Fs=500;
x=-1+(1-(-1)).*rand(100,1);
subplot(2,2,1);
plot(x);
title('stochatic signal');
R=xcorr(x,'coeff');
subplot(2,2,2);
plot(R);
title('auto-correlation');
window=boxcar(length(x)); 
nfft=250; 
[Pxx,f]=periodogram(x,window,nfft,Fs); 
subplot(2,2,3);
plot(f,10*log10(Pxx));
title('power-spectrum');