Chapter 3: Certify Twisted Pair Cabling
Frequency-Domain Results
147
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.
TCL is similar to return loss, except that return loss is measured by
applying a differential-mode signal to one pair and measuring
the differential-mode signal reflected back on the same pair.
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 67 shows the TCL plot.
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