From: DL4YHF@aol.com
Message-ID: <49.187c719c.299d57a4@aol.com>
Date: Thu, 14 Feb 2002 13:10:44 EST
Subject: LF: RE^3: GPS Locked PSK
To: rsgb_lf_group@blacksheep.org
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<HTML><FONT FACE=arial,helvetica><FONT  SIZE=2>Hi Andy and group,
<BR>
<BR>Andy G4JNT wrote:
<BR>
<BR>&gt; Is there a reluctance to build extra hardware these days ? &nbsp;&nbsp;
<BR>&gt; 
<BR>&gt; But I supppose that anyone who doesn't use the Windoze operting system
<BR>&gt; now is behind the times and "Not with it!". &nbsp;An attitude seen more and
<BR>&gt; more these days until the advantage of simple software performing
<BR>&gt; real time decoding is demonstrated to them.
<BR>
<BR>No reluctance to build extra hardware !
<BR>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.
<BR>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 ?
<BR>
<BR>
<BR>Apart from this:
<BR>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. &nbsp;The current version is "V1.67 b4", but this may change very soon.
<BR>
<BR>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'. 
<BR>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 &nbsp;'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). 
<BR>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.
<BR>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 &nbsp;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 :-)
<BR>
<BR>
<BR>Sorry for the lengthy windoze-biased message.
<BR>
<BR>Looking forward for your transmission on friday,
<BR>
<BR>Wolf &nbsp;DL4YHF.
<BR>
<BR></FONT></HTML>