Compact Multi-Layer Bandpass Filter With Wide Stopband Using Selective Feeding Scheme

This brief presents a new approach for designing compact bandpass filters (BPFs) with harmonic suppression using quarter-wavelength stepped-impedance resonators (SIRs). The voltage-null point of the third harmonic on the SIR is theoretically derived and found for the first time. It is used as the exciting location for the selective feeding line so that the third harmonic cannot be excited and is suppressed successfully in the filter design. Meanwhile, the passband of the BPF operating at the fundamental resonant frequency (<inline-formula> <tex-math notation="LaTeX">${f_{ 1 }}$ </tex-math></inline-formula>) of the SIR can be optimized and obtained by tuning other parameters. By combining the proposed simple technique and the SIR merits, the resultant BPF owns several advantages, such as wide stopband and miniaturization. To demonstrate the feasibility of the proposed idea, two design examples of second-/fourth-order BPFs based on multilayer PCB are designed. In both designs, the third harmonic suppression is larger than 30 dB and the first spurious occurs at a frequency beyond <inline-formula> <tex-math notation="LaTeX">${7f_{ 1 } }$ </tex-math></inline-formula>. Their simulated and measured results are presented, showing good agreement.