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A novel continuously tunable lowpass filter (LPF) with compact size, sharp roll‐off and low insertion loss is presented in this paper. The filter employs two varactor diodes, a pair of open‐ended coupled lines and a U‐shape step impedance line (SIL) with an open‐ended stub loaded at the center of the SIL to form a very compact layout. The odd‐ and even‐mode analysis and equivalent circuit model are demonstrated for estimation of the transmission characteristics. Tuning the DC voltage applied on the varactor diodes, the varactor capacitance accordingly changes leading to a varying cutoff frequency fc. The measured results show that the achieved 3‐dB fc tuning range is 60.6% (1.15–2.15 GHz). The measured insertion loss (IL) and roll‐off rate are 0.2‐0.4 dB and 50–73 dB/GHz, respectively. The overall size of the LPF is only 0.005λg2, which shows a competitive advantage comparing with the state‐of‐the‐art work.

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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 194±8 MHz. The insertion loss (IL) varies within the range of 2.1-3.3 dB. The measured isolation level keeps better than 38 dB.INDEX TERMS Element variable coupling matrix, frequency-agile bandpass diplexer, identical constant absolute bandwidth.