Cable Crosstalk

By Christel Amburgey

Cable crosstalk in complex systems must be analyzed and mitigated. EMA3D and MHARNESS software can predict the crosstalk that would occur to determine if mitigation is necessary. This post will work through a simple example to demonstrate how MHARNESS is used to predict crosstalk. This example will also illustrate why twisted pairs are often shielded.

In order to save weight on a vehicle it is common practice to mix different types of cables within the same overbraid. In this example a twisted unshielded pair of power wires operating with 4A current at 5 kHz is placed in the same overbraid as an RF cable of 0.5 A at 400 Hz. The signals operate in differential mode and the lines are balanced. A short circuit is represented by a 2.5 mΩ resistance and an open circuit represented by 1 MΩ.  A load from a box will be represented by 100 Ω. A 1 m overbraid 1 mm above a ground plane is used. For this demonstration, only the two conductors described will be modeled; therefore the crosstalk from the source to the victim can clearly be identified.

Cable Crosstalk, source and victim diagram

 

First we will consider the power cable the “source” and the RF cable the “victim” cable which picks up unwanted signal from the source cable. The plots of the MHARNESS results are shown for the “source” and “victim”. Both short circuit and open circuit boundary conditions are simulated. This result illustrates how having a shield will significantly reduce crosstalk. The “victim” short circuit current is 111 dB below the source.

Cable Crosstalk

 

A plot of the victim open circuit voltage are shown. There is no differential voltage for the victim.

Cable Crosstalk plot of the victim open circuit voltage

 

Adding a 100 Ω load (to represent a real box) to the victim wire does not significantly affect the short circuit current or open circuit voltage for this case. The plots of the MHARNESS results of the short circuit current and open circuit voltage with a load are shown below.

The Cable Crosstalk plots of the MHARNESS results of the short circuit current and open circuit voltage with a load

 

In this example the source is operating in differential mode, however the shield of the victim only picks up a common mode current. The common mode on the victim shield can’t couple to the inner conductors in differential mode. Only the current during turn on and off couples to the victim and there is no differential mode voltage. A similar effect would occur if the source was inside the shield; for example if the RF signal was the “source” and the power signal was the “victim”. This illustrates how a shield on a twisted pair greatly reduces crosstalk.

This is a very simple example shown for demonstration proposes. However, EMA is able to model complex cable systems to predict what crosstalk would occur down to the pin level. An engineer is then able to determine if the mitigation for crosstalk is necessary and simulate any mitigation techniques to evaluate their effectiveness.