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Thans to those who replied to me directly on this
subject, there certainly does appear to be some interest and in 2 - 3 weeks
I'll be in a position to listen for some weak signals.
In the meantine how about this :
A PIC with on board A/D converter (8 bit is more
than adequate) samples an audio waveform at, say, 1kHz (or 800Hz). This
can be downconverted to zero freqeuncy quite easily with a simple bit of
PIC software by multiplying by an 8 step I and Q internally generated
waveform.
Now here's a bit I think ought to work but need to
test the idea. If a GPS pulse every second is used to synchronise the
PIC's internally generated LO at precisely the same point each second this will
effectively lock the LO. About 1000 I/Q samples are accumulated
and output as RS232 words for further processing. With an 8 bit
conversion, the only additional hardware needed apart from the PIC would be
sufficient anti alliassing filtering to eliminate audio components below 500Hz
and above 1.5kHz. Any CW filter in a receiver can do this so
with a very simple interface, GPS locked clock and timing recovery is
possible. An even simple conversion using a square wave internal LO at
4kHz is also possible, but now teh anti allias filter has to prevent frequencies
below 750Hz and above 1250Hz from getting to the A/D
Andy G4JNT
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