5. TDR Measurement Theory
Figure 5.8.
Relationship of voltage standing wave ratio (VSWR) to reflection coefficient
(rho). VSWR for 50 ohm load is shown by the red circle.
relative reflection coefficient calculation. There are menu options in the
Display
menu to toggle
the display of VSWR and ∆VSWR.
VSWR can also be selected as the vertical scale for a trace. From the
Main
menu, go to the
Measurement
menu and select
Vertical Units
.
5.9. Rise Time and Spatial Resolution
Spatial resolution is defined by the ability of the operator to distinguish the presence of two
closely spaced faults on a TDR waveform. An accepted rule of thumb is that the limiting spatial
resolution of a TDR instrument is approximately
1
⁄
2
of the system rise time (10-90%). This is
the time required for the step signal reflected from an open or short termination to transition
from 10% to 90% of the final step amplitude. The estimated spatial resolution
R
is calculated
by the following equation:
R
∼
=
1
2
·
c
·
V
p
·
t
r
where
c
is the speed of light and
t
r
is the system rise time. If the velocity of propagation
V
p
is
taken to be 0.66 and the TDR has a 100 ps system rise time, the calculated resolution is ~10
mm.
Faster system rise times allow a given TDR instrument to more faithfully represent partial cable
faults such as shield nicks or kinks, as shown in Figure 5.9. This figure depicts a simulation of
74
CT100B TDR Cable Analyzers Operator’s Manual
