ICS9173B Datasheet, PDF (3/9 Pages) Integrated Device Technology – VIDEO GENLOCK PLL

IDT9173B
VIDEO GENLOCK PLL
CLOCK SYNTHESIZER
Using the IDT9173B in Genlock Applications
Most video sources, such as video cameras, are
asynchronous, free-running devices. To digitize video or
synchronize one video source to another free-running
reference video source, a video “genlock” (generator lock)
circuit is required. The IDT9173B integrates the analog
blocks which make the task much easier.
In the complete video genlock circuit, the primary function of
the IDT9173B is to provide the analog circuitry required to
generate the video dot clock within a PLL. This application
is illustrated in Figure 1. The input reference signal for this
circuit is the horizontal synchronization (H-SYNC) signal. If
a composite video reference source is being used, the
h-sync pulses must be separated from the composite signal.
A video sync separator circuit, such as the National
Semiconductor LM1881, can be used for this purpose.
The clock feedback divider shown in Figure 1 is a digital
divider used within the PLL to multiply the reference
frequency. Its divide ratio establishes how many video dot
clock cycles occur per h-sync pulse. For example, if 880
pixel clocks are desired per h-sync pulse, then the divider
ratio is set to 880. Hence, together the h-sync frequency and
external divider ratio establish the dot clock frequency:
f_OUT = f_IN x N where N is external divide ratio
Both input pins IN and FBIN respond only to negative-going
clock edges of the input signal. The H-SYNC signal must be
constant frequency in the 12 kHz to 1 MHz range and stable
(low clock jitter) for creation of a stable output clock.
desired dot clock frequency. The primary consideration is
the internal VCO which operates over a frequency range of
10 MHz to 75 MHz. Because of the selectable VCO output
divider and the additional divider on output CLK2, four
distinct output frequency ranges can be achieved. The
following Table lists these ranges and the corresponding
device configuration.
FS0
State
0
0
1
1
Output
Used
CLK1
CLK2
CLK1
CLK2
Frequency/Range
IDT9173B-01
10 to 75 MHz
5 to 37.5 MHz
2.5 to 18.75 MHz
1.25 to 9.375 MHz
Frequency/Range
IDT9173B-15
5 to 37.5 MHz
2.5 to 18.75 MHz
1.25 to 9.375 MHz
0.625 to 4.6875 MHz
Note that both outputs, CLK1 and CLK2, are available
during operation even though only one is fed back via the
external clock divider.
Pin 5, OE, tristates both CLK1 and CLK2 upon logic low
input. This feature can be used to revert dot clock control to
the system clock when not in genlock mode (hence, when in
genlock mode the system dot clock must be tristated).
When unused, inputs FS0 and OE must be tied to either
GND (logic low) or VDD (logic high).
The output hook-ups of the IDT9173B are dictated by the
Figure 1: Typical Application of IDT9173B in a Video Genlock System
IDT™ VIDEO GENLOCK PLL
3
IDT9173B
REV B 111209