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To supply high-speed digital circuits with stable power, power/ground noise, such as simultaneous switching noise (SSN) and ground bounce noise caused in multilayer printed circuit boards (PCBs) and packages need to be sufficiently suppressed. The electromagnetic bandgap (EBG) is widely recognized as a good solution for suppressing the propagation of SSN in the gigahertz range. However, a typical coplanar EBG structure etched onto the power and ground planes may degrade the quality of high-speed signals passing over the perforated reference plane. In this paper, a novel method of arranging EBG unit cells on both the power/ground planes in multilayer PCBs and packages is proposed, not only as a means of sufficiently suppressing the propagation of power noise, but also as a means of minimizing the effect of EBG-patterned reference planes on a high-speed signal. On the assumption that noise sources and noise-sensitive devices exist only in specific areas, the proposed method partially positions EBG unit cells only near these critical areas. The SSN suppression performance of the proposed structure is verified by conducting simulations and measurements in the time and frequency domains. Furthermore, signal quality is investigated to verify whether the proposed EBG-patterned reference planes are superior to conventional EBG-patterned planes in terms of signal integrity.