Signal reflection

Lore & Background

The principles of signal reflection are perhaps easiest to understand when considering an optical fiber, where imperfections in the glass create mirrors that reflect light back along the fiber. Impedance discontinuities cause attenuation, attenuation distortion, standing waves, ringing, and other effects because a portion of a transmitted signal is reflected back to the transmitting device, much like an echo. This effect is compounded if multiple discontinuities cause additional portions of the remaining signal to be reflected back to the transmitter, a fundamental problem with the daisy chain method of connecting electronic components.

When a returning reflection strikes another discontinuity, some of the signal rebounds in the original signal direction, creating multiple echo effects. These forward echoes strike the receiver at different intervals, making it difficult for the receiver to accurately detect data values on the signal, with effects that can resemble those of jitter. The combination of signal attenuation and impedance discontinuities on a communications link is called insertion loss.

Because damage to a cable can cause reflections, instruments such as an electrical time-domain reflectometer (ETDR) or an optical time-domain reflectometer (OTDR) can locate the damaged part by sending a short pulsed signal into the cable and measuring how long the reflection takes to return. If only reflection magnitudes are desired and exact fault locations are not required, VSWR bridges perform a similar but lesser function for RF cables.

Reader's Guide

Signal reflection is a fundamental concern in telecommunications and radio electronics because it directly impacts signal integrity and system performance. 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 suppressors or echo cancellers, or both, can sometimes reduce the problems. The Bergeron diagram method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line. Understanding and managing signal reflection is essential for designing reliable communication links, as uncontrolled reflections can cause data errors, ringing, and other degradation that mimic jitter. The ability to measure reflections using VSWR bridges or locate faults with time-domain reflectometers provides practical tools for maintaining system performance.

Did You Know?

• Signal reflection is caused by impedance mismatches from imperfections or physical variations in a cable.
• Time-domain reflectometers locate cable damage by sending a short pulsed signal and measuring the reflection return time.
• Multiple discontinuities can cause forward echoes that strike the receiver at different intervals, resembling jitter.