A amp-driver, that illustrates the above points.


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 02:15:21 AM
Oh, I should mention on the mixer Z. The reason it is nearly 50 ohms, is partly due to the IF termination, arbitrary selection but convenient, 50 ohms. The longer answer has to do with the nature of the diodes, their impedance operating as switches, and the properties of the transformers. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 30, 2011, 03:23:17 AM
thanks for the advice. i definitely need to learn a lot more about linear amp design and impedance matching. z is something in particular that i struggle to understand.

2n3866 ratings are: Ic=.4A , Ib=2.0A, and Pd=5W.

i just did a lot of tinkering with the circuit and got it producing 6V Pk-Pk output with Ic=140mA and Ib=1.3mA.
i started out with the amp in its own spice file with a sinewave signal input and then translated it back into the oscillator file once i got it looks okay.

heres a screenshot showing all three values: http://i.imgur.com/w4EKh.png

does this circuit look okay or am i at least on the right track towards a working circuit?




Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 06:23:50 AM
Great! Now the poor transistor will be a bit cooler! I assume you set Spice up running first the DC operating point analysis. Incidentally, when you pass the measure probe over the device you can obtain DC current measure, and if you press CRTL (something, need to check), you get power dissipation of that element. You can check in the help directory.   


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 06:37:48 AM
Ah, in transient analysis, not DC analysis, at least in my version. If you pass over an element as if you were to measure current and press the ALT key, the current probe will turn to a TEMP probe. LT does the math for you and calculates the power. So, in you circuit, if you pass over the load R, you should obtain a time signature of the output power. Do check its math to make sure it makes sense. You might try a simple circuit case just to be sure.


Title: Re: Building my first oscillator
Post by: w3jn on August 30, 2011, 11:36:47 AM
100 mW is a bit much for a double balanced mixer with schottky diodes.  A better design point would be 10 dBm, or 10 mW.


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 02:56:33 PM
I agree W3JN. However, as he becomes an expert, he may elect to do a high level mixer. In any case, it is easy to dump the power. The design project that I will send coincidentally is ~ 10 mW. Thanks!

Alan


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 30, 2011, 03:37:26 PM
Ah, in transient analysis, not DC analysis, at least in my version. If you pass over an element as if you were to measure current and press the ALT key, the current probe will turn to a TEMP probe. LT does the math for you and calculates the power. So, in you circuit, if you pass over the load R, you should obtain a time signature of the output power. Do check its math to make sure it makes sense. You might try a simple circuit case just to be sure.

oh awesome! i wish i had known i could do this a while ago.

i guess i was doing my power measurements wrong as well. i thought i was getting ~80mW across the load resistor but when i check its power dissipation i get 180/220mW (i get higher voltage on the trough then the peak, how can i fix that?).  http://i.imgur.com/25HlS.png blue is power dissipation in that image.


what are the situations where you would use a high vs low level mixer?



Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 04:41:33 PM
Good observation. The power reported appears to be the peak power from the work I looked at earlier. Should check the HELP menu/documentation to be sure. So the true RMS magnitude power is still available as you calulated earlier and measured from the voltage (rms) available across the external load, 50 ohms. A combination of phase shift and some distortion could account for your observation. In the file I put together I note that I do not see this alignment of power with voltage, Peak power aligns with peak voltage. In the final application for your work, it is not an issue.

The high level mixer, will provide the ability to handle larger RF input signals with significantly lower distortion. One method for achieving this is to add additional diodes in each branch of the ring. Thus you need increase operating voltage and increased LO power since the impedance level is still nearly the same. Again, for your work, 10 mW is quite sufficient. Point in fact, there are tricks to reduce the LO power even further by slightly forward biasing the diodes in the ring. You can experiment with that idea upon completion. 


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 05:39:50 PM
I hope this helps :)



Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 05:42:06 PM
I guess I need to post added items, sorry ADMIN.


Title: Re: Building my first oscillator
Post by: W4AMV on August 30, 2011, 05:44:07 PM
last items, won't do this again.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on August 30, 2011, 06:14:17 PM
holy cow thats a lot of great info! this will take me a little bit of time to work through, but thank you so much for writing all this out.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 02, 2011, 03:53:03 AM
okay so let me make sure i understand what you are doing in the pdf. on the bottom of page2/top of page3  you are calculating the impedance of a hypothetical transistor amp stage right?  and then after that you create an L-Matching circuit to go from 50ohm to the impedance of that hypothetical transistor amp stage?

do i need to create one of these L-Networks between the buffer and the 2n3866 amplifier? if so, would i assume the buffer is providing a 50ohm output or do i want to calculate its impedance and make a matching network based on the impedance of the two transistors?

thanks again for the info.


and just a little update. i put together the 2n3866 amp stage on the breadboard with the oscillator and buffer and i am getting clipped output. in ltspice the waveform looks great so i guess my model is not accurate. http://i.imgur.com/2dHln.jpg


Title: Re: Building my first oscillator
Post by: W4AMV on September 02, 2011, 05:51:12 AM
In the paper pdf I did not mention finding a load line. So the first part was identifying at least part of a load line. I say part, because I did not identify a current limit just a voltage limit. Recall in the DC calculation, the total emitter current is 25 mA, half the maximum for that transistor. Anyway, if the voltage limit is the only limit than 288 ohms would permit that stated class A power to be realized just prior to clipping. Which brings us to your clipped waveform.  You are clearly being limiting by either the current swing or the voltage swing and to fix the clipping either modify your quiescent bias point or your load impedance. Use your scope, adjust your trace to find the bias point no signals, then see where and why your are limiting and make the appropriate adjustments.

Yes, you could work through an input match as well as you observed. However, if you have sufficient voltage or current gain, additional impedance matching may not be warranted. Since the FET buffer is a source follower, it in effect is a impedance transform network providing a low impedance output for the oscillator signal. Your following stage (3866) has a significantly higher input impedance and therefore at least ALL of the FET output voltage should be available to the 3866 input. Perhaps, that is where the issue of clipping occurs. A key question now, IS THE CLIPPING OCCURRING AT THE INPUT or the OUTPUT of the 3866? You are either providing to large an input voltage or current at the base of the 3866 and/or not providing the appropriate output load for the 3866 collector.       


Title: Re: Building my first oscillator
Post by: W4AMV on September 02, 2011, 06:04:09 AM
and yes something is a miss in the model setup. The clipping should appear particularly if it is a DC bias error.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 02, 2011, 05:56:56 PM
i just noticed that when i have the 2n3866 stage connected to the buffer, the buffer output drops down to 2.2VRMS, gets clipped, and is offset above the center line on my scope (is that dc bias?).

here is a photo of the buffer output connected and disconnected from the 2n3866.
http://i.imgur.com/oYFyj.jpg
http://i.imgur.com/HpziR.jpg

heres some questions:

1. can you explain what you mean by "Use your scope, adjust your trace to find the bias point no signals, then see where and why your are limiting and make the appropriate adjustments," and how i would do this?

2. when you say "..providing to large an input voltage or current at the base of the 3866," do you mean too much bias or too much input from the buffer amp?

3. if there is a problem with a mismatched load on the output of the 2n3866, would that be solved by an l-network between load and collector?


Title: Re: Building my first oscillator
Post by: W4AMV on September 03, 2011, 04:27:35 PM
Just to be sure, upload your current schematic and where you are breaking the connection between the buffer and the 3866. Your buffer output alone looks quite good. So it would appear that it is incapable of properly driving the 3866. Either it cannot source or sink the required base current. As an experiment, you should investigate what impedance your buffer can drive with little or no distortion. AC couple via a 0.1 uF block and then decrease the load R from say 10 K downward.

To your questions,  Use you scope, I mean with the scope set for DC coupling observe the DC+AC signal and see if the voltage swings are such that the 3866 would create distortion with its current DC bias settings. Adjustments could be made first and easy, by placing the 3866 on its own supply voltage and adjust. If you have enough supplies you could place one on the collector and one on the base bias network.

On the second, to much drive from the BUFFER. And that may be a good piece of the issue. Try coupling the buffer to the 3866 base via a small C, say 500 - 1k ohm of series reactance at 3 MHz. 1 k is 53 pF, try a 47 pF series C.

On the third, YES. But for now that IS NOT the issue.


