Reflection (electrical)
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Reflection in an electrical circuit occurs at locations where there is an impedance mismatch. This can occur if there is a change in the material comprising the circuit, or if the physical material ends abruptly (see antenna).
Impedance discontinuities cause attenuation, standing waves, and other effects because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo. This effect is compounded if there are multiple discontinuities causing additional portions of the remaining signal to be reflected back to the transmitter. This is a fundamental problem with the daisy chain method of connecting electronic components.
When this returning reflection strikes the first discontinuity, some of the signal rebounds in the direction of the original signal, creating multiple echo effects. The echoes strike the receiver at different intervals making it difficult for the receiver to accurately detect data values on the signal. This can result in effects similar to jitter.
The ratio of energy bounced back depends on the impedance mismatch. Mathematically, it is defined using the reflection coefficient.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system. When a sufficient degree of impedance matching is not practical, echo cancellation can sometimes ameliorate the problems.
