MATLAB: How to filter correctly

filter fft fourier

Hi,
I have two small signals sampled both on 15360Hz:
1)"almost" a SIN, with fundamental frequency on 60Hz. 2) a noise signal (that would represent a SIN too), with fundamental frequency on 60Hz too.
I want to smooth the second signal, so, I did a 2. order butterworth LP filter, cutting over 200Hz.
So, plotting the 2 signal, I got the noised signal smoothed like I want.
The problem is,
If I took the same filter (LP) on the 1 signal, it will change its magnitude. Why (the first signal just have signals down the 200Hz) ??? Am I missing some theory ??
My butterworth:
function Hd=filtsignal
Fs=15360;
N=2; % order
Fc = 200; % cutoff
h=fdesign.lowpass('N,F3db', N, Fc, Fs);
Hd=design(h,'butter');
On my main code:
% y is the 1 signal
  y=filter(filtsignal,y);
% y2 is the 2 signal
  y2=filter(filtsignal,y2);
Andre

Best Answer

I don’t completely understand. However if you filter out noise, the magnitude of the output signal from the filter will decrease because you’ve removed some of the energy in the signal.
Also, I suggest using filtfilt rather than filter. It eliminates phase distortion.
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