Chapter 1 25
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Equation 11
Equation 12
Equation 13
In Equation 11, Z
in
(ω) is calculated from the measured return loss as described in
Equation 4, previously. The primed values are the new calculation values using the
capacitive compensation. With these equations, the network analyzer can compute
values for the cable impedance and mathematically compensate for the connector
mismatch with a given value of C connector compensation.
Connector Length
The shunt C connector model can be improved with the addition of connector
length. Connector length is used to compensate for the phase shift caused by the
electrical length within the connector. The calibration plane can be moved from one
side of the cable connector to the other side, so that the shunt C is placed exactly at
the discontinuity of the connector and the cable under test.
Measurement Uncertainties
In any comparison of cable impedance or structural return loss data, it is important
to understand the measurement uncertainty involved in each type of measurement.
This is critical for manufacturers, who often use the most sophisticated techniques to
reduce manufacturing guard bands. It is also important in field measurements that
users choose the proper equipment for their needs, and understand the differences
that can occur between manufacturers’ data and field data. Also, note that
measurement uncertainty is usually quoted as the worst-case result if the sources of
error are at some maximum value. This is not the same as error in the measurement,
Z′
in
ω()
Z
in
ω()
1
jwC
----------
⋅
Z
in
ω()
1
jwC
----------
+
---------------------------------
=
Z′
cable
Z′
in
ω()
N
----------------------------
=
ρ′
SRL
ω()
Z′
in
ω() Z′
cable
–
Z′
in
ω() Z′
cable
+
-----------------------------------------
=
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