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Author Topic: Digital (DDS) VFO kit - need 5 watt buffer amp  (Read 38577 times)
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K1JJ
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« Reply #25 on: November 18, 2013, 11:43:00 PM »

After thinking it over, I've decided to build up a 2 stage FET analog VFO. This should be good for spurs down -140dB.

I will still need a 5 watt board to go from a few volts up to 5 watts to drive the quad MRF-150 IPA amp.

I already have a digital readout that will monitor the output.  

Here's the schematic for the band switched VFO (#1) and a buffer stage (right side of page #2) :

T


* VFO1.gif (5.14 KB, 584x315 - viewed 977 times.)

* VFO2.gif (5.21 KB, 574x323 - viewed 915 times.)

* Guru Trogladite.jpg (111.77 KB, 426x640 - viewed 616 times.)
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« Reply #26 on: November 19, 2013, 08:49:26 AM »

From an earlier post, using the DDS VFO with a transformer (and possibly a class A amp) to drive the IDXX(whatever number) would be great for a class E rig.  I'm guessing that to get the 2 phases you would just drive them through another transformer like shown in the picture?

Jon
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* E Rig Driver.jpg (1565.73 KB, 3264x2448 - viewed 759 times.)
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« Reply #27 on: November 19, 2013, 08:52:12 AM »

Are you planning on being 140 dB above the noise floor?


After thinking it over, I've decided to build up a 2 stage FET analog VFO. This should be good for spurs down -140dB.

I will still need a 5 watt board to go from a few volts up to 5 watts to drive the quad MRF-150 IPA amp.

I already have a digital readout that will monitor the output.  

Here's the schematic for the band switched VFO (#1) and a buffer stage (right side of page #2) :

T
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« Reply #28 on: November 19, 2013, 10:39:27 AM »

Are you planning on being 140 dB above the noise floor?


Yep, Whirlwide!   Grin


But really, maybe 100 dB above the noise floor on a late afternoon when the S meter shows S1.  (60dB over +8 Sunits = 108dB)    

The best DDS hobby kits out there are about -70 to -80dB down or so and many are only -60dB.  This is probably plenty in most band noise cases.  I see a number of Chinese boards on eBay that don't even give spur specs.


But with my background blower noise, any phase noise will probably be obscured anyway. And I'll bet there are other things in the rig that will generate noise that will limit it to no better than -70dB.

Noise is cumulative, so I figure for almost free, I can build my own analog version and add another -50dB of suppression over a DDS - and effectively subtract the VFO from the noise maker equation, for what it's worth.

T
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« Reply #29 on: November 19, 2013, 11:56:46 AM »

Tom
et al.

[You can skip the math... and just look at the "bottom line" below].

I think that a clarification may be helpful... in understanding how the phase noise specification translates into noise that would interfere with an adjacent QSO, and which would affect the S-meter reading on a distant receiver.

An ideal VFO would produce a signal that is a pure sine wave: v(t) = A sin [2pi x f x t] volts; where A is a constant, f is the frequency in Hz, and t is the time is seconds.

A VFO having phase noise, n(t), would produce a signal that is a phase modulated sine wave: v(t) = A sin [(2pi x f x t) + n(t)] volts; where n(t) is the phase noise, and where n(t) has the units of radians.

Using the trigonometric identity: sin (B + C) = sin (B) cos (C) + cos (B) sin (C), we obtain the output of the VFO as: v(t) = A sin [2pi x f x t] x cos [n(t)] volts  + A cos [2pi x f x t] x sin [n(t)] volts

But: cos [n(t)] is approximately 1 for small values of n(t); and sin[n(t)] is approximately n(t) for small values of n(t).

Therefore we obtain the following approximation for the output of a VFO with phase noise:

v(t) =  A sin [2pi x f x t] volts  + A x n(t) x cos [2pi x f x t] volts

Therefore, the output of a VFO with phase noise is approximately a pure sine wave (at frequency f) plus a double sideband suppressed carrier amplitude modulated cosine wave (at frequency f) whose modulated amplitude is A x n(t).

The double sideband, amplitude modulated component of v(t) (in red font) is what adds to the noise at adjacent frequencies.

Important: the phase noise, A x n(t) x cos [2pi x f x t], is quantified by its noise spectral density, relative to the carrier (A sin[2pi x f x t] volts), and has the (confusing/somewhat illogical) units of "dBc/Hz".

Bottom line:

More specifically, the noise power produced by the VFO's phase noise, relative to the carrier, referenced to the input of a distant receiver whose RF/IF bandwidth is B Hz, will be 10log[B/(1Hz)] + the specified phase noise in dBc/Hz.

