Wide-stopband bandpass filter with symmetrical loaded-stub resonators

In this study, a fourth-order wide-stopband bandpass filter using loaded-stub resonators is designed and implemented. The proposed single- and dual-stub resonators are realized to shift the harmonic frequencies by choosing different stub lengths. In this filter, the proposed resonators are arranged at the same fundamental frequency and different higher-order modes to suppress the harmonic responses in the upper stopband. The measured results are in good agreement with the simulated predictions, showing that the rejection levels are better than 30 dB in the stopband up to 12.2 f₀. The fundamental frequency is at 1 GHz designed for ZigBee solutions of IEEE 802.15.4 standard.