Dear Steve, Alan, Klaus, LF Group,
Thanks for the comments. This calibration method is a bit fiddly in terms of
doing the calculations, but I think it is simpler than W1TAG's Loran
technique as far as making the frequency measurements goes. Also it gives
you the receiver frequency error as well as the sound card sampling rate. I
think the main difficulty with the Loran method is actually selecting and
identifying a suitable spectral line to measure the frequency of -
especially in the UK where the received Loran signal is "dual rated" with
interleaved spectral components from at least 2 different GRIs in the
demodulated signal. A lot of promising looking peaks in the display are
actually two peaks very close together.
As with the Loran technique, it has the advantage that, as far as hardware
is concerned, it is "something for nothing", with no additional equipment
required (something always appealing to amateurs ;-)). If you are prepared
to make some simple calibration hardware, the calibration process is much
more straightforward and potentially more accurate too. I have used a 10MHz
xtal oscillator divided down to 1kHz (2 x 'HC390) to provide an audio
calibration signal. The harmonics of the 1kHz signal can be compared with
198kHz etc. using the RX and soundcard with spectrogram software - in this
way, the calibrator frequency can easily be set to well within 1ppm. In my
case, I used a salvaged TCXO module for the 10MHz, which is stable enough to
provide in-band 1kHz marker pips for checking the receiver calibration too.
I don't know why the WOLF frequency calibration utility only works within
+/- 1Hz. The trouble is, if the offset is outside that range, it gives you a
sensible-looking value that is actually wrong. So it is important to check
the audio frequency on the WOLF spectrum display to make sure it is within
the right limits.
Cheers, Jim Moritz
73 de M0BMU
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