Inductive coupling of two parallel resonant circuits is still one of the most important structures in the practical bandpass filters. The frequency response of these structures has two passbands; one is desirable and the other one is not. This paper presents a novel scheme to omit the second spurious passband based on theoretical analysis. Thus, a prototype circuit has been designed based on transmission line theory. Simulations show that its behavior agrees well with the ideal equivalent lumped circuit over a very wide range of frequencies. Center frequency tuning has been implemented by using a simple high-Q varactor based on microelectromechanical systems (MEMS) technology. The tuning range of the proposed circuit is 9.8% and the fractional bandwidth is 10.8±0.4 for the 15–16.5GHz frequency range in the Ku-band. The compact size of the filter is 1.15mm×0.7mm and it has a good stop rejection and sharp roll-off frequency compared to its conventional counterpart. It has an insertion loss below 2.5dB and returns loss higher than 10dB in the passband across the whole tuning range. 
A novel analytical technique to omit the spurious passband in inductively coupled bandpass filter structures