EL4585C
Horizontal Genlock, 8 F
SC
Typical LC VCO
Applications Information
Choosing External Components
1. To choose LC VCO components, first pick the
desired operating frequency. For our example
we will use 28.636MHz, with an H-sync fre-
quency of 15.734kHz.
2. Choose a reasonable inductor value (1-5mH
works well). We choose 3.3mH.
3. Calculate C needed to produce F
T
.
OSC
4585–10
1
e
F
OSC
2q LC
LC VCO Component Values (Approximate)
0
e
T
Frequency
(MHz)
L1
C1
C2
1
1
(mH)
(pF)
(pF)
e
e
9.4pF
C
T
2
2
2
2
b
4q F L 4q (28.636e6) (3.3e 6)
4. From the varactor data sheet find C
26.602
27.0
3.3
3.3
2.7
2.2
4.7
3.9
3.3
22
21
22
16
26
22
17
220
220
220
220
220
220
220
@
2.5V,
23pF for our
V
e
the desired lock voltage. C
SMV1204-12 for example.
V
29.5
5. C should be about 10C , so we choose
35.468
21.476
24.546
28.636
2
2
V
e
C
220pF for our example.
6. Calculate C . Since
1
C C C
2 V
1
e
C
T
,
Note: Use shielded inductors for optimum performance.
a
a
(C C )
(C C )
1
(C C )
1 V
2
2 V
Typical Xtal VCO
then
C C C
T V
2
e
C
1
.
b
b
(C C ) (C C ) (C C )
2
v
2
T
T V
e
For our example, C 17pF. (A trim cap may be
1
used for fine tuning.) Examples for each frequen-
cy using the internal divider follow.
4585–11
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
