From Peter G3PLX:
Greetings to the group. I was subscribed here some years ago when we first
got 136kHz and I was transmitting on the band myself but I have been busy
with other projects since.
One of these has been an idea for the ultimate in frequency stability, but
using an ordinary SSB receiver, a GPS receiver with a 1PPS output, and some
soundcard software. Oh, and one more thing is needed - a 47pF capacitor.
I contacted a couple of LFers recently, including Brian CT1DRP, VE7TIL, and
W1TAG/WD2XES. These stations have been transmitting special signals for me
that are locked in frequency to GPS, and these signals have enabled me to
confirm that this new idea is working well. The tests also give some idea of
how narrow in bandwidth we might be able to go if the propagation will
permit. I thought it was time I rejoined the reflector and passed on some of
these ideas.
I first thought of this trick after I connected the 1PPS signal from my GPS
to the antenna of the main receiver, wondering if a narrow spectrum analyser
would show this as a comb of harmonics every 1Hz. It sure did, as many
people will have found, but I also watched the 'click'on a scope connected
to the receiver audio, and noticed that the waveform had a characteristic
'beat' to it. I realised that the beat was the difference frequency between
the receiver dial readout and the nearest multiple of 1Hz, and this could be
exploited as a drift-cancelling idea.
The technique therefore involves connecting the all-important 47pF capacitor
from the PPS pulse of a GPS to the LF receiver antenna. The same 1PPS signal
connects to the right channel of a stereo soundcard. The receiver audio,
with a mix of the distant weak signals and the RF click, connects to the
left channel.
The software first uses the 1Hz waveform on the right channel to calibrate
the soundcard samplerate. The left channel audio is processed through a
precision software frequency-shifter, like a very fine RIT control, working
over +/-0.5Hz. The RF click in this shifted audio is examined by the clever
part of the software and the RIT control is adjusted to zero-beat it. The
result is that the frequency-shifted audio has the receiver frequency offset
cancelled. The output is as stable as if it had come from a receiver with
ALL it's oscillators locked to GPS.
This is a simplified description of course. In fact I don't feed the
drift-cancelled audio back out again, but process it inside the software,
and I think this will be the best way to use this idea for weak-signal LF
work. I have done ultra-narrow waterfalls (+/-2mHz), BPSK demodulation, and
in particular charts showing phase drift over time to see how well the
system is working. Many LF operators have built GPS-locked DDS systems
already but this simple technique means we can now GPS-lock the LF reception
process, and it's all done in software (except for the 47pF capacitor of
course).
One thing that this technique is showing is that the propagation medium may
be a lot more stable that we thought. I found I can stay within 30 degrees
of the LORAN line from Rugby (300km away) on 137.57985440499182885 kHz for
days on end. This means there is no lower limit to how narrow we can try to
receive extreme QRP over surface-wave paths if both ends are GPS-locked. On
the 1500km path from CT1DRP to me (54N 3W) the signal is very stable during
the midday period. Around sunset and sunrise the phase changes more
rapidly - about 200 degrees/hour. That amounts to a Doppler shift of
+/-150uHz at sunrise/sunset. But after the sun rises again, the phase
returns to the exact same value as the previous day. I haven't done much
yet on easy/west paths but I was able to hold WD2XES within 90 degrees over
2 hours the other night, so QRSS-7200 might be workable over the TA path if
we had the patience to do it!
There will be no ready-to-fly downloadable software from me. Anyone who
wants to have a try at this who has already some experience with soundcard
software, is welcome to try it. There's great scope for experiment and new
ideas for using this simple idea, perhaps as added front-ends for existing
software, or for completely new techniques for ulra-narrowband working. This
reflector would be a fine place to discuss these.
73
Peter
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