EL4584CS datasheet(11/16 Pages) ELANTEC

11 / 16 page

EL4584C

Horizontal Genlock 4 FSC

transfer function CveF(VC) where VC is the

reverse bias control voltage and CV is varactor

capacitance

Since F(VC) is nonlinear it is

probably best to build the VCO and measure

KVCO about 25V The results of one such mea-

surement are shown below The slope of the

curve is determined by linear regression tech-

niques and equals KVCO For our example

KVCO e 605 MradSV

FOSC vs VC LCVCO

4584 – 14

5 Now we can solve for C3 C4 and R3

C3 e

KdKVCO

02n

(477e b 5)(605e6)

(910)(5000)2

001

C4 e

0001

R3 e

KdKVCO

(2)(910)(1)(5000)

(477e b 5)(605e6)

315 k

We choose R3 e 30 kX for convenience

6 Notice R2 has little effect on the loop filter de-

sign R2 should be large around 100k and can

be adjusted to compensate for any static phase

error Ti at lock but if made too large will

slow loop response If R2 is made smaller Ti

(see timing diagrams) increases and if R2 in-

creases Ti decreases For LDET to be low at

lock

lTil k 50 ns C4 is used mainly to attenu-

ate high frequency noise from the charge

pump

Lock Time

Let

SeR3C3 As T increases damping increases

but so does lock time Decreasing T decreases

damping and speeds up loop response but in-

creases overshoot and thus increases the number

of hunting oscillations before lock Critical damp-

ing (

ge1) occurs at minimum lock time Because

decreased damping also decreases loop stability

it is sometimes desirable to design slightly over-

damped (

gl1) trading lock time for increased

stability

Typical Loop Filter

4584 – 16

LC Loop Filter Components (Approximate)

Frequency

(MHz)

(kX)(kX)(mF)

(mF)

13301

100

001

0001

135

100

001

0001

1475

100

001

0001

17734

100

001

0001

10738

100

001

0001

12273

100

001

0001

14318

100

001

0001

Xtal Loop Filter Components (Approximate)

Frequency

(MHz)

(kX)(MX)

(pF)

13301

100

135

100

1475

100

17734

100

10738

100

12273

100

14318

100

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