Sunday 14 April 2013

Using the Arduino VFO

I've been playing with the Arduino VFO system, after she made her brief appearance as a blushing debutante in the sumptuous elegance of the Norbreck Castle Hotel. Regular readers will remember the special affection I have for this "magnificent" location.

First experiment was to convert George, g3rjv's classic "Sudden" receiver to digital VFO operation, which was achieved with what HF used to call "laughable ease"...


(details of this conversion will be published later).

Readers may recall that I used up some of the free space on the Arduino DDS shield by deriving a pair of quadrature squarewave signals, à la software defined radio, intending to play with Tayloe detectors. Well - yesterday I started to uphold the intention, brewing up a simple circuit with a 74BCT3253 and a 5532 double op-amp...


It works quite well, with the Arduino VFO liberating the receiver from the rock-bound limitations of simple SDR receivers, like the Softrock I cut my teeth on back in 2008...


 Here's a closer look at the board (where you'll see that I left the Sudden experiment in place)... 


RF from the antenna and the pair of quadrature squarewaves from the VFO system enter stage left, after which there's a filter and a transformer (essentially a Balun to convert the unbalanced feed to a balanced input to the detector). Then the 74BCT3253 sits on one of my SOIC - DIL modules, where it switches the RF to generate two signals which, after amplification by an NE5532, form the I and Q input to an SDR program.

I tried Winrad - but then switched to HDSDR.

Here's a screenshot of me listening to m0roa early this afternoon...


as you see, I set the "offset" between HDSDR's "local oscillator" and "VFO tuning" at 10kHz and tuned using the rotary encoder on the Arduino VFO, which makes the whole experience feel more like radio!

I changed the display on the Arduino VFO to add the 10kHz offset - so the LCD shows the correct frequency my receiver is "hearing" - even though the DDS is generating a signal of four times this (minus the offset). A vivid demonstration of the flexibility of the Arduino-based VFO.

I also experimented with generating some CAT controls for HDSDR in the Arduino - I can change the VFO tuning, but not the local oscillator setting - ironic, given that the Arduino is generating the LO signal! If anybody knows if CAT control of the LO setting in HDSDR is possible, I'd like to hear from them. OK - after a few more minutes playing around, I found how to set the Tune fixed to 'LO<->Tune-Offset' option - now I have the screen on the computer displaying the same as the VFO's LCD screen (by sending the appropriate CAT command to HDSDR).

All of which reminds me of just how much I dislike software defined radio. Never mind - next Sunday its G-QRP Valve day!

 ...-.- de m0xpd

No comments:

Post a Comment