1-8 Option 100 Fault Location and SRL
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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