SL74HC4046 datasheet(8/12 Pages) SLS

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SL74HC4046

System Logic

Semiconductor

SLS

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

feed

external

prescalers

(counters) to enable frequency synthesis.

Figure 5. Logic Diagram for VCO

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