echo on
t = 0:0.001:1;
x = randn+randn*sin(2*pi*50*t)...
         +randn*sin(2*pi*100*t)...
         +randn*sin(2*pi*150*t)...
         +randn*cos(2*pi*50*t)...
         +randn*cos(2*pi*100*t)...
         +randn*cos(2*pi*150*t);
y = x + 0.2*randn(size(t));
plot(y(1:500))
pause

Y = fft(y,512);

Pyy = Y.* conj(Y) / 512;

f = 1000*(0:256)/512;
plot(f,Pyy(1:257))
pause

Pyy(48:53)
Pyy(98:103)
Pyy(148:153)