Which instrument sends a test signal and displays reflections caused by impedance mismatches?

The time domain reflectometer (TDR) is specifically designed to send a test signal down a cable or other transmission medium and to analyze any reflections that occur due to impedance mismatches. When the signal encounters a change in impedance, some of the signal is reflected back to the TDR. The instrument measures the time it takes for the reflections to return, which allows it to create a graphical representation of the cable's characteristics and identify the location and nature of any faults or issues along the cable.

In contrast, while a signal strength meter measures the strength of received signals, it does not provide information about reflections or the condition of the transmission medium itself. A network analyzer, although capable of testing and analyzing network performance, does not focus on reflections from impedance mismatches in the same manner as a TDR. Lastly, a power meter measures the power in the system, but it does not show reflections or provide details about impedance mismatches. Thus, the TDR's unique capability to analyze reflections makes it the correct answer.