I'm trying to bandpass filter the signal between 10 to 450Hz, but something keeps making the output weird. The when the lower cutoff is 30, it still works as expected, but if it gets to 20 or below it get's wonky. Why is this happening?
Fs = 1000;
t = 0:1/Fs:1;
x = [2 10 20 15 2]*sin(2*pi*[5 60 150 250 550]'.*t) + randn(size(t))/10;
Y = fft(x);
L = length(x);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
x = abs(x);
bandpass(x,[30 450],Fs)
