Date: Sun, 24 Oct 1999 08:48:58 -0400
From: "Prof R. Jennison" <boffin1@compuserve.com>
Subject: LF: re Datong converters
To: rsgb_lf_group@blacksheep.org
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  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.