Problem using biquad filter object

Question

Vincent St-Pierre ，2022-11-21

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https://ww2.mathworks.cn/matlabcentral/answers/1857698-problem-using-biquad-filter-object

回答： Askic V ，2022-11-21

I have the following code to demodulate AM signal using frequency shifting and filtering :

clc;clear all;
Fs=44100; Ts=1/Fs;
t=0:Ts:4;
s1=cos(2*pi*60*t);
carrier=cos(2*pi*1000*t);
smod=s1.*carrier;
N     = 2;      % Order
Fpass = 100;     % Passband Frequency
Apass = 1;      % Passband Ripple (dB)
Fs    = 44100;  % Sampling Frequency
h = fdesign.lowpass('n,fp,ap', N, Fpass, Apass, Fs);
Hd = design(h, 'cheby1','SystemObject', true);
sdemod = smod.*carrier;
sdemod2 = Hd(sdemod);

Now, when I use the Signal Analyser to see the result I have this :

The frequency shift succeeded but the filter is not behaving like intended and is just attenuating everything.

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Answer 1

William Rose 2022-11-21

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https://ww2.mathworks.cn/matlabcentral/answers/1857698-problem-using-biquad-filter-object#answer_1106528

@Vincent St-Pierre,

I'm not sure why your code does not work, but a simple 4th order Butterworth works fine. See code below.

Fs    = 44100;  % sampling rate (Hz)
Ts=1/Fs; t=0:Ts:4;
s1=cos(2*pi*60*t);
carrier=cos(2*pi*1000*t);
smod=s1.*carrier;
N=4;            % filter order
Fpass = 100;    % passband frequency (Hz)
[b,a]=butter(N,Fpass/(Fs/2));   % Butterworth filter coefficients
sdemod = smod.*carrier;
sdemod2=filter(b,a,sdemod);     % apply lowpass filter to sdemod