Title: Re: Building my first oscillator
Post by: W4AMV on September 03, 2011, 05:11:03 PM
incidentally, I might add, when we say lightly couple with the series C... If you look at the pdf I sent, you will see that as we make that series C small, the equivalent parallel R transformed from the series R INCREASEs. So you see, the load on the buffer can be "lightened" by either an increase in the series load R, or by a decrease in the series cap connecting that series R (the 3866 base) to that of the source of the buffer. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 03, 2011, 07:56:06 PM
i tried swapping out different load resistors onto the buffer output and noticed that around 1kohms the waveform started to get deformed. below 1000ohms the voltage dropped significantly the deformation gets pretty bad.

heres some images to show you what i mean:
10k: http://i.imgur.com/jyqwm.jpg
5k: http://i.imgur.com/ZONNa.jpg
1k: http://i.imgur.com/CrHgU.jpg
50: http://i.imgur.com/KWtxm.jpg


i got this brilliant idea to replace the 2n3866 voltage divider circuit with 15k pots. tweaking things around quite a bit i was able to get this one sweet spot where the waveform looked nice: http://i.imgur.com/n3yHV.jpg

to get a nice sinewave output i had it set like this:
base-ground : 330ohm
base-V+ : 467ohm
emitter-ground : 15ohm

here is the current schematic: http://i.imgur.com/9J430.png

with these voltage divider values i actually get the same result in ltspice as i do on the circuit board. well except that the spice model reports 5mA base current but in real life i am getting about 1mA


Title: Re: Building my first oscillator
Post by: W4AMV on September 03, 2011, 09:08:37 PM
The MPF102 is not a great FET. That is to say, its gm is quote low and therefore the output Z of the FET buffer is not very low. The output Z of a source follower is ~ 1/gm and so the outcome of your tests with the R loads makes sense. The Zout certainly quite high, say 1500 ohms or so. Very well. Now, your 3866 output eLL network is OFF target. You are essentially loading the collector with 50 ohms! A better value with 12 volts collector would be in the neighborhood of 200-500 ohms. Go design an eLL match now to transform 50 ohms to say 350 ohms at 3 MHz for your 3866 output network.  See how that plays.


Title: Re: Building my first oscillator
Post by: WU2D on September 06, 2011, 08:02:55 PM
I think that you are loading the FET a bit too much and this may be spoiling the waveform. Try adding 5 more parts; a simple emitter follower between the source and the base of the amplifier. Mike WU2D


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 07, 2011, 08:44:18 PM
oh boy the forum is back up. :)

i tried to calculate the values for an l-network using the formulas W4AMV supplied combined with some demonstrations from section 20-10 of the arrl handbook.

using a 14v power supply, Rs=50 and Rl=350 i got:

Q = 2.44 = sqrroot(350/50 - 1)
Xl = 122r = 50 * 2.44
Xc = 143r = 350/2.44
L = 6.47uH = 122/2pi3
C = 370pF = 1/2pifXc

i made the inductor using 5 turns on an ft37-43 toroid. this gave me 8.75uH but its class as i could get without going under according to this calculator: http://toroids.info/FT37-43.php

i had to use a combination of silver mica and ceramic capacitors to get to 370pF.

heres what the waveform looks like with this l-network:
http://i.imgur.com/AL9YA.jpg

it looks a lot better but it still doesnt look right. its more triangular now.

did i do the calculations right for this l-network?

i have a bunch of 22222 transistors so i can try adding an emitter follower between the 2n3866 and the buffer stage.


Title: Re: Building my first oscillator
Post by: W4AMV on September 07, 2011, 10:46:34 PM
Hi. First your scope shows a reasonable Pout, 1.27 V rms or 15 dBm. More than ok for your mixer.

Next, the series reactance to transform 50 to 350 ohms, a reasonable choice, is 122 ohms. You have that and that would be accomplished with a series C of 435 pF at 3 MHz. Lets assume the transform Q is large enough (i.e. GT 1), and so L can be obtained directly from the 122 ohm value, L at 3 MHz, is 6.47 uH.

So you are close. Check your math. I choose 3 MHz for Fo. You may be closer at 2.5 MHz. In any case, the SERIES C is 435 pF and the SHUNT L to the 3866 collector is 6.5 uH. If you build a low pass filter next, YOU ARE DONE. The current waveform is JUST FINE. However, if you are a purist, build a 3 section Butterworth filter with a cutoff frequency of say 4 MHz, you will get 18 dB/oct attenuation of the harmonics, it will look super.

Of-course, without going into the details, I hate to break the news, but a better waveform to drive the mixer is a SQUARE wave! Not to worry, what you have is just peachy. I would not waste time with another bipolar stage, but simply reduce the series C between the buffer source node and the base node of the 3866. Again, the real issue is the gm of the MPF102 is not very high, yes a bipolar would be a better follower and in combination with the FET (a BIFET darlington) wonderful. However, not worth the effort.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 07, 2011, 10:56:19 PM
very exciting! i have built low pass filters in the past and i have all the parts i need to make a low pass for the 80m.

currently the oscillator range is 2.5-3mhz. would it be possible to increase the range to 500khz - 4mhz? i would like to be able to listen to broadcast  band radio as well as 160m and 80m bands if possible.

i know that adjusting the range is a matter of geting the right size coil and balance of fixed and variable capacitance in series and parallel. however so far my experience has been either or a short range of frequencies with fine grained tuning or really wide range of frequencies with really course tuning. are there any tricks using multiple tuning capacitors to get course and fine grained tuning over a wide range of frequnencies?

edit: also i have a question regarding the stability of the oscillator. the reading on my oscilloscope jumps around a little bit. right now i see it fluctuating between 3.049 and 3.067MHz. i'm not sure if this fluctuation is caused by the oscillator being unstable or by the scope not being able to detect the frequencies very precisely. how can i check this? would a dedicated frequency counter be more accurate than using my oscilloscope?


Title: Re: Building my first oscillator
Post by: W4AMV on September 07, 2011, 11:56:46 PM
ok, 2.5 to 3 MHz, is a nice tuning range. More is possible, much-much more is also possible. However, as a general rule, as you increase the tune range, achieving frequency stability become more difficult. There are solutions, but that is a new thread! I would encourage you to finish this oscillator and you need to check its stability with a frequency counter. Ideally one with a stable time base, say 5 ppm would be nice. Your goal should be drift less than 500 Hz upon turn on and say after 10 minutes it would be nice to see less than 50 Hz. Trust me, that is not easy. You are going to have to package this unit with care. Isolate it from fluctuations in temp and mechanical vibration. At this point, get a freq counter, or if you have a short wave RX, tune it in on you Rx, see if it stays put. What FUN. Place it on your desk, lightly rap the desk with a pencil, can you hear the tapping in the Rx?!


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 08, 2011, 01:09:58 AM
i'm super excited to get this receiver operational. i adjusted the tank circuit to drop the range down to 1.3-1.8MHz so that i can use it for broadcast band listening and i recalculated the l-network for the lower frequency band. i'm gonna turn this into a sweet little am radio.

can i use any old diodes for the mixer or must they be schottky diodes? what specs should i look for in the diodes?

does it matter that the amplfication is not linear across the frequency settings? i get 1.5VRMS at 1.3MHz and 1VRMS at 1.8MHz


Title: Re: Building my first oscillator
Post by: W4AMV on September 08, 2011, 10:48:10 AM
i'm super excited to get this receiver operational. i adjusted the tank circuit to drop the range down to 1.3-1.8MHz so that i can use it for broadcast band listening and i recalculated the l-network for the lower frequency band. i'm gonna turn this into a sweet little am radio.

can i use any old diodes for the mixer or must they be schottky diodes? what specs should i look for in the diodes?

does it matter that the amplfication is not linear across the frequency settings? i get 1.5VRMS at 1.3MHz and 1VRMS at 1.8MHz

The diode barrier voltage or knee voltage as low as possible. So, for the Schottky 0.3 V vs. Silicon at 0.7. You can use Silicon, say 1n914 would be a nice choice and add adjustable bias to the ring diode to slightly overcome the barrier or turn on voltage. Then your LO voltage swing will do the rest. Try to match the diodes for forward currrent vs applied diode voltage. Not essential but nice. This type of mixer operation is so-called starved LO. If you need a circuit I can pencil sketch. However, it is not unlike the one you sent several posts prior, but you need to introduce an adjustable DC bias through the diode ring. NOTE. You could do FET or BIPOLAR active mixers as well. However, the ring diode balanced mixer is a classic. You should see conversion gain of -6 to -10 dB. Yes, this mixer is lossy.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 08, 2011, 05:26:16 PM
would i need a bias on the diodes if i am using schottkys or is that just for silicon diodes?

the only diodes i have currently are 1n270, 1n4001, and 1n4004. i might as well go with schottky diodes if i have to go buy new ones anyways.



