74HC4046A Datasheet, PDF (3/34 Pages) NXP Semiconductors – Phase-locked-loop with VCO

Philips Semiconductors
Phase-locked-loop with VCO
Product specification
74HC/HCT4046A
The frequency capture range (2f_c) is defined as the
frequency range of input signals on which the PLL will lock
if it was initially out-of-lock. The frequency lock range
(2f_L) is defined as the frequency range of input signals on
which the loop will stay locked if it was initially in lock. The
capture range is smaller or equal to the lock range.
With PC1, the capture range depends on the low-pass
filter characteristics and can be made as large as the lock
range.
This configuration retains lock even with very noisy input
signals. Typical behaviour of this type of phase
comparator is that it can lock to input frequencies close to
the harmonics of the VCO centre frequency.
Phase comparator 2 (PC2)
This is a positive edge-triggered phase and frequency
detector. 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. PC2
comprises two D-type flip-flops, control-gating and a
3-state output stage. The circuit functions as an up-down
counter (Fig.5) where SIG_IN causes an up-count and
COMP_IN a down-count. The transfer function of PC2,
assuming ripple (f_r = f_i) is suppressed,
is: V_DEMOUT = -V-4---^C-_π--^C-- ( φ_SIGIN – φ_COMPIN)
where V_DEMOUT is the demodulator output at pin 10;
V_DEMOUT = V_PC2OUT (via low-pass filter).
The phase comparator gain is: K_p = -V--4--^C-_π--^C-- (V ⁄ r) .
V_DEMOUT is the resultant of the initial phase differences of
SIG_IN and COMP_IN as shown in Fig.8. Typical waveforms
for the PC2 loop locked at f_o are shown in Fig.9.
When the frequencies of SIG_IN and COMP_IN are equal but
the phase of SIG_IN leads that of COMP_IN, the p-type
output driver at PC2_OUT is held “ON” for a time
corresponding to the phase difference (φ_DEMOUT). When
the phase of SIG_IN lags that of COMP_IN, the n-type driver
is held “ON”.
When the frequency of SIG_IN is higher than that of
COMP_IN, the p-type output driver is held “ON” for most of
the input signal cycle time, and for the remainder of the
cycle both n and p- type drivers are ”OFF” (3-state). If the
SIG_IN frequency is lower than the COMP_IN frequency, then
it is the n-type driver that is held “ON” for most of the cycle.
Subsequently, the voltage at the capacitor (C2) of the
low-pass filter connected to PC2_OUT varies until the signal
and comparator inputs are equal in both phase and
frequency. At this stable point the voltage on C2 remains
constant as the PC2 output is in 3-state and the VCO input
at pin 9 is a high impedance. Also in this condition, the
signal at the phase comparator pulse output (PCP_OUT) is a
HIGH level and so can be used for indicating a locked
condition.
Thus, for PC2, no phase difference exists between
SIG_IN and COMP_IN over the full frequency range of the
VCO. Moreover, the power dissipation due to the low-pass
filter is reduced because both p and n-type 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 and is independent of the
low-pass filter. With no signal present at SIG_IN the
VCO adjusts, via PC2, to its lowest frequency.
Phase comparator 3 (PC3)
This is a positive edge-triggered sequential phase detector
using an RS-type flip-flop. 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. The transfer characteristic of PC3,
assuming ripple (f_r = f_i) is suppressed,
is: V_DEMOUT = -V-2---^C-_π--^C-- ( φ_SIGIN – φ_COMPIN)
where V_DEMOUT is the demodulator output at pin 10;
V_DEMOUT = V_PC3OUT (via low-pass filter).
The phase comparator gain is: K_p = -V-2---^C-_π--^C-- (V ⁄ r) .
The average output from PC3, fed to the VCO via the
low-pass filter and seen at the demodulator output at
pin 10 (V_DEMOUT), is the resultant of the phase differences
of SIG_IN and COMP_IN as shown in Fig.10. Typical
waveforms for the PC3 loop locked at f_o are shown in
Fig.11.
The phase-to-output response characteristic of PC3
(Fig.10) differs from that of PC2 in that the phase angle
between SIG_IN and COMP_IN varies between 0° and
360° and is 180° at the centre frequency. Also PC3 gives
a greater voltage swing than PC2 for input phase
differences but as a consequence the ripple content of the
VCO input signal is higher. The PLL lock range for this type
of phase comparator and the capture range are dependent
on the low-pass filter. With no signal present at SIG_IN the
VCO adjusts, via PC3, to its lowest frequency.
1997 Nov 25
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