[time-nuts] Basic question regarding comparing two frequencies

J. Forster jfor at quik.com
Sun Jul 25 19:54:58 EDT 2010

Hi Neville,

There are plenty of ways to compare frequencies.

I posted the BPSK demod scheme as a simple way to quickly tweek in the
correct direction without Lissajous Figures.




> There is another way to compare two frequencies, relevant when they
> are very close together.
> I divide a reference down to 100KHz and use it to clock a phase
> detector made of a pair of D flip flops.
> The unknown (divided to 100KHz) is fed into the circuit and an output
> that is proportional to the phase
> difference appears on the output as a changing mark-space ratio.
> Using CMOS and a precise power supply (because under no load, CMOS
> output is precisely rail to rail),
>   the averaged output (100ms RC filter) is fed to a strip chart
> recorder.
> The recorder shows the changing phase difference and folds back each
> time a whole cycle passes.
> A 12 bit analog data logger resolves 2.5ns of phase and gives data
> for further analysis.
> There may be a small amount of missing data in the vicinity of the
> foldback, but if life threatening this could be avoided
> by running a second unit with the signals delayed to be near
> quadrature, and using the better data of the two.
> I use a lower frequency version of this system to monitor clocks
> (mechanical ones with pendulums).
> Cheers, Neville Michie

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