Compact fixed and tune-all bandpass filters based on coupled slow-wave resonators

A compact topology for bandpass filters based on coupled slow-wave resonators is demonstrated. A study of fixed bandpass filters leads to design rules and equations. Measurements on a 0.7-GHz fixed bandpass filter, consisting of three coupled slow-wave resonators, demonstrate the validity of the proposed topology and validate the theory, since the agreement between simulations and measurements is very good. Designed for a Q-factor of 5, this filter shows a Q of approximately 5.2. At the center frequency, insertion loss is 0.6 dB and return loss is greater than 20 dB. A 0.7-GHz tune-all bandpass filter is also designed and tested. The performance of this electronically tuned filter, which incorporates semiconductor varactors, is promising in terms of wide continuous center-frequency and bandwidth tunings. For a center-frequency tuning of /spl plusmn/18% around 0.7 GHz, the -3-dB bandwidth can be simultaneously tuned between /spl sim/50 and /spl sim/78 MHz, with an insertion loss smaller than 5 dB and a return loss greater than 13 dB at the center frequency. The surface areas of the fixed and tunable 0.7-GHz filters are, respectively, /spl sim/16 and /spl sim/20 cm/sup 2/.