The AM Forum

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.  :o
I made a video so you can see and hear how Hi the Fi is.

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

Phil

I've heard Phil on this rig. It sounds fantastic.

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!

Thanks Steve! I hope other hams find the information useful.

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

Rob

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..

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

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

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

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 it into an amplifier chain, the chain will never be better than -33... and slightly worse than the amplifier itself. You will not have any splatter problems on the band with it as long as you can keep the whole chain's output  cleaner than -30...  Most ricebox amplifier modules and 3-500Z amps will accomplish this.  However, the cheap SS amps will not make the grade and are usually -25 dB 3rd or worse.

Of course, there are SS low level drivers much cleaner than the DDS. 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

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

Eric: One possibility,

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

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

Although, since SSB and CW are mentioned as the modes, it's probably a 10W PEP or 2.5W AM carrier, a bit short of what I would need to drive the SB-220. 

Eric,

[/quote]
 it's probably a 10W PEP or 2.5W AM carrier, a bit short of what I would need to drive the SB-220.  
[/quote]

Although fun to do, building a kit or building an amplifier from scratch is more effort than necessary. If you can find one, using a PA deck from an old HF rig is a much easier way to attack the problem.  The five transistor types that use a single 2SC1971 as a pre-driver (1st stage) will work great with this DDS. No additional input gain is needed for driving the PA deck to 100 watts PEP. Hook up the band filter relays and you are good to go.

Phil

Thanks for all the well thought out advice, Tom.  You make some very excellent points but I'm looking for a simple, low cost, add-on amplifier to go along with the bargain basement DDS.  

I'm not an RF engineer (by any stretch) so I can't just whip up a solid state RF amplifier out of my head and expect it to work.  Everything I build from scratch is a rip-off of an existing design with my own adaptions if necessary.  I'm looking either for detailed project instructions, a non-SMT kit or an assembled board.  

As far as the vacuum tube option, what exactly is a "single tube driver designed for linear service (not a class C designed tube)" in this power range?  Does such a tube really exist?  Do you have any particular tube in mind?  I think a 6146 is a class C designed tube, isn't it?  Although a 6146 would just about fit in the correct power range, a 6146 type tube is not a choice for grounded grid since the suppressor grid is tied to the cathode.  I would suspect a TV sweep tube might be a better choice but it was not designed for linear RF service.  

Eric

it's probably a 10W PEP or 2.5W AM carrier, a bit short of what I would need to drive the SB-220.  
[/quote]

Although fun to do, building a kit or building an amplifier from scratch is more effort than necessary. If you can find one, using a PA deck from an old HF rig is a much easier way to attack the problem.  The five transistor types that use a single 2SC1971 as a pre-driver (1st stage) will work great with this DDS. No additional input gain is needed for driving the PA deck to 100 watts PEP. Hook up the band filter relays and you are good to go.

Phil
[/quote]

Phil, your advice is well taken, but I think you just got lucky with having the PA deck of an old HF rig at your disposal.  This is not exactly something easily found lying around most ham shacks, except yours :-). 

You could probably buy the hulk of a crapped out FT-757 or similar aged Kenwood or Icom for less than you could build an amp.

Hi Eric -

What I mean by a tube designed for linear service is the series of high-mu, directly heated cathode tubes designed for grounded grid service, like the 3-500Z, 8877, 3CX-3000A7, YC-156, etc.  These tubes came after the popular 813, 4-400A, 4-1000A and other tubes designed to run grid-driven in class C. (And class B service for modulators)  However, the class C series of tubes can be made as clean and even cleaner than the "linear" tubes by adding a loop of negative feedback.  There are anomalies like the 813 and 4-1000A that do extremely well in GG too.  But the 3-500Z, etc., are naturals and so easy to use (less circuitry) and get great results in linear service.

There are other good GG tubes of lower power available.

My intention for this discussion was to give a "heads up" that yes, we can add a cheapo SS amp to the VFO, but if we amplify it further with a big 3-500Z amplifier, etc., watch out for IMD problems caused by a marginal driver. (creating overall less than -30 dB 3rd IMD)  Many of the small SS amplifier QRP kits and even many SS ham commercial amps on the market do not get better than -25dB 3rd, if that, and what happens is after a few bad reports of splatter we end up shelving the rig in fear of making too many enemies... :-)  Notice that almost ALL of the ham solid state  commercial manufacturers never publish their 3rd order IMD products. It takes a call to the factory and reluctantly you may get an engineer there to admit the specs... maybe.  Hopefully things have improved with the new devices on the market and better transformer designs for those $10K SS ham amps. But the little kit, QRP and CB-style amps for $50 are a red flag unless we keep them at QRP levels.

The driver (VFO and first RF IPA amplifier = 10 watts) is absolutely critical to achieving a clean overall signal once a bigger, clean tube amplifier is added resulting in over 100 watts output.

Steve's suggestion of using an older ricebox SS amplifier module will work FB keeping us on the clean edge of -30dB 3rd.  After all, there are plenty of riceboxes successfully driving 3-500Zs on the air. Once we fall into the -20's, the band splatter problems usually start.  We simply must do a 2-tone test on our rigs (complete chain) to know for sure.

