Many thanks to Alan, G3NYK, and Bob, K3DJC, for their comments on the
problem with Datong converters.
In particular Alan's suggestion number 3 hit the nail on the head.
Further checks with an oscilloscope have shown the offending cw components
at the output of the converter (and also at a lower level at the input).
Furthermore, tuning the main receiver over the whole spectrum with the
converter in the antenna socket has now shown that there is a harmonic set
of these powerful cw components precisely in agreement with Alan's
suggestion that it could be associated with the use of a third harmonic
overtone crystal. The signals from one of my two converters are listed
below (the other Datong converter has a similar set differing by a few
kHz). All of the signals listed are at strength 40dB over S9 on the S
meter, each is accompanied by a related set of somewhat weeker modulation
harmonics to either side:
9.309 MHz and 9.345 MHz; 18.654 MHz and 18.690 MHz; 28.072 MHz and 28.360
MHz.
It is the 28.072 harmonic that severely affects my reception on the 73
kHz band but my other converter has a higher harmonic very close to the 137
band, so it may be adviseable for all our colleagues using these converters
to check where their harmonics lie.
I have not yet found a solution to the problem but it may be
possible to shift any offending harmonic sufficiently clear of the ham
bands. I will copy this email on FAX to Dave Tong and I hope that we may
then get an official solution.
73, Roger, G2AJV.
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