Tunable band-pass filter with wide stopband and high selectivity using centre-loaded coupled structure

A tunable band-pass filter using a coupled structure that is centrally tapped with a resonator is proposed for wideband applications. The utilised resonator includes parallel-coupled lines loaded with a short-ended varactor. The resonator enables the independent tunability of two transmission zeroes (TZs), which define the passband of the filter. Moreover, the structure creates multiple TZs in upper stopband which enable realising a wide spurious-free stopband. A thorough theoretical analysis is used to verify the performance and calculate the initial design parameters. To validate the proposed design procedure, a prototype is designed, fabricated and tested. The simulated and measured results agree with the derived theory, and indicate a tuning range for the 3 dB bandwidth from 47.8 to 105.7% with <1.4 dB of insertion loss and more than 12 dB of return loss. It also has more than 18 dB of attenuation across a stopband extending to around four times the centre frequency.