TABLE OF CONTENTS Circuit design is an essential contributor to EMC. In this area we can distinguish the techniques used to control radio frequency emissions from an operating circuit, and those used to control radio frequency and transient immunity of an operating circuit. There are some common points between these two, but there are also some differences which mean that to completely address the compatibility of a product you need to deal with both. Low frequency (mains supply) EMC is also a function of circuit design, principally of the power supply, but does not suffer from the mystery surrounding RF effects and is not considered in this chapter.
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 and enclosure resonance effects are important. Analogue circuits are in general much quieter because high frequency square waves are not normally a feature. A major exception is wide bandwidth video circuits, which transmit broadband signals up to several tens of MHz, or over a hundred MHz for high resolution video. Any analogue design which includes a high frequency oscillator or other high di/dt circuits must follow HF design principles, especially with regard to ground layout.
Some low frequency amplifier circuits can oscillate in the MHz range, especially when driving a capacitive load, and this can cause unexpected emissions. The switching power supply is a serious cause of...
