EMC Question of the Week: December 30, 2024

side view of a display connected to a circuit board through a connecting cable

A 20-cm LCD display assembly is connected to a printed circuit board through a 15-cm cable. The cable carries 8 single-ended, 100-Mbps video data signals plus several low-frequency control signals. A good choice for this cable would be a 

  1. ribbon cable
  2. 2-layer flexible circuit board
  3. ribbon plus 8 miniature coaxial cables
  4. ribbon plus 8 twisted wire pairs

Answer

The best answer is “b.” The high-frequency data signals are single-ended, so their connections need to be unbalanced. This eliminates the ribbon cable and the ribbon plus 8 twisted wire pairs. Connecting through 8 miniature coaxial cables is reasonable from an EMC compliance standpoint, but this would be unnecessarily expensive and unwieldy. A 2-layer flexible circuit board is the best solution. It can be laid out with a solid return plane on one side and all of the signal traces on the other side. The trace widths relative to the plane width can be adjusted to provide the right level of imbalance as well as the right characteristic impedance.

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