Fluke DSP-2000 - Manual (Page 120)

DSP-100/2000
Users Manual
7-8
Minimizing Impedance Discontinuities
Characteristic impedance is usually altered slightly by cable connections and
terminations. Sharp bands 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. The radius of bends in a cable
should be larger than 1 inch (2.54 cm).
•
Handle LAN cable carefully during installation. Do not step on the cable or
pinch it with tight cable ties.