Cavity-Based Helical Resonator Bandpass Filters Designed in NI AWR Software

This application note describes the design process for a low-cost family of ultra high-frequency (UHF) cavity-based helical resonator bandpass filters for a cable television (CATV) component test switched filter bank. Cavity-based filter performance is determined entirely by geometry. Even though the filter structures are tuned, it can be challenging to size all the component geometries such that the tuning elements are effective to meet the desired synthesized response, hence a tool such as NI AWR Design Environment™ - inclusive of Analyst 3D EM simulator - greatly assists.

3D EM geometry creation and simulation for cavity-based fi lters, including tuning elements, is generally very time intensive. With one integrated project, new designs could be rapidly explored through changes in a small number of key global parameters. By leveraging parameterized 3D EM building blocks for cavities and resonators in the fi lter, as well as port tuning techniques, the synthesis and design stages were simplifi ed and the design was then highly re-usable. A key element of the low cost, fast fi lter implementation was the unique construction of the fi lter cavities using double clad printed circuit board (PCB) material. The cavities and enclosure were cut out using an
LPKF PCB milling machine driven from a parameterized set of fabrication fi les and assembled like a planar die cut model airplane.

NI AWR Design Environment software, specifi cally Microwave Offi ce circuit design software, was used to create a complete “synthesis to implementation” process for the entire fi lter design including the generation of fabrication fi les. A hierarchical modeling approach was used for the cavities that included optimization of the circuit model in Microwave Offi ce and verifi cation using Analyst EM simulation. Once the template project was created, the entire process of designing for a new fi lter frequency could be undertaken in less than a day due to the tightly integrated project environment. First pass success was achieved on three different frequency fi lter designs, as well as a large reduction in time for 3D EM structure creation of the fi lter family due to the cascadable building block nature of Analyst PCells.

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