The AM Forum

Hola!

I need a 5 watt digital (DDS) VFO to drive the IPA amp in my 4-1000A plate modulated rig. I just ordered the $79 digital VFO kit below. It puts out about 250 mV:

http://www.pongrance.com/super-dds.html


Here is a 2N2222 buffer amp I will build to bring the VFO drive up to 2 volts into a 50 ohm load. (80 mW)
http://www.pongrance.com/dds-buffer.html


* I need another broadband amplifier (160M - 40M) to take 2V @ 50 ohms (80 mW) and bring it up to about 5 watts at 50 ohms.   What is the easiest way to do this?  Anyone know of a SIMPLE circuit or hobby board available?   I wonder if a class A MOSFET using resistors might be easiest - or is a conventional toroidal core circuit is best...

Thanks.

T

Hi Tom,

Hey, when is that 4-1000 going to be heard on 75 again??  ;)

On your driver question - two possibilities:

1) If the 250mV output from the DDS is into a relatively low impedance, you could step it up to TTL levels with a suitable transformer. Then, drive an IXDD614 driver IC (hi Z input), which will then deliver more than 10A of RF current (at 12V).  That's more than enough power.  You could then step the 12V peak RF output up to whatever you need.

2) If the 250mV output doesn't deliver any power, go through the intermediate amplifier and driver the IXDD614 with that..

Simple, simple, simple  :D

Regards,  Steve

Thanks for the suggestions, Steve.

Yes, I forgot I had some of those IXDD614 driver ICs hanging around and will use them.

The IPA amp in the 4-1000A rig uses quad MRF-150s.   Right now, 2 watts is plenty to drive the MRF-150s to 50 watts out for the 4X1s.     I wonder if the IXDD614 has enuff voltage output to drive the MRF-150s?    Or would I need a step up transformer?   I think the bias voltage on the MRF-150s is about 3.5 volts, so maybe we are fine with 5 volts of IXD TTL level drive, I dunno.

The 2N2222A circuit puts out 2-5V @ 50 ohms.  But you say the IXD is high impedance input. I don't know if the VFO output is suitable to drive the IXD directly.

The Digital VFO output info says:

"The output of my DDS is unbuffered. The voltage is rather low (~300mV P-P) but it drives active mixers (e.g. SA612) with no problem. But it is not sufficient to drive a diode ring mixer, or vintage radio. Consequently a buffer amplifier of some sort is needed. "


Hope to be on soon, once I get this VFO system working.

T

Hmmm... wait a second...

The VFO output is a pure filtered sinewave.   The MRF-150 amp is a linear amp.   

If I use the IXD chip as an IPA, will that convert the sine wave into a TTL square wave?   I realize it's ultimately driving a class C stage, but do I want a clean sine wave pulse or will a square wave  produce the same results once it is filtered through the 4-1000A L/C circuitry?


T

If you are worried about "clean"...
you may be disappointed with your dds purchase.
(or you might have better luck than I did)

I used mine for a day....

have a second new one down at the bottom of the scrap box too never assembled.


As they say... "your mileage may vary".   ;)

I be no expert, but putting square waves into a linear stage will produce more harmonics than a sine wave.

I'd go for a simple follower to boost the current available from the output of the DDS.

If you are using the DDS for just one band, then you could use some transformers in there, where the transformers form part of a bandpass or low pass filter, this will drop the higher out of band harmonics down to nil. With enough current you can make the transformer a step up. Probably not that much current either.

            _-_-

As college electronics project I built a DDS "oscillator" that was driven by a PC that could go up to 15Mhz. When employed as a VFO with my radio stuff there were 2 issues I came across long after. One was impedance matching (i could never seem to get a good match with Johnson V2 with an outboard amp but the other was the noise floor with the DAC. I tried various LPFs but couldn't get the noise down and I attributed it to phase noise. Never proved it. I never took it beyond that or employed it in my radio stuff here so I still use analog all the way for frequency gen. I occasionally use it as an RF generator. I don't know if I was getting noise from the onboard XTAL oscillator or some manifestation in the design. Suffice it to say it was very stable but just not clean.

Vely interesting comments.   Yes, I am always concerned about cleanliness... :-)

I could cancel the order at this point, so I'm flexible...

