HCF4046B_01 Datasheet, PDF (3/12 Pages) STMicroelectronics – MICROPOWER PHASE-LOCKED LOOP

HCF4046B
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
Condition of f_o
3/12