5. TDR Measurement Theory
5.14. Layer Peeling/Dynamic Deconvolution
Layer peeling (or dynamic deconvolution) is a general method to solve the inverse scattering
problem in partially reflective transmission lines such as cables. When applied to TDR traces,
this method attempts to extract the underlying real reflection coefficients and impedance in the
presence of multiple reflections caused by the presence of impedance discontinuities in the cable
assembly under test.
As implemented in the CT100B (see Section 3.11.23), the layer peeling method iteratively
examines the TDR trace at each time step and attempts to correct for forward and backward
signal propagation to extract impedance values at each time step. This is diagrammed in
Figure 5.12 showing measured voltages
V
(
t
) taken from the TDR trace and their relationship to
actual reflection coefficients (Γ) at time (
t
) and impedance values (
Z
x
) at different physical
distances. The diagram shows that for each time step forward in the TDR trace, there are
increasingly complex contributions from forward and backward reflections as they interact with
each new impedance boundary.
For instance, the test signal moving forward from the
Z
0
/Z
1
impedance boundary is reduced by
the energy reflected by the
Z
0
/Z
1
boundary. Likewise the
Z
1
/Z
2
impedance boundary is
“tested” by the original TDR test signal at time
t
1
but also interacts with reflected signals at
every subsequent time point due to internal reflections in the cable. The layer-peeled TDR trace
partially corrects for the presence of these multiple reflections and provides more accurate
impedance values. This effect is most pronounced when there are multiple large impedance
transitions in a cable assembly and at short and open faults.
Figure 5.12.
Layer peeling scattering diagram relating measured TDR trace (
V
[
t
]
) to
actual impedance changes (
Z
[
x
]
) and their associated actual reflection coefficients
(
Γ[
x, t
]
).
CT100B TDR Cable Analyzers Operator’s Manual
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