FFT of signal from text file

Hi, I am trying to plot an FFT for an EMG signal which is saved in a text file. I did manage to plot the actual signal, but when I plotted the FFT, the window remained blank. Can someone please highlight what I am doing wrong. Also, How to determine the sampling frequency value (Fs)?

Attached please find the text file containing the EMG signal data. Thanks in advance.

Ts = mean(diff(t));                                 % Sampling Time
Fs = 1/Ts;                                          % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency

FEMG = fft(EMG)*2/L;                                % Fourier Transform (Normalised)
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                 % Frequency Vector
Iv = 1:length(Fv);                                  % Index Vector

figure(1)
plot(Fv, abs(FEMG(Iv)))
grid

Answer 1

[Star Strider]

I don’t see the error in your code, but the code here I wrote (plotting half of the fft) works:

fidi = fopen('John Smith emg.txt', 'rt');
T_EMG = textscan(fidi, '%f%f', 'Delimiter',' ');
t = T_EMG{1};
EMG = T_EMG{2};
L = length(t);

Ts = mean(diff(t));                                 % Sampling Time
Fs = 1/Ts;                                          % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency

FEMG = fft(EMG)*2/L;                                % Fourier Transform (Normalised)
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                 % Frequency Vector
Iv = 1:length(Fv);                                  % Index Vector

figure(1)
plot(Fv, abs(FEMG(Iv)))
grid

[John Smith]

Thanks. It worked :)

[Star Strider]

My pleasure!

[krn99]

FEMG = fft(EMG)*2/L; in above line why you are using 2/L; can you please tell me

[Star Strider]

@krn99 — See the documentation for the fft (link) function.

The Fourier transform is a sum-of-sinusoids, so dividing by the original length of the data vector normalises the energy in the Fourier transform of the signal.

[krn99]

thanks

[Star Strider]

My pleasure.

[krn99]

sir i have used 10hz cutt of freq(fc) high pass and 1000hz fc for low pass FIR filter(4th order). i have used filtfilt function to avoid zero lag. But my doubt is do i want to do divide my cut off frequency with 0.802 for fourth order which is mentioned in Winters and Robertson book. i dont understand their concept. if you know please tell me. Thanks in advance