Monday, 2 May 2011

Kenwood R-2000 as a front end to a Software Defined Radio

As I hinted at last night, I modified my Kenwood R-2000 to act as a front end to my SDR.

[Update: Someone else did something similar, but used a DRM conversion board and tapped the 3rd IF - this is fine if you don't have an SDR that will cover 9-10MHz like mine:]

The SDR in question is a Cross Country Wireless SDR from Chris, G4HYG.  It was originally bought from a radio rally back in 2010 and was the 40/30m version (7 and 10MHz, for those not into hamradio).  It had two crystal oscillators on board, which allowed you to 'see' around 50kHz of spectrum around their centre frequencies.  All fine and dandy, but I knew what I wanted to use it for - an IF panadaptor.

So I set about working out how to achieve this.  I thought about getting a crystal cut for 42.8MHz (ish) to allow me to feed in one of my V/UHF radios, using its 10.7MHz IF.  But then I figured why not just buy an Si570 based VFO (variable frequency oscillator).  I decided on a Team QRP2000 VFO, from SDR-Kits.  This comes as a kit, and is 90% surface mount.  I had a few problems with those tiny little surface mount capacitors, and a few idiotic things like soldering the electrolytics the wrong way round - in my defence, I was never any good at kit building.

Well after a few trials and tribulations, it works.  I had a few problems with the SDR though, and required me to unsolder the crystal oscillators - I was getting interaction with them from the VFO's output - this didn't solve the problem entirely, so an email to Chris 'HYG sorted me out.

Now I modified the Kenwood R2000 - this was pretty easy - there's a device just before the 2nd IF filter that is perfect for a tap.  I decoupled it with a small capacitor and fed it to a BNC socket. There's a handy steel panel on the back that is designed to be removed when fitting the VHF convertor - this is perfect for popping for a BNC socket.

I think I need to de-couple the circuit a little better though, as there's some drop in sensitivity when switching the SDR on - something to investigate at another date.

Anyway - tuning around takes some getting used to, but I've worked out the 'sweet spot' for tuning the SDR to.  It is 9860.130kHz.  When tuning to 5600kHz on the R2000, this is what the SDR's display looks like.

I've annotated the display - as you can see, 5598 is on the far left of the screen, 5616 a few kHz further up, and 5649 just right of the centre.  Personally I think this is amazing.  I can see well over 100kHz of spectrum (minus the tiny 'dip' in the centre) and aside from some 'cross talk' which I can fix with the skew calibration, all seems well.

This picture shows me receiving Kinloss ARCC on 5680kHz - you obviously have to work out what frequency he's on by 'doing the math'.  I know if I'm tuned to 5600kHz (or any multiple of 100) then the "9820" marker is the band bottom, and "9900" is 80kHz.  If I wanted I could "shuffle" the Si570 oscillator down a few kHz and have the band edge right at the edge of the screen, but its nice to be able to see below it without have to retune anything.  Lots of scope of experimentation and playing around.

Just for fun, here's a screen shot of some Link-11, or TADIL-A.  I think looking at this that it must be in double side-band mode.  What do you reckon?  By the way, that's 5450kHz on the left of the screen (RAF Volmet).

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