A DDS VFO for the TS-520S

While I am waiting for the new DG5 boards to arrive (they shipped yesterday – yay!), I decided to see if I could get the AD9850 DDS module I got on eBay to work as an external VFO.  If you search “DDS AD9850” on eBay you should find lots.

This is mine.


The module is being controlled with an Arduino UNO – it sends the desired frequency serially to the DDS chip.  The Arduino is also attached to a rotary encoder for frequency input, as well as an LCD display.  In this prototype, the LCD display is showing the DDS frequency, not the VFO frequency.  I don’t really need a display on the VFO since the DG5 emulator already displays the dial frequency.  An additional advantage to this is I don’t have to worry about the radio Carrier/BFO and Heterodyne oscillators warm-up drift as the dial frequency is being calculated using these actual frequencies by the DG5 emulator.

I put a 0.01uF cap between the output of the DDS and the VFO signal input on the TS-520 to block any DC component present on the output from the module.   The stock output was a little low, but worked – signals were a bit low- 3-4 S units as compared to my K3.  Looking at the datasheet for the DDS module, there is a 3.9k ohm resistor called “Rset”, which sets the DAC’s full scale output current.  I put a 2.2k resistor in parallel with it (just soldered it on top of the existing 3.9k SMD part, you can see it in the pic above) to lower the Rset value and increase the DAC output – bingo!  Signals are on par with the K3, and signal reports are good.  I’ll need to make sure I am not saturating the output or causing any distortion – I might need a buffer amplifier if there are any problems. Looked OK on the scope.



Next steps will be to get it in a case on a protoboard, and establish communications between the DG5 emulator and the VFO – the DG5 is connected to the computer, and I can have the computer tell the DDS what frequency to go to, and I might be able to use one display.  At this point I am leaning to independent boxes as the DG5 box is best on top of the TS-520, and the VFO would be best off to the side with the VFO knob and at the same general height as the existing VFO knob.  The rotary encoder I am using is a cheap mechanical type with detents, I’ll need to get a higher resolution, smoother encoder (probably optical) for the final design.

Let me know what you think!

Steve – KV6O

P.S. – I have been in touch with Todd at ToddFun, I sent him a couple of the first run PCB’s for the DG5 emulator – he’s building it and will be posting a new video with the DG5 emulator soon!  Keep your eye out for it!

13 thoughts on “A DDS VFO for the TS-520S

  1. William Adams WD8OSJ

    Now the VFO I would be interested in. I want to build it for my other TS-830S which has a port on the back of the unit for external VFO.

  2. admin Post author

    I’ll probably tackle this as a project next, I’ll update the website as I make progress.


  3. william adams wd8osj

    Very good on the ex VFO. Do you need me to send you the paperwork on the 830 or do you have it already?
    The 830 and the 530 are the same in that respect. My VFO-230 works on both radios.

    1. admin Post author

      No, I have the service manual for the VFO-520 and I have figured out (somewhat) what the switch does at the pins. I rigged up my 520 by disconnecting the jumper from 8 to 9, and shorting 8 and 6 (tells the 520 that the external VFO is set for Xmit/Rec), and injecting the signal on pin 1 (gnd at 2), and it worked. When I have time I’ll take it further – perhaps integrating it with the DG5 project as that gives you the ability to show the actual dial frequency.


  4. Pingback: DG5 Emulator boards still available as of 2/8/15 | KV6O

  5. Mike Heneghan

    Hello KV6O,
    I just acquired a TS 520S at the NH Ham Fest. It will be my nostalgia radio as I already own a Kenwood
    590S. Your DG5 Emulator would be a great asset to me. Just got started in Ham Radio Feb 2015. Getting
    started late in life at 72.
    I have some soldering skills, but arthritis in the hands. Think I could do this.
    So, I would need to buy your board, the Arduino and the parts list??
    Your help greatly appreciated.
    Mike Heneghan, USAF (Ret)
    K1MIC “73”

  6. Ian lyckholm

    Hi Steve
    I built the emulator a couple of weeks ago and it works brilliantly. Thanks for producing a high quality kit like that.
    I’m going to try to rewrite the display routines for a 12864 graphics display to make the numbers a bit bigger. It might turn out to be a bigger task than I can Handle.

    Best Regards
    Ian VK5MA, Adelaide South Australia..

    1. admin Post author

      Excellent, glad to hear it! If you do anything that you would like to share, let me know and I can either make a link to your code, or to your page.


  7. Livan leyet

    Soy cm2lyt y tengo un ts130s y quiero saber cómo se puede adaptar un dds al equipo lo hice por la entrada del vco y recibo pero no trasmite en la misma frecuencia Salé desplazado por favor q puedo hacer Livan en Cuba buscarme en Facebook.


Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.