TDR Fundamentals
Instrument Configuration
9-26
Since
then
For a series L discontinuity ±3% when ρ ≤ to 10%
Using a similar derivation for a shunt C interline discontinuity, the
relationship between shunt C and the reflection is:
For a shunt C discontinuity ±3% when ρ ≤ 10%
Cable Loss
As a step travels down a non-ideal transmission line, the higher frequencies are
attenuated by skin effect losses and dielectric losses. This distorts the step,
and is called cable loss. The effect of cable loss is shown in Figure 9-23 Plot A,
which shows the reflection of a short at the end of a 1 meter cable. Since cable
loss degrades the risetime of the TDR step, it can limit the distance resolution
and the accuracy of reflection measurements made at the end of a cable.
If fast risetime TDR measurements are needed, short interconnecting cables
should be used to reduce the effects of cable loss. The same reflection off a
short is shown in Figure 9-23 Plot B except now it is at the end of a very short
cable (approximately 5 cm).
Another way to reduce cable loss effects is to use normalization, if the TDR
system has this capability. Normalization to an ideal (approximating a Gaussian)
step removes the effects of cable loss to the point in the cable where a calibration
is done which establishes the reference plane from which TDR measurements
can be made without suffering effects from the cable. Calibration typically
involves connecting a 50 Ω termination and a short termination at the reference
plane. Figure 9-23 Plot C shows the results of normalizing the reflection of a
short at the end of a 1 meter cable. Normalization can also be used to remove
cable loss effects from transmission measurements.
ρ
v
rL
v
iL
-------=
L 98.4
ρ
t
r
system
=
C 0.0303
ρ
t
rL
=
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