Title: Re: Building my first oscillator
Post by: W4AMV on September 08, 2011, 06:10:05 PM
The diodes you mention are a bit to large for this application (to much capacitance), however at these low frequencies they may work just fine. You can try them out first in LTSpice as you should build the mixer in Spice to see its function. You can remove the bias stuff I added in the view 1 of this schematic. I ran the mixer and in view 2 you see the time waveforms. The inputs are 1.8 MHz, LO, 1 MHz RF, 800 kHz is the IF. The IF port signal shows the sum, the difference, and the and distortion all removed via a LPF (not added here). You can take a FFT snap shot of the IF signal see panel 3. The 800 kHz is the most prominent. Note how the xmfrs are built in Spice. I used 1n914 diodes.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 10, 2011, 02:44:43 AM
i ended up finding some really cheap schottky diodes at the electronics shop: http://www.fairchildsemi.com/ds/1N/1N5818.pdf
mine are the 1n5817 version.

i threw together a diode mixer on my breadboard. my triflar transformers have 11 windings and i used 104 capacitors in place of .4uF ones.

with an input into the mixer stage of 1.62V RMS at 1.6MHz i get 414mV RMS 1.36MHz at the IF output. is that the amount of voltage drop i should expect from the mixer stage?

is there anything i can do to make sure the mixer stage is working properly?

is the next step to add another amplifier and then an envelope detector?


edit:

1.62VRMS = 52.4mW = -12.8dB
.414VRMS = 3.4mW = -24.7dB

that makes it a -12dB gain from the mixer right? is that a little much attenuation? you said it would be around -6 to -10dB.


Title: Re: Building my first oscillator
Post by: W4AMV on September 10, 2011, 09:01:23 PM
hi, I am  a little confused. There should be 3 signals involved, the LO, RF, and IF. The conversion loss is the ratio of the RF level to the IF level. Can you clarify? Is the 1.62 V the RF signal?


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 12, 2011, 02:24:53 AM
i was under the impression that the RF input is just via a wire antenna? is that not correct?

when testing the circuit should i use a rf function generator into the RF Input?


Title: Re: Building my first oscillator
Post by: W4AMV on September 12, 2011, 05:28:28 PM
yes.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 12, 2011, 06:51:29 PM
ahh okay. so i'm not entirely sure what to be looking for here but here is a bunch of numbers:

LO Frequency: 1.5Mhz
VRMS at IF output with no RF Input: 340mV RMS

RF Input (function generator) Frequency: 2MHz
RF Input Voltage: 1VRMS

when i connect the RF Input to the mixer circuit the IF output voltage becomes 1VRMS and the wave looks like this:http://i.imgur.com/SO0v0.jpg

is my RF input voltage way too high? what should the if output from the mixer look like?


Title: Re: Building my first oscillator
Post by: W4AMV on September 12, 2011, 07:41:18 PM
Yes, back off the RF input and watch the IF port output. With no RF input, at the IF you are seeing the LO input blow by. The mixer is not perfectly balanced. If it were that LO signal would be null. Now a LPF would be handy, even an RC network might be fine. You need to suppress as much of the LO bleeding to the IF so you can see the IF with some clarity on your O'scope. The ratio of the IF level to the RF level will be the conversion gain (loss).


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 12, 2011, 09:23:45 PM
ahh okay. i just re-read arrl handbook section on DBMs. At this point my mixer is not balanced.

the arrl handbook says that "diode and transformer uniformity results in equal LO potentials at the center taps of T1 and T2. The LO potential at T1's secondary center tap is zero (ground); therefore the LO potential at the IF port is zero."

the handbook doesn't offer solutions to correct the balance.

are you saying that i can use a low pass filter somewhere to correct the balance? where is it supposed to be used? i'm not sure i understand what you are saying to do with the LPF.


Title: Re: Building my first oscillator
Post by: W4AMV on September 12, 2011, 09:53:28 PM
Hi. Add the LPF at the IF port of the mixer. Lets assume the ports of the mixer are 50 ohms. That is, our signal generator as well as antenna etc... are all 50 ohms. Then we just need to add a 50 ohm LPF at the IF port with the main purpose to pass the IF signal and reject the LO and RF signal. A simple Butterworth filter would be a good start. A 3-section LPF Butterworth has element values of 50, 100 and 50  ohms, Cshunt-Lseries, Cshunt.  Start there add more sections if required.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 01:40:49 AM
looking at the normalized value table for a 3 section butterworth i see g1 = 1 ; g2 = 2; g3 = 1
where do you get 50,100,50 from?

also, what should the cutoff point be set at for this filter? would 1mhz be sufficient?

with the normalized values from arrl handbook and assuming a 1mhz cutoff i get these values for my filter:

C1,2 = 3184pF
L = 15.9uH

i checked my numbers against this online calculator: http://www.pronine.ca/butlf1.htm
i seem to have done everything right but when i tested the circuit in LTSpice it isnt working right. i fed it the filter with a 1.5MHz sinewave at 1V and at the load resistor i still have a 200mV-Pk sinewave.

http://i.imgur.com/c0RB1.png

is there a way to feed the filter with noise so that when i look at the fft view i see the actual cutoff point?


EDIT:

i put the filter together on breadboard got fairly similar results. i had to round up to 7turns on a ft-37-43 torroid which gives 17.15uH and the closest i could match the capacitors was 3166pF using several mica caps in parallel. it starts to attenuate at about 1Mhz but there is still small amounts of signal until 2.5Mhz when it fully cuts off. Using a 1VRMS input i get 180mVRMS output at 1.5MHz.

is there another filter type that has a sharper cutoff? i tried a 3 section chebyshev with L,C values of 2000pF and 7.7uH but the cutoff was even less sharp. i thought chebyshev was supposed to have a sharper cutoff?

i put the filter onto the IF output and i now have a 55mV 1.5MHz output. http://i.imgur.com/YvlbY.jpg

will i see the intermediate frequency once i have completely filtered out the LO?


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 04:17:04 AM
Hi. Because the LO and RF are so close together it will be difficult to build a simple filter, i.e. Low Pass that will provide enough attenuation. A higher Q narrow bandwidth bandpass which is tunable would be one choice. Of course it would be nice to achieve better balance in the mixer. That requires care, matched diodes, etc... I would do the best you can, add a demodulator and recognize that the audio once stripped from the carrier will be easier to filter.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 05:30:06 AM
i was having a really hard time understanding the big picture of how a superhet receiver works but i think i am starting to get it. can you tell me if what i say here is accurate?

1. rf signal is picked up by an antenna, bandpass filtered for the desired frequency and fed into an rf amplifier.
2. the filter is variable via a ganged capacitor which is also controlling a local oscillator tuned typically 455KHz higher than the bandpass filter.
3. the LO signal and the RF signal are fed into a double balanced mixer which produces a new intermediate frequency (455Khz in this example) which is the difference between the LO and RF signals. this IF signal has the same modulation as the original RF signal.
4. the IF signal is always the same frequency (455khz for example) because the difference between the RF signal and the LO is always the same due to them operating on a ganged capacitor.
5. carefully tuned filters and amplifiers can be applied to the IF signal because it is never changes.
6. an envelope follower is used to finally demodulate the IF signal and produce an audio frequency signal.

i didn't mention imaging or any other details but is this essentially how a superhet works?


EDIT:

i did some more post mixer filtering attempts. this time i made a 3section butterworth lowpass at 455KHz. i tuned my LO to 1800KHz and the RF signal to 1345KHz. the output from the filter is a 1800KHz signal at 58mVRMS. perhaps my mixer is not setup correctly and is not producing an IF frequency?

EDIT2: well i just went through the wiring of my mixer really carefully and everything seems to be in order. it is just like this schematic: http://www.qrp.pops.net/images/2008/2008-larger/big_dbm.gif


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 08:03:57 AM
yes, you have it correct. I suspect the mixer is working fine, but having difficulty seeing the IF. Try increasing the LO level if possible. Clearly, with the LO removed and the RF only present, you should see no IF. Now as you add the RF to the mixer you should see at the IF port, at the very least, the LO signal modifying the RF. That composite signal will have both the SUM and DIFFERENCE frequency present. What would be helpful, and this is a good experiment, set the LO frequency AT THE SAME or NEARLY the SAME as the RF Frequency. The DIFFERENCE frequency is DC. You are now seeing the action of the mixer operating as three different functional circuits.
1. RF MIXER
2. PRODUCT DETECTOR
3. PHASE DETECTOR

As you pass the LO frequency pass the RF frequency you will see the IF cross from DC, a beat note, to a higher frequency.

A 455 kHz ceramic filter is inexpensive and building a simple IF amplifier maybe your next best project. For now though you should be able to get the mixer functional and understand its operation. If I have time I will post a BB diode ring mixer and its waveforms at your frequency and what you should expect to see.


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 08:07:55 AM
You did not mention the trifilar transformers... the material used to construct... at these low frequencies to get a working trifilar xmfr is going to require a pretty high permeability, say 850-2200 or higher. With only a few turns of trifilar wire, you will be hard pressed to achieve suitable inductive reactance relative to 50 ohms. The mixer will be very lossy.


Title: Re: Building my first oscillator
Post by: DMOD on September 13, 2011, 02:08:04 PM
Quote
i got this brilliant idea to replace the 2n3866 voltage divider circuit with 15k pots.

My experiences with the 2N3866, 2N5109, 2N3553, or NTE 278 or NTE 311, or NTE 473 buffers/voltage amps and linearity at 12-15 V ARE:

I use a base bias of approx. 2.5 mA composed of a bypassed 3.3K (.05 uFd) from the collector, then go to a 560 ohm to base; and then a 1k from base to ground. This way, you get both AC and DC feedback for stability and improved linearity. Any lower values of base voltage divider resistances and the base impedance gets so low it will load down the previous stage.

