WM380 (675–700 GHz) Bandpass Filters in Milled, Split-Block Construction

Filters play an invaluable role in RF analysis and communications hardware, blocking unwanted signals, limiting bandwidth, attenuating harmonic components, etc. In waveguide construction for mm-wave frequencies, where the powers are generally low, the iris-coupled-resonator bandpass filter has proven to be very practical and amenable to easy machining. At Virginia Diodes, much of our technology has been constructed using standard “split-block” techniques, in which high-precision computer numerical control milling machines are used to mill out the features from solid blocks of metal. Typical machining tolerances are on the order of +/-5-10 μm. Construction of such filters to ~200 GHz has been routine but pushing the operation into the Terahertz regime requires special considerations. Described here is a WM380 (WR1.5) filter for the 500-750 GHz waveguide band having a passband of 675-700 GHz. The filter is proof of concept for a 640 GHz heterodyne polarimeter, designed as a part of a Small Business Innovative Research (SBIR) Grant to develop technology for NASA's planned Aerosol, Cloud, and Ecosystems mission. From the very beginning, filter design and optimizations were carried out assuming the structure would be milled, hence necessitating that all internal features accommodate the finite radii of available machine tools. Construction of this filter required careful attention to machining tolerances to be able to push the milling machines to their absolute limits of +/-2.5 μm or better. Measured results of a batch of filters are shown and compared to simulations to illustrate just how well these filters can be made. Furthermore, the filters can be tuned by simple mechanical means and data are presented to illustrate how easily the filters can be adjusted. Additional modifications to the filter topology to simplify machining, and allow other construction techniques to be utilized are also demonstrated. Ultimately it should be possible to push the split block technology to manufacture iris coupled resonator filters for use at frequencies of well over 1 THz.