Example, if the specified phase noise of the VFO is -140 dBc/Hz, and the RF/IF bandwidth of the distant receiver is 10,000Hz (i.e. +/- 5kHz), then the power relative to the carrier, referenced to the input of a distant receiver, produced by the phase noise of the VFO will be: -140dBc + 10log (10,000) = -100 dBc

Stu
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« Reply #30 on: November 19, 2013, 01:07:27 PM »

Thanks for the explanation, Stu -

In the real whirl, do you think a -70dB down spur spec on a DDS VFO is of any concern  if an AM carrier signal is S9 + 60 over  on a band that is a quiet S1 noise floor?


IE, Does it make sense for me to sidestep one of the DDS VFOs in favor of building an analog VFO?

Talking with Frank/ GFZ, he feels that there are many other factors, such as the modulator, power supply, etc., that will overshadow a very clean VFO. And perhaps the analog VFO itself will have problems doing better than -70dB in the real whirl.


I just don't want to go thru the effort of this VFO project and then discover later there are problems I could have avoided by a little more research and axing the right people the right questions.... Wink.

T

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"Season's Greetings" looks okay to me...


« Reply #31 on: November 19, 2013, 02:14:02 PM »

Tom

To answer your question... let's consider a "realistic" worst case scenario that would apply to your application.

1. You are building a driver for a plate modulated rig. The plate modulated stage will be biased in Class C, and therefore the plate current will have strong harmonics at multiples of the carrier frequency, and cross products at multiples of the carrier frequency +/- multiples of the frequencies of any other signals that reach the grids of the final stage tubes.

2. Each non-linear stage between the VFO and the final stage (if there are any intermediate non-linear stages... like an IXYS driver) will also produce harmonics at multiples of the carrier frequency, and cross products at multiples of the carrier frequency +/- multiples of the frequencies of any other signals that reach the input of that stage

3. In principle, it is possible for all of these cross products (2nd, 3rd, ...nth order cross products) to add up in the plate current in the final amplifier stage, such that the ratio of spurious components (originating in the VFO) to carrier in the final amplifier plate current is larger than the ratio of spurious components to carrier in the output of the VFO. Also, some of these spurious components could be close enough in frequency to the carrier frequency to appear at the antenna (after the tank circuit and the antenna tuner remove far out of band components).

4. In practice (just as in the case of intermodulation distortion products), one would need to measure a sample of the signal at the output of the transmitter to determine how much impact the spurs from the VFO have on the output of the transmitter.

My gut feeling is that if each spur at the output of the VFO is 70dB down from the carrier at the output of the VFO... then each of the spurious signals at the output of the transmitter will be at least 60dB down from the modulated carrier. The frequency offset between some the transmitter's very weak (i.e. more than 120dB down from the modulated carrier) output spurs and the output carrier might be different from the frequency offset of the VFO's output spurs and the carrier (and, in particular, some very weak 3rd or higher order cross products could fall within the passband of the output tank circuit and the antenna tuner, even though none of the VFO's spur frequencies do).

I'd go for the DDS, provided its phase noise is less than -100 dBc/Hz ... i.e. the noise level is 60db below the carrier, when the bandwidth of the receiver is 10kHz.

Stu
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« Reply #32 on: November 19, 2013, 02:41:48 PM »

I would be more concerned about phase noise with that DDS. Even the kit seller comments that it does not perform up to the specs of better DDS chips. There are much better DDS out there than that particular one. It is marketed mostly for QRP use and with a big QRO transmitter, you are going to be dirty. Just my opinion. I have a couple of those kits that I never built because of phase noise. The DDS 60 uses a much better DDS chip but I am not sure about using it with a QRO amplifier.
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« Reply #33 on: November 19, 2013, 03:33:34 PM »

It sure has been spectrally pure recently  ------------------------------------------------------



Throw the big switch before the whirl blows up  Grin
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« Reply #34 on: November 19, 2013, 07:16:16 PM »

T, in your pics, you'll have one hell of a tough time taming VFO1 due to the "bandswitching" and wiring associated with the switch.  It'll never be mechanically nor thermally stable enough to use.  Also, belay that "RIT" crap and the varicap... a big source of phase noise.

VFO2's problem is the clipper diode on the gate of the first FET.  Another big source of phase noise as well as drift.  If you lower the B+ on the FET you won't need the clipper diode.  The reason for that diode is that, with a very high Q tank the voltage very easily could exceed the Vgs and blow the FET.  With 5V or so on that FET's plate I never had a problem, even better that it's a series resonant rather than parallel resonant.

One of the best and easiest oscillators I built used just the osc section of a NE-602, surface mounted to a singled sided PC board with traces routed out with a Dremel.  I had the best luck using stout Miniductor stock, as little inductance as I could get away with and plenty of capacitance in the tank.  Build it in one of those cast aluminum boxes, plenty of attention to mechanical stability, and you'll be golden.  No clipper diode, the damn chip has a built in voltage regulator, and has a buffered output.... but it WILL need an amp for your purpose.