I also prefer a 40-50 mA collector current, so I use a 33-39 ohm resistor in the emitter. And use a top hat heat sink.

In addition, if you isolate the buffer/voltage amp and it tends to want to oscillate at VHF (only had this situation with the 2N5109), connect a series circuit from the collector to ground consisting of a 470 ohm 1/4w watt CC resistor in series with a 15 pf cap.

I know you have a lot of information to digest, but keep up the experimentation.  You will learn a lot from simulating and then building. Kinda like OJT.  :D

Phil


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 03:08:48 PM
Good inputs Phil. Thanks. This form of DC bias is improved over the standard 4-R bias that I believe he is currently using. The AC feedback incorporated into the DC bias system is also a good thing. It would be useful for him to place this configuration into Spice and see the differences between the circuits, both AC and DC.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 04:59:04 PM
as far as the transformer goes: i used FT-37-43 torroids with 11turns on of #28 enamel wire as recomended here: http://www.qrp.pops.net/xmfr.asp



i just checked the diodes on my DMM and replaced one that wasn't matched up very well. i think this made a huge difference.

here is an image with RF=1.8MHz and LO=1.8MHz
http://i.imgur.com/i9JoQ.jpg
here is an image with RF=1.0MHz and LO=1.8MHz
http://i.imgur.com/eYjyP.jpg

is this second image giving me a hint of the sum and difference signals?

thanks DMOD for the biasing tips. i will definetely try your method of biasing once i work through this mixer stuff.


Title: Re: Building my first oscillator
Post by: DMOD on September 13, 2011, 05:14:38 PM
Right out of the mixer you should get 2.8 Mhz and 0.800 Mhz (or 800  Khz), the additive and subtractive frequencies.

So you have two possible IF frequencies from which to select and filter.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 05:28:08 PM
i just increased the voltage of my RF signal and now i have this waveform: http://i.imgur.com/NKFdx.jpg
in that image i have taken the mxier output and ran it through a 3section 1mhz butterworth lowpass.

i moved the cursor manually to two the zero crossing points of the outside edges of the signal and its about 800KHz.

is this image showing the mixing that i am looking for? does it just need to be better filtered? would a bandpass filter set around my desired IF frequency get me the the difference signal i am looking for?


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 05:42:24 PM
From the images, yes, I would say the mixer is functioning. You are just going to need more filtering or as you suggest, a BPF. However again a simple experiment to confirm. Fix the LO frequency. Very carefully walk the RF frequency up and down about the LO frequency. By carefully, I mean if you have the capability say every 10 Hz steps! Oh, it can be more than that, but what you are looking for is a clear BEATING of the 2- frequencies. It will be obvious to you when you see the heterodyning, that the mixer is functioning.  :)


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 06:46:00 PM
i'm using a HP 3312A function generator which only goes up to 13MHz. On the mhz range setting you really dont get much fine tuned control but here is a video of me sweeping the RF oscillator from 0.5mhz to 3mhz and back to 0.5mHz: http://www.youtube.com/watch?v=W4fbKeGuH18



Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 08:48:31 PM
It is not as clear as I had hoped. I believe it is due to your inability to really fine tune control your signal and this is further aggravated by O'scope triggering issues. I suggest you set the LO and RF near each other, say 100 kHz or so difference, LO at 1.8 MHz, RF at 1.7 MHz, then a nice R-C and/or  LC filter following should permit the 100 kHz signal to stand out and the sum signal should be nil.


Title: Re: Building my first oscillator
Post by: W4AMV on September 13, 2011, 11:05:22 PM
Here is a well behaved DBM. The generators are quite clean and settable in 1 Hz steps. Actually, in hindsight, this is not an easy waveform to capture in the time domain, this old scope must be set carefully to trigger so the intensified difference frequency which is clearly seen. The larger sinusoid signals are the LO and RF both set to -6 dBm. That is SMALL signal. Perhaps that is one of your issues, namely distortion. You might consider reducing your signal levels and also LP your signal sources. If they have significant harmonics you will see all the inter mix products. That is really going to cause quite a visual mess! 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 13, 2011, 11:37:08 PM
oh wow that is really helpful. at least now i know what i am trying to achieve. :)

i think i found the difference frequency! i put a 455khz LPF on the mixer output and fed it with a 1.6MHz LO and 1.7MHz RF. i had to crank up the voltage on the RF input to 2.8VRMS but i was able to find a 100KHz signal in the filtered mixer output.

i've attached images of the filtered and unfiltered IF output. (the webhosting ih ave been using is down right this second).

my unfiltered output does not look the same as yours. why is that?


Title: Re: Building my first oscillator
Post by: W4AMV on September 14, 2011, 05:17:40 AM
Your mixer appears to be functioning fine. Although the conversion loss may be higher than normal and the isolation less than desirable. How did you construct this mixer? Even at 1 MHz, it is imperative that good RF layout and construction practice be used. I'll take a pix of a homemade ring diode DBM and post. To do the mixer justice it would be nice to have a spectrum analyzer. Then you can measure accurately conversion loss and isolation as well as mixer balance. I suspect the first main difference between your screen shot and mine, is my difference frequency was much lower, if I recall 5 kHz! These generators are locked to a stable clock,so their is no wandering in frequency which aggravates the scope synch and finally how clean are your sources. If their is significant harmonic content that will further mess with the picture. I assume you are delivering both signals RF and LO from commercial generators? Balance and LO/RF leakage to the IF is also key. Ideally, when the LO or the RF is TURNED off, there is ZERO if output?


Title: Re: Building my first oscillator
Post by: W4AMV on September 14, 2011, 05:10:26 PM
Hi, I went back and looked at your video again. Indeed you can see ZERO beat quite well.  :) Watch the last 5 seconds or so towards the end of the video. You can see a sinusoid whose amplitude DISAPPEARS to zero. Then re-builds. That is the difference frequency which is going to DC.  :D


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 14, 2011, 06:31:55 PM
okay now i am really confused. i was looking the mixer last night and i noticed that one of the ring was not assembled properly. it was really 3 diodes in a ring with the 4th one's cathode not connected to anything. this is one of the many problems with using breadboard.

so i fixed the diode ring but now i cant for the life of me find the difference frequency.

to give you a clear picture of what i am doing here is a series of photos which show exactly what kind of signals i am inputting and what i am getting at the IF:

RF Input (from an HP 3312A functon generator):
http://i.imgur.com/wUwpZ.jpg

LO Input (from my homemade oscillator):
http://i.imgur.com/StVn0.jpg

unfiltered IF output:
http://i.imgur.com/rgald.jpg

IF output with a 455KHz butterworth filter:
http://i.imgur.com/5L05j.jpg

heres some questions:

1. what voltage should i set my RF and LO signals to? should they be around the same voltage or should one be higher?

2. my mixer is on the same piece of breadboard as my oscillator and has tons of jumper cables. could this be a part of the problem?

3. my mixer is exactly like this: http://www.qrp.pops.net/images/2008/2008-larger/big_dbm.gif the schematic you posted a while ago has capacitors and a few chokes. what are those doing and should i be using them?

i feel like i am really close to making this thing work but i am running against a couple of different issues none of which is enough to break the circuit but in combination they make it unreliable and lossy.

just for fun i tried reconnecting it as a modulator with audio in the IF port and the carrier on the RF port and i was able to hear audio on a radio receiver. it was super noisy and undermodulated but it did work.


Title: Re: Building my first oscillator
Post by: W4AMV on September 14, 2011, 08:42:14 PM
Hi, well let me keep it brief. I'll try to get to your question in a latter post. Although you had the ring 3/4 correct, it still performed albeit not very well. Find attached a set of pix on construction, one is homebrew ( a knockoff of the one on the right) and one commercial, on the right. Emphasis is on construction technique. Can't over emphasize how key that is to performance. Please note, the HP mixer on the right has two additional xmfrs and will leave them out of the discussion for now. Note the tidy connections and minimum crossovers of connections. Hope this helps.


Title: Re: Building my first oscillator
Post by: W4AMV on September 14, 2011, 09:54:26 PM
okay now i am really confused. i was looking the mixer last night and i noticed that one of the ring was not assembled properly. it was really 3 diodes in a ring with the 4th one's cathode not connected to anything. this is one of the many problems with using breadboard.

so i fixed the diode ring but now i cant for the life of me find the difference frequency.

to give you a clear picture of what i am doing here is a series of photos which show exactly what kind of signals i am inputting and what i am getting at the IF:

RF Input (from an HP 3312A functon generator):
http://i.imgur.com/wUwpZ.jpg

LO Input (from my homemade oscillator):
http://i.imgur.com/StVn0.jpg

unfiltered IF output:
http://i.imgur.com/rgald.jpg

IF output with a 455KHz butterworth filter:
http://i.imgur.com/5L05j.jpg

heres some questions:

1. what voltage should i set my RF and LO signals to? should they be around the same voltage or should one be higher?

