MC74HC4046A Datasheet, PDF (7/16 Pages) ON Semiconductor – Phase-Locked Loop

MC74HC4046A
DETAILED CIRCUIT DESCRIPTION
Voltage Controlled Oscillator/Demodulator Output
The VCO requires two or three external components to
operate. These are R1, R2, C1. Resistor R1 and Capacitor C1
are selected to determine the center frequency of the VCO
(see typical performance curves Figure 14). R2 can be used
to set the offset frequency with 0 volts at VCO input. For
example, if R2 is decreased, the offset frequency is
increased. If R2 is omitted the VCO range is from 0 Hz. The
effect of R2 is shown in Figure 24, typical performance
curves. By increasing the value of R2 the lock range of the
PLL is increased and the gain (volts/Hz) is decreased. Thus,
for a narrow lock range, large swings on the VCO input will
cause less frequency variation.
Internally, the resistors set a current in a current mirror, as
shown in Figure 5. The mirrored current drives one side of
the capacitor. Once the voltage across the capacitor charges
up to Vref of the comparators, the oscillator logic flips the
capacitor which causes the mirror to charge the opposite side
of the capacitor. The output from the internal logic is then
taken to VCO output (Pin 4).
The input to the VCO is a very high impedance CMOS
input and thus will not load down the loop filter, easing the
filters design. In order to make signals at the VCO input
accessible without degrading the loop performance, the
VCO input voltage is buffered through a unity gain Op–amp
to Demod Output. This Op–amp can drive loads of 50K
ohms or more and provides no loading effects to the VCO
input voltage (see Figure 12).
An inhibit input is provided to allow disabling of the VCO
and all Op–amps (see Figure 5). This is useful if the internal
VCO is not being used. A logic high on inhibit disables the
VCO and all Op–amps, minimizing standby power
consumption.
12
R2
VREF _{+_}
VCOIN 9
11
+_
R1
I1
CURRENT
MIRROR
I1 + I2 = I3
I2
I3
4 VCOOUT
DEMODOUT ₁₀
+_
C1
(EXTERNAL)
6
7
Vref
+
+
INH ₅
Figure 5. Logic Diagram for VCO
The output of the VCO is a standard high speed CMOS
output with an equivalent LS–TTL fan out of 10. The VCO
output is approximately a square wave. This output can
either directly feed the COMPIN of the phase comparators or
feed external prescalers (counters) to enable frequency
synthesis.
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