Self-Packaged Balanced Bandpass Filters With Impedance Transformation Characteristic

In this article, the co-design of novel self-packaged balanced bandpass filters (BPFs) integrated with the functionality of impedance transformation is presented for the first time. The proposed design is based on a modified branch-line structure, which is composed of two different branch lines and two identical coupled-line sections. Analysis results show that the considered filters can not only achieve Chebyshev equal-ripple responses with impedance transformation properties for the differential mode (DM) but also can efficiently improve the bandwidth of the common-mode (CM) rejection with resorting to this modified branch-line structure. Furthermore, according to the specified equal-ripple fractional bandwidth (FBW), in-band return loss (RL) level, and impedance transformation ratio ${m}$ , the DM frequency response can be directly determined. Two design prototypes, the second-order balanced BPF with a 50-75- $\Omega $ ( $m= 1.5$ ) impedance transformation and the third-order one with a 50-200- $\Omega $ ( $m= 4$ ) impedance transformation, are implemented in self-packaged forms by using the multilayer liquid crystal polymer (LCP)-bonded printed circuit board (PCB) technology. Theoretical, simulated, and measured results are recorded in good agreement, well verifying the design concept.