2. my mixer is on the same piece of breadboard as my oscillator and has tons of jumper cables. could this be a part of the problem?

3. my mixer is exactly like this: http://www.qrp.pops.net/images/2008/2008-larger/big_dbm.gif the schematic you posted a while ago has capacitors and a few chokes. what are those doing and should i be using them?

i feel like i am really close to making this thing work but i am running against a couple of different issues none of which is enough to break the circuit but in combination they make it unreliable and lossy.

just for fun i tried reconnecting it as a modulator with audio in the IF port and the carrier on the RF port and i was able to hear audio on a radio receiver. it was super noisy and undermodulated but it did work.

Based on your input signals, which look quite good, your IF would be 200 kHz. So your LP is really to broad. Also, your RF input level is much to large. Attenuate it. I would vary its level until you begin to see some occurence of an IF at 200 KHz. Design an RC low pass with a cutoff of say 100 kHz to suppress the LO and RF signals. Again, if your layout of all this is not clean, you are going to get cross talk and intermixing of signals that will really confuse the result.  Mixer balance is quite helpful and its not clear that your balance (due to layout?) is very good. Yes, osc on same card with tons of cables, UGH, sounds awful. Forget about the mixer design I posted, it was for starved LO, and that is not your issue. Again, clean up your layout, reduce your RF level, reduce your cutoff freq for the current IF and try again.


Title: Re: Building my first oscillator
Post by: W4AMV on September 14, 2011, 10:10:42 PM
ah, a light bulb moment! Since your isolation between LO and RF may not be very good and your buffer for the oscillator not up to par (an assumption), you are aggravating another electrical problem called injection lock. I will spare the details, but do look at your oscillator (via your O'scope) when connected to the mixer. Your oscillator can be moved or pulled in frequency by the RF signal, so reducing the RF level is imperative. This would explain why the sources look fine without connection to the mixe, but there is NO IF to be found! 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 14, 2011, 11:03:46 PM
i rebuilt the mixer on copper clad board in an attempt to clean things up: http://i.imgur.com/xjiSS.jpg
i still have jumper wires but theres way less of them.

heres a difference frequency i was able to isolate: http://i.imgur.com/djiiS.jpg
LO was 1.6VRMS and RF was 3.5VRMS. if i drop the RF voltage then i cant find it anymore.

i can reliably find a difference frequency of whatever value but the signal always looks weird like in that last image and i have to keep the rf voltage cranked up really high.

now that i have rebuilt the circuit on copper clad board, what else can i do to try to achieve a balanced signal?



when the RF osc is connected, the LO output has a similar sort of 'modulated' look to it as the IF output. is that maybe the injection lock you are talking about?

edit:
it is really hard to capture this in photographs but are these bright spots in the unfiltered mixer IF output the difference frequencies? http://i.imgur.com/1CANL.jpg
here is a poorly highlighted version of what i mean: http://i.imgur.com/5K1Iq.jpg

the bright spots expand and contract as i adjust the RF frequency around the LO frequency and seem to correspond to the difference between the RF and LO frequencies.


Title: Re: Building my first oscillator
Post by: W4AMV on September 15, 2011, 05:07:20 AM
Hi, Gasp :o. I looked at your layout. Alligator clips! Fine for DC. Wires! Fine for DC. Patch boards, good for...well I sold all of mine and gave them away. Also, it is not clear in pix, but it looks like only half the ring is connected? Unless the connect is via the copper, cannot tell. Let me see if I can pix an RF breadboard built with Xacto blade knife and/or Demel tool...(moto-drill).


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 15, 2011, 06:08:48 AM
the transformer on the right side has insulated wire attached as jumpers for part of the secondary coil whereas i used scrap component leads as jumpers on the left transformer. the component leads done show up in photos from my junky cellphone camera.

i want to build the oscillator on copperclad board but i am hesitant to solder the parts down while i am still adjusting and tweaking things constantly. it would be nice to have some kind of modular system where each stage (oscillator with a buffer and amp to get to a standard dB gain, mixer unit, various rf amps with standardized gains, demodulator,etc) was in its own enclosure and i used coaxial cable to patch everything together. would that be a good system?


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 15, 2011, 06:59:34 AM
i'm having a lot of problems with my filters. i can't seem to suppress the LO and RF signals. i've tried various cutoff points, different torroids (37-43 and t68-1) and rc filters. they all drop the LO and RF signals down to around 100-300mV but no filters get rid of them entirely. i think i am seeing the difference frequencies in there but i cant isolate them.



Title: Re: Building my first oscillator
Post by: W4AMV on September 15, 2011, 04:43:40 PM
http://www.dartmouth.edu/~sullivan/prototyping.pdf

HI. This is a good site to look over. Again, if I have time I will post pix of some cards built. You can build sub modules  like the mixer, buffer, osc,, etc... and link them via coax cable, the mini 50 ohm, RG 174 or its varations. Next time you are at the Ham Fest, pick up epoxy-glass PC panels, FR-4, 1 oz copper 32 or 64 mil card, double sided and/or single sided. You can build boxes and circuits literally in 3-D, so they become compact amd minimize all the problems you currently face.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 15, 2011, 08:45:06 PM
i would really appreciate photos of your construction technique.

do you think it might be possible that the ft37-43 is the wrong toroid for my application?
http://toroids.info/FT37-43.php

i'm not sure if these specs are relevant but toroids.info says:
Quote
Application Freq Range
Wideband Transformers 5 - 400 MHz
Power Transformers 0.5 - 30 MHz
RFI Suppression 5 - 500 MHz

would the fact that the wideband transformer min frequency is 5mhz mean that this toroid is unsuitable for a 1-2mhz transformer?

also, is the transformer's resonant frequency relevant to this application?
i tried checking the resonant frequency of the trifilar transformers by running my function generator through the primary winding and connecting my scope to the secondary winding. the resonant frequency appears to be above 12mhz (the upper limit of my function generator). the rms voltage was 3.5VRMS at 2mhz and 6VRMS at 12MHz.


Title: Re: Building my first oscillator
Post by: W4AMV on September 15, 2011, 09:22:43 PM
No, 43 material which has a permeability of 850 is fine. What is useful to check is the total start-to-finish inductance of the secondary, which has the center tapped connection. The inductance of this part of the winding should have a reactance of 500 ohms at the lowest application frequency, say at 2 MHz. 39 uH would be nice. If say a third of that, 12 or so uH, still ok. The intent, try to minimize the mixer loss. I really think your main issue is construction technique and stray coupling and significant un wanted radiation. Coax interconnect is great solution, and connectors are not required, just use the coax cable, properly dressed for short run inter connect.   


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 15, 2011, 10:59:27 PM
you are right. my mess of wires was confusing everything. i just ripped apart all my alligator clips and rebuilt them using coaxial. this simplified things massively and i am now able to get a clear difference signal if i filter the IF output.

here is the mixer with coaxial alligator clip connections:
http://i.imgur.com/bK1Y5.jpg
the oscillator and if filter are still on breadboard. i plan to put them on copper and put each component into its own little enclosure with so-239 connectors in the near future.

here is the difference frequency:
http://i.imgur.com/qAxIR.jpg

i am able to turn the RF input down to under 1vRMS and still have a clear signal.

the mixer still is not balanced but at least i am getting a clear difference frequency. i assume that a tuned IF amp will further cleanup the signal, is that correct?


Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 03:51:12 AM
YEAH :D The AL on your toroid is 350, then the calculated L is 42 uH. So no issue here. SO-239 are ok, a bit huge. If you are an audio guy, you can use the ol'  RCA-phono plug and jack. They are easy to solder to coax, make a nice secure connection between male-female, and guess what, they are nearly 50 ohm characteristic impedance.

I measured my homebrew mixer last night, it works very nice with loss at about 6 dB. The IF is easy to see and extract, its balance is AWFUL. That is to say, if you turn either LO or RF off and look at the IF port, the LO or RF is blowing through. That is not an issue, your LPF will take care of that. Of course without any IF filter, you would be in trouble, since looking for the small IF signal in competition with the LO blow by, will saturate your IF amplifier. Thus the need for the IF filter.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 16, 2011, 04:28:46 AM
yeah maybe so-239 is a bit overkill for benchtop gear. i rca would be easy but i want to stick to a connector that is more standard for rf gear so that i dont need to use quite so many adapters. maybe bnc would be a good idea?

i was looking around for info on IF amps in the handbook and was disappointed to find very little information. i searched on the web and it sounds like they are normal transistor amp stages with an LC circuit tuned to the IF frequency on the collector. is that correct?

edit: DMOD, is this the bias circuit you were describing the other day? http://i.imgur.com/lDlx1.png

edit2: just for fun i put together the mixer in ltspice and attached it to the spice mode of my oscillator. in spice the mixer is well balanced. i am going to try rebuilding it with shorter leads. http://i.imgur.com/AbGjf.png http://i.imgur.com/DeekF.png


Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 05:03:54 PM
Very good. To your Spice entry, add another port and attach your LPF here, and port out the IF signal in a clean manner.

