Which of the following is a key testing device associated with communication systems?

The Time Domain Reflectometer (TDR) is essential in communication systems as it allows technicians to assess and troubleshoot transmission lines and cables. By sending a test signal along a cable and analyzing the reflections that return, the TDR can pinpoint issues such as breaks, shorts, or impedance mismatches within the medium. This testing device is particularly valuable for ensuring the integrity and quality of signal pathways, which are vital for maintaining reliable communication.

While other testing devices such as a distortion meter, spectrum analyzer, and signal processor serve important roles within the realm of communication, they focus on other aspects. A distortion meter is used to measure the level of distortion in signals, primarily for audio and video applications. The spectrum analyzer is critical for analyzing the frequency components of a signal, assessing performance and interference issues. Signal processors, meanwhile, are involved in modifying or analyzing signals rather than testing the physical integrity of communication lines. Therefore, the TDR's specific function in identifying faults in transmission lines makes it a key device in communication system testing.