Option 100 Fault Location and SRL 1-7
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
cable, which contributes to SRL, is the sum of all the tiny reflections
along the length of the cable. In terms of cable impedance, the SRL can
be defined mathematically as:
Equation 1
Z
in
is the impedance seen at the input of the cable, and Z
cable
is the
nominal cable impedance.
Cable impedance is a specification that is defined only at a discrete point
along the cable, and at a discrete frequency. However, when commonly
referred to, the impedance of the cable is some average of the impedance
over the frequency of interest. Structural return loss, on the other hand,
is the cumulative result of reflections along a cable as seen from the
input of the cable. The above definitions need to be expressed in a more
rigorous form in order to apply a measurement methodology.
Defining Cable Impedance
Following are three common methods of defining cable impedance.
Although all three methods may be commonly used in your industry,
your network analyzer uses the third method (Z-average normalization)
to define cable impedance.
Z
SRL
ω()
Z
in
ω() Z
cable
–
Z
in
ω() Z
cable
+
--------------------------------------=
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