Philips Semiconductors
Product specification
Phase-locked loop
Phase comparator 2 is an edge-controlled digital memory
network. It consists of four flip-flops, control gating and a
3-state output circuit comprising p and n-type drivers
having a common output node. When the p-type or n-type
drivers are ON, they pull the output up to V
DD
or down to
V
SS
respectively. This type of phase comparator only acts
on the positive-going edges of the signals at SIGN
IN
and
COMP
IN
. Therefore, the duty factors of these signals are
not of importance.
If the signal input frequency is higher than the comparator
input frequency, the p-type output driver is maintained ON
most of the time, and both the n and p-type drivers are
OFF (3-state) the remainder of the time. If the signal input
frequency is lower than the comparator input frequency,
the n-type output driver is maintained ON most of the time,
and both the n and p-type drivers are OFF the remainder
of the time. If the signal input and comparator input
frequencies are equal, but the signal input lags the
comparator input in phase, the n-type output driver is
maintained ON for a time corresponding to the phase
difference. If the comparator input lags the signal input in
phase, the p-type output driver is maintained ON for a time
corresponding to the phase difference. Subsequently, the
voltage at the capacitor of the low-pass filter connected to
this phase comparator is adjusted until the signal and
HEF4046B
MSI
comparator inputs are equal in both phase and frequency.
At this stable point, both p and n-type drivers remain OFF
and thus the phase comparator output becomes an open
circuit and keeps the voltage at the capacitor of the
low-pass filter constant.
Moreover, the signal at the phase comparator pulse output
(PCP
OUT
) is a HIGH level which can be used for indicating
a locked condition. Thus, for phase comparator 2 no phase
difference exists between the signal and comparator
inputs 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
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 2 . Figure 5 shows
typical waveforms for a PLL employing this type of phase
comparator in locked condition.
Fig.5 Typical waveforms for phase-locked loop employing phase comparator 2 in locked condition.
January 1995
5
PHILIPS [ NXP SEMICONDUCTORS ]
