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Amplitude Modulating a DDS VFO




 
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Author Topic: Amplitude Modulating a DDS VFO  (Read 703 times)
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W2PHL
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« on: March 17, 2017, 10:30:19 AM »

Gang,
Over the past few years I've been having fun experimenting with cheap DDS signal generator boards from overseas. The AD9850 chip used in many of them can make quality HiFi AM. After getting many "what did you say you're running?" questions, I thought I'd document some information just in case any of youz guys are interested in experimenting.  The board I focus on in the attached article is the best one I've found so far.  You can use it to build a standalone AM transmitter with a cannibalized PA deck from an old HF rig or you can modify an existing HF rig by unplugging the low level transmit RF cable that goes to the PA stage and replacing it with the output from the modulated VFO. It's important to note that by replacing a transceiver's internal TX signal with the VFO's signal, the rig's ALC and SWR fold-back circuits will not function leaving the final amplifier vulnerable to destruction by over driving or output load problems. Destruction is the operative word.  Shocked
I made a video so you can see and hear how Hi the Fi is.

https://www.youtube.com/watch?v=qziatWoF-co

Phil

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Steve - K4HX
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« Reply #1 on: March 17, 2017, 11:52:56 AM »

I've heard Phil on this rig. It sounds fantastic.
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M0VRF
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« Reply #2 on: March 17, 2017, 03:08:06 PM »

www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

Is the 'Original' article.

Just generate low A.M. and use a linear to amplify.

Nothing new....However look how linear the DDS is, wow, that's linear!
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W2PHL
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« Reply #3 on: March 17, 2017, 04:42:52 PM »

Thanks Steve! I hope other hams find the information useful.
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« Reply #4 on: March 20, 2017, 11:16:44 AM »

As always Phil, your experiments are much cooler than anything else I have seen! Very nicely done!

Rob
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« Reply #5 on: March 20, 2017, 12:29:19 PM »

I ordered one of those DDS's. This seems like a cool little project.. I have an amplifier from some sort of rig that has multiple gain stages. So I believe it should work very well..
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W2PHL
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« Reply #6 on: March 20, 2017, 06:36:32 PM »

I've also used one of those DDS modules to build an external VFO for a Tempo 2020. Here's a link to the enclosure:
http://www.ebay.com/itm/262638765798?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

It's a nice size box for a VFO. The top is thin aluminum and the bottom is sheet metal.
Attached is a pic of the Tempo 2020 VFO project.  I'm waiting on a bezel....and maybe a better way to label the buttons...yikes!


Phil


* tempo2020vfo.jpg (2193.32 KB, 3264x2448 - viewed 55 times.)
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« Reply #7 on: March 20, 2017, 09:43:40 PM »

I ordered one also.  Next I'd like to think about constructing an intermediate linear amplifier to bring the level up to something capable of driving the SB-220.  Less than 10 watts carrier should do it.  Question is... should it be vacuum tube based or solid state?  

I've never built a solid state RF amplifier.  This might be the time to think about trying something new.  

Eric
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K1JJ
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« Reply #8 on: March 21, 2017, 10:45:19 AM »

I ordered one also.  Next I'd like to think about constructing an intermediate linear amplifier to bring the level up to something capable of driving the SB-220.  Less than 10 watts carrier should do it.  Question is... should it be vacuum tube based or solid state?  

I've never built a solid state RF amplifier.  This might the time to think about trying something new.  

Eric

Hi Eric,

Some ideas:

First, building a high power SS amplifier that is CLEAN, that is, -30dB 3rd order or better is difficult. Frank, WA1GFZ spent over a year testing and modifying his big homebrew SS linear amp and found that he needed to optimize the core and transformers extensively to break into the "clean" arena. He wrote some good articles for QEX on the subject.

Personally, I have built a few SS amps, even using the expensive MRF 150 transistors. I failed in the first attempt, as it was -25 dB 3rd IMD or worse. The second attempt was better and I currently use it.

Bottom line is unless we are talking "Pure Signal" software to clean it up -  be prepared to optimize the transformers out of the mainstream ways. Tubes may be the better choice.

The second alternative is to build up a pure class A amplifier using a big transistor, maybe a version of the MRF-150 family. That's an easy 10 watts of -40dB 3rd... clean drive for your SB-220 amp. (and 40+ watts of heat)


That said, if you only need 10 watts, running barefoot you are gaining a 10 dB "clean signal" advantage over the regular 100 watt station, so nobody will really notice if you are a little dirty.  (until you drive an amplifier with it) Bear in mind that the average, modern open loop SS ricebox is usually about -29dB 3rd or worse.

