SIW Filter Doubles as Balun to mmWave Frequencies (.PDF Download)

April 5, 2018
SIW Filter Doubles as Balun to mmWave Frequencies (.PDF Download)

Bandpass filters are among the most useful building-block high-frequency components in modern communications networks, for functions such as providing signal isolation and spectral cleaning. Waveguide bandpass filters provide outstanding electrical performance, but are relatively large in size. However, substrate-integrated-waveguide (SIW) transmission-line technology delivers good performance from compact planar formats such as printed circuit boards (PCBs), and can be readily integrated with other transmission-line formats, e.g. microstrip, as needed for high-frequency component and subsystem design.1

In fact, SIW components can be thought of as traditional rectangular waveguide in planar form. This allows a transition of most classical waveguide components into more compact SIW forms. There are inevitable tradeoffs between the two formats, with conventional waveguide lower in loss but larger in physical size. However, the very good electrical performance of SIW components combines well with the compact, planar formats that can be made to fit many system designs.

SIW technology is the catalyst behind a wide range of passive high-frequency components, including planar SIE phase shifters,2 power dividers,3 circulators,4 directional couplers,5 six-port circuits,6 and low-cost filters.7 A variety of resonator-based filters have been developed with SIW approaches, too, including post, iris, and cavity-type filters.

Overall, the low insertion loss, sharp rejection cutoff, and low cost of these filters has made them invaluable for modern wireless communications systems. To explore the design of SIW bandpass filters, some examples will be shown using circular complementary split-ring resonators (CSRRs) fabricated on readily available commercial PCB material and with design guidance using the High Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from ANSYS.

CSSRs have been proposed for the synthesis of negative-permittivity and left-handed (LH) metamaterials in planar circuit configurations.8 They can serve as useful circuit design elements: CSRRs etched in the ground plane or conductor of a planar transmission-line format, such as microstrip or coplanar waveguide (CPW), provides a negative effective permittivity with resulting stopband behavior in the vicinity of the frequency of the resonant structures. CSSRs have been applied to the design of compact bandpass filters (BPFs) to achieve low bandpass loss with high out-of-band rejection.7