Five-pole wideband quasi-reflectionless interdigital BPF with second harmonic suppression

Abstract In this paper, a 5-pole quasi-reflectionless (QR) interdigital BPF with a wide range of non-reflecting signals is proposed, offering good impedance matching. Two absorptive T-shaped stubs are placed at the input and output ports of the interdigital BPF. The absorptive stubs are optimised to obtain the reflection as low as possible for a wide range. Additionally, we shift the adjacent resonators of the interdigital BPF up and down to improve overall performance. The design follows a specific procedure in order to investigate factors affecting the performance. Firstly, the convetional interdigtial bandpass filter is designed relying on the coupling matrix method. Next, we study the conversion of the stopband filter into an absorptive stub. The input impedance of the stub plays a vital role in determining the reflection bandwidth, so it is optimised as well. Meanwhile, the interdigital BPF and one abosrptive stub are combined, where the bandwidth of reflectionless becomes wider compared to the conventional interdigital BPF. Another absorptive stub is added into the other port of the BPF to make the proposed filter symmetric. Different orders of the interdigital PBF are analysed, where the fifth order has the best response. The proposed filter is fabricated and tested, and an FR4 substrate with a thickness of 1.5 mm and a dielectric constant of 4.5 is utilized. The simulation and measurement results agree well. The realised S11 is less than −10dB from 0.72 GHz to 4.1 GHz (i.e., −10dB reflectionless bandwidth 3.38 GHz), while the realised S11 is less than −3dB from 0.42 GHz to 6 GHz (i.e., −3dB reflectionless bandwidth 5.58 GHz). The S21 response is good for the transmission band. Finally, some slots are etched out from the gorund to reduce the influence of the second harmonic response.