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p-type output driver is maintained ON for a time

corresponding

to

the

phase

difference.

Subsequently,

the

capacitor

voltage

of

the

low-pass filter connected to this phase comparator

is adjusted until the signal and comparator inputs

are equal in both phase and frequency. At this

stable point both p- and n-type output drivers

remain OFF and thus the phase comparator

output becomes an open circuit and holds the

voltage on the capacitor of the low-pass filter

constant. Moreover the signal at the "phase

pulses" output is a high level which can be used

for indicating a locked condition. Thus, for phase

comparator II, no phase difference exists between

signal and comparator input over the full VCO

frequency range. Moreover, the power dissipation

due to the low-pass filter is reduced when this type

of phase comparator is used because both the p-

and n-type output drivers are OFF for most of the

signal input cycle. It should be noted that the PLL

lock range for this type of phase comparator is

equal to the capture range, independent of the

low-pass filter. With no signal present at the signal

input, the VCO is adjusted to its lowest frequency

for phase comparator II. Fig.3 shows typical

waveforms for a CMOS PLL employing phase

comparator II in a locked condition.

Figure 1 : Phase-Comparator I Characteristics at Low-Pass Filter Output.

Figure 2 : Typical Waveforms for CMOS Phase Locked-Loop Employing Phase Comparator I in Locked