Re: Best way to measure precise harmonics?

On Oct 18, 2:07 pm, Tom Bruhns <k7...@xxxxxxx> wrote:
One of the problems you'll have is the length of time over which you
can make your measurement. To nail a frequency as accurately as you
want, if you don't know something a priori about it, you need to
observe it for a long time. Will the strings vibrate for 1000
seconds? If not, then realize that you are not dealing with a single
frequency but a spectral density. The attack and decay envelopes
modulate the string's natural frequency.

I can pretty easily measure frequencies in the audio range to 0.001Hz
resolution, IF they stick around long enough. FFT-based spectrum
analyzers worth having should have "zoom" capability, allowing you to
set essentially any center frequency you want and then set the span
very low. _IF_ you have a priori knowledge that the signal you are
looking at is a pure sinewave (that is, will maintain the same
amplitude for a long time and is not polluted by other signals at
other frequencies), you can very accurately determine its frequency in
a much shorter time. That's because you can measure the period of a
relatively small number of cycles. But any other signals, especially
ones not harmonically related to the tone you're looking at, will mess
up the waveform so that the period from zero crossing to zero crossing
is not constant from one cycle to the next. Ultimately, you'll be
limited by inevitable noise that will cause the same problem.

I have a frequency counter that finds low frequencies by the method of
inverting the waveform's period (or the period of multiple cycles),
but I can still get better accuracy in a given time using an FFT-based
spectrum analyzer, and have the added benefit of being able to observe
the shape of the FFT'd input spectrum, which gives confidence that the
signal is (or isn't) clean enough to use for accurate frequency
measurement. With an Agilent 89410 analyzer, I can get a 3200 point
FFT with 1Hz span centered down to 1 millihertz resolution, but at
that span and than number of points, it takes llllooooong time to
make a measurement. On the other hand, with that a priori knowledge
about the signal, I can resolve easily a tenth of the spacing between
FFT points, knowing the details of the windowing function (which
determines the filter shape that each FFT point represents).


Please read all of my posts in this thread. I have infinite sustain
time and I don't need to use a frequency counter to determine
frequency. The frequencies are much too low for that. I can measure
the period in the time of one vibration of the string (<40ms) and it
is much more accurate.

Don

.

Relevant Pages

  • Re: Best way to measure precise harmonics?
    ... can make your measurement. ... But any other signals, especially ... spectrum analyzer, and have the added benefit of being able to observe ... the harmonics you want to look at more carefully, ...
    (sci.electronics.design)
  • Re: Best way to measure precise harmonics?
    ... can make your measurement. ... But any other signals, especially ... spectrum analyzer, and have the added benefit of being able to observe ... How are you getting infinite sustain time, and how do you keep your ...
    (sci.electronics.design)
  • Re: the way in which colour is seen
    ... What "really is there" is a spectrum. ... of the eye. ... > signals, based upon the brightest signal from each band, combined ... of illumination. ...
    (sci.astro.amateur)
  • Re: time variant or invariant output?
    ... If the system is time invariant, ... signals by some time t_s: ... Any system can _change_ the spectrum of the input. ... I mean by change the spectrum is new frequencies are introduced not just ...
    (comp.dsp)
  • Re: KEN SETO, THE RUNT OF THE AETHERIALISTS, AGREES DOPPLER SHIFT IS A CHANGE IN LIGHT SPEED
    ... Hence whatever star the spectrum M was taken from, ... When you have a good match you'll know the velocity. ... |> futz around with precise measurement and nitpick the details. ... The first bowl is from the hot tap. ...
    (sci.physics.relativity)