LAUSR

This paper reports on, for the first time, the design of filtering couplers with reconfigurable frequency. The reconfigurable filtering transmission lines (RFTLs) with 90° or −90° phase shift are first designed, and then the reconfigurable filtering couplers are constructed by substituting the basic unit of quarter-wavelength ( $\lambda/4$ ) transmission line in the conventional couplers with the proposed RFTL. The tunable capacitor loaded half-wavelength ( $\lambda/2$ ) resonators are used to tune the center frequency of RFTL. By properly controlling the electric or magnetic coupling between the resonators, the phase shift of 90° or −90° of the RFTLs can be realized, respectively, while maintaining a near constant bandwidth. The external coupling along with a pair of transmission zeros is achieved by asymmetric tap feeding of the RFTLs. For verification, a class of frequency-reconfigurable filtering couplers is designed and fabricated. First, a reconfigurable filtering rat-race coupler is designed and fabricated. The measured center frequency is tuned from 700 to 1840 MHz while maintaining a 3-dB fractional bandwidth of 13% ± 0.5%/13.7% ± 0.6%, insertion loss of 8.1–6.9/8.3–5.9 dB, return loss of better than 13 dB, and isolation of better than 21 dB. Second, a reconfigurable filtering quadrature coupler is designed and fabricated. The measured center frequency is tuned from 947 to 1775 MHz while maintaining a 1-dB bandwidth of 75 ± 5/65 ± 10 MHz, insertion loss of 8.3–6.9/8.3–7.1 dB, return loss of better than 14 dB, and isolation of better than 14 dB.