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!