1-16 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
display versus frequency. The mechanical interface typically provides an
increasing slope with frequency and flattens out as the connection is
made better.
The termination connector may also affect the SRL measurement if the
cable termination connector and load provide a significant amount of
reflection and the cable is short enough. As longer lengths of cable are
measured, the cable attenuation provides isolation from the termination
on the far end. Use a fault location measurement technique to observe
the reflection from the termination at the far end of the cable. If the
termination is shown as a fault, the reflection from the terminating
connector is contributing to the reflection from the cable. A more suitable
termination is required or a longer section of cable must be measured.
The cable must provide sufficient attenuation to remove the effects of the
connector and load for a good SRL measurement. Performing a good
measurement on a short length of cable is quite difficult and requires
connectors with very low reflections to be effective.
Fixed Bridge with Connector Compensation
The analyzer employs the fixed-bridge method and instrument software
to emulate the traditional variable-bridge method. Vector error
correction is used to provide the most accurate measurements up to the
calibration plane defined by the calibration standards. Additional
corrections can also be used to minimize the effects of the test-lead
connector on the measured SRL response.
The error corrections done for a fixed bridge can also include connector
compensation. The fixed bridge method with connector compensation
technique mathematically removes the effects of the test-lead connector
by compensating the predicted connector response given by a connector
model.
Shunt C Connector Model
One model that can be used for the cable connector is the shunt C
connector model. With this model, the adjustment of the C value given in
a variable impedance bridge can be emulated. The shunt C connector
model assumes the discontinuity at the interface is abrupt and much
smaller than a half wavelength of the highest frequency of measurement.
With this assumption, the discontinuity can be modeled as a
single-shunt twisted pair, where C = C
0
+ second and third order terms.
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