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FFT with sub-Hz resolution?

Started by me4dtrade January 7, 2009
Newbie question:

I am learning FFT and wandering how to achieve sub-Hz resolution. If
the sampling rate is Fs, and you take N points out of the samples to
do FFT, then the maximum resolution is 1Hz. Or can you take more
points than sampled in a second to achieve sub-Hz resolution?

Maybe the zoom FFT can achieve sub-Hz resolution?

Can somebody enlighten me on this? Thanks!

me4dtrade asked:
> can you take more > points than sampled in a second
Yes -jim
On Jan 7, 11:56&#2013266080;am, me4dtrade <me4dtr...@gmail.com> wrote:
> Newbie question: > > I am learning FFT and wandering how to achieve sub-Hz resolution. If > the sampling rate is Fs, and you take N points out of the samples to > do FFT, then the maximum resolution is 1Hz. Or can you take more > points than sampled in a second to achieve sub-Hz resolution? > > Maybe the zoom FFT can achieve sub-Hz resolution? > > Can somebody enlighten me on this? Thanks!
If you need to detect a 1/2 hz sinewave, then the FFT needs 2 seconds worth of data to see a complete 1/2 hz sinewave. Likewise if you need to resolve a 1,000,000Hz and 1,000,000.5 Hz sinewaves, you need 2 seconds worth of data before the FFT can see the difference between the two signals.
me4dtrade wrote:
> Newbie question: > > I am learning FFT and wandering how to achieve sub-Hz resolution. If > the sampling rate is Fs, and you take N points out of the samples to > do FFT, then the maximum resolution is 1Hz. Or can you take more > points than sampled in a second to achieve sub-Hz resolution? > > Maybe the zoom FFT can achieve sub-Hz resolution? > > Can somebody enlighten me on this? Thanks!
Where do you get 1Hz??? The frequency resolution is the reciprocal of the time span of the samples. For Sub-Hz resolution this means it must be greater than 1 second. So, if the sample rate is fs, then the sample interval is 1/fs. And the number of samples "N" must be such that N/fs > 1 second. Thus N > 1 second * fs hz a dimensionless number. Fred
Fred Marshall wrote:
> me4dtrade wrote: >> Newbie question: >> >> I am learning FFT and wandering how to achieve sub-Hz resolution. If >> the sampling rate is Fs, and you take N points out of the samples to >> do FFT, then the maximum resolution is 1Hz. Or can you take more >> points than sampled in a second to achieve sub-Hz resolution? >> >> Maybe the zoom FFT can achieve sub-Hz resolution? >> >> Can somebody enlighten me on this? Thanks! > > Where do you get 1Hz??? > > The frequency resolution is the reciprocal of the time span of the > samples. For Sub-Hz resolution this means it must be greater than 1 > second. > So, if the sample rate is fs, then the sample interval is 1/fs. > And the number of samples "N" must be such that N/fs > 1 second. > > Thus N > 1 second * fs hz a dimensionless number. > > Fred
And, I might have added, that selecting N is a function of how "sub" you want to go. Fred