Trying to add Butterworth filter after fft to obtain only some of the original signal.

Hello, I have a group of signals that I've added together and done the FFT. Now I'm trying to add a Butterworth low pass filter to only obtain the first three signals with a cutoff of 74 Hz. However, I'm not sure how to add this filter. I included the butter function but it didn't provide the correct graph.

sample_rate = 4e6;

duration = 1.5;

t = linspace(0,duration,sample_rate*duration);

amp_sum = amp_1 + amp_2 + amp_3 + amp_4 + amp_5 + amp_6 + amp_7 + amp_8;

Y = fft(amp_sum);

L = length(amp_sum);

P2 = abs(Y/L);

P1 = P2(1:L/2+1);

P1(2:end-1) = 2*P1(2:end-1);

f = sample_rate*(0:(L/2))/L;

[b, a] = butter(2, 74/(sample_rate/2), 'low'); %Cutoff frequency of 74 to only obtain the first three signals

P = filter(b, a, P1);

Answer 1

[Paul]

Hi Alejandro,

The input to filter should be the time domain sequence to be filtered, not its Fourier transform.

So I guess that should be

amp_sum_filtered = filter(b,a,amp_sum);