Packaged Tunable Combline Bandpass Filters

This article describes the design of a packaged tunable bandpass filter used for the Student Filter Design Competition held during the 2014 International Microwave Symposium (IMS2014) in Tampa, Florida. The focus of the 2014 competition was to demonstrate a tunable bandpass filter between two wide separated bands. The first band is 4.25-4.4 GHz, and the second band is 6.25-6.4 GHz. The tunable filter must be realized on a printed circuit board with planar and lumped surface-mounted components and required to be assembled in an open-tooled FP118118-1 package [1] securely sealed with a copper tape lid. The entire package was mounted on an evaluation board, with 50-X traces connecting the radio-frequency (RF) input and output pins to 50-X female subminiature version A (SMA) connectors. The evaluation board also had a trace that connects the dc control pin of the package to a pad on the perimeter of the evaluation board. The filter performance was measured at the SMA interfaces with a dc voltage level from 0 to 20 V to shift the passbands of the tunable bandpass filter using a varactor [Microwave Associates (M/A) Com 46H202].