Fluke Networks CertiFiber Pro Users Manual - Manual (Page 135)

Chapter 3: How to Certify Twisted Pair Cabling
Twisted Pair Autotest Results
113
the same amount of power in differential mode because the
common-mode radiation from the wires does not cancel when the
wires are close together.
TCL and the related measurement of ELTCTL are important for
evaluating the performance of links installed in industrial premises
because of the high level of EMI caused by industrial equipment, and
the problems that EMI causes in networking equipment. Figure 58
shows the TCL plot. The tester puts the cursor at the worst margin.
GPU67.EPS
Figure 57. The Effects of EMI on Balanced and
Unbalanced Twisted Pairs
+1 V
-1 V
+1 V
-1 V
+0.5 V
noise spike
+0.5 V
noise spike
+1.5 V
-0.5 V
When the wires have equal impedance,
EMI adds the same amplitude of noise to
each wire, so no differential voltage is
induced.
When the wires have different impedances,
EMI adds different amplitudes of noise to
each wire. This induces differential noise
that interferes with the data signal.
Unbalanced pair
Balanced pair
+1.0 V
noise spike
+0.5 V
noise spike
+2.0 V
-0.5 V
Differential-mode
signal remains at 2 V
0.5 V spike of
differential noise
added to the signal
+2.0 V
+2.5 V