Perhaps something like this —
x = randn(2000, 1);
Fs = 250;
L = numel(x);
t = linspace(0, L-1, L).'/Fs;
figure
plot(t, x)
grid
xlabel('Time')
ylabel('Amplitude')
filt_x = lowpass(x, 60, Fs);
filt_x3040 = filt_x(30:40); % Select Segment Of Signal
Fn = Fs/2;
Lx = numel(filt_x3040);
NFFT = 2^nextpow2(Lx);
FTfilt_x3040 = fft(filt_x3040 .* hann(Lx), NFFT)/Lx;
Fv = linspace(0, 1, NFFT/2+1)*Fn;
Iv = 1:numel(Fv);
figure
plot(Fv, abs(FTfilt_x3040(Iv))*2)
grid
xlabel('Frequency')
ylabel('Magnitude')
title('Fourier Transform Of ‘filt\_x’ (30:40)')
The bandpass call as originally stated is actually a lowpass call. The bandpass passband must be greater than 0 and less than the Nyquist frequency.
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