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Figure 81: Poor SNR
Power Influence
•
The measurement of the electromagnetic field from AC power induced into a cable pair
•
PI < 90 dBrnc = OK
Longitudinal Balance
For efficient transmission, the
impedance
(Z) to earth of each conductor must be as near to equal as possible.
This is called the longitudinal balance of the line. Any imbalance will cause noise as the “cancelling effect” of
a twisted pair is reduced.
PI – N = LB > 60 dB = OK
Typical causes of an Imbalanced Cable Pair
•
One side of the pair is longer than the other
•
One side of the pair is crossed with another wire, or ground