16 Chapter 1
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Method 1. One definition of cable impedance is that impedance which results in
minimum measured values for SRL reflections over the frequency of interest. This
is equivalent to measuring a cable with a return loss bridge that can vary its
reference impedance. The value of reference impedance that results in minimum
reflection, where minimum must now be defined in some sense, is the cable
impedance. Mathematically, this is equivalent to finding a cable impedance such
that:
Equation 2
where ρ is some mean reflection coefficient. Thus, cable impedance and SRL are
somewhat inter-related: the value of SRL depends upon the cable impedance, and
the cable impedance is chosen to give a minimum SRL value.
Method 2. An alternate definition of cable impedance is the average impedance
presented at the input of the cable over a desired span. This can be represented as:
Equation 3
The value found for Z
avg
would be substituted for Z
cable
in Equation 1 to obtain the
structural return loss from the cable impedance measurement.
∂ρωZ
cable
(, )[]
∂ Z
cable
()
-----------------------------------
0=
Z
avg
F
min
F
max
Z
in
F()Fd
F
max
F
min
–()
--------------------------------------------------
=
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