How do I measure the crosstalk in differential lines?
The crosstalk is the induced voltage on one conductor due to a changing current in another. The line that provides a coupling signal is called the “aggressor,” and the line where you measure the crosstalk is the “victim.” The crosstalk is usually measured at the victim’s ends. The induced noise at the near end of the victim is called a Near End crosstalk (NEXT), and the noise induced at the far end is called a Far End crosstalk (FEXT).
Since we measure the differential crosstalk in the victim, we need to measure a differential response produced due to the vicinity of the aggressor. Therefore, to measure the NEXT we need to stimulate the aggressor differentially, measure the responses at both the near ends of the victim, and then subtract the obtained victim's waveforms. Similarly, to measure the FEXT we need to measure the responses at the far ends of the victim and subtract the acquired victim's waveforms. Please, keep in mind that when you measure the NEXT, the far ends of the victim must be differentially matched, otherwise, the FEXT will be reflected back to the near end.
To obtain the crosstalk as a percentage of the offender 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 offender. Finally, to get the frequency dependent crosstalk in IConnect, use the difference of the induced voltages on the victim as the DUT waveform and the difference of the aggressor voltages as the Step waveform in IConnect S-parameter computation.
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