Dear Dick, Andy, LF group,
At 15:20 30/07/2002 +0200, PA0SE wrote:
It would be interesting to know what other amateurs find? Or provide an
explanation of the anomaly?
I made some rough measurements of the FS of Droitwich and Donnebach over
the weekend using an SPM-19 with 25Hz filter, and saw similar fluctuations
in level to Dick's.
All these signals have phase modulation of some sort applied to their
carriers - it occurs to me if the RX bandwidth is narrower than the
modulation sidebands produced by the PM, the result of filtering out some
components of the PM signal will be an amplitude-varying signal - The
higher the modulation index is, the greater the signal power in frequencies
further from the carrier - a bit like the technique for calibrating FM
deviation using a spectrum analyser, where with a mod index of 2.4 the
carrier power drops to zero, in spite of the total signal power remaining
constant. The filtered PM signal would then have an amplitude which
generally decreased as the phase modulation index increased. Perhaps with
the wider bandwidth of Andy's W-J receiver, all the sidebands are passed,
and this does not occur? A simple experiment would be to use the
spectrogram software to see how wide the phase modulation sidebands are.
I also checked the France Inter signal on 162kHz, which did not show this
effect - but I recall from playing around with my vectorscope circuit a
while ago that the phase modulation on this signal is much slower,
therefore narrower bandwidth than the Droitwich phase modulation.
I will check up on this when I get home - assuming I can hear anything
through the lightning QRN!
Cheers, Jim Moritz
73 de M0BMU
Cheers
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