Problem using biquad filter object

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.

Answer 1

[William Rose]

@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

Here is a screen shot of the power spectra of the signals, from signalAnalyzer:

Try it. Good luck.

Answer 2

[Askic V]

Even better, I would use filtfilt function in order to have zero lag:

clc;clear all;
Fs=44100; Ts=1/Fs;
t=0:Ts:0.2;% to beeter see it on plot
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)


[b,a] = cheby1(N, Apass, Fpass*2/Fs);
sdemod = smod.*carrier;
sdemod2 = 2*filtfilt(b,a, sdemod);

subplot(311)
plot(t, s1);
subplot(312)
plot(t, smod);
subplot(313)
plot(t, sdemod2);