Digital Circuit Boards: Mach 1 GHz

2.10 Nonlinear Terminations

When a transmission line is terminated in a high impedance, the reflected wave (2) adds to the forward wave (1). When the full voltage is transmitted down an unterminated line, the reflected wave is clamped (diode action) by the hardware. Since energy cannot be dissipated in a short circuit, a reflected wave propagates back to the source, canceling the voltage. The result is wave action that keeps reflecting between the source and the termination until losses dissipate the excess energy supplied to the line. In general, it is a good practice to design transmission lines so that clamping action does not occur. It is possible that clamping action can fire a four-layer Shockley junction that can result in a catastrophic failure. In some cases, the conduction path through the clamping diode can impact logic operations resulting in logic errors. For these reasons, it is wise to design the logic so that protecting clamps are not used to limit logic levels.