Digital circuits are prolific generators of electromagnetic interference. High-frequency square-waves, rich in harmonics, are distributed throughout the system. The harmonic frequency components reach into the part of the spectrum where cable resonance effects are important. Analogue circuits are in general much quieter because high frequency squarewaves are not normally a feature. Nevertheless any analogue design which includes a high frequency oscillator or drives wideband signals must follow HF design principles, especially in regard to ground layout.

Because the microprocessor is a state machine, processor-based circuits are prone to transient corruption. Care is necessary to prevent any clocked circuit from being susceptible to incoming interference. Analogue circuits tend to be more affected by continuous RF disturbances, and their immunity is improved by minimizing amplifier bandwidth, maximizing the signal level, using balanced configurations and electrically isolating or filtering I/O that will be connected to “dirty” external circuits.

More detailed circuit design practices follow:

Power switching circuit design including the switching emission coupling paths , variable speed motor drives and AC supply harmonics

Decoupling

Clock distribution

Transmission line practice

Digital immunity

Analogue immunity

Switches and motors

Power switching circuits