A Comparison of Vertically Integrated Combline Filter Approaches in PCB

Vertical stacking of planar line resonators in multilayer media produces filters with a smaller footprint than horizontal coplanar positioning of the same resonators, especially for high order filters. Strong broadside line coupling, however, makes it difficult to synthesize filters of moderate or narrow bandwidth. In this paper, a new method of positioning vertically stacked planar combline resonators is proposed, whereby the resonators are rotated around the common short-circuit termination rather than horizontally displaced to control coupling. The coupling is found to be mixed-mode, but predominantly inductive. This method is shown to produce filters of moderate bandwidth. Four variations of a 5
^th
order filter are demonstrated, achieving 1.92 - 3.3 dB IL over 10% FBW around 10 GHz while occupying between 6.62 - 22.47 mm
²
surface area on a 7 layer PCB.