In this paper the authors present the application of functional vias to narrowband load matching in the frequency range from 2 up to 20 GHz. The utilization of single and double stub matching is demonstrated. Corresponding network parts basically contain one or two via stubs and transmission line elements, e. g. striplines and microstrips. It is also shown that in most of the given frequency range vias and via stubs can be characterized as transmission lines. Especially the influence of the via configuration on the characteristic impedance and the propagation constant are analyzed in order to investigate their feasibility for matching purposes. Influences are the via length, the number of adjacent ground vias and their distance to the signal via. The analysis is done for open and shorted vias and is based on measurements, fullwave simulations and physics-based via models. Examples include matching networks with inductive loads up to several nH.
IntroductionVias are essential components of multilayered printed circuit boards (PCBs) for both digital and analog applications. They provide intralevel electrical interconnections for signal and power distribution networks. It is well known though that at frequencies when the dimensions of a PCB get into the range of the wavelength, vias may show severe parasitic effects that can have detrimental impacts on e.g. the bandwidth of digital links [1]. A major contributor to these parasitic effects is the so-called via stub -any part of a via that is not an element of the direct signal path -since via stubs show a resonant behavior similar to open transmission lines due to wave effects. As a consequence open via stubs may act as electric shorts at their opposite ends resulting in near-zero signal transmission. However, in another perspective open via stubs may be considered as vertically integrated transmission lines that can be used for typical RF applications such as stub matching [2].Based on measurements, full-wave simulations and physics-based via models [3], a general characterization of vias and the via stub is made in the first part of this work in order to establish the equivalence to transmission lines below 20 GHz as reported in other publications [4]. Arguing from this equivalence the application of single and double via stubs in a matching network will be shown. Open and shorted stub matching and the impact of the via geometry will be demonstrated to match inductive loads up to 3 nH. Figure 1 illustrates the basic geometry of such a matching configuration.