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I am new to FIR filter design. I have a question about the MATLAB filter designer. I notice that the attenuation at the cut-off frequency is fixed at 6dB. So if I want my attenuation to be 3dB, what should I do?

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  • $\begingroup$ That's an inherent condition for FIRs. You can consider a different value, but you'll only complicate with calculations. Better choose an IIR, instead. $\endgroup$ Commented May 8, 2022 at 8:13
  • $\begingroup$ Discussed here. Choose any method other than the window method. $\endgroup$
    – Matt L.
    Commented May 8, 2022 at 12:52

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This is a direct result in using the windowing approach for FIR filter design. With the windowing approach we multiply in time the ideal impulse response (a Sinc function for a brick-wall low pass frequency response for example) with a window that has desirable properties for filtering, notably low side-lobe levels and minimum time-bandwidth (such as the Kaiser and DPSS windows). Multiplying in time is convolution in frequency, thus with a brick-wall frequency response that step in frequency from one to zero at the transition frequency after convolution will be at the half magnitude location or -6dB. This is not the case for all FIR filter design approaches; for example with the Parks McClellan equiripple design we specify passband and stopband frequencies and the cutoff that can be specified directly from the algorithm is the edge of passband where the response first exceeds the passband ripple requirement.

In both cases a simple approach to design to a 3 dB cutoff is to simply iterate; change the target passband accordingly while monitoring the resultant 3 dB cutoff.

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