Compact Tunable Bandpass Filter With Wide Tuning Range of Centre Frequency and Bandwidth Using Short Coupled Lines

A new compact microstrip tunable bandpass filter with continuous control of center frequency and bandwidth is proposed in the paper. The proposed design is based on short parallel-coupled lines, which are tuned by properly loading varactors. The structure exhibits a pair of transmission poles and one-side passband edge transmission zero that can be flexibly adjusted for required positions. A thorough theoretical analysis is derived to estimate the performance of the proposed filter and verify the initial values of design parameters. To verify the design concept, a prototype is fabricated and measured. The fabricated circuit has an extreme compact size (smaller than 0.03λ
_g
× 0.1λ
_g
). The simulated and measured results agree well with the derived theory, and indicate wide tunable center frequencies (0.56-1.15 GHz) and 1-dB bandwidth (65-180 MHz).