LAUSR

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 centre of the SIL to form a very compact layout. The oddand evenmode 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 to 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 0.005λg, which shows a competitive advantage comparing with the state-of-the-art work.