On the IF strip, most folks buy a commercial IC. There are a number of old great ones, like the MC1590, but there are very excellent new ones from several companies. In fact, todays operational amplfiers have significant gain-bandwidth and could be used in an IF. You certainly could build your own with bipolar, MOSFET, or other semi-conductor choices. The major design challenge there is achieving stable gain, a gain value of at least 90 dB, a reasonable noise figure, AGC, and a good grasp of impedance matching. The filters used in the IF strip will have to properly matched if you are to preserve their frequency response and selectivity.

Now you can get by with alot less, say 20-40 dB of gain, no AGC, and some simple ceramic filters, centered at 455 kHz, with bandwidths of 6 or so kHz for AM. The remaining gain can be achieved at audio after demodulation. I would poke around the library, the ARRL handbook and other texts. There is plenty of reference material available.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 16, 2011, 06:50:35 PM
the filtered IF output in spice looks really nice. in fact it is almost incomparably nicer than my real life results. my mixer has too much loss.

http://i.imgur.com/i9qds.png
http://i.imgur.com/s5oDJ.png

i bought some ferric chloride and am in the process of etching a very simple pcb for the mixer. i used a sharpie to draw the circuit layout. hopefully this will improve my real life results.


Do you recommend the MC1590 or do you think i should try something more modern?


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 16, 2011, 08:27:05 PM
here is my new mixer: http://i.imgur.com/IDN4W.jpg
there is a missing jumper in that photo. i added it after i took the picture but before i tested the circuit.

i still do not get a balanced output but there is slightly less loss: http://i.imgur.com/xmmcn.jpg

is this the best i can hope for? LTSpice still gives way better results. :(


Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 09:17:30 PM
I agree, you should be able to do much better. Again, I looked at the HB mixer that I posted, the IF signal and conversion loss is in line with a quality commercial mixer. Yes, the balance is not great, but that is not due to layout. It is due to the nature of that particular circuit design. There are modern IC's, the older Motorola IC's work fine, but devices from Analog devices and TI are extraordinary leap forward. Switch to coaxial interconnect and minimize the use of wire jumpers. Here are some pix of the technique I use. The amplifier and filter card are all built on FR-4, dremel tool and took about 2 hours to complete.

Here is a hint to construction. Draw your schematic out in a manner this is electrically proper to minimize stray coupling, parasitic C and L, and just flows properly. Use the schematic as a guide to laying out the card. With your parts in hand, transfer the parts from the schematic to the card, pencil outline their position on the card and trace the part outline. Then with the dremel tool, begin cutting your circuit pads. You will see the filter card and the power FETS are sitting on dremel cut outs, all point to point wire connection and you can build your circuits in 3-dimensions! Yes, you have length, width and height available. I'll post a converter module built in that manner.


Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 09:24:05 PM
Here is a converter module, and a Tx exciter, ALL built with the same technique. There is not one PCB in the lot. All cut with either knife or dremel tool. Once you get the hang of it, you can think a schematic in your head and transfer to the PC card in one motion.


Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 09:27:59 PM
oops here is the 3-D converter card. A clever idea for a VFO is also shown, this one from Rick Campbell who writes for QST and QEX. A clever application for a SOUP CAN!



Title: Re: Building my first oscillator
Post by: W4AMV on September 16, 2011, 09:35:03 PM
boy, sorry, here is the converter which shows the stacking of modules. The whole assembly slides into a JPL extruded case that cost Uncle SAM a fortune from the Apollo days!!


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 16, 2011, 10:34:14 PM
wow really nice circuits. so the advantage to your dremel method is that you can put all the wiring in 3d space and have a giant groundplane? i noticed you have baluns on all your jumper cables. is that to prevent them from radiating and causing intereference? will coaxial with grounded shielding achieve result?

drawing the circuit with a sharpie and then etching was really easy for me and i would like to keep working making my circuits in that way. i'm worried that i will be sloppy with a dremel.


i looked at my ltspice model again and i realized that i added the mixer to the wrong file. the file i used didn't have an amp stage on the LO and i forgot to include a load.

here is a corrected spice model: http://i.imgur.com/uuhUZ.png

i think i actually get comparable if voltage. the main difference seems to be in the filtering. my butterworth filter doesn't clean up the signal nearly as much as in LTSpice. how do i go from a composite wave to a clean sinewave like in ltspice?

edit: i put the IF filter together on copper with very short leads. the board is fused onto the mixer's board. it looks goofy but it enabled me to get rid of one coaxial connector. i get a clearer difference frequency and am able to detect it with the RF signal as low as 130mVRMS. http://i.imgur.com/bYf9H.jpg

now i am curious to see what kind of improvements i will see if i get my oscillator off of the breadboard and onto copper. i am hesitant to do this until i know exactly how i want to setup my bias the 2n3866 and how i want to setup the tuning on the colpitts oscillator.


Title: Re: Building my first oscillator
Post by: W4AMV on September 17, 2011, 05:23:09 AM
Hi. Early AM! Good questions. First, I suspect that if you drove your mixer with a set of perfect sig gens it would look quite nice. I still suspect cross talk and some level of injection locking or pulling of your osc is present. Also, NOT terminating the mixer IF port into 50 ohms, put terminating in a larger impedance, will provide a larger output IF voltage. Now their are other issues that occur if you do not terminate into 50 ohms, however for your purposes this is not a severe issue. So, that said, you might at least look at your mixer IF output by connecting it directly to the O'scope through a short (3 foot) coax cable. No need to use the 10:1 scope probe divider and/or terminating the IF into 50 ohms.

The filter performance will be significantly hampered with poor layout and construction, you are only using a 3 section, 18 dB/octave rate of roll off beyond the cutoff point. So you can't afford fouling up the construction. The Spice simulator has no stray coupling. Its perfect! You can add in electrostatic and magnetic coupling, a lot of work and not needed.  If you can sweep your filter on its own, do that, use your scope and sig gen and measure the cut off freq, -3 dB down point and roll off, make sure the filter is functioning.

The single 1/2 turn wire through the toroid or bead is a simple RF choke. It provides a broadband reactive impedance that helps isolate two nodes. In many cases, an RF node from a DC node. To a limited value it provides some shielding but not nearly as well as coax. However, coax would not provide the required impedance disconnect between nodes, EXCEPT under certain conditions.

The main thing about these ferrite beads is their lack of low frequency resonance. They are lossy RF paths (providing several hundred ohms) of series reactance with a small DC component of resistance. You could use "RF chokes" so called lossy inductor, or just a good quality inductor to accomplish the same job. Just be careful about their self resonant point! In any case, the bead is easy to use. Don't put them in unless you find a need, they are easy to place,  cut a trace and run a pig tail wire through. They can introduce their own set of problems, so experiment first.

Practice with the dremel moto drill. It is faster, but your technique is fine. Grab some scrap PC card material and try cutting some simple rectangles, then fancy islands. The small grinder tipped dremel bits are great and they are inexpensive.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 17, 2011, 07:03:55 PM
dropping the 50ohm load increased the voltage (as expected). at higher voltage the composite wave dissipears and the if frequency looks much smoother it is still not perfect but is a lot better. this is a good sign.

i was looking again at the handbook. would replacing the butterworth filter with a diplexer on the IF output improve my IF signal?

would one of these work as part of a diplexer? http://www.kitsandparts.com/455KHz.htm

maybe before i start working on the IF stage i should try to beef up my oscillator. is injection locking a symptom of an inadequate buffer stage? what could i do to improve that?




Title: Re: Building my first oscillator
Post by: W4AMV on September 17, 2011, 10:43:51 PM
The diplexer is a network that provides a constant resistive termination, typically 50 ohms. A bandpass diplexer is an appropriate approach, although a low pass is fine as well. The purpose of this network is to minimize the INCREASE in the IF termination impedance for the mixer, which will occur if a FILTER is used, for example a xtal filter. The problem that occurs, is the voltage gain (as you see) of the mixer rises off channel (i.e. from the center frequency of the filter) and the distortion products of the mixer will increase as well. That said, don't worry about it. It is not your current issue. Yes, adding more buffer, another stage would help. A pad for example, a 6 dB pad, if you can afford some loss, would help. If you scope your oscillator when not connected to the mixer and then connected to the mixer, say at the oscillator source terminal of the FET, do you see the signal change in any manner? I'll take a pix of my homebrew mixer which has no IF filter and the IF goes directly into a O'scope via 3 feet of coax for reference. As I recall, the IF frequency is quite easy to detect, as well I will RC lowpass the IF and scope that as well.


