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Characteristic Impedance
Characteristic impedance, Z(0), must be measured under balanced conditions.
Measurements should be taken with an impedance measuring device and high quality
balun. For each frequency at which a measurement is made, three initial conditions
must be measured. These are:
•
Impedance with secondary balun terminated in a matched load
•
Impedance with secondary balun shorted
•
Impedance with secondary balun open.
Each measured impedance must then be converted to a reflection coefficient using
the following equation:
(
)
( )
Z m
Z b
–
p
=
--------------------------------- -
(
)
( )
Z m
Z b
+
Where:
Z(m)
measured impedance
Z(b)
output impedance of balun.
The three reflection coefficients, p(match), p(short), and p(open) are then substituted
into the following equations to determine the scattering parameters of the test setup:
(
)
S11
p match
=
×
(
2 p match
S22
=
--------------------------------------------------------------------------------------------------- -
(
p short
×
(
(
S12 S21
p match
=
After the scattering parameters are calculated, the cable to be tested must be
connected to the secondary of the balun with the cable's braided shield connected to
instrument earth. The cable is then tested under two conditions:
•
With the far end of the cable shorted
•
With the far end of the cable open.
Each result is then entered into the following equation as p(meas), along with the
scattering parameters for the particular frequency of measurement in order to
determine the actual reflection coefficient of the cable
(
)
p actual
=
-------------------------------------------------------------------------------------------------------- -
×
(
S22
p meas
)
(
)
(
)
p short
p open
–
–
)
(
)
p open
–
)
(
)
)
×
(
p short
1 S22
–
+
(
)
p meas
S11
–
(
)
)
S11
S12 S21
–
+
Chapter 5. Planning for Twisted Pair Cable Connections
)
×
45
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