[time-nuts] Basic question regarding comparing two frequencies

[Neville Michie]

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