Effect of logic family on peak radiated emissions from circuit boards

Radiated emissions from simple digital circuits were found to depend both on the circuit layout and the choice of logic family. However, the difference in peak emissions between two logic families was found to be independent of circuit layout. The greatest difference in peak emissions was between high-speed 74 ACT logic and low-speed 4000 CMOS logic devices, with a mean value of approximately 20 dB. Emissions from a more complex circuit were compared with the measurements on simple loop circuits. For each logic family, test circuits were used to measure the rise and fall times, the maximum operating frequency and the transient switching currents between two successive logic gates. Empirical formulae have been derived which relate relative peak emissions to these switching parameters. These are more accurate than an analytical solution that assumes the switching waveform to be a perfect trapezoid