EL4585
For our example, C
1
=17pF. (A trim cap may be used for fine
tuning.) Examples for each frequency using the internal
divider follow.
Typical Application
TYPICAL LC VCO
LC VCO COMPONENT VALUES (APPROXIMATE) (NOTE)
FREQUENCY
(MHz)
26.602
27.0
29.5
35.468
21.476
24.546
28.636
L
1
(µH)
3.3
3.3
2.7
2.2
4.7
3.9
3.3
C
1
(pF)
22
21
22
16
26
22
17
C
2
(pF)
220
220
220
220
220
220
220
Horizontal genlock provides clock for an analog to digital
converter, digitizing analog video.
XTAL VCO COMPONENT VALUES (APPROXIMATE)
FREQUENCY
(MHz)
26.602
27.0
29.5
35.468
21.476
24.546
28.636
R
1
(kΩ)
300
300
300
300
300
300
300
C
1
(pF)
15
15
15
15
15
15
15
C
2
(µF)
0.001
0.001
0.001
0.001
0.001
0.001
0.001
NOTE: Use shielded inductors for optimum performance.
The above oscillators are arranged as Colpitts oscillators,
and the structure is redrawn here to emphasize the split
capacitance used in a Colpitts oscillator. It should be noted
that this oscillator configuration is just one of literally
hundreds possible, and the configuration shown here does
not necessarily represent the best solution for all
applications. Crystal manufacturers are very informative
sources on the design and use of oscillators in a wide variety
of applications, and the reader is encouraged to become
familiar with them.
TYPICAL XTAL VCO
COLPITTS OSCILLATOR
C
1
is to adjust the center frequency, C
2
DC isolates the
control from the oscillator, and V
1
is the primary control
device. C
2
should be much larger than C
V
so that V
1
has
9
FN7175.3
July 1, 2005
INTERSIL [ INTERSIL CORPORATION ]
