Re: FIR filter question



Jamie Morken wrote:

Hi,

When using a lowpass FIR filter, with 100kHz sampling and 1kHz -3dB cutoff frequency, is there a minimum number of coefficients required for the filter to operate properly similar to a Nyquist minimum? I was thinking that there would be 100 or even 200 coefficients required since the cutoff frequency is 100 times lower than the sampling frequency so that the low frequency data can remain in the coefficient data long enough, but I am not sure if this is correct..


First, having a FIR filter with 100kHz sampling and 1kHz cutoff doesn't look like a good idea. For the cases like that, you should use IIR and/or make the sampling rate lower.

Very roughly, the required length of the FIR LPF can be estimated as a period of the cutoff frequency. There are more exact formulae for the FIR length depending on the passband flatness and stopband attenuation.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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