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