22 Chapter 1
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
SRL and Discrete Cable Faults
In addition to a set of periodic bumps, a cable can also contain one or more discrete
faults. For this discussion, discrete imperfections will be referred to as “faults,” and
periodic imperfections will be referred to as “bumps.”
Reflections from discrete faults within the cable will also increase the level of SRL
measured. The energy reflected from a fault will sum with the energy reflected from
the individual bumps and provide a higher reflection level at the measurement
interface. Examining the cable for faults before the SRL measurement is a
worthwhile procedure. The time required to perform the fault location measurement
is small compared to the time spent in performing an SRL measurement scan.
A fault within the cable will provide the same type of effect as a bad connector. If
the fault is present within the end of the cable nearest to the analyzer, the effect will
be noticed throughout the entire frequency range. As the fault is located further into
the cable, the cable attenuation will reduce the effect at higher frequencies. The
reflected energy travels further through the cable at lower frequencies where the
cable attenuation per unit distance is lower.
Techniques for Removing Connector Effects
Connector Effects on SRL
To remove the unwanted effects of worn connectors, the SRL measurement uses a
built-in connector model. The connector model consists of compensation for
connector length and compensation for connector capacitance (connector C).
The “connector C” compensation emulates the C trim value of a variable impedance
bridge.
The connector length is used to compensate for the effects of an electrically long
connector and extends the calibration reference plane.
NOTE A calibration reference plane is established at the point where the short, open, and
load standards have been measured.
The analyzer can automatically measure the optimum values for your connector
model, or you may enter them manually.
The default values for the connector model are 0.00 mm length, and 0.00 pF
capacitance (no compensation).
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