# Re: Length of signal vs. frequency resolution

• From: Jerry Avins <jya@xxxxxxxx>
• Date: Wed, 22 Mar 2006 17:59:19 -0500

M.L. wrote:
Hi NG,

I would like to have someone confirm (or not), that I have understood
the following correctly:

I have a signal which is sampled at .125 Hz. In this signal, a pulse
lasting for 130 samples (i.e. 1040 s) is present. I would like to
analyze this pulse and see the frequency content of it. I therefore
window it with some window (Hanning or something like it) and calculate
the FFT. In my spectrum I can now resolve frequencies up to .125/2
(Nyquist), and I will have 130/2 frequency bins in my spectrum which
will govern the frequency resolution. I can therefore resolve 65
different frequencies between 0 Hz and 0.0625 Hz.

So, by shortening the window I am looking at, I decrease the number of
frequency bins (i.e. resolution), but I do not change the limits of the
frequencies I can resolve?

Is this correct? And if not, can somebody set me straight? Should be
simple..

Yes, if.... Is it possible for the signal you sampled to have component frequencies higher than 1/16 second? If there are any, they will contaminate the components you're looking for. As an example, it would be impossible after sampling to distinguish a component at .070 Hz from one at .055.

Jerry
--
Engineering is the art of making what you want from things you can get.
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.

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