Virtual ground fence: A methodology for GHz power filtering on printed circuit boards

Abstract

Today; with both RF and digital devices using the same voltage source on a PCB, power filtering is needed more than ever to isolate the noise between these two types of devices for proper operation. Especially the power/ground planes in packages or boards can be a major factor for noise coupling. Simultaneously switching drivers cause supply voltage fluctuations which can propagate both horizontally and vertically between the power/ground planes. The sensitive RF/analog signals have to be isolated from this digital switching noise, which gets coupled through the shared power distribution system. Hence, accurate estimation and improvement of the performance of power/ground planes is critical in a mixed-signal system. This paper introduces a new methodology to minimize the transfer impedance of the power distribution system. This will be achieved by a new design methodology, which we will call the virtual ground fence. At its basic level, the virtual ground fence consists of quarter-wave transmission-line stubs that act as short circuits between power and ground planes at their design frequency. An array of such stubs can then be considered as a ground fence. Power filtering is currently achieved mainly by using discrete decoupling capacitors at low frequencies. The virtual ground fence design is the distributed analog of this methodology at GHz frequency regime.