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| To: | [email protected] |
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| Subject: | Re: LF: Phase meter for propagation experiment |
| From: | [email protected] |
| Date: | Thu, 11 Apr 2002 13:34:50 EDT |
| Reply-to: | [email protected] |
| Sender: | <[email protected]> |
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Hello Jim, Brian, and group,
Jim wrote: >>Signals like MSF have on-off keying. Feeding this signal into a digital divider , the counting process will come to a stop when the signal level falls below some threshold, so output from the divider will start and stop at points in the cycle which depend on the amplitude of the input. So the effect of dividing this signal to an audio frequency will be to introduce more or less random jumps in phase each time the signal is keyed, even though the original signal is phase coherent. Can the calibration software cope with this? << No, the calibration software cannot cope with phase jumps. I am not sure if MSF uses 100% on-off keying. If so, an additional PLL and a stable 60kHz oscillator with a narrow tuning range must be added before the digital frequency divider. This PLL then fills out the "gaps" if there are any. DCF77 does not use 100% on-off keying, the amplitude is only reduced to 25% during the short "gaps". But a PLL help here also as a narrow input filter.The loop filter should have a lowpass filter with < 1 Hz bandwidth so the PLL does not unlock. >> Also, what signals do you plan to monitor? I expect the FSK and PSK signals would be quite difficult, because both receiving stations would have to track all the phase variations at the modulation rate, and store a vast amount of data to allow the relative phase to be compared - a high degree of timing accuracy (<1ms) would also presumably be required. Stations like DCF39 would be much easier since they are CW most of the time, but even here there is a jump in the phase every few seconds, so timing accuracy of the order of 1 second would be required for the two RX stations to compare the phase - still needing some work to achieve. << Not only difficult but almost impossible to observe FSK and PSK signals this way, because the phase meter uses a very narrow bandwidth (the phase meter's bandwidth can be set between 10 Hz to 41 milliHertz). So it only makes sense for a signal with a 'coherent' carrier. Maybe a well-equipped amateur can provide such a signal at the upper end of the band. To check the system without the need for such a test signal, observing DCF77, MSF, or one of the AM longwave broadcasters like Droitwich or DLF (Deutschlandfunk) may be a start (though I don't know about DLF's carrier accuracy). To monitor DCF39 for a long time and compare the results from several receiving sites will indeed be complicated because of the random phase jumps. The data would need to be logged in a file (not too complicated) with an electronic timestamp and a program (possibly a spreadsheet) would have to be written to subtract the phase values collected by a receiver close to DCF39 from the logged phase values recorded by "DX" stations. To me, this sounds a bit like overkill. For a start, take some phase plots (with phase jumps) and compare them visually. If they look good and 'say something', a more complex system like the one mentioned above can be attempted... Regards, Wolf. P.S: I visited the Trimble website today looking for the low-cost "Lassen SK II" GPS receiver. There seem to be no distributors in this region, only two in the Frankfurt/Main area. Anyone in northern DL / PA know other sources for this interesting modules, and thinks about buying one ? |
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