Signal reflection

Signal reflection

When a signal is transmitted along a transmission medium, such as a copper cable or an optical fibre, there is the possibility that some of the signal power is reflected back to its origin, rather than being carried all the way along the cable to the far end. This happens because of imperfections in the cable causing impedance miss matching and non linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected. This is perhaps easiest to understand when considering an optical fibre. Imperfections in the glass create mirrors that reflect the light back along the fibre.

The most likely places for this to occur are at any joints in the cable, or at points of damage.

Because damage to the cable can cause reflections, an instrument called an Electrical Time Domain Reflectometer ETDR (for electrical cables) or an Optical Time Domain Reflectometer OTDR (for optical cables) can be used to locate where the damage to the cable has occurred. These instruments work by sending a short pulsed signal into the cable and measuring how long the reflection takes to return.