how to find frequency content of original, interpolated and filtered data in matlab

main.m
% Generate original signal
t = linspace(0, 1, 1000);
x = sin(2*pi*5*t) + sin(2*pi*10*t); % signal with 5 Hz and 10 Hz components

% Interpolate signal
tq = linspace(0, 1, 5000); % new time vector with more samples
xq = interp1(t, x, tq, 'linear');

% Filter signal
fs = 5000; % sampling frequency
f_nyquist = fs/2; % Nyquist frequency
Wn = [6, 11]/f_nyquist; % passband from 6 Hz to 11 Hz
n = 200; % filter order
b = fir1(n, Wn); % filter coefficients
xf = filter(b, 1, xq); % apply filter to interpolated signal

% Compute FFT of signals
X = fft(x);
Xq = fft(xq);
Xf = fft(xf);

% Compute frequency axis
f = (0:length(X)-1)/length(X)*fs;

% Plot results
figure
subplot(3,1,1)
plot(t, x)
title('Original Signal')
xlabel('Time (s)')
ylabel('Amplitude')
subplot(3,1,2)
plot(tq, xq)
title('Interpolated Signal')
xlabel('Time (s)')
ylabel('Amplitude')
subplot(3,1,3)
plot(tq, xf)
title('Filtered Signal')
xlabel('Time (s)')
ylabel('Amplitude')

figure
subplot(3,1,1)
plot(f, abs(X))
title('Original Signal')
xlabel('Frequency (Hz)')
ylabel('Magnitude')
xlim([0, 20])
subplot(3,1,2)
plot(f, abs(Xq))
title('Interpolated Signal')
xlabel('Frequency (Hz)')
ylabel('Magnitude')
xlim([0, 20])
subplot(3,1,3)
plot(f, abs(Xf))
title('Filtered Signal')
xlabel('Frequency (Hz)')
ylabel('Magnitude')
xlim([0, 20])
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