EL4584CS datasheet(10/16 Pages) ELANTEC

10 / 16 page

EL4584C

Horizontal Genlock 4 FSC

Xtal VCO Component Values (Approximate)

Frequency

(MHz)

(kX)

(pF)

(uF)

13301

300

001

135

300

001

1475

300

001

17734

300

001

10738

300

001

12273

300

001

14318

300

001

The above oscillators are arranged as Colpitts os-

cillators and the structure is redrawn here to em-

phasize 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 in-

formative sources on the design and use of oscil-

lators in a wide variety of applications and the

reader is encouraged to become familiar with

them

Colpitts Oscillator

4584 – 12

C1 is to adjust the center frequency C2 DC iso-

lates the control from the oscillator and V1 is the

primary control device C2 should be much larger

than CV so that V1 has maximum modulation

capability The frequency of oscillation is given

F e

0LCT

CT e

C1C2CV

(C1C2) a (C1CV) a (C2CV)

Choosing Loop Filter

Components

The PLL VCO and loop filter can be described

4584 – 13

Where

Kdephase detector gain in Arad

F(s)eloop filter impedance in VA

KVCOeVCO gain in radsV

Neinternal or external divisor

It can be shown that for the loop filter shown

below

C3 e

KdKVCO

C4 e

R3 e

KdKVCO

Where

0n e loop filter bandwidth and g e loop

filter damping factor

1 Kd e 300 mA2qrad e 477e-5Arad for the

EL4584C

2 The loop bandwidth should be about H-sync

frequency20 and the damping ratio should be

1 for optimum performance For our example

0n e 15734 kHz20 e 787 Hz

5000 radS

3 Ne910 from table 1

N e

VCOfrequency

H-SYNCfrequency

1431818M

1573426k

910

4 KVCO represents how much the VCO frequen-

cy changes for each volt applied at the control

pin It is assumed (but probably isn’t) linear

about the lock point (25V) Its value depends

on the VCO configuration and the varactor

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