Effects of skewing output driver switching on the electromagnetic emission

In today´s high speed designs we often have to deal with clock rates in the GHz range and rise and fall times in the order of a few pico seconds. At these speeds it can no longer be assumed that a design is immune to parasitic effects. A lot of problems originate from high speed signals as they cause high electromagnetic emissions. These high emissions may lead to serious degradations to the electromagnetic compatibility of an electronic system. In this paper, the reduction of the electromagnetic emissions by skewing CMOS output driver is described. This technique is explained in detail and it is shown how skewing can be used to reduce ground bounce as well as the ringing of an integrated circuit. Using the TEM-cell method (IEC 61967-2), the radiated electromagnetic emissions of an EMC test chip was measured. It is shown that preventing the output drivers from switching simultaneously reduces the radiated electromagnetic emission. As most of the electronic systems designers are interested in the reduction of the electromagnetic emission of their designs it is also shown how this technique can be used to reduce the emission of an electronic system. For this purpose an EMC test printed circuit board was designed and the effects of skewing on its electromagnetic emission are described.