I want to model phase noise. Code below shows how I am adding phase noise in the signal. However, i am not getting correct output. That means, the signal sprectrum shoud be spreading in the frequency domain however, its not changing the phase of the signal. please, anyone can tell me how to model phase noise.

```
sigma = 0.5;
t = 0:0.01:1;
sig  = cos(2*pi*4*t);                                                        % original signal
A = sigma *randn(1,length(t));                                          % standard deviation of phase noise
sig2 = (A) .* cos(2*pi*4*t + sigma*randn(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))));    %%% takin absolute value of fft and zero padding
figure, plot(abs(fft(sig2,10*length(sig2))));
return

```