[time-nuts] Basic question regarding comparing two frequencies

Bob Camp lists at rtty.us
Mon Jul 26 19:43:16 EDT 2010


Hi

I've used the 6790 for this sort of thing before. It's a good choice since the whole signal chain is synthesized (if I remember correctly ..).  It's still going to be tough to hit the originally requested accuracy with one. 

Bob


On Jul 26, 2010, at 6:34 PM, 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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