December 1990 27
Philips Semiconductors Product specification
Phase-locked-loop with lock detector 74HC/HCT7046A
APPLICATION INFORMATION
This information is a guide for the approximation of values of external components to be used with the 74HC/HCT7046
in a phase-lock-loop system.
References should be made to Figs 27, 28 and 29 as indicated in the table.
Values of the selected components should be within the following ranges:
R1 between 3 kΩ and 300 kΩ;
R2 between 3 kΩ and 300 kΩ;
R1 + R2 parallel value > 2.7 kΩ;
C1 greater than 40 pF.
SUBJECT
PHASE
COMPARATOR
DESIGN CONSIDERATIONS
VCO frequency characteristic
VCO frequency
without extra
offset
PC1, PC2 With R2 = ∞ and R1 within the range 3 kΩ<R1 < 300 kΩ, the characteristics
of the VCO operation will be as shown in Fig. 23.
(Due to R1, C1 time constant a small offset remains when R2 = ∞.)
Fig. 23 Frequency characteristic of VCO operating without offset:
f
o
= centre frequency; 2f
L
= frequency lock range.
Selection of R1 and C1
PC1 Given f
o
, determine the values of R1 and C1 using Fig.27.
PC2 Given f
max
and f
o
, determine the values of R1 and C1 using Fig.27,
use Fig.29 to obtain 2f
L
and then use this to calculate f
min.
