DDS VFO Question????

[wb2jix]

I would like to hear any recommendations or experience, using some of the DDS VFO kits or pre-builts, to drive a power amp.
I will also need about 2-5W to drive the amp.

Thanks!

Brian/WB2JIX
www.trueladderline.com

[W3GMS]

I don't know of any DDS that will do that.  Saying that, you can certainly build an intermediate amp to give you the required drive.  Plenty of circuits out there to do that.  I know that the N3ZI DDS VFO has only a couple hundred mV of output voltage and that is even at a relatively high impedance.  So you will need a gain stage where the first stage has a relatively relatively high input impedance with voltage gain.  From that point the following stage(s) will develop power by delivering current to drive 50 ohms required by your final amplifier.

Joe-GMS   

[w8khk]

You might want to look at Jim Hagerty's site, WA1FFL at http://www.wa1ffl.com/

He offered kits to build a nice VFO, http://www.wa1ffl.com/ordering.html

The kit was featured in a QEX article some years ago.  I met him and bought the kit in Dayton in 2006, and ordered the amplifier by mail later.

He also offers a companion amplifier to drive tube transmitters: http://www.wa1ffl.com/VFO_driver_amp.pdf

I built the kit and the amplifier and it worked very well.  His home page has pictures of several of his kits built by various amateurs.  The kits do not include an enclosure and some of the controlling parts, so you  can get creative and customize the packaging to meet your own tastes.

Good luck on your project

Rick

[K1JJ]

Hi Brian,

I run a pre-built $50 Chinese DDS here and it works very well.  They are on e-Bay. There are some problems to overcome, however.  As  Joe said, it has a low output and most DDS boards will require an IPA to bring it up to 5 watts.  I used a simple  11N90 MOSFET to do the job.

Also, the Chinese version has side channel garbage that is down maybe -60 dB or so. You need it to be cleaner.  But by using a tuned circuit somewhere in the chain, it solved this problem and brought the spurious down below -90 dB, which is acceptable.   I use the inexpensive Chinese DDS / 11N90 to drive a 100 watt SS amp which drives the a class C  4-1000A plate modulated rig.  I use a small 50 ohm tuner between the 100 watt amp and the 4-1000A, so that kills the DDS spurious signals very well and provides a good 1:1 swr match for the SS amp..

The WA1FFL DDS board is very clean and does not need any more spurious suppression, but is much more expensive.  It will also require an IPA amp if you want 5 watts out.

Tom, K1JJ

[ka1tdq]

I bought this for $70 and it goes to 25 MHz as a sine wave:

http://www.ebay.com/itm/New-Digital-DDS-Dual-channel-Signal-Generator-Source-Frequency-Meter-25MHz-KC4W-/252183431116?hash=item3ab74dc7cc:g:dUIAAOSw7FRWU8n9

The output is 5 volts+ when run into a high impedance.  I use it to drive my CW transmitter.  It's Chinese, so there's hash involved.  Every time I turned it on, my HF receiver got wiped out with 20db over 9 noise.  I found that wrapping the DC power connector 4 times around a large ferrite core totally eliminated the problem.

Jon
KA1TDQ

[w4bfs]

Mar '16 QST has a related article

[w5gw]

I have built several DDS VFOs using the AD9850 boards that are on eBay for about $10 now. As others have said, you need to up the output power with a small buffer amp. I used a couple of 2N2222s to create an impedance matcher/emitter follower to drive a low impedance VFO input in my Viking II and it does quite well. The March issue of QST has info on using such a VFO to be the heart of a transceiver and they sell the board and parts for the VFO at a reasonable price. Here is the link to the VFO. http://www.farrukhzia.com/k2zia/

73 Gary W5GW

[WBear2GCR]

Not being a QST subscriber, and not seeing it on the website, wondering what the "features" of the kit VFO might be? Like does it "remember" where it was last after power off... bands covered/freq covered, etc... specs re: spurious noise, etc...

Looks cool, especially if with the source code I could modify it to do things like put my callsign up on power up, etc..

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[w5gw]

Go to website and contact them. It remembers last freq. The AD9850 boards are only as clean as the power you put into them. Many switching power supplies will result in some spurs. See my paper on a crystal bank I built using a AD9850 - I did some testing and found the best way to eliminate spurs was by using a 'clean' power supply and incorporating some filtering in the power leads. Here is a link to the paper http://w5gw.com/images/xtal.pdf

[WBear2GCR]

just for reference, regarding clean power supplies, in audio there is a significant need for really excellent "regulation/filtering". There was a recent article in an obscure publication called Linear Audio http://linearaudio.net(some of the articles are online), where there was an in depth comparison of ultra high performance discrete (some SMD layouts - if small is important) some IC, iirc, some using opamps in the loop, iirc, all that reach below numbers like -120dB (again iirc) noise floors. I may have some other citations if there is interest. In general the best performance seems to be with shunt regulators, and sometimes series followed by shunt (probably the best way).

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