48
Twisted pair cable with a bad splice – Figure 3-33. Too much untwist at the
splice.
Figure 3-33
Wet twisted pair cable – Figure 3-34. Wet sections always show a dramatic dip
in the impedance. Additionally the VF in the wet zone drops to some unknown
value so distances from the start of the wet zone, 95ft (29m), on are not valid
even after the wet section as the cable dries out and impedance returns to
normal. Note the rise in impedance after the right cursor (CRSR1) as an example
of the drying of the pair. Only the distance from the TDR to just before the wet
zone can be trusted (CRSR2).
Figure 3-34
Bridged Taps are an auxiliary connection or drop cable from a main line. – Figure
3-35. Bridged taps are characterized by a downward dip in impedance to almost
half the cable’s Z
0
. This is the results of the TDR receiving reflections from two
100 Ohm pairs running in parallel. At the end of the tap the trace will return to
normal impedance. This permits an accurate measurement of the tap’s length.
See figure 3-36 for a line drawing of a bridged tap.
