
ICS3771-18
DTV, STB CLOCK SOURCE
SYNTHESIZERS
IDT / ICS DTV, STB CLOCK SOURCE
3
ICS3771-18
REV B 111307
Application Information
Series Termination Resistor
Clock output traces should use series termination. To
series terminate a 50
trace (a commonly used trace
impedance), place a 33
resistor in series with the
clock line, as close to the clock output pin as possible.
The nominal impedance of the clock output is 20
.
Decoupling Capacitors
As with any high-performance mixed-signal IC, the
ICS3771-18 must be isolated from system power
supply noise to perform optimally.
Decoupling capacitors of 0.01F must be connected
between each VDD and the PCB ground plane. To
further guard against interfering system supply noise,
the ICS3771-18 should use one common connection to
the PCB power plane as shown in the diagram on the
next page. The ferrite bead and bulk capacitor help
reduce lower frequency noise in the supply that can
lead to output clock phase modulation.
PCB Layout Recommendations
For optimum device performance and lowest output
phase noise, the following guidelines should be
observed.
1) Each 0.01F decoupling capacitor should be
mounted on the component side of the board as close
to the VDD pin as possible. No vias should be used
between decoupling capacitor and VDD pin. The PCB
trace to VDD pin should be kept as short as possible, as
should the PCB trace to the ground via. Distance of the
ferrite bead and bulk decoupling from the device is less
critical.
2) The external crystal should be mounted next to the
device with short traces. The X1 and X2 traces should
not be routed next to each other with minimum spaces,
instead they should be separated and away from other
traces.
3) To minimize EMI and obtain the best signal integrity,
the 33
series termination resistor should be placed
close to the clock output.
4) An optimum layout is one with all components on the
same side of the board, minimizing vias through other
signal layers (the ferrite bead and bulk decoupling
capacitor can be mounted on the back). Other signal
traces should be routed away from the ICS3771-18.
This includes signal traces just underneath the device,
or on layers adjacent to the ground plane layer used by
the device.