MilliHertz Bandwidths

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The Proper Approach?

HF Doppler work requires a very stable receiver and effective bandwidths in the milliHertz range. This tends to rule out standard transceivers. My IC737 turned out to be very stable, drifting about 800mHz each day in a cyclical fashion - probably related to the temperature in the shack. However, even low drift like this was too much for meaningful Doppler results. A glance at the circuit diagram of the radio revealed that the synthesiser was referenced to a 30MHz crystal. I fed the 10MHz output from my HP source (HP Sauce?) into a simple tripler, ripped out the crystal from the IC737 and fed in a super-stable 30MHz. My first tests were incredible - incredible that is for the amount of drift I was seeing. The radio took a day to settle after which the diurnal drift was about 7Hz - nearly ten times what it was before. Much headscratching and careful measurements followed. Then the penny dropped. The IC737 synthesiser does not provide all of the frequencies in the radio; the BFO/passband tuning has separate crystals. But why was the drift now so large? I can only think that somehow the drift in the BFO was cancelled by the drift in the synthesiser - and once that had gone, I was seeing all of the BFO drift. How depressing was that!

Two options now presented themselves:

bulletgenerate another stable signal for the BFO/passband tuning
bulletgive up on the IC737.

I decided on the later course because it would require major surgery in the radio to remove the BFO/pbt signals and replace them - I was also uncertain as to whether I could get away with just the BFO or would need to replace the PBT oscillator too.

The KISS Approach

What I needed was a very stable receiver that could be driven by my DDS. Then I remembered The Cracker. I had a lot of fun with this little radio but it has been in a drawer fir ages. Its simple DC receiver was just what I needed. A little simple surgery and my new RX was all ready to go - with my super-stable DDS as its LO. A few minutes later and I had it hooked up to the PC and ready for some serious narrow band work. I left it running through the night and came in to a screen full of straight lines. Hmmm, very odd. A listen to the AF showed the problem. It was pick up from the PC - or to be more accurate from the laptop's switched mode PSU/charger. I ditched this and built a 5 minute 20v linear supply. Bliss!

A few tests today (26 October 2002) have showed that this is fine but rather insensitive. More work to do but at least I have an operational Doppler system. I have just completed a run using an effective bandwidth of 7mHz and it looks good. The signals that I was following were all completely inaudible but showed up at 10dB over the noise - that works out to about 34dB below the noise in a 250Hz bandwidth so it is not surprising that I could not hear them!

Next job is to improve the sensitivity of the DC RX with an MMIC preamp...


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