9

FN7174.3

May 9, 2008

The resistance needed will depend on VCO design or VCXO

module selection.

Applications Information

Choosing External Components

1. To choose LC VCO components, first pick the desired

operating frequency. For our example, we will use

2. Choose a reasonable inductor value (10µH to 20µH

works well). We choose 15µH.

Equations 1 and 2.

example.

our example.

then:

for fine tuning). Examples for each frequency using the

internal divider follow.

Typical Application

Horizontal genlock provides clock for an analog to digital

converter, digitizing analog video.

FIGURE 11.

F

OSC

1

2

π LC

T

-----------------------

=

(EQ. 1)

C

T

1

4

π

2

F

2

L

---------------------

1

4

π

2

14.318e6

()

2

15e

6

–

()

---------------------------------------------------------------------

8.2pF

==

=

(EQ. 2)

C

T

C

C

()

C

()

C

()′

++

----------------------------------------------------------------------------

=

(EQ. 3)

C

1

C

C

() C

()

C

()

–

–

′

--------------------------------------------------------------------------

=

(EQ. 4)

TABLE 3. LC VCO COMPONENT VALUES (APPROXIMATE)

(NOTE)

FREQUENCY

(MHZ)

(µH)

(pF)

(pF)

13.301

15

18

220

13.5

15

17

220

14.75

12

18

220

17.734

12

10

220

10.738

22

20

220

12.273

18

17

220

14.318

15

14

220

NOTE: Use shielded inductors for optimum performance.

TABLE 4. XTAL VCO COMPONENT VALUES

(APPROXIMATE)

FREQUENCY

(MHz)

(k

Ω)

(pF)

(µF)

13.301

300

15

0.001

13.5

300

15

0.001

14.75

300

15

0.001

FIGURE 12. TYPICAL LC VCO

FIGURE 13. TYPICAL XTAL VCO

EL4584