7. TDR Measurement Theory
7.8. 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 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 7.8. VSWR is a unitless, scalar value.
Figure 7.8. Relationship of voltage standing wave ratio (VSWR) to reflection coefficient
(rho). VSWR for 50 ohm load is shown by the red circle.
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 ∆mρ, relative reflection
coefficient calculation. There are menu options in the Display menu to toggle the display of VSWR
and ∆VSWR.
102 CT100B TDR Cable Analyzers Operator’s Manual
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