EL4585C
Horizontal Genlock, 8 F
SC
5 volts, and it can source or sink a maximum of
about 300mA, so all frequency control must be
accomplished with variable capacitance from the
varactor within 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 incom-
ing horizontal 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 300ppm, an LC
oscillator should be considered, as crystal oscilla-
tors are very difficult to pull this far. When H-
sync input frequency is greater than CLK fre-
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 110nS, 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 EL4585C, 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.
These modules are very forgiving of PCB layout,
but cost more than discrete solutions. The VCO
or VCXO is used to regulate 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 the VCO module or va-
ractor, 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 the
CLK out pin, divided by two, then sent to the
divide by N counter. The divisor N is determined
by the state of pins 1, 2, and 16 and is described
in table 5 above. The divided signal is sent, along
with the delayed H-sync input, to the phase/fre-
quency detector, which compares the two signals
for phase and frequency differences. Any phase
difference is converted to a current at the charge
pump output, (pin 7). A VCO with a positive fre-
quency deviation with control voltage must be
used. Varactors have negative capacitance slope
with voltage, resulting in positive frequency de-
viation with increasing control voltage for the os-
cillators in figures 10 and 11 below.
d
quency 2N, charge pump output (pin 7) sources
current into the filter capacitor, increasing the
voltage across the varactor, thus tending to in-
crease VCO frequency. Conversely, charge pump
output pulls current from the filter capacitor
d
when H-sync frequency is less than CLK 2N,
forcing the VCO frequency lower.
Loop Filter
The loop filter controls how fast the VCO will
respond to a change in phase comparator output
stimulus. Its components should be chosen so
that fast lock can be achieved, yet with a mini-
mum of VCO ‘‘hunting’’, preferably in one to two
oscillations of charge pump output, assuming the
VCO frequency starts within capture range. If
the filter is under-damped, the VCO will over and
under-shoot the desired operating point many
times before a stable lock takes place. It is possi-
ble 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-damp-
ing is also characterized by an easily unlocked
system because the filter can’t respond fast
enough to perturbations in VCO frequency. A se-
verely over damped system 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 experimentation to determine the
best values to use for any given situation. Use the
component tables as a starting point, but be
aware that deviations from these values are not
out of the ordinary.
VCO
The VCO should be tuned so that its frequency of
oscillation is very close to the required clock out-
put 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
necessary if using one of these units. The output
range of the charge pump output (pin 7) is 0 to
6
ELANTEC [ ELANTEC SEMICONDUCTOR ]
