Phil Jobson Consulting Designs Family of UHF Cavity-Based Helical Resonator Bandpass Filters with NI AWR Software
Based in Santa Rosa, California, Phil Jobson Consulting provides RF/microwave system and component design solutions, including linear, nonlinear, EM, system modeling and simulation, system/component characterization and measurement, and prototyping.
The Design Challenge
Phil Jobson Consulting was asked to design a family of low-cost, ultra high-frequency cavity-based helical resonator bandpass filters for the CATV test market that could be combined into a switched filter bank. Cavity-based filter performance is determined entirely by geometry. Even though the devices are tuned, it can be challenging to size all the components such that the tuning elements are effective to meet desired synthesized response. 3D EM geometry creation and simulation for cavity-based filters is generally very time intensive. The designer wanted to create a complete “synthesis to implementation” process for helical cavity-based bandpass filters in a single, integrated project that could be quickly applied to new designs by changing a small number of key global parameters. He also wanted to be able to leverage reusable parameterized 3D EM building blocks for cavities and resonators in the filter synthesis and design stages to simplify design capture and re-use. Finally, he wanted to leverage layout parameterization to automatically create fabrication files for cavities for each new design based on a few key global parameters.
The designer used NI AWR Design Environment™ Microwave Office circuit design software and Analyst™ 3D electromagnetic (EM) simulator to capture and automate the entire filter design process within a single Microwave Office project. The tightly integrated environment enabled a single, simple flow from synthesis to design to verification and then to implementation.
The design and manufacture of the filter presented a number of challenges, as is typical of this type of filter design. First, the filter was designed using ideal elements and traditional filter theory. Optimizations were carried out to get the required response. The first challenge was to get the ideal filter response into an actual, physical cascaded, cavity filter topology. Analyst was used for the EM simulation because of its ability to create 3D parts as PCells that can be repeatedly used in the final layout. This made model creation easier, as the steps from individual cavity design to the final five cascaded sections essentially involved connected similar shapes. The shapes were created using parameters so that the actual geometries could be quickly changed without having to redraw the structures in a 3D layout editor. The final filter was created using only three basic 3D components: the cavity with the coil, the end plates with the coaxial feeds, and the interior separation walls with the coupling slots.
The actual manufacture of the filter was carried out quickly and inexpensively by first creating the basic parts without the coils, and then winding the coils to the predetermined specifications. As noted above, the actual layout was done in Microwave Office and exported. The resulting board was then cut, and the filter assembled. The coil was wound on a plastic cable former that was created using a 3D printer. Final assembly was then carried out by positioning the coils and tuning screws. A wooden dowel was used for mechanically supporting the coils.
The designer achieved first pass success on three filter designs, despite never having designed cavity-based filters before. His success also yielded at least a 5x reduction in time for 3D EM structure creation of the filter family due to the cascadable building block nature of Analyst 3D finite element method (FEM) EM PCells. He noted that the innovative integration of Analyst is an exceptional feature within NI AWR Design Environment that differentiates it from the competition. Agreement between simulated and measured results was considered excellent. First pass design, construction, and verification of the filters was achieved.
Why NI AWR Design Environment
Phil Jobson Consulting chose NI AWR Design Environment because he was an early adopter and expected that the integrated environment would fulfill his needs. He has always enjoyed the ease of use, customer care and constant innovation of AWR (now National Instruments).
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