Design of Compact Low-Pass Elliptic Filters Using Double-Sided MIC Technology

A novel implementation of stepped-impedance low-pass elliptic filters is presented in this paper. The filters are based on the well-known technique of cascading high- and low-impedance sections to simulate the ladder LC lumped-circuit prototype. We propose in this study a new approach to build up the constitutive circuit elements by taking advantage of the use of both sides of the substrate. The use of double-sided technology yields both design flexibility and good circuit performance. High-impedance sections are achieved by using slots in the backside of the substrate, whereas low-impedance sections are obtained with parallel-plate capacitors. In order to achieve the transmission poles corresponding to the elliptic design, these capacitors are series connected to the ground plane by means of high-impedance coplanar-waveguide lines, which mainly act as inductors. As a final step, meandering techniques have been applied to the high-impedance sections of the filter to reduce the overall circuit size. The measurement of several fabricated filters shows fairly good agreement between theory and experiment

I. Introduction

Low-Pass filters (LPFs) are very important components used to eliminate unwanted harmonics, as well as spurious bands in microwave and millimeter-wave systems. The conventional stepped-impedance low-pass filter (SI-LPF) consists of a cascading of electrically short high- and low-impedance sections to approximate the corresponding ladder LC lumped-circuit prototype. SI-LPFs fabricated using conventional microstrip lines are among the most commonly used LPFs owing to simple design and low fabrication cost [1]. However, this type of filter inherently presents two important practical problems. The first one is the degradation of the out-of-band rejection level because of the frequency-distributed behavior of the finite section lines beyond the cutoff frequency. The second problem comes from the limits imposed by the microstrip technology to achieve very narrow strip widths and, consequently, high-impedance section lines.