Correction :-(
I looked at the Droitwich carrier again after reading Jim's posting. Using
the same setup as described there definitely is a dB or two of variation -
had Gram on 90dB dynamic range yesterday which explains why I didn't see
anything then.
Try this if you have a suitable Rx :
Select CW mode with a wide bandwidth to encompass all the AM signal - like
6kHz. Then set the BFO to 3kHz, or half the IF bandwidth, so the entire
transmission gives an audio output ranging from DC to 6kHz with the carrier
appearing as a 3kHz tone in the middle. Look at this downconverted signal
on Gram using 22kHz sampling at 512 or 1024 FFT size, viewing the line
spectrum, not the colour waterfall / spectrogram.
Any voice/music sidebands appear equally, either side of the carrier exactly
as AM theory predicts they will, BUT the amplitude does indeed drop slightly
as the sidebands appear. It almost suggests the transmitter is adjusted to
give a constant output power - there's a thought ! One way to check would
be measure the total power in the signal in a 6kHz (better still 8kHz)
bandwidth and plot this out over a few seconds. Not unsurprisingly, none
of the PD software available includes this option so I've recorded a few
seconds of .WAV file of this downconverted signal and can have a go at
processing that.
It just may be my imagination, but the power appeared to reduce a fraction
of a second BEFORE the sidebands popped up. This is at variance with Jim's
plot which appeared to show simultaneity. But if he was receiving the
carrier in a narrow bandwidth there would be a filter delay which might
account for the discrepancy. My technique delays all components equally
BTW, the WJ Rx is a superb piece of kit for monitoring and measuring RF. DC
to 30MHz coverage with around 60 different IF bandwidths possible from 56Hz
to 16kHz. Not that user friendly, though, if all you want to do is to
actually listen to the signals. Only trouble is, the WJ-8711 costs
several thousand pounds and is subject to export restrictions from the US !
Must get back to doing some proper work now.
Andy G4JNT
Dear LF Group,
I looked at the Droitwich signal using Spectrum Lab last night - the phase
modulated signal occupies a BW of about 80Hz centered on the carrier -
however, measuring the carrier level in 100Hz Bandwidth instead of 25Hz
made no difference to the fluctuations in carrier level, so that blew my
theory of yesterday out of the water...
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