# noise in signal

I am trying to add phase noise in signal. However, when i see the graph in frequency domain the spectrum is not broading. Can anyone tell me what mistake i am making?

sigma = 0.5;      %%% standard deviation,
t = 0:0.01:1;
sig = cos(2*pi*4*t); % original signal
A = sigma *randn(1,length(t)); % standard deviation of phase noise, increase
sig2 = (A).*cos(2*pi*4*t + sigma*rand(1,length(t))); %signal with phase noise & amplitud4e noise
figure,plot(t,sig,'r',t,sig2, 'b');
figure, plot(abs(fft(sig,10*length(sig))));
figure, plot(abs(fft(sig2,10*length(sig2))));
return

• Of course the spectrum changes a lot, look again. – Matt L. Jul 16 '15 at 14:41
• Hi Matt L, since i am intrerested in phase noise only, i am expecting broading of pulse in frequency domain. – Urban_Yogi Jul 16 '15 at 14:43
• OK, so you think that too much is changing, right? First of all, note that you also add a lot of amplitude noise, and second, decrease the amount of phase noise. – Matt L. Jul 16 '15 at 14:48
• i mean, if i change sigma t0 3 or 4 than only phase noise is changing . However, in that case fft peaks are not visible. Moreover, i need to measure the phase noise. – Urban_Yogi Jul 16 '15 at 14:54