Minimizing Impedance Discontinuities - Fluke DSP-4100 User Manual

Minimizing Impedance Discontinuities

Characteristic impedance is usually altered slightly by cable connections and
terminations. Sharp bends or kinks in LAN cable can also alter the cable's
characteristic impedance. Networks can operate with small discontinuities because
the resulting signal reflections are small and are attenuated in the cable. Larger
impedance discontinuities can interfere with data transmission. Such
discontinuities are caused by poor electrical contacts, improper cable terminations,
mismatched cable or connector types, and by disturbances in the twisting pattern
of twisted pair cable.
You can avoid problems with impedance discontinuities by observing the
following precautions during installation:
•
Never mix cables with different characteristic impedances (unless you use
special impedance-matching circuitry).
•
Always terminate coaxial cables with a resistance equal to the cable's
characteristic impedance. The terminating resistance prevents signal
reflections by absorbing the signal's energy.
•
When untwisting cable pairs to install connectors or make connections at
punch-down blocks, make the untwisted sections as short as possible.
•
Do not make sharp bends or kinks in the cable. Check the cable manufacturer's
specifications for the minimum bend radius.
•
Handle LAN cable carefully during installation. Do not step on the cable or
pinch it with tight cable ties.
Crosstalk
Crosstalk is undesirable signal transmission from one cable pair to another nearby
pair. Like electrical noise from outside sources, crosstalk can cause
communication problems in networks. Of all the characteristics of LAN cable
operation, crosstalk has the greatest effect on network performance.
The test tool makes two crosstalk measurements: NEXT and ELFEXT.
Basic Cabling Testing
Minimizing Impedance Discontinuities
7
7-11