Re: Best way to measure precise harmonics?

On Oct 19, 1:37 am, Martin Brown <|||newspam...@xxxxxxxxxxxxxxxxxx>
wrote:
On Oct 18, 2:22 pm, eromlignod <eromlig...@xxxxxxx> wrote:

Hi guys:

I need to find the component harmonic frequencies of an AF wave and I
need for it to be pretty precise (+/- .001 Hz or so). I have access
to a spectrum analyzer, but it just doesn't seem to be precise enough
(or I'm using it wrong). It gives me peaks in a frequency domain, but
they are not pinpoint lines, ostensibly due to a limited-sample FFT.

That goes with the territory. To obtain a +/- 0.001 Hz frequency
resolution you would have to measure the signal for ~1000 seconds (you
might get away with 250s using some mathematical symmetry tricks if
you can control the initial conditions well enough).

For an upper frequency limit of A at 440Hz and oversampled by x2 that
is roughly 1,000,000 samples and should be doable with relative ease
in software on a PC. Your required precision seems to be severe
overkill, but let that pass. To go beyond the edge of human hearing at
20kHZ and oversampled x2 it is 40,000,000 samples which is still
doable but a fair bit slower - and you might want to go for 40kHz for
added headroom.

BTW You probably want to measure your signal for a fixed number of
cycles of the fundamental.



Are there any other devices or methods to obtain accurate frequencies
of each harmonic to three decimal places? Thanks for any suggestions
you might have.

FFT will do it if you can supply enough data in the time domain
(choose a 2^N FFT). Practical implementations will require potentially
anti-alias regridding and a few other tweaks to sort out boundary
conditions.

Some hardware FFT based analysers can zoom in on a region of interest,
but non of them can get around the uncertainty principle. A short
burst of pure tone decaying in amplitude always contains a range of
frequencies around the fundamental.

Regards,
Martin Brown

Question: How good are todays A to D Converters?
Will the conversion introduce serious artifacts?

I'm thinking that once the wave form is in digital
form it's just software to compare that to a perfect
sine wave at various frequencies. I'm sure that's
be done.
Regards
Ken

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