Compact Quasi-Elliptic and Highly Selective AFSIW Filter with Multilayer Cross-Coupling

A quasi-elliptic filter topology based on high performance air-filled substrate integrated waveguide (AFSIW) technological platform is presented. This new topology takes advantage of the AFSIW multilayer property to implement a cross coupling between non-adjacent resonators while maintaining the self-packaging characteristics of AFSIW filters. By etching inductive windows on the top copper layer of the AFSIW bottom substrate, a second signal path is created using an SIW transmission line. Thus, the primary path of the signal is implemented using a lossless medium (air in the milled inner substrate) and the second path is implemented in the bottom substrate. The multipath coupling diagram is constructed, considering a fourth order filter. Then, the relative phase shifts of the primary and secondary path are determined to obtain transmission zeros. For experimental validation, a quasi-elliptic fourth order filter demonstrator operating at 21 GHz with a 350 MHz (1.66%) bandwidth is designed, fabricated and measured. Furthermore, a study shows its high robustness against printed circuit board (PCB) process manufacturing, considering standard tolerances. In and out band measured results are in a good agreement with simulated results. The fabricated filter achieves an insertion loss as low as 0.7 dB with an excellent unloaded Q-factor of 1478.