5. TDR Measurement Theory
Again, this relationship is valid if the cable is terminated with a short or open. One drawback to
this technique is that the round-trip insertion loss reduces the length of cable that can be tested
over a given bandwidth relative to a 2-port measurement because the attenuation of the test
signal is doubled. However, for many applications it is adequate.
5.12. Smith Charts
The impedance Smith chart is a useful graphical tool for visualizing complex impedances and
scattering parameters as shown by Figure 5.11. The CT100B includes Smith chart functionality
that can be used to simplify transmission line troubleshooting and impedance matching as
described in Section 3.11.20.
Figure 5.11.
Impedance Smith chart relationships.
5.13. Normalized TDR Traces
The S-parameter OSL calibration process mathematically models the TDR pulser-sampler
system as a two-port error network and subtracts systematic pulser-sampler errors from the
S
11
matrix. Convolution of the
S
11
matrix with an idealized Gaussian step or other idealized
excitation signal allows the CT100B to create and display a normalized TDR trace. The
normalized TDR trace has lower aberration and improved impedance accuracy compared with
the original TDR trace.
Although cable fault detection is the most sensitive when using an excitation signal with the
fastest rise time possible, by changing the rise time of the Gaussian step, the importance of a
given cable or connector fault can be estimated at different signal rise times and bandwidths.
For instance if the cable under test is typically used with 1 ns rise time signals, the severity of a
cable fault can be determined by using a 1 ns rise time normalized TDR trace. Localized cable
faults will always appear less severe when examined using slower rise time excitation signals.
78
CT100B TDR Cable Analyzers Operator’s Manual
