how to use fft to plot the average values and their standard deviations in the frequency domain?

Hello,

I would like to plot a signal (av) containing average values in the frequency domain, having a vector T containing time values.

But i would also like to put in the same plot the curve of av+ the vector of standard deviations sd

Here's the code i have used:

when i plot only av in the frequency domain i get a curve like this:

However when I plot both, i only get one curve and the other one has null values:

Any ideas of how to do it?

Best regards.

Answer 1

[Sulaymon Eshkabilov]

First of all, both signals need to be in one domain. If the freq domain is chosen then both need to be in the freq domain.

In your shown part of your code, it is in freq domain. That is ok.

Presumably as shown here the magnitude of the two signals differ significantly and thus, they are not visible. To make both signals visible, use yyaxis l;eft and yyaxis right - see example:

fs = 2000;    % Sampling frequency  
t =0:1/fs:5;  % Time 
F1 = 5;       % Lowest frequency
F2 = 17;      % Mid frequency
F3 = 55;      % Highest frequecy    
X1 = 0.25*sin(2*pi*t*F3)+randn(size(t))*.5;
X2 = 3*sin(2*pi*t*F1)+cos(2*pi*t*F2)+randn(size(t))*.5+X1;
yyaxis left
N = 2^nextpow2(length(X1));
Y1 = fft(X1, N);               % FFT of the signal x1(t)
Y2 = fft(X2, N);               % FFT of the signal x2(t)
freq = fs*(0:(N/2))/N;         % Freq values
Yp1 = abs(Y1/N);               % Absolute value or magnitude of the signal spectrum
Yp2 = abs(Y2/N);             
yyaxis left
plot(freq,Yp1(1:N/2+1)) 
ylabel('Magnitude of signal = |Y1(f)|')
yyaxis right
plot(freq,Yp2(1:N/2+1))
title('FFT analysis of a signal')
xlabel('Frequency, [Hz]')
ylabel('Magnitude of signal = |Y2(f)|')
grid on
xlim([0, 65])

[Matthew Worker]

It worked! Thank you so much.

[Sulaymon Eshkabilov]

Most welcome. Glad to help!