How to Calculate the transfer function and plot it after making fourier transform in frequency domain in terms of magnitude and phase?

Question

I have a trouble of plotting the transfer function in terms of magnitude and phase after making the Fourier transform of two signals.

First, I have used excel xlxs to read the column to plot in time domain and frequency domain, then I calculate the transfer function and this goes fine.

But for magnitude and phase, I have trouble for plotting them. I tired to plot them, but this is totally incorrect. Would someone help me with this?

Here is the code and excel file also.

solarcell1 = xlsread('solarcell.xlsx','A2:C100005');
t=solarcell1(:,1);
N=length(t);
Channel1V = solarcell1(:,2);
Channel2V = solarcell1(:,3);
sig1=Channel1V;
sig2=Channel2V;
fs=1/((solarcell1(3)-solarcell1(2))*1e-3);
FA1=fs/length(sig1);
FA2=fs/length(sig2);
frange1=-fs/2:FA1:fs/2-FA1;
frange2=-fs/2:FA2:fs/2-FA2;
subplot(3,2,1);
plot(t,sig1);
hold on
plot(t,sig2);
title('Input and Output of Solar cell');
xlabel('Time');ylabel('Amplitude');
subplot(3,2,2);
plot(t,sig2);
title('Output');
xlabel('Time');ylabel('Amplitude');
z1=fftshift(fft(sig1))/length(t);
subplot(3,2,3);
plot(frange1,abs(z1));
title('Input');
xlabel('Freq.');ylabel('Amplitude');
z2=fftshift(fft(sig2))/length(t);
subplot(3,2,4);
plot(frange2,abs(z2));
title('Output');
xlabel('Freq.');ylabel('Amplitude');
TFC=z2./z1;
magnitude=20*log(abs(TFC));
phase=atan2(imag(TFC),real(TFC));
subplot(3,2,5);
bode(frange1(1002:N),magnitude(1002:N));
title('Magnitude');
xlabel('Freq.');ylabel('Magnitude');
subplot(3,2,6);
semilogx(frange1(1002:N),phase(1002:N));
title('Phase');
xlabel('Freq.');ylabel('Phase');

alpereira7 alpereira7 · Accepted Answer · 2019-01-15T14:22:34

I can see three issues in the code you provided.

First, your usage of bode is incorrect. This function takes a dynamic model system as argument, and you gave two vectors frange1(1002:N)and magnitude(1002:N). For now, I suggest you simply use plot instead.

subplot(3,2,5);
plot(frange1(1002:N),magnitude(1002:N));
title('Magnitude');
xlabel('Freq.');ylabel('Magnitude');

Then, your usage of semilogx is also risky when you have negative values for the x-axis. With frange1=-fs/2:FA1:fs/2-FA1; you shouldn't ask for semilogx of it. For now, I suggest plot.

Finally, I suggest you use unwrap to plot the phase.

subplot(3,2,6);
plot(frange1(1002:N),unwrap(phase(1002:N)));
title('Phase');
xlabel('Freq.');ylabel('Phase');

It plots:

How to Calculate the transfer function and plot it after making fourier transform in frequency domain in terms of magnitude and phase?

1 Answers