EFFECT OF LOGIC FAMILY ON RADIATED EMISSIONS FROM DIGITAL CIRCUITS

Radiated emissions were measured for simple digital circuits designed to operate with various logic families. Emissions in the near and far field were found to depend both on the circuit layout and the choice o f logic family. However, the difference in peak emissions between any two logic families was found to be independent of circuit layout. The greatest difference in peak emissions was between high-speed 74ACT log ic and low-speed 4000 CMOS logic devices, with a mean value of approxi mately 20 dB. Emissions from a more complex circuit were compared with the measurements on simple loop circuits. Test circuits were used to measure the propagation delay, the rise and fall times, the maximum op erating frequency and the transient switching currents between two suc cessive logic gates for each logic family. Empirical formulas have bee n derived that relate relative peak emissions to these switching param eters. It is hoped that these will assist designers to assess the effe ct of choice of logic family on electromagnetic compatibility.