Dear LF Group,
As a "confidence check" of the GPS time/frequency standard stability, I
monitored the phase of MSF relative to the standard's 10MHz output. MSF was
received using an SPM-19 selective level meter, with the 10MHz standard
driving the external reference input, and the 10kHz IF output of the SLM
was compared on a 'scope with the 10MHz signal divided by 1000. This allows
you to see a phase change of a few degrees. So far, over about 24 hours,
there has been no overall phase change, indicating that the difference in
frequency is no more than a few parts in 10^12. During daylight, there were
practically no visible variations in phase, but at night, fluctuations of
about +/- 10 degrees occurred over a period of several minutes, accompanied
by the amplitude of the MSF signal varying over a range of about 3dB. My
QTH is only about 100km from MSF, so I expect the ground wave signal
largely swamps variations caused by changing sky-wave.
I think a DSP scheme like G4JNT is using is the best way of doing these
phase measurements in the long run, but in the mean time, I intend to
resurrect the "vectorscope" circuit I built a while back - this should also
be able to remove the PSK modulation from the R4 and DLF carriers, with
suitable choice of time constant.
Cheers, Jim Moritz
73 de M0BMU
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