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[SOLVED] does fft function in matlab have gain?

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yamid

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hello all,
i wrote this code:

N=10000; %number of data bits
data=randn(1,N)>=0; %Generate uniformly distributed random data
Fs=20*10.7e+6; %Sampling frequency
Rb=Fs*(1/10);
Tb=1/Rb;
Fc=10.7e+6;
snrdb=10; % Eb/N0 range in dB for simulation
time_bpsk=0:(1/Fs):Tb*(length(data))-(1/Fs);
carrier=cos(2*pi*Fc*time_bpsk);
figure; plot(abs(fft(carrier)));

can someone explain me why the harmonies amplitude is so high? (5*10^4!!!!)
(i know that by theory the harmonies amplitude should be 1/2 if you insert signal with unot amplitude.)
 

Hiya yamid,

No, it doesn't have gain - it's just MATLAB's implementation of the FFT doesn't renormalise the result by 1/N as some other approaches do.
Your code works fine if you either:

a) use the ifft function instead (it DOES renormalise... but bear in mind that the phases of your frequency components will all flip signs), or
b) change the last line to: figure; plot(abs(fft(carrier)/length(carrier)));

Cheers :)
 
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