EMC Question of the Week: July 29, 2024

a coaxial cable connected to a circuit board

Two modules communicate with each other at 100 Mbps over a 2-meter coaxial cable. Common-mode current on the cable causes the system to fail radiated emissions at 300 MHz. One possible solution is to   

  1. filter the signal
  2. use a common-mode choke
  3. float the cable shield at one end
  4. put a ferrite core around the cable

Answer

The best answer is “d.” While a ferrite core on a cable is never a particularly satisfying fix, the other options listed are not practical.

Filtering is not an option because the failure is occurring at the third harmonic of the data rate. This frequency can't be reduced significantly without affecting the integrity of the digital signal.

Common-mode chokes are not an option, because a common-mode choke would cause half the signal voltage to drive the cable shield relative to the circuit ground. Common-mode chokes should never be used to filter unbalanced signals.

Floating the cable shield to stop the flow of high-frequency current prevents the signal current from returning on the shield. Floating the cable shield at one end through a capacitor to eliminate low-frequency current doesn't help. Coaxial cable shields should always be connected to the EMC ground at both ends.

A ferrite core made with the appropriate material could introduce a significant resistance to the shield current at 300 MHz without affecting the signal current at all. It is the best option of those presented.

Nevertheless, it is important to note that something in the circuit is driving the cable shield relative to the module chassis or circuit EMC ground. This generally can't happen when the cable shield is well-connected to the EMC ground and the circuit is reasonably well laid-out. It would generally be better to identify the design mistake that is allowing current to be driven onto the cable shield. Fixing the problem at the source is almost certainly less expensive and more reliable than putting a ferrite core on the shield.   

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