A novel compact interdigital bandpass filter using multilayer cross-coupled folded quarter-wavelength resonators

In this letter, a novel compact cross-coupled interdigital bandpass filter (BPF) is developed. By using multilayer folded quarter-wavelength resonators, the size of the filter is reduced greatly, meanwhile a cross-coupling is introduced to produce transmission zeros and thereby improve the stopband characteristics of the filter. As an example, a four-pole BPF centered at 2.25GHz with a fractional bandwidth of 31% is designed, fabricated, and measured. The simulated and measured results show an excellent agreement.