Cable Impedance And Structural Return Loss Measurement Theory; Cable Impedance - Agilent Technologies 8712ET User Manual

Introduction and Measurement Theory

Cable Impedance and Structural Return Loss Measurement Theory

Cable Impedance and Structural Return
Loss Measurement Theory
The SRL feature is designed to measure cable impedance and structural
return loss. Cable impedance is the ratio of voltage to current of a signal
traveling in one direction down the cable. Structural return loss is the
ratio of incident signal to reflected signal in a cable, referenced to the
cable's impedance.
The network analyzer uses a synthesized RF signal source to produce an
incident signal as a stimulus. A reflection measurement is made and
then used to compute the cable impedance. The structural return loss
measurement is displayed referenced to the measured cable impedance.
For CATV cable, the cable is measured from 5 MHz to 1000 MHz at
narrow frequency resolutions down to 125 kHz. The analyzer will
automatically scan the cable, then report the worst-case responses.

Cable Impedance

The analyzer automatically computes the cable impedance (Z). However,
if you wish to turn off this "auto Z" function and input your own value of
impedance, you can. See "Connector Model for Short Cables" on
page 4-13.
In coaxial cable, the value of the impedance will depend upon the ratio of
the inner and outer conductor diameters, and the dielectric constant of
the material between the inner and outer conductors. The cable
impedance will also be affected by changes in conductivity. These
changes are a natural consequence of RF currents that flow near the
surface of a conductor. This effect is known as the "skin effect." Also, the
construction of the cable can change along the length of the cable, with
differences in conductor thickness, dielectric material and outer
conductor diameter changing due to limitations in manufacturing. Thus
the cable impedance may vary along the length of the cable.
The extent to which manufacturing imperfections degrade cable
performance is characterized by a specification called structural return
loss (SRL). SRL is the ratio of incident signal to reflected signal in a
cable. This definition implies a known incident and reflected signal. In
practice, the SRL is loosely defined as the reflection coefficient of a cable
referenced to the cable's impedance. The reflection seen at the input of a
1-6 Option 100 Fault Location and SRL