A Simple and Effective Method for 1.9–3.4-GHz Tunable Diplexer With Compact Size and Constant Fractional Bandwidth

In this paper, a simple and effective method is proposed for designing tunable diplexers with constant fractional bandwidth using dual-mode resonators. The proposed method directly connects two independently designed dual-mode bandpass filters and forms a tunable diplexer without any additional matching network at the common port, thus bringing significant design flexibility to the diplexer design process. Also, an alternative design process is proposed to characterize the coupling in the dual-mode resonator and external quality factors at the input/output, making it much easier for constant bandwidth designs. A second- and fourth-order diplexer consisting of dual-mode stepped-impedance microstrip resonators are designed and fabricated to demonstrate this method. The second-order diplexer is tuned using silicon varactor diodes, and covers 1.9-3.4 GHz with an insertion loss of 3.0-5.0 dB. Constant fractional bandwidths of ~ 6% and ~ 4.5% are obtained for the two frequency channels, respectively. In addition, due to the employment of the stepped-impedance dual-mode resonator, the second-order tunable diplexer occupies a compact area of 0.037λ
₀
×0.158λ
₀
, where λ
₀
is the free-space wavelength at the lowest frequency that the diplexer can be tuned. Simulations and measurements are presented with excellent agreement. Application areas are in multi-standard communication systems.