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RF MEMS Tunable Filters

As the electromagnetic environment becomes more populated, frequency-agility and narrow operating bandwidths are becoming highly desirable characteristics for bandpass filters in RF front ends. As such, bandpass filters having a high unloaded quality factor (Qu) are a requirement for maintaining low insertion loss and minimum noise figure. To simplify and reduce the cost of today’s communications systems, tunable filters are being used to replace banks of switched filters. Tunable filters can significantly improve communication systems, especially for satellite-based or unmanned air vehicle applications where weight and volume are a primary concern.


Tunable filters based on radio frequency micro-electromechanical systems (RF MEMS) technology provide the ability to create low-loss, high-linearity tuning while simultaneously reducing volume, weight, and parts count. On DARPA’s UltraComm Program, the use of MEMS tunable filters in the 80 MHz to 2.8 GHz frequency range demonstrated the functionality of switched filter banks while reducing the volume by 60x, the parts count by 14x, and the weight by 150x [1].


MEMtronics has developed an innovative concept for implementing MEMS-tuned circuitry with high-Q resonators to construct tunable microwave and millimeter-wave filters. This involves integrating MEMS circuitry within the confines of compact dielectrically loaded metal waveguide cavities and using the MEMS to perturb (tune) their natural resonant frequency. MEMtronics’ capacitive MEMS switch technology possesses some of the absolute lowest loss switches available in the industry with effective series resistances less than 0.25 W, enabling ultra-low-loss tuning of filters. Similarly, metal waveguides represent the absolute lowest loss transmission media available at microwave and millimeter-wave frequencies. Mating these two technologies together yields the highest-Q, lowest loss filters which can be constructed.




Photo of Tunable MEMS Filter Prototype


A prototype two-pole/two bit MEMS-tuned filter (incorporating four tuning stubs and four RF MEMS switches) was shown to operate at X-band with bandwidths of less than 1% with Insertion loss ranging from 4.7 dB to 5.8 dB.  This equates to unloaded quality factors, Qu, ranging from 170-320. Further measurements indicate the possibility for Qu ~ 500 at X-band and higher.  Work is currently ongoing to develop MEMS-based Ka-band tunable filters with 1.5% bandwidth, less than 3 dB loss, in a surface mountable form factor of approximately 20 mm x 5 mm x 1 mm.


X-band Tunable Filter Insertion Loss


If you have microwave or millimeter-wave applications similar to these, please feel free to contact a MEMtronics representative.  We are always interested in talking with potential customers and collaborators on how MEMtronics tunable filter technology may benefit their application. 

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