9

FN7175.4

September 3, 2009

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 (1µ to 5µH works

well). We choose 3.3µH.

our example.

then:

for fine tuning.) Examples for each frequency using the

internal divider is shown in Figure 8.

Typical Application

Horizontal genlock provides clock for an analog-to-digital

converter, digitizing analog video.

The oscillators are arranged as Colpitts oscillators

(see Figure 8), 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.

LC VCO COMPONENT VALUES (APPROXIMATE) (Note)

FREQUENCY

(MHz)

(µH)

(pF)

(pF)

26.602

3.3

22

220

27.0

3.3

21

220

29.5

2.7

22

220

35.468

2.2

16

220

21.476

4.7

26

220

24.546

3.9

22

220

28.636

3.3

17

220

NOTE: Use shielded inductors for optimum performance.

F

OSC

1

2

π LC

T

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

=

C

T

1

4

π

2

F

2

L

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

1

4

π

2

28.636e6

2

() 3.3e 6

–

()

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

9.4pF

==

=

(EQ. 1)

C

T

C

C

()

C

()

C

()

++

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

=

(EQ. 2)

C

1

C

C

()

C

()

–

C

()

–

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

=

(EQ. 3)

FIGURE 8.

TYPICAL LC VCO

XTAL VCO COMPONENT VALUES (APPROXIMATE)

FREQUENCY

(MHz)

(k

Ω)

(pF)

(µF)

26.602

300

15

0.001

27.0

300

15

0.001

29.5

300

15

0.001

35.468

300

15

0.001

21.476

300

15

0.001

24.546

300

15

0.001

28.636

300

15

0.001

FIGURE 9.

TYPICAL XTAL VCO

EL4585