EL4585

FN7175 Rev 4.00

Page 11 of 15

September 3, 2009

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PCB Layout Considerations

It is highly recommended that power and ground planes be

used in layout. The oscillator and filter sections constitute a

feedback loop and thus care must be taken to avoid any

feedback signal influencing the oscillator except at the

control input. The entire oscillator/filter section should be

surrounded by copper ground to prevent unwanted

influences from nearby signals. Use separate paths for

analog and digital supplies, keeping the analog (oscillator

section) as short and free from spurious signals as possible.

Careful attention must be paid to correct bypassing. Keep

lead lengths short and place bypass caps as close to the

supply pins as possible. If laying out a PCB to use discrete

components for the VCO section, care must be taken to

avoid parasitic capacitance at the OSC pins 3 and 5, and

FILTER out (pin 7). Remove ground and power plane copper

above and below these traces to avoid making a capacitive

connection to them. It is also recommended to enclose the

oscillator section within a shielded cage to reduce external

influences on the VCO, as they tend to be very sensitive to

“hand waving” influences, the LC variety being more

sensitive than crystal controlled oscillators. In general, the

higher the operating frequency, the more important these

considerations are. Self contained VCXO or VCO modules

are already mounted in a shielding cage and therefore do

not require as much consideration in layout. Many crystal

manufacturers publish informative literature regarding use

and layout of oscillators which should be helpful.

LC LOOP FILTER COMPONENTS (APPROXIMATE)

FREQUENCY

(MHz)

(k

)

(k

)

(µF)

(µF)

26.602

100

39

0.01

0.001

27.0

100

39

0.01

0.001

29.5

100

43

0.01

0.001

35.468

100

51

0.01

0.001

21.476

100

30

0.01

0.001

24.546

100

36

0.01

0.001

28.636

100

43

0.01

0.001

FIGURE 12. TYPICAL LOOP FILTER

XTAL LOOP FILTER COMPONENTS (APPROXIMATE)

FREQUENCY

(MHz)

(k

)

(M

)

(pF)

(pF)

26.602

100

4.3

68

6.8

27.0

100

4.3

68

6.8

29.5

100

4.3

68

6.8

35.468

100

4.3

68

6.8

21.476

100

4.3

68

6.8

24.546

100

4.3

68

6.8

28.636

100

4.3

68

6.8