Synthesis and Design of Wideband Dual-Band Bandpass Filters With Controllable In-Band Ripple Factor and Dual-Band Isolation

In this paper, an exact synthesis and design procedure has been proposed for a class of wideband dual-band bandpass filters (BPFs) with both controllable in-band ripple factor and isolation between the dual passband. Two multi-mode resonators (MMRs) with short-circuited stubs are parallel connected to form the basic structure of the proposed dual-band BPFs. Up to two pairs of transmission zeros (TZs) could be created with reference to the dual wide passbands. By choosing the impedances of the two paralleled MMRs, one pair of TZs could be moved from imaginary to real frequencies, whereas the second pair of TZs could be adjusted by folding one of the MMRs to form coupled lines. For these two prototypes, short-circuited stubs are utilized to excite the two paralleled MMRs, but not adequate to obtain a determined coupling degree. To provide an additional freedom for controlling the in-band ripple factor, a combination of short- and open-circuited stubs is used. A generic synthesis procedure is developed for the filter design. First, a generalized Chebyshev transfer functions with one TZ at the origin and symmetrical with respect to the origin has been discussed and frequency transferred to form a dual-band filtering function. Next, by equating of the dual-band circuit response and the filtering function, the circuit parameters could be obtained. Three filter prototypes have been designed and measured with good matching between these two sets of results.