How do I set my script for FFT of a physiological signal?

Hi everyone,

I sampled a physiological signal every 20ms (50Hz) and I have a vector of 4000 bins. I want to know which is the spectrum of frequencies of this signal. There should be two main frequencies. The one with a deeper sliding should be respiratory frequency and the smaller one on the top of signal is the cardiac pulse artifact. I want to know what are the respective frequencies.

This is my code:

Fs = 50; % Sampling frequency

T = 1/Fs; % Sampling period

Lgp = length(godp); % godp is my physiological signal

tgp = (0:Lgp-1)*T; % Time vector

Ygp = fft(godp);

P2gp = abs(Ygp/Lgp);

P1gp = P2gp(1:Lgp/2+1);

P1gp(2:end-1) = 2*P1gp(2:end-1);

fgp = Fs*(0:(Lgp/2))/Lgp;

figure(1); hold on

subplot(2,1,1)

plot(1000*tgp,godp,'black')

subplot(2,1,2)

plot(fgp,P1gp,'black'); hold on

xlim([-1 10])

title('Single-Sided Amplitude Spectrum of X(t)')

xlabel('f (Hz)')

ylabel('|P1(f)|')

This is what I obtein:

Am I correct? Is the result I get reliable? It seems to me I obtein three peaks: 1) at zero, 2)at 0.46hz 3)0.88hz and maybe one another at 1.4hz.

Answer 1

[Swetha Polemoni]

Hi

It is my understanding that you want to plot single sided amplitude spectrum of a signal.

As per my understanding, the information extracted by you from the attached graph is correct.

Refer to this answer for better understanding of plotting single sided amplitude spectrum.