11

FN7174.3

May 9, 2008

smaller, t

θ (see “Timing Diagrams” on page 5) increases,

lock, |t

θ| < 50 ns. C

frequency noise from the charge pump.

Lock Time

lock time. Decreasing t decreases damping and speeds up

loop response, but increases overshoot and thus increases

the number of hunting oscillations before lock. Critical

damping (

ζ = 1) occurs at minimum lock time. Because

decreased damping also decreases loop stability, it is

sometimes desirable to design slightly overdamped (

ζ > 1),

trading lock time for increased stability.

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 capacitors 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 “handwaving” 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.

The VCO and loop filter section of the EL4583, EL4584,

EL4585 demo board can be implemented in Figures 18, 19

and 20.

TABLE 5. LC LOOP FILTER COMPONENTS (APPROXIMATE)

FREQUENCY

(MHz)

(k

Ω)

(k

Ω)

(µF)

(µF)

13.301

100

30

0.01

0.001

13.5

100

30

0.01

0.001

14.75

100

33

0.01

0.001

17.734

100

39

0.01

0.001

10.738

100

22

0.01

0.001

12.273

100

27

0.01

0.001

14.318

100

30

0.01

0.001

C

3

K

N

ω

2

n

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

4.77e

5

–

() 6.05e6

()

910

() 5000

()

2

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

0.01µF

==

=

(EQ. 9)

C

4

C

3

10

-------

0.0001µF

==

(EQ. 10)

R

3

2N

ζω

n

K

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

2

() 910

() 1

() 5000

()

4.77e

5

–

() 6.05e6

()

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

31.5k

Ω

==

=

(EQ. 11)

FIGURE 17. TYPICAL LOOP FILTER

TABLE 6. XTAL LOOP FILTER COMPONENTS

(APPROXIMATE)

FREQUENCY

(MHz)

(k

Ω)

(M

Ω)

(pF)

(pF)

13.301

100

4.3

68

6.8

13.5

100

4.3

68

6.8

14.75

100

4.3

68

6.8

17.734

100

4.3

68

6.8

10.738

100

4.3

68

6.8

12.273

100

4.3

68

6.8

14.318

100

4.3

68

6.8

EL4584