7. TDR Measurement Theory
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.
7.9. 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 7.9. This figure depicts a simulation of the
difference between a 90 ps rise time TDR and an 800 ps rise time TDR (typical of low-cost TDR
instruments) for characterizing two 1 cm long, 75 ohm faults spaced 1 cm apart on a 50 ohm cable.
The 800 ps rise time TDR detects only a single fault and grossly underestimates the severity of the
fault.
CT100B TDR Cable Analyzers Operator’s Manual 103
To Next Page
Loading...