PSD calculation using Matlab{help needed}

[qslazio]

calculate psd

I'm writing a Matlab m-file to calculate PSD of sinusoid signal.
However the output puzzle me a lot.

Below is the m-file:
------------------------------------------------------------------
fs =1e10;
fi =1e8;
amp =1;
nbpts =4e5;
t =[0:1/fs:nbpts/fs];
sig =amp*sin(2*pi*fi*t);
Hs = spectrum.periodogram('rectangular');
Hdsp2 = psd(Hs,sig,'Fs',fs,'NFFT',nbpts,'SpectrumType','onesided');
Pow = avgpower(Hdsp2,[0.9*fi,1.1*fi]);
plot(Hdsp2);
------------------------------------------------------------------


Here, I intended to calculate psd of this sinusoid of which the frequency is 100Meg Hz and amplitude is 1. I expected to see this 0.5*δ(w-100Meg) impulse on this one-sided PSD plot.
But instead, there is a impulse of which the amplitude is around -50dB, which I cannot explain it at all.

Can anyone tell me what's wrong with the m-file?
Thank in advance!!

 

[lassmat]

psd calculation

There is function in matlab call psd().This will calculate the psd of any signal you pass to it and plot it.Try help psd and will get all the details.You can even check the code of that function.

 

[sohkha]

Re: psd calculation

psd() there is no such function in matlab

 

[JoannesPaulus]

yes, there is. Try help psd (no parenthesis).

 

[sohkha]

thanks mate i got it.

 

[stevenchen1979]

The following convertion leads to correct power value 0.5:

10^(-47/10)*(5e9/200001) = 0.5

-47: The dB value of PSD maximum point.
5e9: Sample Rate.
200001: Point in the PSD plot.