Broadband Reflectometry: Theoretical Background - Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications - page 26

reflectometry - FDR). Depending on the specific application, one approach may be
more suitable than the other.
TDR instrumentation is usually less expensive than instruments operating in fre-
quency domain. Nevertheless, instruments operating directly in frequency domain,
such as vector network analyzers (VNAs), despite being more expensive, usually
provide higher measurement accuracy. An optimal trade-off (in terms of low cost
and measurement accuracy), can be achieved through a time domain/frequency do-
main (TD/FD) combined approach: in this case, measurements are carried out, for
example, in TD and the corresponding FD-information is retrieved through ap-
propriate processing of the acquired data, thus discovering additional information
[11, 36].
It is worth pointing out that a substantial advantage in operating in FD relates to
the possibility of performing calibration procedures (as will be detailed in Sec. 3.4),
thus enhancing the final measurement accuracy. This procedure can be implemented
in the TD/FD approach; consequently, the combination of the two approaches is
highly regarded as a powerful tool for guaranteeing at the same time low cost of
the setup and adequate measurement accuracy. This aspect, along with the detailed
description of the specific implemented procedure, will be addressed in Sect. 3.4. In
the following subsections, the basic principles behind all the three approaches are
recalled, and pros and cons are discussed.

3.2 Time Domain Reflectometry (TDR)

In TDR measurements, the EM stimulus is usually a step-like signal that propagates
along the probe, through the SUT. The basic block diagram of a typical TDR system
is shown in Fig. 3.1. It consists of a step-pulse generator, a sampling scope, and a
signal analyzer. The step generator produces a positive-going incident wave that is
applied to the SUT. The step travels down the transmission line at the velocity of
propagation of the line. Any impedance variation causes the partial reflection of the
propagating signal. If the load impedance is equal to the characteristic impedance
of the line, no wave is reflected, and the oscilloscope displays only the incident
voltage step. If a mismatch exists at the load, part of the incident wave is reflected

Fig. 3.1 Functional block diagram for a time domain reflectometer [4]