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. In Radio Frequency (RF) practice, this is often measured in a dimensionless ratio known as VSWR with a VSWR bridge. Although the principles are the same, this concept is perhaps easiest to understand when considering an optical fiber. Imperfections in the glass create mirrors that reflect the light back along the fiber.

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 damage to a cable has occurred. These instruments work by sending a short pulsed signal into the cable and measuring how long the reflection takes to return. If only reflection magnitudes are desired, however, and exact fault locations are not required, VSWR bridges perform a similar but lesser function for RF cables.