Here's the published specs:

•DDS SPECIFICATIONS

◦Total Harmonic Distortion: -66db dBcf
◦Mirror Spur supression -60 db to 30MHz, -45 db to 34MHz.
◦Spurious-Free Dynamic Range (SFDR)
◾Wide Band (0 to Nyquist) -60 dBc
◾Narrow Band (±200 kHz) -74 dBc
◾Performance details

•Main output is a filtered sine wave
◦250mV p-p, when driving 1K load.
◦Secondary unfiltered analog output


I know that cleanliness is critical for a receiving system, but what can I expect to see when transmitting a dead carrier?  Would there be S9 side crud hash noise when I am S9+60dB over?  What does a "mirror spur" sound like?  Maybe the specs should be more like -80dB down.


I have a homebrew FET analog VFO left over from the class E rig which I could use again.  But the DDS seems so slick.

T

Over on the Viking II VFO thread there is s a link to the WA1FFL DDS, on his site he shows -140dB noise floor... fwiw.

KF1Z, wondering which one you found not up to par?
What were the issues, and how old were these, what chipset?
PM please if you don't want to post negative stuff.

                  _-_-

It appears that the WA1FFL DDS is cleaner, like down to -70 to -80dB when clocked at a rate of 400Mhz.

http://www.wa1ffl.com/index.html

Can anyone confirm this?    I may cancel and replace this order.


"A 14-bit D/A converter at the output, and
a much higher rated clock speed than the
98xx family allows a frequency-dependent
SFDR in excess of 70 dB, and approaching
80 dB with a 400 MHz internal clock frequency."


Here's the full article:

http://www.wa1ffl.com/Hagerty_QEX.pdf


** Here is an excellent review write-up:
http://www.eham.net/reviews/review/107788

I met Jim / FFL in Dayton when he displayed his kit.  He was operating it with HP oscilloscope and spectrum analyzer displays.  I cannot recall the exact numbers, but it was impressive, so I bought the kit. 

If I recall correctly, the display is not a frequency counter; rather it is a computed value to display the synthesized frequency output.

One of the features I liked was the ability to program an IF offset so that it can be used not only as the transmit vfo, but also as the receiver local oscillator for transceive operation with separate transmitter and receiver.

The kit was a breeze to build and operated perfectly when first turned on.  I was going to use it with the viking, as well as the small wonder but have been too busy with other projects to finish these.

If in doubt, contact Jim directly, he is quite a gentleman to deal with.

Hi Rick!

Good to hear from you again.

Thanks for the info. Yes it does seem like a nice clean kit. There is also a buffer/driver board available to ramp up the output, but it may not be enough to give me 2 watts out that I need.

I cancelled the other order and may place an order for the FFL DDS.

I'm now toying with the idea of building up a 2 tube VFO. (osc and buffer)  But will probably take the easy way out and go wid the DDS... :-)

T

Hi Tom,

I think the issue boils down to your earlier question.   If you are just driving a buffer and/or a class C amplifier that is modulated, how critical is the phase noise issue, if at all?  Surely if this was a signal that you were mixing with the output of a balanced modulator for sideband, it would be an issue.  Surely some of the gurus on the forum could shine a light on the subject and help us understand whether it is really an issue for your desired application.

73, Rick

Yes, good point Rick.   I don't know the answer myself.  Maybe driving the non-linear class C stage is a different animal with less stringent requirements vs: the mixing and IMD problems a linear amp incurs.  After all, a class E rig uses a square wave for RF drive... :-)

I'll bet someone like Stu would know...

I sent an email to Jim / FFL with my requirements and we'll see what he says.

T

I think your biggest issue with phase noise applied to the transmitter is if you have a very strong signal you are raising the noise floor close to your signal, think spread spectrum.   I think the only time this could be an issue in the ham bands would be during a contest with a lot of high power transmitters with significant phase noise and in the typical contest QRM is so intense the increase in noise level would not be significant.

Clean oscillators are far more critical for commercial service.

