Description: 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
o er better noise cancelation, a loop lter of order 2 or more are used in most of the
critical application PLL circuits.
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