el4584 Intersil Corporation, el4584 Datasheet - Page 7

• Manufacturer Part Number: el4584
• Description: Horizontal Genlock, 4fsc
• Manufacturer: Intersil Corporation

Block Diagram
Description Of Operation
The horizontal sync signal (CMOS level, falling leading
edge) is input to H
about 110ns, the falling edge of which becomes the
reference to which the clock output will be locked. (See
timing diagrams.) 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 EL4584, LC and crystal controlled.
Additionally, each type can be either built up using discrete
components, including a varactor as the frequency
controlling element, 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 jitter. The “pullability”
of the circuit refers to the ability to “pull” the frequency of
oscillation away from its center frequency by modulating the
voltage on the control pin of a VCO module or varactor, 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 divided signal is
sent, along with the delayed Hsync 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. Varactors have negative capacitance
slope with voltage, resulting in positive frequency deviation
with control voltage for the oscillators in figures 10 and 11.
VCO
The VCO should be tuned so its frequency of oscillation is
very close to the required clock output frequency when the
voltage on the varactor is 2.5 volts. VCXO and VCO
SYNC
input (pin 10). This signal is delayed
7
EL4584
modules are already tuned to the desired frequency, so this
step is not necessary 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 300µA, 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 incoming horizontal sync signal is known to be
very stable, then a crystal oscillator circuit can be used. If the
H
than about 300ppm, an LC oscillator should be considered,
as crystal oscillators are very difficult to pull this far. When
H
charge pump output (pin 7) sources current into the filter
capacitor, increasing the voltage across the varactor, which
lowers its capacitance, thus tending to increase VCO
frequency. Conversely, filter output pulls current from the
filter capacitor when H
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
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 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 characterized by an easily unlocked system
because the filter can’t respond fast enough to perturbations
in VCO frequency. A severely over damped system will
seem to endlessly oscillate, like a very large mass at the end
SYNC
SYNC
signal experiences frequency variations of greater
input frequency is greater than CLK frequency ÷ N,
SYNC
frequency is less than CLK ÷ N,
July 25, 2005
FN7174.2