T

solution !

http://online.sfsu.edu/hl/tmc2.html

Agreed!  I owned one a few years back. The first thing I did was increase the internal RF negative feedback until the IMD improved to -50 dB 3rd. Only 100 mW input drive required for a couple of hundred watts out carrier. It's one of the best made amplifiers you can buy.  Mil-spec, heavy duty.

I built an external power supply and installed PTT, etc. What an amp for the VFO project.  The only problem is the VFO is -33 dB 3rd while the amp is 10-15 dB cleaner...  Generally, the opposite relationship is desirable.


T

Thinking about building one of these.  I have an old ft757 for parts. What will I need for a mic?  All I have right now is some old cb base mics(d-104 both styles and some turner plus threes)  Will I need to add a mic preamp? 

The FT 757GX RF deck will work OK fine with this VFO, but it isn't plug and play. Any RF deck or amplifier will require modifications (switching of bias, band and TX/RX relays etc...) to make it work. As for the audio,  I'm using line level from an Inovonics 222. I haven't tried using an amplified microphone directly to the VFO. I would be hesitant to go that route anyway as there would be no control over peak limiting and bandwidth. There are some excellent articles here on AMfone and on the web that discuss audio processing for AM.

Phil

I don't see an Inovonics 222 in my future but would a BEHRINGER Ultragain Pro MIC2200 and a condeser mic work ok as a starting point?

As a starting point? It will work but far from ideally.

Looking at the specs, the Mic2200 is a just microphone preamp with EQ. Without peak limiting it will be difficult to prevent over-modulation and distortion.

A common modification for many HF rigs is to feed audio directly to the balanced modulator. Because all of the transceiver's audio processing (gain/compression/limiting) is bypassed, a broadcast style audio chain is required. Same scenario with modulating this DDS VFO.

In that silly YouTube demo I made, the audio to the transmitter is being processed by Breakaway Broadcast. Without that software processor the transmitted audio would sound bad...really bad! If it were on the air, it would cause splatter up and down the band!!!

This is a great topic! Thanks to all and please keep advancing it!
It could be useful as a signal generator as well, many uses!

THIS!!! and the NPL for the Flex Steve did make a basic, awesome, hi fi building block.

--Shane
KD6VXI

You're right Shane. Steve's circuit should be perfect for this gadget.

I've been measuring the IMD incorrectly.  I was looking at the 2nd order level instead of the 3rd! I throw myself on the mercy of the court. In a thread on measuring distortion, WD5JKO posted a very helpful spectrum pic with notes. Thank you Jim!
The pics below are from an Elad FDM-DUO. The FDM-DUO uses Elad's SW2 software which has a built in measurement function (who knew?)
This DDS is much cleaner than originally thought. The top pic is the barefoot DDS. The bottom pic is using the TS450s amplifier deck at 50watts PEP.

Here's a link to a neat and super cheap amplifier kit.
http://www.ebay.com/itm/45W-SSB-Linear-Power-Amplifier-DIY-Kits-For-Transceiver-Radio-HF-FM-CW-40db-Q3Q0-/401193735950?hash=item5d6902970e:g:x74AAOSw8gVX44bI

Scroll down the page for the schematic and links to the manual.

It doesn't have an output filter so watch out for the harmonics. I've been using it to drive an Ameritron Al-1500. I imagine any amplifier with tuned input filters will be sufficient to suppress the harmonics. Untuned/ broadband input amplifiers will probably explode if you use this little guy as a driver.

Another issue is the lack of a temperature compensated bias circuit. The gain increases about 3-4dB as it gets warm.
But hey...for $15 bucks....

Phil

Most linear amp input tuned circuits only have a Q of 2. That's not going to reduce the harmonics much.

Thanks for the heads up Steve.
The spectrum view of the unfiltered driver amp is obviously full of harmonics but the spectrum view from the output of the AL-1500 looks the same whether using the unfiltered driver amp or a filtered one. It passes muster without a problem but I've only tried it on 75m. Perhaps the AL-1500 will complain at higher frequencies and/or the output will become dirty.

The output network of your amp should kill any harmonics. The amp may be doing some "extra" work by amplifying the 2nd harmonic. It's probably not a big deal though. How many dB down is the 2nd harmonic coming out of your driver amp?

 With the DDS VFO and the ebay amp transmitting on 3.875 MHz, 7.75MHz is 35dB down and 11.625MHz is 8dB down. Running that combo through the AL-1500, 7.75 MHz is 44 dB down and 11.625 MHz is 54 dB down.

Good to go!  ;D

Wow, that is some VERY Hi Fi...  great results!!    And a nice range of modulation you ran there,  I was liking that too!   ;D

Been meaning to get off the fence and buy one of these,  would be a nice RF sig gen as well as a good core VFO.

So,  I know it's probably been a while,  but could you say which vendor you bought it from?  There seems to be at least 2 or 3 out there now, with varying prices.  I know they're likely a clone design, even the PCB's look..  well...  similar at least if not identical.  Wanting to avoid a 3d or 4th generation knock off of a good working design,  as well as YMMV a lot with vendors on how quickly it shows up once you pay them.

I've purchased several of these VFOs from two different vendors, tomtop_w and kissbuyer. Delivery took about two weeks from either seller. All of the VFO boards I received are identical. My guess is these things are surplus from a radio that was discontinued or never manufactured.

BTW I'm waiting on 4x3 keypad module to try with the VFO. The schematic shows pins for a keypad. Direct frequency entry would be 8)