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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