Crosstalk

The Company, Inc. Model Monitoring System Control Unit was experiencing lightning-induced transient susceptibility failure modes. The control unit was disassembled and compared with schematics and drawings for good EMC-engineering practice. The layout of the internal wiring, printed circuit board traces, and EMI protection components was shown to have multiple deficiencies:

1. The internal wiring from the circular connector to the PCB header is >10 cm.
2. The input-power traces on the PCB meander several centimeters before reaching the EMI protection components.
3. EMI protection components must be installed properly to be effective:
   a. They should be physically located near the enclosure penetration.
   b. The combined length of internal wiring and PCB trace before reaching the EMI protection components should be <3 cm.
   c. The circular connector would be an ideal location for a 1-nF capacitor and a properly rated MOV.
4. The long internal wiring and PCB traces defeat the EMI protection and induce externally created signals onto the sensitive electronics. This effect is known as crosstalk (see Figure 1 below).

Figure 1 – Current EMC Layout vs. Ideal EMC Layout
The lightning bolt represents the extent of penetration of external signals into the enclosure.