74VHC4046CW Просмотр технического описания (PDF) - Fairchild Semiconductor

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74VHC4046CW

CMOS Phase Lock Loop

Fairchild Semiconductor 

Detailed Circuit Description

VOLTAGE CONTROLLED OSCILLATOR/SOURCE

FOLLOWER

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. R1 controls the lock range. As R1’s resistance

decreases the range of fMIN to fMAX increases. Thus the

VCO’s gain increases. As C1 is changed the offset (if used)

of R2, and the center frequency is changed. (See typical

performance curves) R2 can be used to set the offset fre-

quency with 0V at VCO input. If R2 is omitted the VCO

range is from 0Hz. As R2 is decreased the offset frequency

is increased. The effect of R2 is shown in the design infor-

mation table and typical performance curves. By increasing

the value of R2 the lock range of the PLL is offset above

0Hz and the gain (Hz/Volt) does not change. In general,

when offset is desired, R2 and C1 should be chosen first,

and then R1 should be chosen to obtain the proper center

frequency.

FIGURE 1. Logic Diagram for VCO

Internally the resistors set a current in a current mirror as

shown in Figure 1. The mirrored current drives one side of

the capacitor once the capacitor charges up to the thresh-

old of the Schmitt Trigger the oscillator logic flips the

capacitor over and causes the mirror to charge the oppo-

site side of the capacitor. The output from the internal logic

is then taken to pin 4.

The input to the VCO is a very high impedance CMOS

input and so it 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 a

source follower transistor is provided. This transistor can

be used by connecting a resistor to ground and its drain

output will follow the VCO input signal.

An inhibit signal is provided to allow disabling of the VCO

and the source follower. This is useful if the internal VCO is

not being used. A logic high on inhibit disables the VCO

and source follower.

The output of the VCO is a standard high speed CMOS

output with an equivalent LSTTL fanout of 10. The VCO

output is approximately a square wave. This output can

either directly feed the comparator input of the phase com-

parators or feed external prescalers (counters) to enable

frequency synthesis.

PHASE COMPARATORS

All three phase comparators share two inputs, Signal In

and Comparator In. The Signal In has a special DC bias

network that enables AC coupling of input signals. If the

signals are not AC coupled then this input requires logic

levels the same as standard 74VHC. The Comparator input

is a standard digital input. Both input structures are shown

in Figure 2.

The outputs of these comparators are essentially standard

74VHC voltage outputs. (Comparator II is 3-STATE.)

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