IN74HC4046AD 查看數據表（PDF） - Integral Corp.

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IN74HC4046AD

PHASE-LOCKED LOOP

Integral Corp. 

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

Phase Comparators

All three phase comparators have two inputs,

SIGIN and COMPIN. The SIGIN and COMPIN have

a special DC bias network that enables AC

coupling of input signals. If the signals are not AC

coupled, standard IN74HC input levels are

required. Both input structures are shown in

Figure 6. The outputs of these comparators are

essentially standard IN74HC outputs (comparator

2 is TRI-STATEABLE). In normal operation VCC

and ground voltage levels are fed to the loop filter.

This differs from some phase detectors which

supply a current to the loop filter and should be

considered in the design.

7

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