amplitude of FFT output high compared to input

I'm generating a unipolar PWM of 50Hz for triggering the MOSFETs in a single phase inverter. The output voltage of the inverter is given to a magnitude FFT and its output is given to SpectrumScope. But the amplitude of the output FFT is very high compared to input. For eg: if input = 50V FFT output at 50Hz = 6*10^5 in magnitude squared mode.

Sampling freq given 9.76*10^-6 ,FFT length = 2^11, buffer size = 2^11 and simlation time = 0.02s, buffer overlap =0.

Please help with this problem

Answer 1

[Arturo Moncada-Torres]

Remember that you must scale the output of the FFT, for example:

fs = 250;   % Sampling frequency [Hz].
Ts = 1/fs;  % Sampling period [s].

t = 0:Ts:(10-Ts);           % Time axis.
x1 = sin(2.*pi.*t.*10);     % Sinusoidal with f = 10 Hz.
x2 = 5*sin(2.*pi.*t.*50);   % Sinusoidad with f = 50 Hz.
x = x1 + x2;

N = length(x);                  % Number of samples.

% Plots
figure();
subplot(3,1,1);
plot(t, x);
title('Original signal');
xlabel('Time [s]');
ylabel('Amplitude');

subplot(3,1,2);
plot(abs(fft(x)));
title('Magnitude spectrum with unscaled FFT');
ylabel('Amplitude');

subplot(3,1,3);
plot(abs(fft(x)/N));           % Scaling is done here.
title('Magnitude spectrum with scaled FFT');
xlabel('Frequency [Hz]');
ylabel('Amplitude');

In your case, the difference is bigger because you are using the square magnitude.

Answer 2

[Honglei Chen]

Hi rekh,

You may find the following tech note useful:

http://www.mathworks.com/support/tech-notes/1700/1702.html

HTH