Hi, Visjnoe,
For a PFD used in a charge-pump PLL, the gain of the PFD (K
p) has not a VDD term in its numerator. On William F. Egan's
Frequency Synthesis by Phase Lock, 2nd ed., the author gives a very good description about K
p.
Quote:The gain of the phase detector is equal to the charge-pump pulse current Ip divided by on cycle, since, if there wre a 1-cycle phase error, the average current woule equal Ip. This can be stated in terms of voltage by multiplying the current by resistance, usually part of the filter, and then adjusting the filter function to obtain the correct product. .... having Kp=Ip/cycle, ....
Obviously, the gain of the PFD is proportional to the charge pump pulse current, not VDD.
Thanks
Yawei