Philips Semiconductors
Product specification
Phase-locked-loop with VCO
74HC/HCT4046A
provided at pin 10 (DEMOUT). In contrast to conventional
techniques where the DEMOUT voltage is one threshold
voltage lower than the VCO input voltage, here the
DEMOUT voltage equals that of the VCO input. If
DEMOUT is used, a load resistor (RS) should be connected
from DEMOUT to GND; if unused, DEMOUT should be left
open. The VCO output (VCOOUT) can be connected
directly to the comparator input (COMPIN), or connected
via a frequency-divider. The VCO output signal has a duty
factor of 50% (maximum expected deviation 1%), if the
VCO input is held at a constant DC level. A LOW level at
the inhibit input (INH) enables the VCO and demodulator,
while a HIGH level turns both off to minimize standby
power consumption.
FEATURES
• Low power consumption
• Centre frequency of up to 17 MHz (typ.) at VCC = 4.5 V
• Choice of three phase comparators: EXCLUSIVE-OR;
edge-triggered JK flip-flop;
edge-triggered RS flip-flop
• Excellent VCO frequency linearity
• VCO-inhibit control for ON/OFF keying and for low
standby power consumption
• Minimal frequency drift
• Operating power supply voltage range:
VCO section 3.0 to 6.0 V
digital section 2.0 to 6.0 V
The only difference between the HC and HCT versions is
the input level specification of the INH input. This input
disables the VCO section. The sections of the comparator
are identical, so that there is no difference in the
SIGIN (pin 14) or COMPIN (pin 3) inputs between the HC
and HCT versions.
• Zero voltage offset due to op-amp buffering
• Output capability: standard
• ICC category: MSI.
GENERAL DESCRIPTION
Phase comparators
The 74HC/HCT4046A are high-speed Si-gate CMOS
devices and are pin compatible with the “4046” of the
“4000B” series. They are specified in compliance with
JEDEC standard no. 7A.
The signal input (SIGIN) can be directly coupled to the
self-biasing amplifier at pin 14, provided that the signal
swing is between the standard HC family input logic levels.
Capacitive coupling is required for signals with smaller
swings.
The 74HC/HCT4046A are phase-locked-loop circuits that
comprise a linear voltage-controlled oscillator (VCO) and
three different phase comparators (PC1, PC2 and PC3)
with a common signal input amplifier and a common
comparator input.
Phase comparator 1 (PC1)
This is an EXCLUSIVE-OR network. The signal and
comparator input frequencies (fi) must have a 50% duty
factor to obtain the maximum locking range. The transfer
characteristic of PC1, assuming ripple (fr = 2fi) is
The signal input can be directly coupled to large voltage
signals, or indirectly coupled (with a series capacitor) to
small voltage signals. A self-bias input circuit keeps small
voltage signals within the linear region of the input
amplifiers. With a passive low-pass filter, the “4046A”
forms a second-order loop PLL. The excellent VCO
linearity is achieved by the use of linear op-amp
techniques.
V
suppressed, is: VDEMOUT
=
CC (φ
– φCOMPIN)
SIGIN
----------
π
where VDEMOUT is the demodulator output at pin 10;
DEMOUT = VPC1OUT (via low-pass filter).
V
VCC
˙
The VCO requires one external capacitor C1 (between
C1A and C1B) and one external resistor R1 (between
R1 and GND) or two external resistors R1 and R2
(between R1 and GND, and R2 and GND). Resistor R1
and capacitor C1 determine the frequency range of the
VCO. Resistor R2 enables the VCO to have a frequency
offset if required.
The phase comparator gain is:Kp
=
(V ⁄ r) .
----------
π
The average output voltage from PC1, fed to the VCO
input via the low-pass filter and seen at the demodulator
output at pin 10 (VDEMOUT), is the resultant of the phase
differences of signals (SIGIN) and the comparator input
(COMPIN) as shown in Fig.6. The average of VDEMOUT is
equal to 1⁄2VCC when there is no signal or noise at
SIGIN and with this input the VCO oscillates at the centre
frequency (fo). Typical waveforms for the PC1 loop locked
at fo are shown in Fig.7.
The high input impedance of the VCO simplifies the design
of low-pass filters by giving the designer a wide choice of
resistor/capacitor ranges. In order not to load the low-pass
filter, a demodulator output of the VCO input voltage is
1997 Nov 25
2
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