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Chapter 3: Certify Twisted Pair Cabling
Frequency-Domain Results
137
If the DC resistance of the wires is not the same, the DC voltage
on each wire is not the same. This causes a voltage across the
wires, which causes an offset, or bias, current in the transformer
at the far end. The offset raises or lowers the mid-point of the
transformer’s operation. If the offset is large enough, the flux
level in the transformer’s core saturates during the peaks of the
data signals. During saturation, the flux level in the core cannot
increase further in response to the change in current caused by
the data signal. Since voltage on a transformer’s output is present
only when the flux field is changing, the output voltage droops.
This causes distortion of the data signal, as shown at the bottom
of Figure 61.
Resistance unbalance within pairs can be caused by poor-quality
terminations, damaged connectors, corroded contacts, and
damaged or poorly-designed cable.
Figure 62 shows test results for resistance unbalance within pairs.