Multiple Power Spectral Density spectra on one plot

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Andy
Andy 2011-12-15
评论： Ken Saw 2014-3-24
Hello,
I have calculated the PSD spectrum of a signal and displayed it from 0-500Hz using the following code:
periodogram(M3A,[],'onesided',512,1000)
I would like to display the spectrum up to 2000Hz, and then plot another spectrum on the same graph for comparison.
Please could someone point me in the right direction?
Best Regards
Andy

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采纳的回答

Wayne King
Wayne King 2011-12-15
Hi Andy, sorry now I understand. You can do this:
% I'll just create an example
Fs = 44100;
t = 0:1/Fs:1-1/Fs;
M3AnO = cos(2*pi*1000*t)+randn(size(t));
M3A = cos(2*pi*500*t)+filter(1,-0.9,rand(size(t)));
[Pxx1,Fxx] = periodogram(M3AnO,[],length(M3AnO),Fs);
[Pxx2,Fxx] = periodogram(M3A,[],length(M3A),Fs);
plot(Fxx,10*log10(Pxx1)); hold on;
plot(Fxx,10*log10(Pxx2),'r'); hold on;
set(gca,'xlim',[0 2000]);
legend('M3AnO','M3A','Location','NorthEast');
xlabel('Hz'); ylabel('dB/Hz');

更多回答（1 个）

Wayne King
Wayne King 2011-12-15
Hi Andy, from the code you have shown above, your sampling frequency is 1 kHz. That means your spectrum is periodic with a period equal to 1 kHz. It then sounds like you want to compare it against the power spectral density of another process with a higher sampling rate?
You don't gain anything by extending your process spectrum to 2 kHz because it's periodic. You're better off (if the other process has a higher sampling rate) limiting the part of the spectrum you consider in the process with the higher rate.
Also, the sampling interval and the length of the input time series are part of the scaling factors for the periodogram.
And both the length and sampling interval affect frequency resolution (spacing of the DFT bins).
  2 个评论
Andy
Andy 2011-12-15
Hi Wayne,
The sampling frequency of both signals is 44100. I think i must be reading the demo page wrong!
By changing the 1000 to 44100, it produces the frequency range i require.
I have tried the following code to produce both results on the same graph...
[pxNo, freq] = periodogram(M3AnO);
[px, freq] = periodogram(M3A);
plot(freq, px, freq, pxNo)
However, there doesnt appear to be two plots as i think one may be obscuring the other. The frequency also ranges between 0 and 3.5 as opposed to 0-2000Hz.
When i plot using psd instead of periodogram, both lines are visible, but the frequency axis still does not show Hz.
Thanks for your help.
Regards
Andy
Ken Saw
Ken Saw 2014-3-24
load sunspot.dat
year=sunspot(:,1);
relNums=sunspot(:,2);
plot(year,relNums)
title('Sunspot Data')
Y = fft(relNums);
Y(1)=[];
n=length(Y);
power = abs(Y(1:floor(n/2))).^2;
nyquist = 1/2;
freq = (1:n/2)/(n/2)*nyquist;
plot(freq,power)
xlabel('cycles/year')
title('Periodogram')
period=1./freq;
plot(period,power);
axis([0 40 0 2e+7]);
ylabel('Power');
xlabel('Period (Years/Cycle)');
hold on;
index=find(power==max(power));
mainPeriodStr=num2str(period(index));
plot(period(index),power(index),'r.', 'MarkerSize',25);
text(period(index)+2,power(index),['Period = ',mainPeriodStr]);
hold off;

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