Compact 3-D Stacked Stripline Bandpass Filter with High Selectivity for 5G BTS Applications

A novel compact stripline bandpass filter (BPF) with 3D stacked structure is proposed in this paper. The 3-D stacked structure is made up of several metal layers to form a folded stripline configuration. Then, conventional inline structure is transformed to the U-type one, so that the filter size is reduced dramatically. By introducing cross coupling, the proposed BPF can realize a sixth-order general Chebyshev bandpass response with four transmission zeros among which each two are at each side for improving frequency selectivity. The coupling matrix is presented to reveal its coupling topology and extract coupling coefficients so that design process can be facilitated. For demonstration, an example operating at 3.5 GHz was designed, fabricated and measured. Sheet metal and soldering processes are applied to realize the 3-D stacked structure. Apart from its compact size, the measured results of the example show that the BPF has excellent in-band and out-of-band performance.