EMI reducing interdigital slot on reference planes of the PCBs

Abstract

In some very low noise mixed-signal circuits, designers may use intentional slots in order to create a better isolation between a noisy digital ground and a very quiet analog ground, especially in low-frequency applications ($< $1 MHz) when the current spread in the return plane may interfere with the quiet analog ground. Traces on the signal planes, crossing over these slots, are well-known sources for electromagnetic interference (EMI) due to discontinuities on the return current path. This article presents a novel interdigital slot structure used in the reference planes. The interdigital structure increases the coupling and, hence, provides a low impedance path for the return current of the signal line. This, in return, significantly reduces the radiated emission level in the printed circuit boards (PCBs). Furthermore, due to the periodic nature of transmission lines and the slow-wave effect provided by the interdigital structure, the periodically repeating resonance frequencies of the interdigital defect can be tuned by simply changing the geometric parameters such as digit length and the number of digits. It is shown that the electromagnetic radiation from these slots exhibits peak levels at these antiresonance frequencies, which are also the transmission zeros of the circuit. Conversely, radiation levels are minimized at passbands, where a good signal transmission is achieved. Therefore, this relationship would allow designers to tune the slot length to minimize radiation at the frequency of the signal passing through the trace. Four PCBs with three different-size interdigital structures and a traditional slot structure are first simulated and then fabricated. Well-agreed simulation and measurement results show that the EMI is suppressed by as much as 30 dB within the frequency range of 250 MHz-1 GHz. The radiation peaks and dips match with the $S_{21}$ minima (transmission zeros) and maxima, respectively, in measurement results as well.