[Lf] [Fwd: LF: Phase meter for propagation experiment]

[Andre Kesteloot]

DL4YHF at aol.com wrote:

> Dear LF group,
>
> I am planning an experiment based on an idea from Alan G3NYK. The idea
> is to monitor the phase variation of strong signal on LF over day and
> nighttime. It may be interesting to find out how much phase variation
> is there; how much the propagation path length varies etc.
>
> At least two receiving sites are required: One close to the
> transmitter, and another far away from it. On both ends very accurate
> phase meters must be used. Of course, the phase of the transmitted
> signal must connected to an atomic clock like MSF or DCF77.
>
> I think I found a possibility now how the required accuracy can be
> achieved, here is the basic principle. Most is done by software, only
> a decent hardware is required:
>
> - The RX stations have clock sources locked to GPS, MSF, DCF77 or TV
> sync (15625 Hz) of certain broadcasters.
>
> - The reference clock (from the locked source) is divided down to a
> frequency which can be handled with a standard soundcard. For reasons
> explained below, an audio tone of 1...3kHz is required. Assume
> 60kHz/24=2.5 kHz,  or 77.5kHz/31=2.5kHz, or
> 15.625kHz/6=2.604166666kHz. This audio frequency must be entered in
> the software's "sample rate calibrator".
>
> - The divided reference frequency (or the 15625 Hz signal) is used in
> the software to PERMANENTLY monitor the soundcard's sample rate. This
> is important because the sample rate may drift by a few millihertz
> which is unacceptable here.
> The software can already detect the sample rate from a very weak
> reference signal, so it is enough to add a small fraction of the
> reference frequency to the receiver's audio output because it is in
> another audio frequency band (longwave RX: 100...2000 Hz, reference:
> 2.5kHz or 15625 Hz). So there is no need for a stereo soundcard !
>
> - The 2.5kHz reference is formed into a square wave like a 'frequency
> marker generator'. Odd harmonics are the result. One harmonic must be
> in the longwave receiver's passband, for example 55*2.5kHz = 137.5
> kHz, or 53*2.604166666kHz = 138.020833333kHz. A small fraction of this
> harmonic is added to the antenna signal which goes into the receiver.
> We need this to compensate the VFO drift of a "normal" shortwave- or
> longwave receiver via software as explained below.
>
> - Assume your SW receiver runs in USB, the VFO tuned to 136 kHz. For
> the VFO drift compensation (which is completely done in software), the
> received audio should contain a weak 'audio peak' at 137.5-136=
> 1.50kHz, or 138.02083333-136 = 2.02083333kHz. This audio frequency
> must be entered in the software's "frequency offset calibrator".
>
> With this system of two "calibrators" (one for the PC's audio sample
> rate, the other for the longwave receiver's slightly drifting VFO) it
> is possible to make very accurate long-term phase measurements.
> I have such a system running now, but not perfect yet, because my
> DCF77-locked source sometimes unlocks for a few seconds which spoils
> everything. I tried to convince my pocked GPS receiver to produce a
> 1-pps-signal which could drive G4JNT's GPS locked source but no
> success. At the present time I use the german ZDF TV broadcaster which
> has a precise 15625 kHz signal.
>
> If someone likes to participate in this experiment, he may try to get
> the last version of SpecLab running. The two 'calibrator' routines are
> implemented but not explained in the manual yet, if there is interest
> in this experiment I will continue development and tell you how to use
> it. Or offer the calibration routines (written in C) to anyone who can
> program nice and clean user interfaces...  ;-)
>
> Regards,
>  Wolf DL4YHF.
>



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