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LF: RE^3: GPS Locked PSK

To: [email protected]
Subject: LF: RE^3: GPS Locked PSK
From: D[email protected]
Date: Thu, 14 Feb 2002 13:10:44 EST
Reply-to: [email protected]
Sender: <[email protected]>
Hi Andy and group,

Andy G4JNT wrote:

> Is there a reluctance to build extra hardware these days ?   
>
> But I supppose that anyone who doesn't use the Windoze operting system
> now is behind the times and "Not with it!".  An attitude seen more and
> more these days until the advantage of simple software performing
> real time decoding is demonstrated to them.

No reluctance to build extra hardware !
But consider how difficult is to get some of the DSP's, CPU's, evaluation boards etc these days for those non-professionals. Waiting a couple of weeks for that 'very special semiconductor', paying a lot of pounds/dollars/euros for it, and -worst case- blowing the chip before the first sign of life can be as annoying as writing windoze software.
Don't let us go into a fruitless debate of 'pro' and 'contra' using modern PCs, my very personal opinion is this: If we have these pentium PCs etc, why not try to make the most use of it, with or without extra hardware ?


Apart from this:
The phase-over-time plot works (the polar phase plot not yet), so if anyone is interested to spend a few hours on a building-site called 'Spectrum Lab' try it. It is available at www.qsl.net/dl4yhf.  The current version is "V1.67 b4", but this may change very soon.

For a quick start, to receive Andy's PSK CW transmission, open the 'Time domain scope' window (from 'View/Window'), select the preset "Phase meter for 15625 Hz", overwrite the 'L.O.' frequency with the precise audio frequency from of your receiver, usually 650Hz..800Hz if the receiver is set to 'CW'.
Before Andy's test transmission, use a known stable frequeny (like 77.5kHz) to 'calibrate' your RX if it's not GPS-disciplined. Either use a high-resolution spectrum or waterfall to find the correct value for the phase meter's  'L.O. frequency', or adjust the L.O. frequency by "trial and error" until the phase graph gives a horizontal line (the phase plot for the LEFT audio input will be green, the phase for the RIGHT audio input red).
The decimation factor can be set on the "Acquisition+Trigger" tab (subject to change), the preset value is 4096 which gives a bandwidth of less than 44100 Hz / (2*4096) = 5.38Hz. You can set the decimation factor MUCH higher for lower bandwidth, for 10 second dots a good value may be 20736, resulting bandwidth less than 1 Hz, and the time scale covering about 300 seconds if the screen is maximized.
About the stability of the despised soundcard: Mine is stable enough to observe the phase of a TV sync signal (15625Hz) over 5 minutes with less than 45 degrees phase drift, but the PC must be running for a couple of hours before. The error at lower audio frequencies should be  less. If this is still too much: Permanently determining the soundcard's clock error with a GPS-derived signal on one of the stereo input channels may be a solution, though it sounds like 'overkill' to me. Better build a stand-alone high-precision audio generator and use it to replace the 'L.O.' for the phase meter. Now we are back in the hardware business, hooray :-)


Sorry for the lengthy windoze-biased message.

Looking forward for your transmission on friday,

Wolf  DL4YHF.

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