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https://designers-guide.org/forum/YaBB.pl Measurements >> Phase Noise and Jitter Measurements >> Matlab script periods to Sphi https://designers-guide.org/forum/YaBB.pl?num=1170678430 Message started by seho on Feb 5th, 2007, 4:27am |
Title: Matlab script periods to Sphi Post by seho on Feb 5th, 2007, 4:27am I have a question concerning the matlab code from Ken's Paper "Predicting the phase noise and jitter of PLL-based frequency synthesizers" that computes the phase noise PSD from periods data. During my simulations and analysis I found a difference of 3dB in the phase noise spectrum (Sphi) from the matlab output and the expected value. I used the parameters from the example in Ken's paper on page 44. VCO: L(100kHz)=-48dBc/Hz and thus Sphi(100kHz) =-45db/Hz fc=2GHz using the equation J2=Sphi/2*(100kHz)2/(2GHz)3 this leads to a period jitter of 4.45ps. I generated periods with accumulating jitter with the matlab skript: Code:
and calculated the phase noise PSD Sphi with the skript: Code:
The output gives a period jitter of 4.45 ps (same as input), but a Sphi of -68 dB/Hz @ 10^6Hz -48 dB/Hz @ 10^5Hz and so on. Oviously there is a difference of 3dB (factor 2) in the PSD compared to the analytical equation. Maybe this is due to the normalization to resolution bandwidth which is not correct. Does anybody have an idea where the 3db (factor 2) are missing? Thank you very much |
Title: Re: Matlab script periods to Sphi Post by abs on Feb 8th, 2007, 1:35am The psd or pwelch gives single single-sided psd if you give real signal. It thinks all energy distrubuted in 0~1/2T. However, we The Sphi equation has already count single-sided L(f). So, you should divide the Sphi by 2. |
Title: Re: Matlab script periods to Sphi Post by seho on Feb 9th, 2007, 12:34am Thanks for your answer, I'm a little bit confused now. To my opinion in the given formulas single sideband noise spectra are used to compute the jitter variance. The psd() function also gives single sideband PSD for real input signals. So why is the PSD from the formula (for a given jitter variance) not the same that matlab computes (for the same jitter variance)? In my case the matlab output is 3dB below the calculated value. So I should multiply the Sphi from matlab with two. But why? Kind regards Sebastian |
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