9 - Time domain reflectometers

Abstract

This chapter describes time domain reflectometers as one of the measuring equipment in optical communication networks. Borne of the non-real-time storage oscilloscope principle, in which a single occurring measurement is to be displayed, the time domain reflectometer (TDR) is used to test and measure parameters in communications networks, with the specific purpose of identifying and, more important, locating faults. In simple terms, it does this by transmitting a defined signal into the network and displaying the returned signal. There are two main categories of time domain reflectometer: (1) metallic time domain reflectometers (MTDRs) – used in metallic cable communications systems; co-axial, twisted pair and so on. (2) Optical time domain reflectometers (OTDRs) – used in optical cable communications systems; usually optical fiber networks, giving rise to the alternative name fiber optical time domain reflectometers (FOTDRs). Considerably different techniques are used in each category. Furthermore, metallic time domain reflectometers vary in two main operational ways: single type and display type.