No, it isn't restricted to an integral number of periods, but you
will have a higher variance in your fits unless there are, due to
variations of the fit with respect to initial phase.
I don't want to send this thread in the wrong direction... but we
have to agree on apples vs apples comparisons. How many samples per
second are being counted, versus what is the sample rate in the ADC?
Offhand, I totally agree with you, but that is a gut instinct. And
what I'm trying to do is force a showdown between gut instinct and
facts. Are there facts that would back this assertion?
Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ 85750
email: [email protected]
phone: 1.520.390.3995
web: http://refined-audiometrics.com
On Oct 14, 2010, at 06:16, jimlux wrote:
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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