Hi Wolf and all,
With some additional programming, it should be possible to
eliminate most or all of the special hardware. That might
let you get a number of folks to participate in reception.
One possibility is to use the LORAN sidebands already present
in the received signal. They can calibrate both receiver LO
and soundcard errors. Of course, if you are not close enough
to the selected LORAN station for its groundwave to always
be dominant, that might not provide an adequate reference
for your needs. But that's also true if you use hardware
based on MSF or DCF77, and you're not really close to them.
I don't know how the TV broadcasters lock their sync, but
if the signal is suitable, and the test signal is near the
high end of the band, then just turn on the TV and the flyback's
ninth harmonic of 15625 will be at 140.625 kHz. It may
be somewhat out of the receiver's passband, but should be
strong enough to be trackable. For calibrating the sample
rate, use the field rate modulation inevitably present.
If one has a GPS with a 1 PPS output, you can get really
accurate results. Again, no special hardware is needed.
Couple the 1 PPS directly into the Rx antenna input (along
with the desired signal) and do the rest in software
(G3PLX algorithm).
73,
Stewart KK7KA
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, April 10, 2002 7:36 PM
Subject: LF: Phase meter for propagation experiment
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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