Quote:Both in section 5 and 9.1.2, which described the method of extracting the phase noise or jitter in PFD/CP, it was said that I should drive the PFD/CP with periodic signals with offset phase, and measure the output noise current spectrum. Isn't the output noise current spectrum correlated to the offset phase? If I drive the two input port of PFD with zero phase deviation, the output spectrum could go wrong? If so, why?
My feeling is that in most cases the noise will not vary substantially with the phase difference of the two input signals. In doing so I am implicitly assuming that the noise of the PFD/CP is dominated by jitter, which is independent of the phase difference between the inputs. But this is a judgment that can only be made by knowing the phase detector being used. For example, it may be that the jitter in the edges of the PFD/CP output pulses changes considerably between when the CP produces positive and negative pulses. If this is a concern, you should extract the noise for both cases and either use the worst case, or use more sophisticated models that includes the effect of both. At zero phase deviation, either positive and negative pulses are present simultaneously, or neither are present. Whichever, this is yet another condition that should be considered. Finally, if the noise of the PFD/CP is dominated by the noise in the amount of current produced rather than the jitter in the edges of the pulses, then you can expect the noise power to vary in proportion to the phase difference in the input signals. Again, in this case, a more sophisticated model is needed.
Quote:And in section 5, you said it is necessary to capture the noise during the switching process, it seemed you’re suggesting the use of a strobed noise analysis, but in section 9.1.2, you said that we should treat PFD/CP output noise as a continuous noise current. What is the difference between these two cases? Or is it just because I made some mistakes in section 5?
A charge pump when off generally produces very little noise at all. When on you also get the noise associated with the current it produces. And when turning on or off, you also see the effect of jitter. In section 5 where I said it is important to “capture the noise of the switching process”, what I as suggesting is that it is important to measure the noise at the output while the charge pump is producing pulses. If it is not producing pulses, you are not getting a realistic estimate of the noise it produces in practice. I was not suggesting that you use the strobed noise feature.
Quote:Finally, I would like to ask what's the bandwidth of PFD/CP that was mentioned in section 9.1.2?
It is the noise bandwidth of the PFD/CP. Thus, I assume that the input frequency is fixed, and I am measuring the bandwidth of the noise at the output of the CP.
Quote:If I drive the PFD/CP at a different frequency to extract the noise spectrum or jitter, should I take the frequency into consideration? For example, the input referred noise floor of PFD/CP at certain frequency could somehow differ from that driven at another frequency.
I did not consider the effect of frequency changes at the input of the PFD. It was my assumption that it would always operate at
Fref, which was constant. If in your application
Fref is variable, you might want to run experiments to see how the noise produced by the PFD/CP changes with input frequency.
Quote:If the noise floor of PFD/CP won't change with the frequency of the driving signal (or reference frequency), then the output phase noise of PLL in the loop bandwidth would probably be the input referred noise floor PFD/CP plus 20*log(N), where N means the main divider ratio. While documentations declared the input referred PFD/CP noise floor should be proportion to 10*log(Fref) in dB.
I’m afraid I do not understand the second statement.