TABLE OF CONTENTS The PCI Express Base Specification requires that each lane of a PCI Express link be AC-coupled between the driver and receiver. The actual AC coupling capacitors can be located either on or off the die/component. In most cases, the AC coupling is separate from the component, and you must use discrete capacitors on the PCB board itself to satisfy the AC coupling requirement of the specification. The following guidelines apply to AC coupling capacitors.
For add-in cards and ExpressCard modules, the AC coupling capacitors are located on the card for each of the TX pairs originating from the add-in card PCI Express device. You do not need to place any AC capacitors on the system or host board for these signals.
For the system board, the AC coupling capacitors are required on the TX pairs originating from the system board PCI Express device and traveling to the respective connector interface.
For chip-to-chip connections, all PCI Express differential pairs should have AC caps located on them somewhere along the interconnect.
Because most four-layer system boards allow only single-sided, top layer component placement, all TX pairs originating on the system board are likely to be routed on the top layer. Knowing this, you should plan to route all the RX pairs on the bottom layer of your board. This separation of the RX and TX pairs also helps to reduce near-end crosstalk (NEXT), which can severely limit the performance of PCI Express signals. If necessary, TX signals routed...
