IN74HC4046A Просмотр технического описания (PDF) - IK Semicon Co., Ltd

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IN74HC4046A

Phase-Locked Loop High-Performance Silicon-Gate CMOS

IK Semicon Co., Ltd 

IN74HC4046A

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.

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).

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).

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.

Figure 5. Logic Diagram for VCO

Rev. 00

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