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« Reply #35 on: November 19, 2013, 08:02:28 PM »

All kidding aside, your concern is legitimate for larger frequency offsets. For stuff very close to the carrier, who cares? Take a look close-in on the carrier of some AM sigs sometime with your SDR. Use a small frequency span to get  a very small resolution bandwidth (something less than 10 Hz). On many sigs you will see spurs at 60 and 120 Hz (and often multiples thereof). Often the spurs at 60 or 120 Hz are only 30-40 dB down from the carrier.
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« Reply #36 on: November 20, 2013, 10:55:00 AM »

Some very good suggestions and info, thanks!

Stu, from what I read, you feel a DDS VFO should be OK. At least one of the better ones  that are showing spurs -70 to -80dB down.     Yes, the phase noise is a concern too.

John, you made some good points.  I worried about that bandswitch too, for mechanical and thermal stability. I've built a few VFOs in the past and always used an isolated coil with no bandswitch.

Your idea about using an NE-602 chip, with the regulator, osc and buffer all built in is a cool idea. I have a few here an may try that.  I need it to cover 160 - 40M, so wonder if I will need to switch the inductor somehow. It seems like a big range for one cap to cover without switching L.  Maybe two variable caps in parallel, one big and one small would do it.

I have a spare FT-102 that Frank feels would be a perfect driver, being a clean VFO and crystal.  But I already have a MRF-150 amp built and just need a 2-5 watt VFO driver to finish it.  



I'll look at the NE-602 and go from there.

Here's a circuit I found on the web.  Looks like it uses a toroid for the coil form.

Leads 1 and 2 are not used.

http://www.pan-tex.net/usr/r/receivers/ra01091.htm

T


* NE-602 VFO.gif (4.62 KB, 470x345 - viewed 763 times.)
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« Reply #37 on: November 20, 2013, 11:55:27 AM »

5 watts for $18.00.

http://kitsandparts.com/qrp_amp2.php

Last night I found some broadband amplifier boards on the web, but had to go to work and did not bookmark it....


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« Reply #38 on: November 20, 2013, 12:12:32 PM »

Solid state rigs can make nice drivers, but the tube ones are a bit of a pain.
You have the grid tuning, the tube rig dip and load, drive levels, it gets old quickly if you move around much.

I used an Icom 756 pro for a while, nice as it had stable output power, 2 different antenna outputs (one for each big rig), and a receiver input so the band scope would work (the receiver sucked).

I use a 32v3 to excite the big rig on 80 meters, but sometimes tune it up on 40 and it takes 10 minutes to change everything. I have to tune the 32v, then the big rig, and if I used one, an antenna tuner.

I like knobs, I just don't want to have to turn them all the time, so I have seperate antenna's for 80 and 40 with no tuner, and set one rig up on 40, the other on 80, and have no tune of anything.
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« Reply #39 on: November 20, 2013, 01:10:39 PM »

Another one, more money...
http://www.ebay.com/itm/RF-POWER-LINEAR-AMPLIFIER-AM-HF-HAM-RADIO-/321254983869?pt=US_Ham_Radio_Amplifiers&hash=item4acc4a08bd
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« Reply #40 on: November 20, 2013, 06:18:16 PM »


That may do the trick - thanks Brett!  

I don't see any specs on input drive, but I'm hoping the NE-602 VFO can drive it directly.

Looks like a great deal for only $18.

T
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« Reply #41 on: November 20, 2013, 06:44:02 PM »

Use a pair of those 5V mini DPDT relays to switch.  Or better yet, the damn NE602s are cheap enough, just build and oscillator for each band and use a 3-section variable for the tooner.  One section for each VFO.

Or, multiply up.

I'm not a fan of using toriods in oscillators.  Never was able to tame 'em thermally.  Polystyrene or air cores are the best, but then they're hard to adjust to get the tracking you want.  Toroids, being non-adjustable, present a similar PITA.

This site has a nice osc tracking program.  Plug in your upper and lower freq limits and it does it all automagically.  I just tried it and it timed out, so I dunno if it still works....

http://ironbark.bendigo.latrobe.edu.au/~rice/superhet.js/superhet.html
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« Reply #42 on: November 20, 2013, 10:15:02 PM »

And I am sure you can play with things to get more power or lower drive out of it.




That may do the trick - thanks Brett!  

I don't see any specs on input drive, but I'm hoping the NE-602 VFO can drive it directly.

Looks like a great deal for only $18.

