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DL4YHF@aol.com wrote:
<blockquote TYPE=CITE><font face="arial,helvetica"><font size=-1>Dear LF
group,</font></font>
<p><font face="arial,helvetica"><font size=-1>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.</font></font>
<p><font face="arial,helvetica"><font size=-1>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.</font></font>
<p><font face="arial,helvetica"><font size=-1>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:</font></font>
<p><font face="arial,helvetica"><font size=-1>- The RX stations have clock
sources locked to GPS, MSF, DCF77 or TV sync (15625 Hz) of certain broadcasters.</font></font>
<p><font face="arial,helvetica"><font size=-1>- 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".</font></font>
<p><font face="arial,helvetica"><font size=-1>- 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.</font></font>
<br><font face="arial,helvetica"><font size=-1>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
!</font></font>
<p><font face="arial,helvetica"><font size=-1>- 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.</font></font>
<p><font face="arial,helvetica"><font size=-1>- 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".</font></font>
<p><font face="arial,helvetica"><font size=-1>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.</font></font>
<br><font face="arial,helvetica"><font size=-1>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.</font></font>
<p><font face="arial,helvetica"><font size=-1>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...
;-)</font></font>
<p><font face="arial,helvetica"><font size=-1>Regards,</font></font>
<br><font face="arial,helvetica"><font size=-1> Wolf DL4YHF.</font></font>
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