[time-nuts] Basic question regarding comparing two frequencies

[Hal Murray]

> 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.

I like it.  Thanks.

How did you pick 100 KHz?

> 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.

Has anybody checked the edge cases and/or linearity of a setup like this?

> 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. 

Is 2.5 ns good enough?  What would you gain by using a 16 bit DAC?



If 2.5 ns is good enough, I'll bet you can do the whole thing in digital 
logic.  Just get a fast FPGA/CPLD.  I haven't done a serious design, but a 
quick check at some old data sheets shows it's not silly.  You could probably 
bump it up by another factor of 2 with some external (p)ECL chips.



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