Balance Measurements; Differential And Common-Mode Signals; Effects Of Unbalanced Impedance In A Wire Pair - Fluke Versiv Series Technical Reference Handbook

Chapter 3: Certify Twisted Pair Cabling
Frequency-Domain Results

Balance Measurements

Balance measurements show you how well the impedance and
resistance of the wires in a pair match each other. Balanced wire
pairs are used with differential mode signaling to make a
transmission system that is less susceptible to electrical noise and
emits less noise itself.

Differential and Common-Mode Signals

To increase noise immunity and decrease the emissions of twisted
pair cabling, network equipment transmits and receives signals in
differential mode. Differential-mode uses two signals that have
equal amplitudes and opposite polarities. The transmitter applies
one signal to each wire in a pair, and the receiver measures the
voltage between the wires. Because the voltages on the two wires
have equal amplitudes and opposite polarities, the
electromagnetic fields around the wires cancel each other out.
The twisting of the pairs improves the cancellation. Because the
pair emits almost no electromagnetic radiation, it causes almost
no noise in other wire pairs.
Differential-mode signaling also reduces electrical noise on the
pair from outside sources such as electric motors and nearby pairs.
Electromagnetic fields from outside sources cause voltages that
have the same amplitudes and polarities on each wire in the pair.
These are common-mode voltages. Because the difference in the
noise voltage between the wires is zero, the receiver rejects
common-mode signals, and the noise does not show in the
network signals on the pair (Figure 60, top).

Effects of Unbalanced Impedance in a Wire Pair

If the wires in a pair have unequal impedances to ground, the
common-mode voltages induced on the pair by nearby
electromagnetic fields converts to differential-mode noise that
interferes with the data signal. (Figure 60, bottom).
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