EMC Question of the Week: October 2, 2023

Two coaxial cable connections to a circuit board

A GNSS signal (1.6 GHz) is conveyed from an antenna to a circuit board on a coaxial cable. On the circuit board, the center conductor of the cable is connected to the receiver via a 5-cm trace. The coaxial cable shield should be connected  

  1. to a 5-cm trace grounded at the receiver
  2. to a 5-cm trace grounded at both ends
  3. to the circuit board ground plane(s)
  4. through capacitors to circuit ground


The best answer is “c.” The coaxial cable is unbalanced, so it is very important to keep the transmission line on the circuit board unbalanced. This requires a direct connection of the cable shield to the circuit board ground planes. Any attempt to route the shield's signal return current on a trace will temporarily balance the transmission line resulting in mode conversion that steals power from the signal and drives common-mode current onto the coaxial cable shield. 

Any impedance in the shield connection (for example, capacitors inserted in the current path) also produces a voltage that drives common-mode current onto the cable shield. For signals with significant frequency content at 100 MHz and higher, coaxial cable shields ALWAYS connect directly to the circuit board ground planes. 

Note: For noise immunity, it is often important to make a good high-frequency connection between the coaxial cable shield and the chassis ground as well.

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