Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 13 Jul 2005 17:08:58 +0100
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Date: Wed, 13 Jul 2005 16:07:58 +0200
From: Wolf DL4YHF <dl4yhf@freenet.de>
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Subject: Re: LF: Rugby lines
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Hi Dave,


You wrote:

> That was done by measuring the "strength" of the tone when analysing 
> the wav files with Spectrum Lab. The receiver AGC made the peak noise 
> level the same I could read off the difference in tone level (in a 
> tiny bandwidth) .

Did you turn your receiver's AGC off (or to "manual") when recording the 
files ? If not, the increase of the audio signal will not tell you the 
true story.

Besides this, I played with some recent recordings which Jim sent me. 
Trying to subtract the spectrum of the Loran signal from the "wanted" 
signal, but this didn't work too well. And the automatic multi-notch 
function (which also acts upon the FFT'ed audio stream) also kills 
CW-type signals.. too bad. So the removal of the Loran noise must be 
done in the time domain, maybe similar to a classic "QRM eliminator" on 
the rcvr's antenna input. Here is how such a circuit could work:

1.) Build a broadband receiver for 100 kHz, which recovers the waveform 
of the Loran pulses
2.) Regenerate the pulses (constant amplitude) by means of a 
Schmitt-Trigger.
3.) Use these pulses to drive an analog switch to "blank" the 137 kHz 
receiver input.

Or (a bit more complicated due to the need to demodulate "positive" and 
"negative" pulses):
Add the harmonics of the regenerated Loran signals, with amplitude and 
phase carefully adjusted, to the 137 kHz signal (no blanking but 
subtraction). This may produce less unwanted sidebands, but requires an 
SSB-type receiver for the Loran signal with controlled 100 kHz L.O. .

Well these are just a few thoughts... not verified the principle because 
Loran is rather weak here (or other kinds of QRM too strong ! ).

Cheers,
  Wolf  DL4YHF .