Screening design for EMC in digital systems

Electromagnetic compatibility (EMC) covers the need for adequately low emissions of electromagnetic energy from electronics equipment and corresponding good immunity to any electromagnetic disturbance. Applied to high integrity systems the electromagnetic immunity must not only be good, it must be known to be good. The author explains how this requires both testing to demonstrate immunity and a process of design-usually `worst case design´-to ensure that the equipment will pass the test. Any high integrity instrument, in fact any instrument connected to trip or alarm circuits, will need to have immunity to transient disturbance of duration comparable with its response time. For a digital system a single spurious digit might be unacceptable so it is good practice to ensure that, at least for high integrity systems, no external transient disturbance could upset a single gate