Title: Re: Building my first oscillator
Post by: W4AMV on September 17, 2011, 10:46:55 PM
meant to say measure the osc NOT connected to the mixer, then connected, and note any difference in the oscillator waveform as measured at the source terminal of the FET osc.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 19, 2011, 04:13:42 PM
when measuring directly off the oscillator FET i get 5VRMS with it disconnected from the mixer. connected to the mixer with the RF osc lturned on, i get a 4.2VRMS. in both cases the waveform was a nice sincewave.

a 6db attenuator after the LO lowpassfilter fixed the voltage drop on the osc. i used this calculator to make the attenuator: http://chemandy.com/calculatorsmatching-pi-attenuator-calculator.htm

when using the attenuator i cant find the beat frequency when running the mixer into a 50ohm dummy load. do i need more amplification of the LO to makeup for the attenuator?

i also noticed that at the LO filter output the signal is 70-80khz higher than when measuring directly off the LO FET. this is when disconnected from the mixer. why does this happen and is it a problem?


Title: Re: Building my first oscillator
Post by: W4AMV on September 19, 2011, 08:51:17 PM
You have more than enough LO even with the attenuator to permit mixing action. I assume you are looking at the IF (beat) after a LPF. The shift in LO frequency is minor and is due to the difference in load impedance presented to the oscillator, in one case your LO filter vs. no filter. Again, be sure your IF is below your LPF cutoff, check your LO and RF frequencies to be sure. I ran my mixer posted earlier with just 0 to -10 dBm LO, while the RF is at -20 dBm. The IF out and IF filtered out is very clearly seen. The conditions were LO at 2 MHz, RF at 1.5 MHz, the IF at 0.5 MHz. You can clearly see the RF and LO in pix below, and if you look carefully, the IF. However, through the IF filter, it is very clear. The IF filter used ~6500 pF and ~32 uH L. I'll post next.   


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 19, 2011, 09:23:56 PM
my filter is essentially the same as yours: 6300pF caps and 32.3uH (53 turns on a T68-1) toroid. i set my function generator to -20dBm and my LO to 0dBm and hooked them to the mixer. i used a potentiometer in series to reduce the LO down to 0dBm, could that cause problems? i also measured the dBm using a 50ohm load, is that correct?

i get out of the mixer's LPF a 5mV signal that the oscillator has a really hard time reading the freqencyon. it appears to be in the 100s of MHz.  ???


i am really tempted to order one of these $8 diode mixer kits: http://kitsandparts.com/drm.php
i want to use a homemade one in my receiver but i would like to have a working mixer that i can compare my homemade stuff too.


Title: Re: Building my first oscillator
Post by: W4AMV on September 19, 2011, 09:46:38 PM
First, here is the pix's. I am beginning to suspect there is something amiss in your mixer. From the screen shots of your oscillator, it looks fine. The mixer is behaving now as though you have a considerable amount of conversion loss. Make sure your interconnect is clean. In may case, I just bolt the connectors (BNC) from the mixer card to the filter card. I BNC TEE off the IF direct from the mixer and then via the LPF and these go to the 2-channel inputs of the scope. No probes, just coax to the vertical channel inputs of the scope.


Title: Re: Building my first oscillator
Post by: W4AMV on September 19, 2011, 09:52:48 PM
The scope horizontal is 0.5 usec / division, so the IF is 500 kHz, 4 divisions. If you look carefully the RF and LO are seen. I amplitude modulated the RF signal with low level AM and it helps (sort of) mark the RF signal (a bit of a blur from the modulation), while the LO is not modulated. Again, the sources are sig gens, so they are decent sources. However,  so are yours,  unless something is significantly upsetting your LO, and we I don't see that issue.


Title: Re: Building my first oscillator
Post by: W4AMV on September 19, 2011, 09:57:28 PM
Yes, get a commercial mixer. Or, if you want, mail me yours, I would be glad to check it out. The problem could be as simple as your diode selection. You said that are hot carrier. It would be useful to measure one. See if the forward drop on your VOM is indeed 0.3 and not 0.7 V. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 19, 2011, 10:14:40 PM
oh wow that would be really amazing if you could checkout my mixer.

i dont have any bnc connectors so i cant set everything up as you have it but tomorrow i will go to the shop and some up. if the connectors dont help then it would be great to send you the device. can you private message me your address for shipping?

i'll also order a ring modulator kit online for comparison. the one i am looking at is really cheap and it has an included diplexer. to make comparisons to my mixer could i just bypass the diplexer? http://kitsandparts.com/drm.php


edit:

i think i blew out an mpf102 messing around with a clapp oscillator. http://i.imgur.com/V0YQ7.png
at first it was working fine and then it would only work if i turned the power supply off and on and then it stopped working. i tried putting the transistor back into the oscillator design you helped me with and it didnt work.

while it was working i was tried various values for C2 and i noticed that i needed at least 150pF to get it to oscillate. in ltspice increasing C2 that causes an increased spike in gate current when the oscillator is first turned on. do you think that is what burnt out the transistor?

now that i need a new transistor, would you recommend i go back to radioshack for another mpf102 or is there a better option i can order online? it would be nice to have a "go to" stable transistor i can use in a variety of HF situations.


Title: Re: Building my first oscillator
Post by: W4AMV on September 20, 2011, 05:05:22 AM
Yes, you can bypass the diplexer. Just need the mixer, that is 4 diodes and the set of xmfrs. On the FET, I have taken out a MPF102 many a time, it is a small geometry device, not a very big FET. Excessive gate current in forward bias mode will take it out. A good GP FET is the J309 or the J310. Either in plastic or metal case. If you want to mail, just look up my ham call on QRZ.COM. I have BNC etc... no need to buy them.


Title: Re: Building my first oscillator
Post by: W4AMV on September 20, 2011, 05:10:50 AM
The W8DIZ kit is fine. A bit more than you need. You might look into mixers from Mini Circuits Labs, not sure what their one up price is, probably less than the postage to mail!


Title: Re: Building my first oscillator
Post by: W4AMV on September 20, 2011, 05:27:07 AM
http://www.minicircuits.com/products/fm_pic_level_7.shtml

The SBL-1+ is 8 bucks. Same price, but just 1 part. The folks at MCL are very open to assisting experimenters, hams, etc... You might give them a call, tell that what you are up to and see if they would sample you one unit. You pay postage. It does not cost to ask.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 20, 2011, 11:25:07 AM
thanks for the tip on getting samples from mini-circuits. i'll give them a call and try to get a test unit but i like the idea of the w8diz kit because it is discrete components. i am imagining it will be a better comparison to show what is possible with a home made kit. i also ordered the IF Amp kit from w8diz at the same time which is convenient.

the 9310 and 9309 FETs are super cheap. i'm gonna buy a big bag to keep lying around. i'll be ordering them from mouser. any other transistors or little discrete components you would recommend i order from mouser at the same time? i have a bunch of 2n3866 which i guess are good for amplifiers under 1W but it would be cool to have some sort of transistor i can use for QRP amps up to 5W.

what are these style connectors called? http://kitsandparts.com/iconnect.php
i want to buy a bunch that have 2pins but mouser has so many different connectors that i am having a hard time narrowing my search down.


Title: Re: Building my first oscillator
Post by: W4AMV on September 20, 2011, 02:17:26 PM
Hi. They may be MOLEX, but the exact part no. etc... not sure. You might email his address and ask.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 20, 2011, 07:21:43 PM
i just shipped my mixer. hopefully it will arrive relatively soon. hopefully the w8diz mixer will arrive relatively soon as well.

what do you think i should work on the in the mean time? i need to put the LO oscillator together on copper but i want to make sure i have the right dBm output and the right frequency range. should i aim for 0dBm?

if in the future i wanted to make oscillators that didn't require large coils what would be my options? crystal oscillators, pll synthesizers? anything else?


Title: Re: Building my first oscillator
Post by: W4AMV on September 20, 2011, 08:19:53 PM
ok, fine I will look for it, set it up, and report back. On the LO, goal should be 7 dBm nominal and good stability and of course reasonable spectral purity. Not sure what your desired receive band is and goals. I would keep it simple. Say the broadcast band! However, if you want to go shortwave, pick a favorite frequency range. Your initial 160,80,75 meters seemed reasonable. If you want to start investigating a  PLL, synthesizer, etc... I suggest you start a new thread! I am sure there are experts on the forum who will provide a lot of thoughts in this area.

Alan


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 20, 2011, 10:11:39 PM
maybe i should stick to one project at a time.  :) a pll synthesizer can wait.

i would like to make a broadcast band receiever but i have noticed that the lower i drop the frequency range, the less total tuning range i get. with it operating around 1-2mhz i only get about 0.5Mhz of range. perhaps i dont have a high enough inductance coil but i haven't even gotten it to oscillate under 1.3mhz.