T
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« Reply #43 on: December 11, 2013, 04:18:23 PM »

After a few false starts, I finally have a working DDS VFO. It uses an inexpensive Chinese  DDS VFO that drives a 20 mW to 3 watt linear amplifier board. This drives a pair of class A  11N90 FETS in parallel at 13.8 volts. The complete unit will put out about 20 watts with a clean sine wave.

This 20 watts is more than enuff to drive the MRF-150s that in turn drive the 4-1000A plate modulated rig. I really need only about 4 watts out of the VFO, so when backed off, it is pretty clean.

I tried an on-air test yesterday with Rob/ W1AEX's SDR spectrum analyser and he could not see any spurs or phase noise on my signal, down at least -65dB.  This was with the DDS VFO system driving the 4X1 rig.

This is the coolest VFO I've ever built and is extremely stable and accurate out to 4 decimals.  I'll post the links for the DDS and 3 watt linear board later. The pair of 11N90s are homebrew and give it that extra power needed. This would make a neat 20 watt CW QRP rig!

It's been running at 20w on the bench for a few hours now.

T


* DSCF0005.JPG (329.69 KB, 1280x960 - viewed 720 times.)

* DSCF0002.JPG (326.17 KB, 1280x960 - viewed 727 times.)

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« Reply #44 on: December 11, 2013, 04:23:26 PM »

Here's a shot on the scope into a 50 ohm load.  This is WITHOUT a tuned circuit, broadband.

The power output is variable from about 1 to 20 watts.  I am running everything at 13.8 volts.

Thanks to Jeff/ W2NBC for alerting me to the DDS VFO board.  He is testing one too.

I think once the VFO system goes thru THREE tuned circuits in the 4-1000A rig, it is probably even cleaner. (matching tuner input, grid tuning, plate tank output)   Jeff's tests show the spurs down more than -70dB when driving his rig.

I plan to hook up my own SDR soon and run some detailed spur tests using the complete chain.

T


* DSCF0003.JPG (328.67 KB, 1280x960 - viewed 657 times.)
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« Reply #45 on: December 11, 2013, 04:28:09 PM »

Tom,

You realize why its working so well, its that old Johnson knob you used on the front.  A little old and a little new! 

How flat output power wise over your frequency span is the output from your PP amplifier?    I don't think your going up to 10 meters with it, but the DDS and I assume the IPA within your DDS box would probably go that high.  I am thinking of some other applications, hence the question. 

Looks good!

Joe, GMS
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« Reply #46 on: December 11, 2013, 04:33:16 PM »

Here is the Chinese DDS VFO:

http://www.ebay.com/itm/10MHz-DDS-Function-Signal-Generator-Module-Sine-Triangle-Square-Wave-TTL-SWEEP-/321157000855?ssPageName=ADME:L:OC:US:3160


Here is the 20 Mw to 3 watt linear amp board:

http://www.ebay.com/itm/RF-POWER-LINEAR-AMPLIFIER-AM-HF-HAM-RADIO-/321262782220?ssPageName=ADME:L:OC:US:3160
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« Reply #47 on: December 11, 2013, 04:37:45 PM »

Tom,


How flat output power wise over your frequency span is the output from your PP amplifier?    I don't think your going up to 10 meters with it, but the DDS and I assume the IPA within your DDS box would probably go that high.  I am thinking of some other applications, hence the question.  

Looks good!

Joe, GMS

Hi Joe,

The DDS is good up to 10 Mhz.  That's all I needed for this application; 160- 40M for the 4X1 rig.

I don't remember  how much the DDS rolls off, but the 3 watt board drops maybe 3dB up to 7 Mhz.    With the 11N90 IPA amplifier, it is not an issue - plenty of power.

Yes, the Johnson knob is the secret.... Wink  

I needed to mount the DDS board on the front cuz there are four buttons that must be accessed for QSY.   It has memories and fast QSY ability too.

T
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« Reply #48 on: December 11, 2013, 05:05:19 PM »

Thanks Tom for the extra information.   I currently have the N3ZI DDS board that a friend passed along to me.  I am building an output stage for it to deliver some more soup!  I was just curious as to what the IPA and final in the DDS enclosure would do. 

When you go to receive are you just programming the DDS to go to some other frequency so you don't hear it in the receiver?  It appears that's what a lot of folks are doing.

Joe, GMS 
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« Reply #49 on: December 11, 2013, 05:22:56 PM »

When you go to receive are you just programming the DDS to go to some other frequency so you don't hear it in the receiver?  It appears that's what a lot of folks are doing.
Joe, GMS  


On receive I have a relay that disconnects the 11N90s from the SO-239 output.  I also kill the 13.8V to the 3 w board and 11N90s.     The DDS stays running at 5 volts, but does not appear to put out any stray RF affecting the receiver.

If I have any future problems I will try your idea of switching the output freq on RX.  Tnx for the idea.

T
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