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View Full Version : Re: Fourrier Analysis of EMG



Maillard Patrick (fv/sla)
03-21-2000, 09:43 PM
> Nathan,
>
> How to choose the sampling frequency fs ?
> The Nyquist Theorem says, that only the signal whose frequency components
> are smaller than fs/2 is correctly sampled. Every signals whose freq is
> above fs/2 will be sampled also, but its frequency component is translated
> to another frequency in the band DC-fs/2.
>
> For example, if you sample at fs=3KHz a sinusoid of 1V at 1KHz, after
> sampling its Fourier Transform will be a peak of 1V at 1KHz (that is OK).
> But if you sample at 3KHz a sinusoid of 1V at 2KHz, after sampling its
> Fourier Transform will be a peak of 1V at 1KHz also ! That is false, and
> you cannot remove it from your signal (aliasing).
>
> So, if you want to sample a signal at fs, you need to pass your analog
> signal through an anti-aliasing filter, in order to remove any component
> (signal and noise) higher than fs/2.
> For example, an 8th-order elliptic filter with a cutoff frequency at 1KHz
> will remove any signal whose freq component is at and above 1.5KHz.
> (between 1KHz and 1.5KHz the signal will be attenuated). Therefore you can
> sample it at 3KHz.
> Maybe your acquisition system has already such a filter build-in it, maybe
> not. You should verify that very important point.
>
> In http://www.teleemg.com/Chapters/jbr011.htm you can read: "Sensory
> studies are performed with the low frequency setting between 32 and 50 Hz
> and the high frequencies between 1.6-2 and 3 KHz."
> Therefore, if you consider that the signal you want to measure reaches
> 1.6KHz, you need to put your anti-aliasing filter cutoff frequency at
> 1.6KHz. This filter will remove any signal above 2.4KHz then you can
> sample at 5KHz.
>
> If you want to sample at 3KHz, then put your filter at 1KHz. But check
> carefully the quality of the EMG after filtering.
>
> Patrick
>
>
> Patrick Maillard
> Institut de Recherches Robert BOSCH SA
> PO BOX 12, Route de Denges 2
> CH-1027 LONAY, SWITZERLAND
> phone: ++41 21 804 72 17
> fax: ++41 21 804 72 05
> e-mail: Patrick.Maillard@ch.bosch.com
>

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