Title: Re: Building my first oscillator
Post by: W4AMV on September 21, 2011, 01:49:34 AM
yes. Here is a good bedtime read.

http://dspace.mit.edu/handle/1721.1/15991


Title: Re: Building my first oscillator
Post by: W4AMV on September 22, 2011, 09:13:18 PM
Hi, I received your unit in good shape. I checked your mixer and it does function, however, there is a problem. Your loss is high and the balance is poor. This is the reason you are having difficulty in observing the IF. So, where is the problem? I checked your diodes, they are fine. I believe the issue is your transformer connections. Since you did not use colored wire, a trifilar xmfr is easy to get wrong. So, I am going to start there. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 23, 2011, 02:50:05 AM
weird. i'm pretty sure i put the transformers together correctly but maybe i mixed something up in the secondarys (bifilar side).

thanks for checking it out.


Title: Re: Building my first oscillator
Post by: W4AMV on September 23, 2011, 09:49:02 AM
Most likely or used one of the windings meant to be a secondary as the primary. The balance and feedthrough of the LO and the RF to the IF is pretty significant. In a reasonably balanced mixer, these are doubly balanced, the LO and the RF level are reduced significantly with respect to the IF. So, shuting down either the LO or the RF, will force the IF signal to goto nearly zero. There would be no output whatsoever. Not in the case of yours, the LO is quite well coupled to the IF port. The diodes are spot on. The Vd nearly 0.3 V at 2.5 mA and they are well matched.   


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 23, 2011, 05:25:31 PM
the way i checked the transformers was to do a continuity check on the ends. the two wire ends that have continuity with each other but not the other 3 wires are the primary side and center point of the secondary side is where there are two wires soldered together.

are the two non-center tap wires of the secondary coil identical electrically or do they need to be distinguished from one another in the circuit?

edit:
my W8DIZ diode ring mixer kit arrived today. i just assembled it. i cant test it until i get a new fet for my LO oscillator. hopefully that will arrive tomorrow.

i also got an IF amp stage kit which is assembled. these W8DIZ kits are pretty nice. if only they had an VFO kit, then i would be able to build an entire receiver from kits. 


Title: Re: Building my first oscillator
Post by: W4AMV on September 25, 2011, 10:04:19 AM
Hi. I will try to check the xmfrs today and let you know. It may be latter this week as I have a couple of other items. 


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 25, 2011, 02:15:24 PM
okay. take your time and let me know how it goes. i got a cheap frequency counter at the swapmeet (dsi 5600A). I'm waiting for the 9310 fets to arrive in mail before i can do any more work with the LO or test my W8DIZ mixer.


Title: Re: Building my first oscillator
Post by: W4AMV on September 25, 2011, 03:15:49 PM
Hi. I looked at the xmfr on the LO/IF side first. The SINGLE and ONLY center tap as you recall is the IF output. The start-finish winding of one set of wires is the LO primary. The other TWO sets of remaining wires, properly connected create the CENTER TAP IF AND the two remaining wires drive one half of the diode ring. For some reason, you connected ON THE SAME SIDE of the start-finish winding the center tap for the IF. In other words, you did not connect the START FINISH of wire pair but in fact the START-START of a set of wire pair. The main bad result of this is incorrect phasing of the LO voltage to the diode ring and thus poor balance of the LO signal to the IF port. This is an easy error to make. By START FINISH I mean observing the wire entrance to the toroid (the START) and its exit on the OTHER SIDE of the toroid (the FINISH). You had them connected on the SAME SIDE...oops. So, I will make a better drawing than what you used and post. Make the changes to the units and see the resulting improvement.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 25, 2011, 05:08:27 PM
i was worried about that. my question "are the two non-center tap wires of the secondary coil identical electrically or do they need to be distinguished from one another in the circuit?" was my attempt to ask about the start/finish ends of the coils but i did not have the right terminology.

at least i now know what i did wrong. :)


when i made the transformers, i didn't realize there was a difference between the start/finish ends of the coils. i guess i need to be way more careful with my windings and keep better track of each wire and its start/finish ends.


Title: Re: Building my first oscillator
Post by: W4AMV on September 25, 2011, 08:56:02 PM
YES, indeed there is a difference in the START-FINISH ends of the coil and the resulting transformer. SO, I will prepare a drawing and I hope an understandable explanation why this is so.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 26, 2011, 03:37:35 AM
i was looking at various oscillator schematics in the arrl handbook and 'experimental methods in rf design' and noticed that a lot of designs use toroids instead of large air coils. i replaced the large ceramic coil form in my LO with a T94-6 and it seems to perform perfectly fine.

what is the reason to use air coils instead toroids in oscillator tank circuits?


Title: Re: Building my first oscillator
Post by: W4AMV on September 26, 2011, 12:25:04 PM
A good question and you probably should post this as a new thread item to gather more views.

A quick answer, the TORUS and the SOLENOID are just two different physical geometries. So on the surface, there is a difference only due to their geometry. The torus is self shielding, therefore, the influence of the enclosure on inductance change would be less. The solenoid can achieve a very large Q and thats a benefit in oscillators for various reasons. However, the toroid can do quite well in achieving high Q, 200-400 is not uncommon. Toroid permeability must be carefully selected -6 material etc... check their temp. coefficient. Air inductors (solenoid) can achieve a very low TC (but it still has a TC) and maybe easier to temp compensate.

Again, there is more to this, but ping the general audience for more replies via a new thread.


Title: Re: Building my first oscillator
Post by: W4AMV on September 26, 2011, 11:08:17 PM
Hi. Your mixer works perfect. Now, both xmfr's were incorrect.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 26, 2011, 11:38:45 PM
oh wow. is it balanced well enough to operate without the low pass filter? is it comparable to the hp mixer of yours?

i got my bag of J310 transistors in the mail today. i get a lot more gain. using j310 in the oscillator and the buffer amp i get 5.1VRMS signal into a 50ohm dummyload from the buffer amp output! why am i getting such a high voltage output? using a j310 in the oscillator and an mpf102 in the buffer amp stage i get 2.44VRMS output from the buffer amp.

arent these values really high? should i really be getting 500mW output from a j310 buffer stage? the transistor itself is not hot or anything.

the output from the 2n3866 stage when using the j310 transistors is 7.3VRMS.


in other news i put together some basic test equipment. here is a photo of a 10db attenuator, a return loss bridge, and a 50ohm 4W dummy load. i used bnc connectors and these neat little 2x2 enclosures. i dont have enough bnc cables to test the return loss bridge just yet. http://i.imgur.com/Wl28M.jpg sorry my desk is covered in paint and junk which makes it a terrible backdrop for photos.

edit: oh i am really dumb. i totally messed up when i put together my dummy load. i spaced out when soldering it and made it 50ohms pad in series with the circuit. oops. i'm fixing it right now.

okay so i fixed my dummy load and checked my outputs again. now i am getting 4.1VRMS from the 2n3866 amp stage. this seems a lot more accurate.


Title: Re: Building my first oscillator
Post by: W4AMV on September 27, 2011, 04:14:12 AM
Yes, the balance is quite good and no LPF is needed to see the IF. Both LO and RF signals are suitably suppressed. This is sort of a nice mark to qualify a mixer. I would need to measure balance value on a spectrum analyzer in order to bench mark it against the HP. The HP has additional xmfrs to further improve balance, so I would not be disappointed in a slightly worse number. The j310 is a nice device with decent gm.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 27, 2011, 07:23:45 PM
very cool. i'm excited to get build a complete superhet receiver. for now i am going to use the w8diz IF amp i bought but i would like to try building one from scratch myself in the near future.


Title: Re: Building my first oscillator
Post by: W4AMV on September 27, 2011, 09:43:03 PM
RF-IF and LO-IF rejection (balance) is 50 dB, quite good. This is not surprising as the diodes are well matched. The xmfrs are proper now. If I have a chance, I will measure vs. frequency and that should do it.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 27, 2011, 10:06:09 PM
i picked my diodes by using the diode test mode on my multimeter and picking diodes that gave the most matched value. is that a suitable way to match them or did i just get lucky?


Title: Re: Building my first oscillator
Post by: W4AMV on September 28, 2011, 09:53:34 AM
yes. I used a TEK curve tracer and the matching was very good from off state through 2.5 mA of forward current. Of course AC matching could be off. That would be another measurement.


Title: Re: Building my first oscillator
Post by: ssbothwell SWL on September 29, 2011, 12:02:55 AM
in the test equipment section of the arrl handbook there is a schematic for a 3.7mhz oscillator which they recommend for receiver testing. here is an image of the schematic: http://i.imgur.com/LOU7L.png

it is supposed to put out a +4dbm signal into a dummy load.

i have all the components other than the 2n5486 FET. i used an MPF102 and put the circuit together on breadboard (i know its bad) and am only getting 20mVRMS output. the signal looks okay but its really low power. in the schematic they note that you can replace the 2n5486 with a mpf102 but to expected reduced output.

would you expect to get 20mVRMS output? i tried a J310 and i get 25-30mVRMS. i was hoping to get atleast 1mW output. could the breadboard and long component leads be producing major loss?



EDIT: NEVERMIND. i made the exact same mistake as in my diode mixer. i mixed up the start-end leads of the transformer T1. its working perfectly now. :)
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