Of course the other case where minimal phase noise is important is in your receiver synthesizer system.  Here phase noise effectively creates mixing products from stations far removed from the desired signal that will be fed into the IF chain and greatly degrades your effective noise floor.  Some of the early PLL synthesized rigs (like my FT-980) suffer greatly from this issue.   One of the "cheap" techniques for reducing phase noise was to accept the tradeoff of slower lock time thus the "chuffing" tuning of some of the lower cost early receivers.  DDS definitely improved the state and I doubt that phase noise is going to be an issue for you unless you are planning to use a lot of big tubes with big handles in your final :)

here they use an ef184 with el84 tube.

I'm going out on a limb here because my info is 20+ years old and I havn't seen the schizmatic of the dds vfo ....

If it is a lookup method of direct synthesis (counter to prom to adc) then there is a phase discontinuity between each entry in the lookup table .... raising the speed of the adc and adding steps per cycle (more samples) lessen this discontinuity and lower undesired products ..... this of course is dependent of the layout and quickness of the parts chosen

even a crystal oscillator has a phase noise pedestal .... its just more like -140 db

Despite all of the hi-tech stuff I build, the oscillators (VFOs) for all of my transmitters are simple, free running oscillators because there is little phase noise.  Yes, they drift a bit but so what  ;D  After all, it's AM.  On the plus side, in addition to the low phase noise, they're simple and inexpensive to build and are very trouble free.

Tom, you show me your schematic and I'll show you mine. :D

Anyway, below is one potential IPA driver implementation using a bipolar circuit.

Q1 will need a heatsink since it would be a class A circuit.


Phil - AC0OB

Thanks for the suggestion, Phil.

Jim/FFL has an old 4 watt QRP handbook circuit that I may build up to go from 250mV to 4 watts.   I may go that route wid his DDS VFO board and display.

Then there is the possibility of using my old FT-102, three 6146s in parallel, as a dedicated driver to drive the 4X1 directly.   Still trying to decide.

T

FFL says on his site that the current product is lower distortion than the QEX article, if I read the site properly... fyi.

                   _-_-

I built a vfo and exciter last winter.
6C4 osc using B+W coil stock and a cap, feeds a buffer amp, into a 6146.
Has a digital freq counter as a display, but no digital stuff in it other then that.

I was thinking of building another one for the big rig using an old Collins pto that I have.
Its amazingly stable, I was shocked.

The exciter is very small, but has a remote power supply.

I never liked cheap digital stuff, or cheap modern analog stuff, it all seems to generate a lot of noise.

 

Yeah, I also have a Collins PTO. I pulled out the tube and put in a small FET at 12V.  As you say, it is very stable.   At one point I used it to drive a little 5 watt QRP board for 80M.  That may be another solution.   (And the spur crud wud be down -140dB)

T

I was unclear, the homebrew vfo is very stable.
Its the same circuit as used in my home brew receivers as the LO (go with what you know).
From turn on to about 60 seconds it gets stable, and then drifts no more then 100 Hz

My biggest problem was the change of about 100 hz between keyed and unkeyed.
I got it down to about 50 Hz.

Its got the digital display, plus I look on the sdr, and for junk box parts, I think its quite amazing.
The vfo box is too small, as is the coil, but it seems to work....

90 volts on the plate of the 6C4, regulated with VR tubes....

I do not think the pto in my 32v3 is as stable, but its OLD...
 

If Jim is talking about the one below, it can only put out about 1.5W into 50 ohms according to P = V^2/50.

I think this circuit was originally speced to output the 12V Peak into > 2.2k loads.

I added a slight voltage supply mod to up the output voltage a bit.

Phil -AC0OB

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

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
KA1TDQ

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

Yep, Whirlwide!   ;D


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

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

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.... ;).

T

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

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.

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



Throw the big switch before the whirl blows up  ;D

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.

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.

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

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....

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.

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

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

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

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

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

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

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

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

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.... ;)  

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

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 

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

Tom,

I was listening to you and Rob doing the testing, looking for spurs, etc.  You both sounded terrific.  I would have jumped in but didn't want to disrupt your work.

Fred

I suspect that you have long since worked out the details for your DDS VFO but if not, you might look at the DDS amp on Nick Kennedy's page at:
http://pages.suddenlink.net/wa5bdu/dds_amp.htm

Its purpose is to drive tube gear with the AD9850/51 DDS modules that USED to be cheap and plentiful on ebay but are now around $20 and no guarantee they are usable.