PSD question

What are the units of the Y axis of this code? I think it's dB/Hz but I'm not sure.

fs=2000;

Hs=spectrum.periodogram; % Use default values

psdest = psd(Hs,Signal,'Fs',fs);

semilogx(psdest.Frequencies,10*log10(psdest.Data));

grid on;

Answer 1

[Wayne King]

Yes, it is dB/Hz.

psdest.Data are the power estimate, proportional to magnitude-squared of the DFT. So 10*log10() is in dB

Answer 2

[Wayne King]

Hi, below I provide an example using just the DFT and then applying the proper scaling and the spectrum.periodogram object, so you can see how to do it the "brute force" way.

    dt=1/10000; 
    t=0:dt:.1024-dt; %time vector of length 1024  
    y=cos(2*pi*1000*t)+randn(size(t)); 
    h=spectrum.periodogram; %spectrum object (periodogram)
    psd_est_1side=psd(h,y,'spectrumtype','onesided','Fs',10000);
    y_fft=abs(fft(y)).^2;
    Y_fft=y_fft(1:513); % we only keep the positive frequencies.
    Y_fft=(dt/1024)*Y_fft;
    Y_fft(2:end-1)=2*Y_fft(2:end-1);    
    %compare 
    plot(psd_est_1side.Frequencies./1000,10*log10(Y_fft));
    xlabel('kHz'); ylabel('Power/Hz (dB)'); grid on;
    figure;
    plot(psd_est_1side);