As a two-tube self-contained 100 W tube project: - Running a single tube designed for linear service, set up properly in class AB1, will give an easy -30 to -35dB 3rd IMD. Pretty clean. Ten dB in IMD improvement makes a huge difference in cleanliness. Make it a two-stage class A  grid driven - driving an AB1 GG amplifier and you may be able to drive it directly with the VFO.  Anything between 10 watts to 100W carrier is easily do-able with tubes. The cool thang about this design is if you make both the driver and final grid driven, the driver and final are close to each other on the same chassis so that you can add some RF negative feedback to make it -40 to -45 3rd  - an ultra clean overall system!  Grounded gird configuration NFB can also be done with a single tube too thru a filament/cathode winding.


And then, a single tube driver designed for linear service (not a class C designed tube) running class A should make a great 10 watt driver.  You will be running about 30 watts of heat to generate 10 watts carrier out, but very clean.

The key thing to remember is that your driver should be at least 10 dB cleaner than the final amplifier - so that it will not degrade and pull down the final amplifier IMD numbers significantly. IE, a -40 dB 3rd IMD driver is adequate for a -30dB IMD  final amplifier so that the final will drop from only -30dB to -29dB 3rd IMD.

Your 3-500Zs SB-220 may be good for at least -35 dB 3rd, so you will need to really pay attention to details to get a -45dB driver or better.

I assume the VFO modulation system is super clean, but it will need to be verified first or all the other work is for not.

T
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« Reply #9 on: March 21, 2017, 08:07:41 PM »

Here are some non laboratory grade test results. The two tone IMD test was generated on a laptop running Spectralab: 700Hz and 1900Hz tones @ 95%modulation and viewed on a Flex 1500 (10dB per division). One pic is the DDS output barefoot the other is at 50 watts PEP using a Kenwood TS450s PA deck. The harmonic spectrum was measured on a Agilent E6380A communications test set.

Looks like 3rd order IMD is about -33 barefoot and -30ish with the TS450s amp @ 50 watts PEP
 
 
Phil


* dds output.jpg (155.97 KB, 1280x960 - viewed 17 times.)

* 50watt PEP.jpg (202.03 KB, 1280x960 - viewed 19 times.)

* -0dBMm harmonics.jpg (175.16 KB, 1280x961 - viewed 23 times.)
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K1JJ
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« Reply #10 on: March 21, 2017, 08:46:15 PM »

Here are some non laboratory grade test results. The two tone IMD test was generated on a laptop running Spectralab: 700Hz and 1900Hz tones @ 95%modulation and viewed on a Flex 1500 (10dB per division). One pic is the DDS output barefoot the other is at 50 watts PEP using a Kenwood TS450s PA deck. The harmonic spectrum was measured on a Agilent E6380A communications test set.

Looks like 3rd order IMD is about -33 barefoot and -30ish with the TS450s amp @ 50 watts PEP
 
 
Phil

Very good, Phil -

-33 dB 3rd IMD as a driver is OK.   If you drive an amplifier chain, the chain will never be better than -33... and slightly worse than the amplifier itself. You should not have any splatter problems on the band with it as long as you can keep everything higher than -30..

Of course, there are SS low level drivers much cleaner. High end test generators usually have better numbers running AM. I tap off of my FT-1000D pre-driver as a source for my linear amp chain. It has 3rd IMD of about -70 dB 3rd ! at that point. The FT-1000D power amplifier deteriorates to -30dB after that.  I use this 70 dB 3rd source into a lab amp, to the 3CX-350J, etc.

I have a similar DDS that I use as a VFO/driver source for my 4-1000A plate modulated rig. Running as a VFO it had some spurs up and down the band that were about -60 dB down bare bones. I was able to cure this adding another -30 dB simply with the existing input tuned circuits to the 4X1 rig. The 4X1 grid tuning Q had enough selection to kill the spurs.

Looking at your spectrum I see a blip down about 3877 or so... is that a spur that stays without modulation? Maybe not.   I worry about them because if my signal were 60 over at a local location, then the spur would be S9 with modulation - enough to bother or at least annoy an adjacent QSO... :-)  So if spurs become a problem, try a sharp tuned circuit after the DDS and see what it does.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

A Night in Tunisia:  https://www.youtube.com/watch?v=baMsQeQpUvw
W2PHL
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« Reply #11 on: Yesterday at 10:08:36 AM »

Interesting comments Tom! That blip seems to be a PowerSDR/Flex anomaly. It disappeared after a restart of the SDR. Yet another reason for me to buy a real spectrum analyzer. ha!  73

 Phil
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« Reply #12 on: Yesterday at 11:26:41 PM »

Eric: One possibility,

http://www.qrpkits.com/10wamp.html


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« Reply #13 on: Today at 06:06:10 PM »


Thanks Steve.  It would be a great possibility if there were more band options. 
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