EL4584C Elantec Semiconductor, Inc. (acquired by Intersil), EL4584C Datasheet - Page 7

• Manufacturer Part Number: EL4584C
• Description: Horizontal Genlock, 4FSC
• Manufacturer: Elantec Semiconductor, Inc. (acquired by Intersil)

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 1 2
and 16 and is described in table 1 above The di-
vided signal is sent along with the delayed
H-sync input to the phase frequency 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 2 5
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
7
can source or sink a maximum of about 300 A
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-
cy
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 CLK
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
d
N charge pump output (pin 7) sources cur-
Horizontal Genlock 4 F
d
N forcing the VCO frequency lower
EL4584C
SC