[time-nuts] Basic question regarding comparing two frequencies

Chuck Harris cfharris at erols.com
Mon Jul 26 19:08:49 EDT 2010


I suppose that you could always cheat?  Since you know where the
transmitter is going to be, if you could get a timenut near to the
transmitter to give you a beacon to measure 24hrs prior to the event,
you could use the diurnal variations that you observed (observe?) on
the beacon to predict the skywave offset due to Doppler at the time
of the event.

-Chuck Harris

Murray Greenman wrote:
> 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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