Hello Jim, LF,
this is interesting! Yesterday I did basically the same experiment using an
audio filter, and saw no difference at all in decoding
ability between full SSB bandwidth and a 250 Hz filter. I concluded (perhaps
prematurely) that there is no blocking effect present.
Today I did another test by injecting an audio carrier from the Speclab
generator within the WSPR search bandwidth. The "wanted
signal" was F5WK signal at 1577 Hz with -57 dB below ADC fullscale, displaying
-15 dB SNR at the time. I found that at a 1450 Hz
carrier at -20 dBFS completely blocked decoding, even though the spectrogram
traces of the WSPR signal and the carrier both looked
perfect. Reducing the blocking carrier to -30 dBFS seemed to reduce Michel's
SNR to -20 dB.
In hindsight, and with the information from Jim, I assume that the out-of-band
interference levels in my first experiment simply
have been too small to cause an effect. DCF39, the strongest daytime carrier,
is attenuated by the SSB IF filter edge to about -34
dBFS at originally 3330 Hz, which is then downconverted to 2830 Hz for the WSPR
computer. So I now also believe that decoding might
possibly be prevented by a 25 dB larger signal anywhere in the audio band.
I guess I will need to play more with this tomorrow.
Kind regards,
Markus, DF6NM
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Saturday, January 17, 2009 8:57 PM
Subject: LF: Re: WSPR does not work
Dear Andrey, LF Group,
Here is an observation that might be relevant, (or perhaps not!) to the
problem of WSPR displaying a clear spectrogram trace of a signal, but
failing to decode it. I noticed it while looking for OH1LSQ's QRSS earlier,
and have now checked a bit more carefully.
First, I changed the BW from 250Hz to 2.4kHz so that I would be able to see
OH1SQ at 800Hz audio frequency (carrier frequency set to 136.000k) using
Spectrum Lab, and at the same time decode F5WK at around 1500Hz audio. This
required reducing RF/IF gain to prevent overloading by the DCF39 carrier at
2.83kHz audio. I could then see OH1LSQ OK, but although there was a good
trace from F5WK in WSPR, no decoding occured. (Changing the gain does not
alter the spectrogram appearance much, so presumably it is displaying SNR
rather than absolute signal level. ).
Reducing the bandwidth to 250Hz, with the gain still reduced, restored
normal decoding.
Increasing the bandwidth to 2.4kHz again, adjusting the passband shift
control, and changing the loop orientation to reduce DCF39 and Lakihegy to a
minimum while maintaining F5WK's signal, with the gain still reduced, also
gave normal decoding, although there was now assorted additional noise in
the passband.
It seems to me most likely that the failure to decode was due to the
presence of the strong tone at high audio frequency from DCF39, although
this tone was outside the WSPR frequency range, and not saturating the sound
card A/D converter or overloading the analogue RX channel.
So if there are other tones in your RX passband while receiving WSPR, it
appears they may prevent decoding occuring, even though a good signal is
displayed on the WSPR spectrogram.
This seems to be a reproducible case where a strong WSPR signal does not
decode on a receiving system that is known to work well. If others can
reproduce this, it may be worth contacting K1JT to see if he is aware of
this or can shed light on the matter.
Cheers, Jim Moritz
73 de M0BMU
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