You are wrong when you say the output from strobed noise is only valid over a range of -f₀/2 to +f₀/2. Rather the noise is periodic with a period of f₀, so you only need to know the noise over the range -f₀/2 to +f₀/2. Once you know if over that range, you know it everywhere.

-Ken

Ken Kundert wrote on Jun 3^rd, 2008, 8:50am:



Thanks Ken, David
As far as I know, all of the noise power concentrate between -fo/2 ~ +fo/2, and for a simple RC network, the power would be kT/C. If the noise is periodic with a period of fo, the total power would be infinite in the whole frequency range because there are infinite shares of kT/C with a period of fo? I fail to figure out where the extra noise power comes from or do I misunderstand something?
There still is another problem. As you said, the strobed noise is periodic, so there should exist replicas of the noise spectrum between -fo~+fo. For my PLL under test mentioned above, the input clock buffer before PFD operate at 40MHz, so I should get a spectrum of strobed phase noise with replicas at 40MHz, 80MHz, 120MHz, ..., etc.. When I use this spectrum as reference clock phase noise in Matlab, I would see strong spurs at these frequency offsets for the noise at these points runing to infinite. Am I right?

-Ryan

David,

Thanks for your reply!
So you mean I should use 'noise type=sources' in pnoise to simulate on buffer+PFD+charge pump+loop filter? But this type of simulation gives time averaged noise. Could this correctly describe the phase noise behavior if a digital buffer (with discrete noise attribute) is included?

Another problem. What if I want to simulate the phase noise of a digital buffer at the PLL output. I have to add this discrete-time noise with the phase noise of the PLL (continuous one), and the same problem is unavoidable. Any suggestion is appreciated!  :-[

-Ryan

Thanks,

Yeah, you are right. But my PFD here is a special architecture and we have a tight spec., so any noise source cannot be ignored. So I am very concern about the phase noise contributed from blocks like buffer, PFD, and charge pump, which may be igored in traditional design.
So, if I run PFD+charge pump with 'noise type=sources', and the PFD jitter is dominant, could the simulator give me a good estimation of current noise at loop filter?

-Ryan

David,

Thank you very much! I have done the same thing as you said. But the simulator always gives a warning saying that the 'maxsamples' need to be given a large number. What does the 'maxsamples' option mean?

-Ryan