Datasheet

19
Application Information
This information is a guide for the approximation of values of
external components to be used with the CD74HC4046A
and CD74HCT4046A in a phase-lock-loop system.
References should be made to Figures 12 through 16 and
Figures 28 through 33 as indicated in the table.
Values of the selected components should be within the fol-
lowing ranges:
HC/HCT4046A C
PD
CHIP SECTION HC HCT UNIT
Comparator 1 48 50 pF
Comparators 2 and 3 39 48 pF
VCO 61 53 pF
R1 Between 3kΩ and 300kΩ
R2 Between 3kΩ and 300kΩ
R1 + R2 Parallel Value > 2.7kΩ
C1 Greater Than 40pF
SUBJECT
PHASE
COMPARATOR DESIGN CONSIDERATIONS
VCO Frequency
Without Extra Offset
PC1, PC2 or PC3 VCO Frequency Characteristic
With R2 = ∞ and R1 within the range 3kΩ < R1 < 300kΩ, the characteristics of the VCO op-
eration will be as shown in Figures 12 - 16. (Due to R1, C1 time constant a small offset remains
when R2 = ∞.)
PC1 Selection of R1 and C1
Given f
o
, determine the values of R1 and C1 using Figures 12 - 16.
PC2 or PC3 Given f
MAX
calculate f
o
as f
MAX
/2 and determine the values of R1 and C1 using Figures 12 -
16. To obtain 2f
L
: 2f
L
≈ 1.2 (V
CC
- 1.8V)/(R1C1) where valid range of VCO
IN
is 1.1V < VCO
IN
< V
CC
- 0.9V
VCO Frequency with
Extra Offset
PC1, PC2 or PC3 VCO Frequency Characteristic
With R1 and R2 within the ranges 3kΩ < R1 < 300kΩ, 3kΩ, < R2 < 300kΩ, the characteristics
of the VCO operation will be as shown in Figures 28 - 33.
PC1, PC2 or PC3 Selection of R1, R2 and C1
Given f
o
and f
L
, offset frequency, f
MIN
, may be calculated from f
MIN
≈ f
o
- 1.6 f
L
.
Obtain the values of C1 and R2 by using Figures 28 - 31.
Calculate the values of R1 from Figures 32 - 33.
FIGURE 45. FREQUENCY CHARACTERISTIC OF VCO OPERATING WITHOUT
OFFSET: f
o
= CENTER FREQUENCY: 2f
L
= FREQUENCY LOCK RANGE
f
MAX
f
VCO
f
o
f
MIN
MIN
1/2 V
CC
V
VCOIN
MAX
2f
L
FIGURE 46. FREQUENCY CHARACTERISTIC OF VCO OPERATING WITH OFFSET:
f
o
= CENTER FREQUENCY: 2f
L
= FREQUENCY LOCK RANGE
f
MAX
f
VCO
f
o
f
MIN
MIN
1/2 V
CC
V
VCOIN
MAX
2f
L
CD74HC4046A, CD74HCT4046A