Figure 5-25D.Time-domain reflectometer display of transmission line problems.
A conventional method of evaluating high-frequency transmission systems and components has been
through the use of standing wave ratio (swr) measurements to obtain an overall indication of transmission
line performance. This method involves feeding a sine-wave signal into the system and measuring the
maximum and minimum amplitudes of the standing waves that result from system discontinuities or load
mismatches. The ratio between the minimum and maximum swr values is then taken as the system
FIGURE OF MERIT. The swr measurement, however, does not isolate individual discontinuities or
mismatches when multiple reflections are present; it only indicates their total effect. Time-domain
reflectometry measurements, on the other hand, isolate the line characteristics in time (location). As a
result, multiple reflections resulting from more than one discontinuity or impedance variation that are
separated in distance on the line are also separated in time at the monitoring point and can be individually
Prior to the advent of time-domain reflectometers, time-domain reflectometry was performed with
the aid of sampling oscilloscopes and pulse generators with very fast rise times.
Figure 5-26 shows the earlier type of test setup, which is still an option. However, today's time-
domain reflectometers have several advantages over the old pulse-generator and oscilloscope methods.
Modern time-domain reflectometers are compact, lightweight, are often supplied with battery pack
options for field use, and provide a direct readout of distances instead of time. Some equipments provide
a paper-tape recording for a permanent record. Figure 5-27 shows a typical time-domain reflectometer.