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New for May 2021. Waveguide filters are deep into the black arts of microwave engineering. We have no particular expertise in this topic, perhaps some readers will contribute some content to help our cavity filer content along...
Early research in cavity filters was performed at the MIT Radiation Laboratory during WWII. References at the bottom of this page are resources if you want to learn about the history of microwave filters in general and the early work in waveguide filters specifically.
Cavity filters almost always require mechanical tuning, which makes them expensive. To get an idea about how to tune a filter by hand, we posted the video below. Here, an old Farinon X-band filter is retuned for a specific application (basically, shifting teh passband down about 3%). The filter is made of four cavities in WR90 waveguide, with irises and tuning screws that form tunable resonators. Tuning the four screws took about four minutes, and the tuning merely peaked up the band in one spot. Imagine what it would take to fit a specific passband? Tuning is a low-wage, boring job, maybe some filter manufacturers are using artificial intelligence (AI) to speed the process and reduce the payroll. Hold that thought...
In the video the presenter shows how to game an old Frequency Sources Inc. X-band oscillator to provide a specific frequency, which is interesting but some people might consider it abusing an antique, like when your Uncle made a lowrider out of Grandma's 1963 Impala. Then he connects the source to the filter and into a power meter, and minimizes loss at at that frequency. Then he shows the passband using an old sweep oscillator, with a "works in a drawer" X-band plug-in that dates back to the 1970s or 1980s, before Hewlett Packard figured out how to put a wideband source into a single, 10 MHz to 26.5 GHz 75 lb (34 kg) HP8340B, affectionately as the "boat anchor".
Tuning a 10GHz Waveguide Cavity Filter by bradthx
You can buy HP8350 sweep oscillator plug-ins here. They last forever.
Now let's look at a video of an AI setup tuning a filter, thanks to Space Forest.. This set-up include multiple screws being turned at the same time. Say, can you speed that thing up? Time is money!
Filter Tuning Software & Multiple Head Tuning Robot, by Space Forest
References
1. R. Levy and S. B. Cohn, "A History of Microwave Filter Research, Design, and Development," in IEEE Transactions on Microwave Theory and Techniques, vol. 32, no. 9, pp. 1055-1067, September 1984, doi: 10.1109/TMTT.1984.1132817.
2. Microwave Transmission Circuits, M.I.T. Rad. Lab. Series, vol. 9, G. L. Ragan, Ed. New York: McGraw Hill, 1948. See chapters 9 and
10, by R. M. Fano and A. W. Lawson..