MM74HC4046 查看數據表（PDF） - Fairchild Semiconductor

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MM74HC4046

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.

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 capaci-

tor over and causes the mirror to charge the opposite side

of the capacitor. The output from the internal logic is then

taken to pin 4.

VCO WITHOUT OFFSET

R2 = ∞

VCO WITH OFFSET

Comparator I

Comparator II & III

R2= ∞

•Given: f0

•Use f0 with curve titled

center frequency vs R1, C

to determine R1 and C1

R2≠∞

•Given: f0 and fL

•Calculate fMIN from the

equation fMIN = fo − fL

•Use fMIN with curve titled

offset frequency vs R2, C

to determine R2 and C1

•Calculate fMAX/fMIN from

the equation fMAX/fMIN =

fo + fL/fo − fL

•Use fMAX/fMIN with curve

titled fMAX/fMIN vs R2/R1

to determine ratio R2/R1

R2= ∞

•Given: fMAX

•Calculate f0 from the

equation fo = fMAX/2

•Use f0 with curve titled

center frequency vs R1, C

to determine R1 and C1

R2≠∞

•Given: fMIN and fMAX

•Use fMIN with curve titled

offset frequency vs R2,

C to determine R2 and C1

•Calculate fMAX/fMIN

•Use fMAX/fMIN with curve

titled fMAX/fMIN vs R2/R1

to determine ratio R2/R1

to obtain R1

to obtain R1

FIGURE 1.

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