[time-nuts] Basic question regarding comparing two frequencies

Murray Greenman Murray.Greenman at rakon.com
Mon Jul 26 18:34:25 EDT 2010


You guys are trying to crack a nut with a sledgehammer!

For a start, as Didier says, you can't possibly read the frequency of a
sky-wave signal to 0.01Hz in any short time frame since the Doppler on
the signal can be as much as 1ppm (i.e. 10Hz at 10MHz). You can only
infer it closer than that by studying the frequency in the very long
term.

In addition, you'll never know how much of the daily variation is
ionospheric, and how much is due to thermal changes at the source.

For what it's worth, the method I use for HF frequency measurements is
much simpler. I use a receiver which I can lock to my GPSDO (RACAL
RA6790/GM and HP Z3801A), and thereafter calibration is simply an issue
of getting the sound card sampling rate correct at the software spectrum
analyser, which you can do with a 1kHz reference from the GPSDO. No
complicated signal generators, signal injection, or AM mode with AGC
problems.

I use Peter G3PLX's SBSpectrum as the analyser, where you can trim the
sample rate in tiny steps. It also has a frequency resolution of 25mHz,
which is more than adequate for HF. My combination has won FMCs, but I
still can't resolve 0.01Hz off-air.

Whatever you do (with a sky wave signal) must be done over a long time
frame in order to be sure of getting closer than 1ppm.

73,
Murray ZL1BPU





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