Adjustable Triple-Band Triple-Mode Dielectric Waveguide Filter With Multiple Transmission Zeros

In this design, an adjustable triple-band triple-mode dielectric waveguide (DW) bandpass filter with multiple transmission zeros (TZs) is presented. The design involves creating blind holes and a slot in the dielectric resonator to control the operating frequencies of the three transmission modes (TM010, TM020, and quasi-TM), each forming a passband. By modifying the slot height to convert the third resonant mode into a spurious mode, a dual-band dual-mode filter can be obtained. Besides, two TZs are introduced between adjacent passbands, significantly improving out-of-band rejection. To demonstrate the concept, a triple-band filter utilizing the DW resonator is manufactured, post-processed, and measured. The center frequencies for the three bands are 2.24, 2.94, and 3.52 GHz, with the in-band insertion loss of 0.49, 0.36, and 0.73 dB, respectively. The in-band return losses are better than 16 dB.