MC74HC4046A
http://onsemi.com
9
Phase Comparator 2
This detector is a digital memory network. It consists of
four flip−flops and some gating logic, a three state output
and a phase pulse output as shown in Figure 6. This
comparator acts only on the positive edges of the input
signals and is independent of duty cycle.
Phase comparator 2 operates in such a way as to force the
PLL into lock with 0 phase difference between the VCO
output and the signal input positive waveform edges. Figure
8 shows some typical loop waveforms. First assume that
SIG
IN
is leading the COMP
IN
. This means that the VCO’s
frequency must be increased to bring its leading edge into
proper phase alignment. Thus the phase detector 2 output is
set high. This will cause the loop filter to charge up the VCO
input, increasing the VCO frequency. Once the leading edge
of the COMP
IN
is detected, the output goes TRI−STATE
holding the VCO input at the loop filter voltage. If the VCO
still lags the SIG
IN
then the phase detector will again charge
up the VCO input for the time between the leading edges of
both waveforms.
If the VCO leads the SIG
IN
then when the leading edge of
the VCO is seen; the output of the phase comparator goes
low. This discharges the loop filter until the leading edge of
the SIG
IN
is detected at which time the output disables itself
again. This has the effect of slowing down the VCO to again
make the rising edges of both waveforms coincidental.
When the PLL is out of lock, the VCO will be running
either slower or faster than the SIG
IN
. If it is running slower
the phase detector will see more SIG
IN
rising edges and so
the output of the phase comparator will be high a majority
of the time, raising the VCO’s frequency. Conversely, if the
VCO is running faster than the SIG
IN
, the output of the
detector will be low most of the time and the VCO’s output
frequency will be decreased.
As one can see, when the PLL is locked, the output of
phase comparator 2 will be disabled except for minor
corrections at the leading edge of the waveforms. When PC
2
is TRI−STATED, the PCP output is high. This output can be
used to determine when the PLL is in the locked condition.
This detector has several interesting characteristics. Over
the entire VCO frequency range there is no phase difference
between the COMP
IN
and the SIG
IN
. The lock range of the
PLL is the same as the capture range. Minimal power was
consumed in the loop filter since in lock the detector output
is a high impedance. When no SIG
IN
is present, the detector
will see only VCO leading edges, so the comparator output
will stay low, forcing the VCO to f
min
.
Phase comparator 2 is more susceptible to noise, causing
the PLL to unlock. If a noise pulse is seen on the SIG
IN
, the
comparator treats it as another positive edge of the SIG
IN
and will cause the output to go high until the VCO leading
edge is seen, potentially for an entire SIG
IN
period. This
would cause the VCO to speed up during that time. When
using PC
1
, the output of that phase detector would be
disturbed for only the short duration of the noise spike and
would cause less upset.
Phase Comparator 3
This is a positive edge−triggered sequential phase
detector using an RS flip−flop as shown in Figure 7. When
the PLL is using this comparator, the loop is controlled by
positive signal transitions and the duty factors of SIG
IN
and
COMP
IN
are not important. It has some similar
characteristics to the edge sensitive comparator. To see how
this detector works, assume input pulses are applied to the
SIG
IN
and COMP
IN
’s as shown in Figure 10. When the
SIG
IN
leads the COMP
IN
, the flop is set. This will charge the
loop filter and cause the VCO to speed up, bringing the
comparator into phase with the SIG
IN
. The phase angle
between SIG
IN
and COMP
IN
varies from 0° to 360° and is
180° at f
o
. The voltage swing for PC
3
is greater than for PC
2
but consequently has more ripple in the signal to the VCO.
When no SIG
IN
is present the VCO will be forced to f
max
as
opposed to f
min
when PC
2
is used.
The operating characteristics of all three phase
comparators should be compared to the requirements of the
system design and the appropriate one should be used.
Figure 9. Typical Waveforms for PLL Using
Phase Comparator 2
V
CC
GND
SIG
IN
COMP
IN
PC2
OUT
VCO
IN
PCP
OUT
HIGH IMPEDANCE OFF−STATE
Figure 10. Typical Waveform for PLL Using
Phase Comparator 3
VCC
GND
SIG
IN
COMP
IN
PC3
OUT
VCO
IN
