EL4584CS datasheet(7/16 Pages) ELANTEC

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EL4584C

Horizontal Genlock 4 FSC

Description Of Operation

The horizontal sync signal (CMOS level falling

leading edge) is input to H-sync input (pin 10)

This signal is delayed about 110 ns the falling

edge of which becomes the reference to which the

clock output will be locked

(See timing dia-

grams) The clock is generated by the signal on

pin 5 OSC in There are 2 general types of VCO

that can be used with the EL4584C LC and crys-

tal controlled Additionally each type can be ei-

ther built up using discrete components includ-

ing a varactor as the frequency controlling ele-

ment or complete self contained modules can be

purchased with everything inside a metal can

The modules are very forgiving of PCB layout

but cost more than discrete solutions The VCO

or VCXO is used to generate the clock An LC

tank resonator has greater ‘‘pull’’ than a crystal

controlled circuit but will also be more likely to

drift over time and thus will generate more jit-

ter The ‘‘pullability’’ of the circuit refers to the

ability to ‘‘pull’’ the frequency of oscillation away

from its center frequency by modulating the volt-

age on the control pin of a VCO module or varac-

tor and is a function of the slope and range of

the capacitance-voltage curve of the varactor or

VCO module used The VCO signal is sent to a

divide by N counter and to the CLK out pin The

divisor N is determined by the state of pins 12

and 16 and is described in table 1 above The di-

vided signal is sent

along with the delayed

H-sync input to the phasefrequency detector

which compares the two signals for phase and

frequency differences Any phase difference is

converted to a current at the charge pump output

FILTER (pin 7) A VCO with positive frequency

deviation with control voltage must be used Va-

ractors have negative capacitance slope with

voltage resulting in positive frequency deviation

with control voltage for the oscillators in figures

10 and 11 below

VCO

The VCO should be tuned so its frequency of os-

cillation is very close to the required clock output

frequency when the voltage on the varactor is 25

volts VCXO and VCO modules are already tuned

to the desired frequency so this step is not neces-

sary if using one of these units The range of the

charge pump output (pin 7) is 0 to 5 volts and it

can source or sink a maximum of about 300

so all frequency control must be accomplished

with variable capacitance from the varactor with-

in this range Crystal oscillators are more stable

than LC oscillators which translates into lower

jitter but LC oscillators can be pulled from their

mid-point values further resulting in a greater

capture and locking range If the incoming hori-

zontal sync signal is known to be very stable

then a crystal oscillator circuit can be used If the

h-sync signal experiences frequency variations of

greater than about 300 ppm an LC oscillator

should be considered as crystal oscillators are

very difficult to pull this far When H-SYNC in-

put frequency is greater than CLK frequen-

cydN charge pump output (pin 7) sources cur-

rent into the filter capacitor increasing the volt-

age across the varactor which lowers its capaci-

tance thus tending to increase VCO frequency

Conversely filter output pulls current from the

filter capacitor when H-SYNC frequency is less

than CLKdN forcing the VCO frequency lower

Loop Filter

The loop filter controls how fast the VCO will

respond to a change in filter output stimulus Its

components should be chosen so that fast lock

can be achieved yet with a minimum of VCO

‘‘hunting’’ preferably in one to two oscillations

of charge pump output assuming the VCO fre-

quency starts within capture range If the filter is

under-damped the VCO will over and under-

shoot the desired operating point many times be-

fore a stable lock takes place It is possible to

under-damp the filter so much that the loop itself

oscillates and VCO lock is never achieved If the

filter is over-damped the VCO response time will

be excessive and many cycles will be required for

a lock condition Over-damping is also character-

ized by an easily unlocked system because the

filter can’t respond fast enough to perturbations

in VCO frequency A severely over damped sys-

tem will seem to endlessly oscillate like a very

large mass at the end of a long pendulum Due to

parasitic effects of PCB traces and component

variables it will take some trial and error experi-

mentation to determine the best values to use for

any given situation Use the component tables as

a starting point but be aware that deviation from

these values is not out of the ordinary

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