Message-ID: <000501c19777$e9882280$05e9fea9@g3aqc>
From: "Laurie Mayhead" <laurie@g3aqc.freeserve.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20020107102830.00b0ecb0@gemini.herts.ac.uk>
Subject: LF: Re: Re: GPS Coherent PSK Transmission
Date: Mon, 7 Jan 2002 12:35:36 -0000
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----- Original Message -----
From: James Moritz <j.r.moritz@herts.ac.uk>

Subject: LF: Re: GPS Coherent PSK Transmission


> Dear Andy, LF Group,
>
> I copied the G4JNT PSK beacon last night; assuming I have the start
> times/polarity correct the sequence was:
>
> 011 010 111 001 011
>
> I found two methods of demodulating the signal using general-purpose
> equipment - the first is simply to produce a spectrogram, using parameters
> for 30s dots. The "key clicks" produced by the phase transitions are
> visible as a spreading of the trace. Setting Spectrum Lab to produce 30s
> time markers made it fairly easy to identify the sequence. I noticed a
> period around 2250 when there were no phase transitions for several
> minutes, but then the sequence returned to normal.
>
> The second method was to look at the phase directly - to do this I fed the
> RX audio into one channel of a 'scope, and fed a stable audio frequency
> equal to the BFO offset into the other channel. The frequency error of the
> RX and locally generated tone was small enough so that the phase
difference
> between the two traces only changed a fraction of a cycle in several
> minutes, so it was easy to see an abrupt phase change when it occurred.
Due
> to QRM/QRN, I found that to make the beacon signal clearly visible on the
> scope, a very narrow bandwidth audio filter (<10Hz) was required; this was
> also implemented using Spectrum Lab. Even then, the apparent phase of the
> signal bounced around by some 10s of degrees due to noise, and noise
spikes
> could cause a temporary phase inversion, so a lot of concentration was
> required to get the sequence. With a strong signal, the traces are quite
> stable, so you could use this method to detect an abrupt phase change, or
> measure frequency to millihertz resolution quite easily.
>
> Although good fun, neither of these methods is really viable for DX
> reception, since they don't integrate the signal over the whole bit
period,
> let alone over several sequences - a bit like decoding QRSS by ear. Andy's
> signal was about S3 at my QTH, but hard to actually hear due to a
permanent
> S6 carrier I get on about 136.65kHz, a real nuisance for manual CW
> operation - does anyone else hear this?

Yes Jim I have a permanent carrier (Loran line ?) on 136.647 by my
measurment  which I call the south coast mystery signal ! since people
further inland dont seem to hear it. I once tried to DF it and belive it
originates from E.France,but I could be wrong. It makes a good calibration
marker since the freq. is very constant. 73s Laurie.
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>