74vhc4046 National Semiconductor Corporation, 74vhc4046 Datasheet - Page 8

• Manufacturer Part Number: 74vhc4046
• Description: Cmos Phase Lock Loop
• Manufacturer: National Semiconductor Corporation

– Given f
– Use f
Detailed Circuit Description
VOLTAGE CONTROLLED OSCILLATOR SOURCE
FOLLOWER
The VCO requires two or three external components to op-
erate 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 f
decreases 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 frequency 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 ef-
center frequency vs R1 C
to determine R1 and C1
0
with curve titled
0
R
2
e
min
%
to f
max
Comparator I
increases Thus the VCO’s gain
– Given f
– Calculate f
– Use f
– Calculate f
– Use f
equation f
offset frequency vs R2 C
to determine R2 and C1
the equation f
f
titled f
to determine ratio R2 R1
to obtain R1
o
a
f
min
max
L
max
0
f
o
with curve titled
R
and f
min
f
b
f
min
max
min
2
min
FIGURE 2 Logic Diagram for VCO
i
f
e
max
L
%
L
from the
with curve
vs R2 R1
f
f
min
o
f
FIGURE 1 (Continued)
b
min
from
f
L
e
(b)
8
– Given f
– Calculate f
– Use f
equation f
center frequency vs R1 C
to determine R1 and C1
fect of R2 is shown in the design information table and typi-
cal performance curves By increasing the value of R2 the
lock range of the PLL is offset above 0Hz and the gain
(Volts rad ) does not change In general when offset is de-
sired 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
0
with curve titled
max
R
o
0
2
e
e
from the
f
%
max
2
Comparator II
– Given f
– Use f
– Calculate f
– Use f
offset frequency vs R2
C to determine R2 and C1
titled f
to determine ratio R2 R1
to obtain R1
TL F 11675 – 18
III
min
max
max
min
with curve titled
R
f
f
max
min
2
min
and f
i
%
with curve
vs R2 R1
f
min
max