SIGNAL REFLECTIONS

3/30/96

Signal Reflections: Signal reflections occur when a varying electric current reaches a discontinuity where optimum voltage to current ratios (impedance) changes. This situation occurs to some extent whenever the characteristics of a wire along which a current is flowing change. Reflections are primarily a problem at high frequencies such as those required to swing digital signals from low to high or high to low rapidly. The higher the frequency and the more severe the mismatch, the more likely the reflections are to cause spurious peaks or nulls that may be erroneously detected as real signal changes. Reflections can be minimized by minimizing mechanical connections or by using terminating resistors that are matched to the driving devices and the wiring. Spurious phenomena caused by reflections quickly fade once voltage and current stabilize at the new level. Circuits often include a deliberate "settling time" to allow signal levels to stabilize. These settling times may limit how fast components can be run. There is a trade off between maximum operating speed and the use of pluggable components such as bus cards or memory modules because the pluggable components introduce more reflections. Increasing the number of cards or memory modules in use can require that response times (wait states) be adjusted upwards. In general, the best performance comes when the fewest components are connected by the shortest feasible paths with the fewest pluggable connections.

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