How to design a FIR filter by the -3db bandwidth?

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斌政刘 2022-7-25

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评论：斌政刘 2022-7-25

When I try to design a FIR low-pass filter by using -3db bandwidth as the cutoff frequency, I find that the amplitude of the cutoff frequency cannot reach -3db, and I design the filter by the FDAtool, how to solve this problem or is my design method wrong? Thanks for any help you can provide.

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Answer 1

Chunru 2022-7-25

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编辑：Chunru 2022-7-25

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Generally speaking, you don't have to worry about -3dB attenuation at the cut-off. You just specify the pass-band and stop band with certain transition band.

However, if you do want to have a deired -3dB response. One appraoch is to use generalized digital Butterworth filter (IIR not FIR) as show below. There are other approaches for FIR (such as using least square methods to fit the freq response).

num = 10; den = 2; wn = 0.2;

[b, a] = maxflat(num, den, wn);

[h, w] = freqz(b, a);

plot(w/pi, 20*log10(abs(h)));grid

ylim([-10 0])

xline(0.2); yline(-3)

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Chunru 2022-7-25

You are right. Very often, the important design parameters of a filter (such as LPF) are given in the following figture, where -3db attenuation at cut-off freq (fpass) is not specified.