Measuring crosstalk in differential signals

When current-carrying wires or PCB traces come in close proximity to other wires or traces, the potential for crosstalk exists. Crosstalk can occur when a changing current in a conductor induces a current in an adjacent conductor that, in turn, produces a voltage. Crosstalk occurs in differential transmission pairs such as those used to carry serial data streams.

The line that provides a coupling signal is called the aggressor; the line where you measure the crosstalk is the victim. You measure the crosstalk at the ends of the victim wires or traces. The induced noise at the near end of the victim (Figure 1a) is called near-end crosstalk (NEXT), and the noise induced at the far end (Figure 1b) is called far-end crosstalk (FEXT). In digital circuits, NEXT and FEXT can cause receivers to incorrectly interpret bit polarities.

Figure 1. To measure differential crosstalk, drive the aggressor with a differential signal and measure the victim with a differential-input instrument.

To obtain the crosstalk as a portion of the aggressor voltage, take the difference of the induced voltages at the near (NEXT) or far (FEXT) end of the victim, and divide it by the difference between the positive and negative voltages on the aggressor.