The ltering operation of the error voltage (coming out from the Phase Detec-

tor) is performed by the loop lter. The output of PD consists of a dc component

superimposed with an ac component. The ac part is undesired as an input to the

VCO, hence a low pass lter is used to lter out the ac component. Loop lter is

one of the most important functional block in determining the performance of the

loop. A loop lter introduces poles to the PLL transfer function, which in turn is a

parameter in determining the bandwidth of the PLL. Since higher order loop lters

oer better noise cancelation, a loop lter of order 2 or more are used in most of the

critical application PLL circuits.

8