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Simulation of Fractional N PLL

Question asked by ROB_LEFEBVRE Employee on May 27, 2003
Simulation of Fractional N PLL's is very straightforward. Simply enter a fractional number for N.

In a normal PLL, the phase/frequency detector "fires" every N cycles. In a fractional N PLL, the detector still fires every N cycles, but N is not an integer. The fractional part is normally represented by extra bits, so the fractional part is something like 5/16 or 27/256. For example, if N is 10.25, then the detector would fire on cycle 10, 20, 30, 41, 51, 61, 71, 82, etc. PLL will simulate fractional-N with no problem, simply specify N as a non-integer number and look at the time-domain simulation. You can even see the behavior of firing after N or N+1 cycles.