N=2000;
L=3000;
k=0:N-1;
f1=0.21;                                                                   % frequency of signal 1
f2=0.22;                                                                   % frequency of signal 2
phase1=2*pi*rand(1);                                                       % random phase 1
phase2=2*pi*rand(1);                                                       % random phase 2
Wk=randn(1,N);
SNR1=10;
SNR2=15;
a1=sqrt(2*10^(SNR1/10));                                                   % amplitude of signal 1
a2=sqrt(2*10^(SNR2/10));                                                   % amplitude of signal 2
xk=a1*sin(2*pi*f1*k+phase1)+a2*sin(2*pi*f2*k+phase2)+Wk; 
window=boxcar(length(xk));
[pxx,f]=periodogram(xk,window,L,Fs);
figure(1)
plot(f,10*log10(pxx))