A 1–1.5 GHz broadband tunable bandpass filter

This paper presents a broadband tunable bandpass filter based on varactor-loaded meandered combline topology. A new coupling path is introduced between the input and output multi-section stepped-impedance coupled lines such that the transmission zeros near the passband can be simultaneously controlled to achieve the desired stopband rejection. The measured performances show that a broad 40% tuning range from 1 GHz to 1.5 GHz was achieved. Across the whole tuning range, the insertion loss is 2.1-2.8 dB and the return loss is better than 15 dB and stopband rejection greater than 32 dB.