A novel 3D metal printing in-line bandpass filter with multiple controllable transmission zeros

In this paper, a novel three-pole bandpass filter with a pair of controllable transmission zeros (TZs) is proposed first. The proposed structure is based on asymmetric dual-postresonators and two external resonators. The fundamental mode and the first higher mode of the asymmetric dual-postresonators are used as the nonresonant mode and the resonant mode separately. The nonresonant mode generates direct source to load negative coupling, thus providing one TZ located below the passband. The asymmetric dual-postresonators and one of the external resonators R3 form a magnetically coupled cascaded triplet unit to produce the other TZ above the passband. Therefore, these two TZs are independently controlled. Moreover, the filter has a good quality factor (Qu ${Q}_{u}$), of around 2700, compact size and an excellent out-of-band performance. For further suppressing the harmonic response and improving the processing accuracy, a six-pole bandpass filter with harmonic suppression is designed and fabricated by using metal 3D printing technology. The measured result matches well with the simulated one, which verifies the accuracy of the method.