Wide-stopband compact pseudo-interdigital over coupled bandpass filter with SIR and open-stub structure

In this study, a wide-stopband compact pseudo-interdigital overcoupled (PIOC) bandpass filter with a stepped impedance resonator (SIR) and open-stub structure was investigated and implemented. We developed a method that could be used to embed the SIR and open-stub lines in the compact PIOC filter suppressing harmonics in the stopband without evidently increasing the circuit size. The SIR and open-stub lines were determined to improve the signal attenuation in the stopband from 3.5 to 4.5 GHz and 4.5 to 5 GHz, respectively. The proposed filter was designed at an operating frequency of 2.4 GHz on an FR4 substrate. In the passband, the measured 3-dB bandwidth was from 2.18 to 2.46 GHz and the minimum insertion loss was 2.22 dB at 2.31 GHz; the return loss was above 15.2 dB. The measured attenuation in the stopband was above 20 dB from DC to 1.74 GHz and from 2.8 to 7 GHz. The proposed filter is very compact with an optimal size of 18.2 × 18.8 mm
²
. Moreover, the filter can be easily integrated with 2.4 GHz wireless communication systems because it can be implemented using standard printed circuit board etching processes without any lumped elements, bonding wires, or via holes.