Chapter 1 15
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
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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