Low-Loss Wide-Tuning-Range Three-Pole Frequency-Agile Bandpass Diplexer With Identical Constant Absolute Bandwidth

This paper presents a low-loss, high-isolation and wide-frequency-tuning-range (FTR) (45) implementation of the three-pole frequency-agile bandpass diplexer (FA-BPD) with identical constant absolute bandwidth (IC.ABW). The FA-BPD includes two three-pole channels and each channel has three transmission zeros (TZs). To develop such a FA-BPD, first, two bandwidth (BW) -identical and -constant frequency-agile bandpass filters (FA-BPFs) with the FTRs of 1.35-2.25 GHz and 1.9-3 GHz are designed individually. Both two FA-BPFs simultaneously employ the cascaded triplet (CT) topology and source-load coupling to introduce three TZs. All couplings of two filters are arranged opposite in sign, thus two TZs of one channel are placed on the side of the other channel. Finally, two FA-BPFs are fed by a common coupling line, thus forming the three-pole 1.35-3-GHz FA-BPD. The design example is fabricated and measured. The experimental results show that the FTRs are 1.35-2.25 GHz (50) and 1.9-3.0 GHz (44.9). The ABW of the two channels keeps nearly a constant of 1948 MHz. The insertion loss (IL) varies within the range of 2.1-3.3 dB. The measured isolation level keeps better than 38 dB.