IN74HC4046A
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 10).
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 12). 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. 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.
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.
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.
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 change the opposite side of the capacitor. The output
from the internal logic is then taken to VCO output (Pin4).
Figure 5. Logic Diagram for VCO
Rev. 00
IKSEMICON [ IK SEMICON CO., LTD ]
