mm74hc4046 Fairchild Semiconductor, mm74hc4046 Datasheet - Page 8

• Manufacturer Part Number: mm74hc4046
• Description: Cmos Phase Lock Loop
• Manufacturer: Fairchild Semiconductor

www.fairchildsemi.com
•Given: f
•Use f
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 f
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
center frequency vs R1, C
to determine R1 and C1
0
with curve titled
0
R
2
VCO WITHOUT OFFSET
Comparator I
MIN
R2 =
to f
•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
MAX
f
L
MIN
MAX
/f
MAX
increases. Thus the
o
0
with curve titled
/f
and f
MIN
/f
MIN
MIN
MAX
f
L
R
MIN
2
MAX
L
with curve
from the
/f
vs R2/R1
MIN
f
o
/f
MIN
from
f
L
FIGURE 1.
8
•Given: f
•Calculate f
•Use f
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.
equation f
center frequency vs R1, C
to determine R1 and C1
0
with curve titled
MAX
o
0
R
from the
2
f
MAX
/2
VCO WITH OFFSET
Comparator II & III
•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
MIN
MAX
MAX
MIN
/f
with curve titled
MIN
/f
MAX
MIN
R
and f
2
with curve
/f
vs R2/R1
MIN
MAX