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Re: LF:GPS Locked signalling at LF

To: [email protected]
Subject: Re: LF:GPS Locked signalling at LF
From: "Andy Talbot" <[email protected]>
Date: Tue, 29 Jul 2003 20:13:40 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Thans to those who replied to me directly on this subject, there certainly does appear to be some interest and in 2 - 3 weeks I'll be in a position to listen for some weak signals.
In the meantine how about this :
A PIC with on board A/D converter (8 bit is more than adequate) samples an audio waveform at, say, 1kHz (or 800Hz).  This can be downconverted to zero freqeuncy quite easily with a simple bit of PIC software by multiplying by an 8 step I and Q internally generated waveform. 
Now here's a bit I think ought to work but need to test the idea.  If a GPS pulse every second is used to synchronise the PIC's internally generated LO at precisely the same point each second this will effectively lock the LO.  About 1000 I/Q samples are accumulated and output as RS232 words for further processing.   With an 8 bit conversion, the only additional hardware needed apart from the PIC would be sufficient anti alliassing filtering to eliminate audio components below 500Hz and above 1.5kHz.   Any CW filter in a receiver can do this so with a very simple interface, GPS locked clock and timing recovery is possible.  An even simple conversion using a square wave internal LO at 4kHz is also possible, but now teh anti allias filter has to prevent frequencies below 750Hz and above 1250Hz from getting to the A/D
Andy  G4JNT
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