EMI Reduction in Multilayer PCBs Using Planar Interdigital Slot Structures on the Reference Planes

In mixed-signal circuits, the return currents of sensitive analog and noisy digital signals must be carefully isolated in order to protect the integrity of the analog signals. To address this issue, a common approach is to utilize slots on the reference plane between digital and analog circuits. However, the slots so created cause the discontinuity in the return current path for a signal passing over these slots which causes unintentional radiation and results in unwanted electromagnetic interference (EMI). In this regard, we propose the planar interdigital slot structures for use in reference planes of multilayer PCBs in order to provide a low impedance path for return current. The interdigital structure alleviates the return current path discontinuity and hence reduces the unintentional radiation. In addition to this, a relationship between radiated emission and transmission response is obtained, allowing the designer to tune the slot parameters to minimize radiation at the signal frequency. PCBs with different size planar interdigital slot structures are simulated in Ansys HFSS and is found that the longer digit length corresponds to better EMI reduction, especially at higher frequencies.