Developing a filter to separate the industrial-scientific-medical (ISM) frequency band at 2.4 to 2.5 GHz from the wireless avionics interconnections (WAIC) band at 4.2 to 4.4 GHz was yet another challenge for contestants in the recent IMS 2016 student design competition. While one part of the competition involved designing a suitable filter layout, another part was the integration of commercial microelectromechanical-systems (MEMS) switch technology to select one of the frequency bands.
A pair of students from the University of Waterloo in Canada, Desireh Shojaei-Asanjan and Raafat R. Mansoor, succeeded in developing several approaches to accomplish the desired results. These included switching between two separate filters and using embedded switches to route signals between different resonant circuits. Although the design with embedded switches represented the smallest approach, a slightly larger circuit design was chosen for the competition for its superior filter characteristics across the bands of interest.
The final design employs a dual-mode bandpass filter design fabricated on 50-mil-thick 3210 printed-circuit-board (PCB) material from Rogers Corp., with dielectric constant of 10.2 in the z-axis (thickness) at 10 GHz. The single PCB holds both a lower-frequency and a higher-frequency bandpass filter. The small sizes of the commercial MEMS switches made it possible to integrate multiple switches with the filter circuitry and, by controlling the correct combination of switches, only one of the filters can be selected.
See “The Sky’s the Limit,” IEEE Microwave Magazine, Vol. 18, No. 1, January/February 2017, p. 100.