A novel analytical technique to omit the spurious passband in inductively coupled bandpass filter structures

• The second spurious passband in the conventional inductively coupled based bandpass filter structures reduces the quality of the circuit.
• This paper theoretically presents a novel technique to omit this second spurious passband and proposes a new tunable lumped-based filter structure.
• Then a new tunable prototype MEMS-based tunable circuit has been designed based on transmission line theory.
• Simulations show that it has a good specifications and its behavior agrees well with the ideal equivalent lumped circuit over a very wide range of frequencies.

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 15–16.5 GHz frequency range in the Ku-band. The compact size of the filter is 1.15 mm × 0.7 mm and it has a good stop rejection and sharp roll-off frequency compared to its conventional counterpart. It has an insertion loss below 2.5 dB and returns loss higher than 10 dB in the passband across the whole tuning range.