From: abe riazi (email@example.com)
Date: Wed Feb 14 2001 - 09:36:20 PST
Eddie Suckow Wrote:
".., it is important to remember the effect of loading the backplane if it is of parallel configuration. With the insertion of each
card a new capacitance is introduced, thus resulting in a decrease in the effective impedance. As multiple cards are inserted it is not uncommon to see the characteristic impedance of the backplane drop
by 25%. Naturally, this calls for a lower termination resistance Rt to keep the impedance matched. The challenge then becomes finding the optimum Rt for both unloaded and loaded backplane
The effective impedance of a transmission line diminishes due to loading effects as you pointed out. Of course, capacitive loading can also reduce signal velocity and hence lengthen propagation delay. The loading phenomenon may be broadly classified as distributed or lumped. Fortunately, many simulator programs possess capability to automatically take into account loading effects caused by various devices (such as
receiver chips, vias, socket and connector pins, etc), provided each device is accurately modeled.
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