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 Cumulative Phase and PhaseNoise (Read 6810 times)
 cheap_salary Senior Member Offline Posts: 162 Cumulative Phase and PhaseNoise Jul 13th, 2014, 12:05am   I have two questions about PSD calculation of phase noise in LISTING-17 at page-46 in the following.http://www.designers-guide.org/Analysis/PLLnoise+jitter.pdf Quote:% output estimates of period and jitterT=mean(periods);% compute the cumulative phase of each transitionphases=2*pi*cumsum(periods)/T;% compute power spectral density of phase[Sphi, f] = psd(phases, nfft, 1/T, winLength, overlap, "linear"); (1) Why do you apply above "phases" for evaluating PSD of phase noise ?http://www.designers-guide.org/Forum/YaBB.pl?num=1066269978/1#1If we use the above "phases", result of phase noise can not be consistent for completely clean signal.For evaluating phase noise, I think phases has to be :phases=2*pi*cumsum(periods-T) / T;The following is same as my opinion.http://www.designers-guide.org/Forum/YaBB.pl?num=1196964298/6#6 Quote:% correct for scaling in PSD due to FFT and windowSphi = winNBW*Sphi / nfft;% plot the results (except at DC)K = length(f);semilogx( f(2:K), 10*log10(Sphi(2:K)) );title("Power Spectral Density of VCO Phase");xlabel("Frequency (Hz)");ylabel("S phi (dB/Hz)");rbw = winNBW/(T*nfft); (2) Why is a unit of Sphi (dB/Hz) ?Has it to be (dB/nbw) ?Here "nbw" is a normalized Noise Bandwidth defined as following.nbw = rbw * T Back to top IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Cumulative Phase and PhaseNoise Reply #1 - May 25th, 2015, 8:52am   If we evaluate Sphi by "phases=2*pi*cumsum(periods)/T", it gives "cyan curve".On the other hand, if we evaluate Sphi by "phases=2*pi*cumsum(periods-T)/T", it gives "green curve".Do you think which result is reasonable ?Of course, green curve is reasonable.This is a reason why I think that phases has to be "phases=2*pi*cumsum(periods-T) / T".Currently pwelch() is recommended for Welch's method of estimation rather than psd(). Quote:winNBW = 1.5; % Noise bandwidth given in binsNfft = 512; % should be power of twowinLength = Nfft;overlap = Nfft/2;nbw = winNBW / Nfft;fs = 1 / T;NBW = fs * nbw;rbw = NBW;% compute power spectral density of phase[Sphi, f] = pwelch(phases, hanning(winLength), overlap, Nfft, fs);Sphi = NBW * Sphi; Back to top IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Cumulative Phase and PhaseNoise Reply #2 - May 25th, 2015, 9:04am   http://www.mathworks.com/matlabcentral/answers/100251-what-is-the-difference-bet... Quote:[Sphi, f] = psd(phases, Nfft, fs, winLength, overlap, 'linear');Sphi = nbw * Sphi; % correct for scaling in PSD due to FFT and window gives yellow curve. Quote:[Sphi, f] = pwelch(phases, hanning(winLength), overlap, Nfft, fs);Sphi = NBW * Sphi; gives green curve.We can see 3dB difference between them, since pwelch() returns a single sided spectrum by default (for real signals).I don't know why they give far different result for very low frequency. Back to top IP Logged
 designer_dude New Member Offline Posts: 5 Re: Cumulative Phase and PhaseNoise Reply #3 - Jul 27th, 2016, 5:49pm   Hi, You need to substract out the nominal period for every period you use. Consider 0 noise clock signal. If you don't substract the nominal period out, the phase will be 0, 2pi, 4pi.... to infinity,  A phase noise plot would have infinite DC component for an ideal, clock.  THis is obviously wrong. Back to top IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Cumulative Phase and PhaseNoise Reply #4 - Jul 27th, 2016, 6:54pm   designer_dude wrote on Jul 27th, 2016, 5:49pm:You need to substract out the nominal period for every period you use. Can you understand my appends surely ?Surely read a start description of this thread. Back to top IP Logged
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