Determining Repeater Margin with a Stub and Attenuator

The margin of a high-speed digital regenerative repeater can be determined easily by adding a misterminated stub at the repeater input. The stub adds an interfering signal which, if large enough, will cause the repeater to make errors. The amplitude of the interfering signal is determined by the termination, so the termination impedance at which errors begin is a good measure of repeater margin. No calibration procedure is required.