MATLAB code for Power Spectral Density

>ngeva0 wrote:
>
>> ... Can you explain what does the "2 volts
>> peak to peak" mean? I don't get that. Thank you.
>
>Consider the function y = A*sin(wt). the positive peak value is +A and 
>is easy to see on an oscilloscope. The value for y = zero is hard to 
>estimate accurately, but the negative peak, -A, is easy to see. We 
>measure the waveform as having a peak-to-peak value of 2*A and often 
>describe it that way. For any function, peak to peak is the entire range

>that it needs.
>
>Jerry
>-- 
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������
>

Thank you.

Hi,

If you're looking to learn how to compute accurate spectral estimates, a 
particular favorite text around here is Stoica  & Moses (Intro to Spectral 
Analysis).

If you want to use preprogrammed software for this, I encourage you to use 
two methods in the Signal Processing Toolbox (if you have it).  Type "help 
spectrum/psd" and "help spectrum/msspectrum" to learn how to compute PSD and 
mean-square spectrum estimates.  These are two well-defined estimates that 
are useful in practice.  Quick examples taken from the methods cited above 
to get you started:

PSD EXAMPLE (if you're looking for a spectral density on, say, a noisy comms 
signal or a general continuous spectral distribution):

     %  Spectral analysis of a signal that contains a 200Hz cosine plus 
noise.
       Fs = 1000;   t = 0:1/Fs:.296;
       x = cos(2*pi*t*200)+randn(size(t));
       h = spectrum.welch;                  % Instantiate a welch object.
       psd(h,x,'Fs',Fs);                    % Plot the one-sided PSD.

MSS EXAMPLE (if you have distinct sines, e.g. discrete spectra, and want 
amplitude levels - sounds like your example):

       % In this example we will measure the power of a deterministic
       % power signal which has a frequency component at 200Hz. We'll use a
       % signal with a peak amplitude of 3 volts therefore, the theoretical
       % power at 200Hz should be 3^2/2 volts^2 (watts) or 6.53dB.
       Fs = 1000;   t = 0:1/Fs:.2;
       x = 3*cos(2*pi*t*200);
       h = spectrum.welch;           % Instantiate a welch object.

       % Plot the one-sided Mean-square spectrum.
       h.FFTLength = 'UserDefined';
       msspectrum(h,x,'Fs',Fs,'NFFT',2^14);

Regards,
--Don


"ngeva0" <ngeva0@hotmail.com> wrote in message 
news:ne-dnR4SaOiNswfenZ2dnUVZ_tqdnZ2d@giganews.com...
> Can anyone please check if my code is correct? SINE512 is just a file 
> name.
> This file has 512 data points and it's a sine wave. Thank you!!
>
> %Sampling frequency
> Fs=50000;
>
> %# of samples in the data
> datasize=size(SINE512);
> numsample=datasize(1);
>
> %Windowing
> H=hann(numsample);
> W=H.*(SINE512(:,2));
>
> %Fourier Transform
> FFTX=fft(W,numsample);
>
> %Power: magnitude^2
> X=FFTX(1:floor(numsample/2)).*conj(FFTX(1:floor(numsample/2)));
>
> %Bandwidth
> BW=1.5*Fs*numsample;
>
> %PSD=magnitude^2/bandwidth
> PSD=X/BW;
>
> %Computing the corresponding frequency values
> Omega=Fs*(0:numsample-1)/numsample;
> Omega=Omega(1:floor(numsample/2));
>
> %Plot PSD
> H=plot(Omega,PSD);
> set(H,'Color','BLACK');
>