Using PEX 8648 SMA Based (SI) Card White Paper – Version 1.3
© 2010 by PLX Technology, Inc. All Rights Reserved
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Figure 3. PEX 8648 SMA Card Clocking
The default card configuration accepts the 100 MHz PCIe system clock from the bottom PCIe connector,
similar to a standard add-in card. Care should be used to ensure that any system clock used is of good
quality – reference clock jitter is a primary source of degraded signal performance.
If a clock other than the PCIe connector system clock is desired, the PCB can be modified to accept an
external, differential clock via SMA J17/18 and relocation of zero ohm resistors R133 and R134 to
location R135 and R136. (Schematic picture shown below) Any clock feeding the PCB is AC coupled on-
board and internally terminated within the chip. Signal swing should be typically 400mV, differential, pk-pk
(range 250mV min – 1600mV max). Frequency value should be 100 MHz +/- 300ppm max. The reference
clock feeding the chip is also made available for output via J24/25.
