Fluke DSP-2000 - Manual (Page 119)

Basic Cable Testing
Characteristic Impedance
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LAN cables act as antennas that can pick up noise from fluorescent lights, electric
motors, electric heaters, photocopiers, refrigerators, elevators, and other electronic
devices. Coaxial cable is far less susceptible to noise than twisted pair cable
because it is shielded by a conductive sheath. The sheath is electrically grounded
to prevent noise from reaching the inner conductor.
The test tool measures impulse noise on the cable under test. Impulse noise is
abrupt “spikes” of interference. This noise is caused by electronic devices that run
intermittently, such as elevators, photocopiers, and microwave ovens. You can
monitor impulse noise in the test tool’s MONITOR mode. The test tool counts
impulse noise spikes that have an amplitude greater than the selected impulse
noise threshold.
Characteristic Impedance
Characteristic impedance is the impedance that a cable would have if the cable
were infinitely long. Impedance is a type of resistance that opposes the flow of
alternating current (ac). A cable’s characteristic impedance is a complex property
resulting from the combined effects of the cable’s inductive, capacitive, and
resistive values. These values are determined by physical parameters such as the
size of the conductors, distance between conductors, and the properties of the
cable’s insulation material.
Proper network operation depends on a constant characteristic impedance
throughout the system’s cables and connectors. Abrupt changes in characteristic
impedance, called impedance discontinuities or impedance anomalies, cause
signals reflections, which can distort signals transmitted through LAN cables and
cause network faults.