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This paper describes an analytic design procedure for microstrip tunable filters. Step-impedance resonators are employed and loaded with varactors for achieving agility in the filter response. Fixed lumped capacitors are utilized as admittance inverters in order to minimize the number of varactors in the filter. An analytic approach for filter design makes it possible to achieve a tunable filter showing the same frequency response when the center frequency is adjusted. A two-pole microstrip tunable filter whose center frequency can be adjusted from 1.1 to 1.5 GHz is designed to demonstrate the validity of design theory. A prototype tunable filter operating from 2.1 to 2.7 GHz is also designed and measured. A good agreement between the measured and simulated results is shown. Finally, three- and four-pole tunable filters are designed to show straightforward application of the presented design method to higher order tunable filter design. Index Terms--Microwave filters, tunable filters, varactor.

This paper introduces a synthesis method for triplepassband microwave filters. A frequency transformation is developed for finding the locations of poles and zeros of the triple-passband filter. The poles and zeros obtained as such are optimized to achieve a transfer function with a reduced number of transmission zeros in order to reduce the number of cross-couplings. Six and 12-pole triple-passband filters are synthesized for validation of this proposed method. A 12-pole triple-passband filter is fabricated with a microstripline structure and shown to provide good agreement between synthesis and measurement results. Finally, the frequency transformation for asymmetric triple-passband filters is briefly discussed. Index Terms--Circuit synthesis, microwave filters, triple-passband filters.

This paper describes a synthesis method for symmetric dual-passband microwave filters. The proposed method employs frequency transformation techniques for finding the locations of poles and zeros of a desired filter. This method can be used to design dual-passband filters with prescribed passbands and attenuation at stopbands directly without the need for any optimization processes. To validate the procedure a dual-passband stripline filter is designed and fabricated. The stripline dual-passband filter is designed with passbands at 3.90-3.95 and 4.05-4.10 GHz, and 30-dB attenuation at the stopband. This measured results show a good agreement with the theoretical ones. The frequency transformation for symmetric dual-passband filters is also extended to include asymmetric dual-passband responses. This flexible frequency transformation preserves the attenuation characteristics of the low-pass filter prototype. Examples are shown to discuss the flexibility of this transformation. Index Terms--Circuit synthesis, dual-passband filters, microwave filters.