el4585 Intersil Corporation, el4585 Datasheet - Page 7

• Manufacturer Part Number: el4585
• Description: Horizontal Genlock, 8fsc
• Manufacturer: Intersil Corporation

EL4585 Block Diagram
Description Of Operation
The horizontal sync signal (CMOS level, falling leading edge)
is input to H
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
EL4585, 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. 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 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 the
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 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 1. The divided signal is sent, along with
the delayed H
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
frequency deviation with control voltage must be used.
Varactors have negative capacitance slope with voltage,
resulting in positive frequency deviation with increasing
control voltage for the oscillators in Figures 10 and 11.
VCO
The VCO should be tuned so that its frequency of oscillation
is very close to the required clock output frequency when the
voltage on the varactor is 2.5V. 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
SYNC
SYNC
input (pin 10). This signal is delayed about
input, to the phase/frequency detector,
7
EL4585
the charge pump output (pin 7) is 0V to 5V, 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
signal experiences frequency variations of greater 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, thus
tending to increase VCO frequency. Conversely, charge
pump output pulls current from the filter capacitor when
H
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 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 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
SYNC
SYNC
input frequency is greater than CLK frequency ÷ 2N,
frequency is less than CLK ÷ 2N, forcing the VCO
SYNC
July 1, 2005
FN7175.3