HP 54753A User Manual page 176

Introduction
The most common method for evaluating a transmission line and its load
has traditionally involved applying a sine wave to a system and measuring
waves resulting from discontinuities on the line. From these
measurements, the standing wave ratio (SWR) is calculated and used as
a figure of merit for the transmission system. When the system includes
several discontinuities, however, the SWR measurement fails to isolate
them. Consider a case where the load is well matched to the transmission
line (i.e., = ) but several connector joining segments of the line act
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as minor discontinuities. This is a realistic situation since BNC
connectors, for example, will typically look like small inductors in series
with the line. The SWR measurement does not single out the component
or components causing the discontinuity; it only indicates their
aggregate effect. Any attempt to improve the system, becomes a trial
and error method of component substitution. In addition SWR
techniques fail to demonstrate whether one discontinuity is generating
a reflection of the proper phase and magnitude to cancel (at a particular
frequency) the reflection from a second discontinuity. When the
broadband quality of a transmission system is to be determined, SWR
measurements must be made at many frequencies, and this method soon
becomes very time consuming and tedious.
TDR avoids all of these disadvantages of the SWR method. TDR employs
a step generator and an oscilloscope in a system best described as
"closed- loop radar." A voltage step is propagated down the transmission
line under investigation, and the incident and reflected voltage waves
are monitored by the oscilloscope at a particular point on the line.
This echo technique (see Figure 9-1) reveals at a glance the
characteristic impedance of the line, and it shows both the position and
the nature (resistive, inductive, or capacitive) of each discontinuity
along the line. TDR also demonstrates whether losses in a transmission
system are series losses or shunt losses. All of this information is
immediately available from the oscilloscope's display. TDR also gives
more meaningful information concerning the broadband response of a
transmission system than any other measuring technique.
Since the basic principles of time domain reflectometry are easily
grasped, even those with limited experience in high frequency
measurements can quickly master this technique. This chapter attempts
9-2