Supply the sampling frequency (here‘Fs’) as an argument tofreqz to get the plot in terms of actual frequency (from0 Hz to the Nyquist frequency), where‘NFFT’ is the length of the Fourier transform you want to use (a good choice being2^16):
freqz(sos, NFFT, Fs)
or:
freqz(b, a, NFFT, Fs)
or others, depending on how you implemented your filter.
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