David McClain wrote:
Or, now that I think about it, it's similar to what we do when
measuring ADEV.. you can do a crude "how many zero crossings in the
time window" or you can do a "fit a sinusoid to a series of ADC
samples". One has an uncertainty of "one count/epoch", the other can
be substantially better.
How could it be substantially better for the same analysis period?
Unless the frequency under test is an integral number of periods during
the analysis period, you will have a variation in the sine fitting due
to starting phase.
Say you have N>3 samples, evenly spaced spanning some reasonable number
of cycles of the unknown+noise.
You fit f(i) = A*cos(B*i+C) {where i is the sample #} to the samples
using any of a variety of techniques (least squares).
You can see that B isn't restricted to particular values that are
multiples of 2*pi/N.
OTOH, as admonished in Horowitz & Hill, if the frequency to be counted
is substantially below your counter timebase, then you should count zero
crossings of the higher timebase frequency in the period of the lower
frequency under test.
that's the "reciprocal counter" approach.
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