Wednesday, September 3, 2008

Trace Length and Length Matching

Trace length greatly affects the loss and jitter budgets of the interconnection. The PCB trace may introduce 1 ps to 5 ps of jitter and 0.35 dB to 0.50 dB of loss per inch.

For an add-in card, the trace length from the edge finger to the Philips PCI Express PHY pads should be limited to 4 inches. For the system board, the trace length is recommended to be less than 12 inches.

Long distance traces should be routed at an off-angle to the X-Y axis of a PCB layer, in order to distribute the effects of fiberglass bundle weaves and resin-rich areas of the dielectric.
The two traces of a pair should be symmetrically routed.

The length difference between a differential pair should be limited to 5 mils maximum. Length matching is required per segment, and any length added (typically a “serpentine” section) for the sake of matching a pair should be added near the location where the mismatch occurs.
There is no length match requirement between transmit and receive pairs.

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