Miniaturized Tunable Filters with T-Coil Design

This paper proposes a tunable filter made of T-coil design with its tuning range as large as 50%. There are three sets of external capacitors to get wider tuning range so that the center frequency, external quality factor and bandwidth can be suitably tuned. The T-coil tunable filter is fabricated in 20mil Ro4003 board and SMD MLCC capacitors. It demonstrates tenability over 1.25 to 2.5 GHz with 5% FBW. The measurement results show a -15dB return loss and 1.7-2.66 dB insertion loss over the entire tuning range. The circuit size is 16.4mm×11.8mm.