Three-Pole Reconfigurable 0.94–1.91-GHz Diplexer With Bandwidth and Transmission Zero Control

This paper proposes two compact three-pole reconfigurable diplexers. The first diplexer demonstrates both controllable channel frequencies and bandwidths and the second diplexer, furthermore, allows for additional transmission zero control to improve stopband suppression and channel isolation. To reduce diplexer footprint, the topology of a common resonator at the input and two dual-mode resonators responsible for the respective passbands was applied. The study of the proposed dual-mode resonators and the design procedure for diplexer synthesis are detailed. The experimental results of the first prototype show that the lower and higher passbands can be tuned from 0.94 to 1.21 GHz with the 1-dB constant bandwidth tunable from 60 to 90 MHz and from 1.51 to 1.91 GHz with a 120-160-MHz bandwidth tuning range, respectively. All tunable states achieved greater than 14 dB in return loss and greater than 36 dB in output isolation. With the transmission zero tuning scheme, the second diplexer achieved isolation greater than 51 dB in both passbands for all tunable states. The measured and simulated results are in good agreement.