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The performance of a cavity-based filter is determined by its geometry. Although these filter structures can be tuned, it can be a challenge to obtain the proper geometric dimensions that allow the tuning elements to attain the desired synthesized response. Simulation software is a valuable tool that can assist with the design of such filters. In the application note, “Cavity-Based Helical Resonator Bandpass Filters Designed With Parameterized Project Template in NI AWR Software,” National Instruments describes the design process for a cavity-based, helical resonator bandpass filter at UHF frequencies. The design of this filter type can be aided with the NI AWR Design Environment, which includes the Analyst three-dimensional (3D) electromagnetic (EM) simulator.
The design process began by using ideal elements and traditional filter theory. Optimization techniques were utilized to achieve the required response. The Analyst software platform was used to perform EM simulations, thereby streamlining the task of creating models. An ideal prototype model was constructed in a linear schematic to prove that the derived coupling coefficients were correct.
Starting-point dimensions were determined for the helical resonator and cavity. These dimensions were then fine-tuned with a 3D model that also included a variable-length tuning screw and a coil-support structure. Simulation results are presented for a 380-MHz filter with the tuning screw inserted at both minimum and maximum depths. Another 3D model was created that was comprised of two cavities and a coupling slot, enabling inter-resonator coupling bandwidths to be characterized for various dimensions of the coupling slot. Finally, the entire filter model was created, demonstrating that it could achieve the desired Chebyshev response.
The fabrication files needed to build the filter were generated by using the hierarchal parameterization elements. A 215-, 380-, and 540-MHz bandpass filter were each assembled. The helical resonator coils were wound on 3D-printed plastic that was formed from a solid-conductor household wire. The application note concludes by presenting the measured results of the 380-MHz filter, which correlated with the simulation results.
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