5. TDR Measurement Theory
The CT100B is also able to measure complex frequency-specific S
11
return loss and cable loss
values such as those produced with Vector Network Analyzer (VNA) and Frequency Domain
Reflectometer (FDR) equipment. This is discussed in further detail on page 76.
Figure 5.7.
Relationship of return loss (dB) to reflection coefficient (rho).
5.8. VSWR
Voltage standing wave ratio (VSWR) is a way of displaying reflection coefficient in a nonlinear
way that emphasizes changes in cable impedance. VSWR is the voltage standing wave ratio and
is related to the reflection coefficient Γ according to:
VSWR =
1 +
|
Γ
|
1
− |
Γ
|
VSWR measures the ratio of the maximum-over-time amplitude of the nodes and anti-nodes of
the standing wave off of a reflection. If there’s no reflection (e.g., 50 ohm termination), VSWR
will be 1. If all energy is reflected (e.g., short or open fault), VSWR goes to infinity. This
relationship is shown in Figure 5.8. VSWR is a unitless, scalar value.
The CT100B displays two different VSWR values at cursor. The first, VSWR, is calculated
from the total reflection relative to the CT100B test port source impedance (50 ohms). The
second, ∆VSWR, is calculated from the reflection between the two cursors. In other words,
VSWR is based on the millirho reflection coefficient, while ∆VSWR is based on the ∆m
ρ
,
CT100B TDR Cable Analyzers Operator’s Manual
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