IMS2015 MicroApps - The Design of an RF 5-Cavity Helical Filter for Cable TV Testing Applications

This presentation shows how a 5 cavity bandpass waveguide filter operating at 380 MHz is designed, constructed, and measured. The successful design was completed with a single design pass. The filter was designed in Microwave Office, making extensive use of Analyst, its 3D electromagnetic simulator. Each step of the design methodology is explained, from the beginning ideal specification to physical realization. The filter works by starting with a helical, metal coil inside of a rectangular cavity. The cavities are then cascaded together, with a rectangular tuning screw placed in each cavity and at each aperture.

The challenge is to quickly create the final design without excessive electromagnetic simulation and optimization, which is time consuming and difficult to setup. The solution is to use an intermediary step in the design process, where the optimization is carried out using circuit models of the tuning stubs. The actual tuning stub performance is then fit to the model.

Advantage was taken of Microwave Office’s integrated layout, and simulation features, especially parameterized layout as a time saving tool. The finished design was built and tested using parts created with a 3D printer and off the shelf inexpensive RF components. The measured results agree to within a fraction of a percent of the simulated prediction.

Presented by: Dr. John Dunn, EM Technologist, AWR Group, NI
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