Fluke Networks CertiFiber Pro Users Manual - Manual (Page 134)

Versiv Cabling Certification Product Family
Technical Reference Handbook
112
TCL (Transverse Conversion Loss)
Transverse conversion loss is the ratio (in dB) of a common-mode
voltage measured on a wire pair relative to a differential-mode
voltage applied to the same end of the pair. The TCL value shows
you how well the impedances of the pair’s conductors are balanced.
A high TCL value means that the impedances of the conductors
relative to ground are almost equal. High TCL values correspond to
better noise immunity and lower emissions.
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 57, 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 57, bottom).
Conversely, an unbalanced pair radiates unwanted electromagnetic
radiation. This occurs because the opposing, differential signals are
not equal, so the they are partially converted to common mode.
Common-mode power on a wire pair produces more radiation than