Ultra-Low-Loss Millimeter-Wave LTCC Bandpass Filters Based on Flexible Design of Lumped and Distributed Circuits

The design of ultra-low-loss millimeter-wave (mm-wave) low-temperature cofired ceramic (LTCC) bandpass filters is proposed by merging the lumped and distributed circuits in this breif. By inserting a ground plane inside the filter, the cross talk between quasi-lumped capacitors and distributed microstrip lines can be highly reduced. Accordingly, the lumped or distributed circuits can be analyzed and designed flexibly for further performance improvements. To alleviate the design complexity, a lossy equivalent circuit model is introduced and verified for the proposed LTCC bandpass filter. Based on the equivalent circuit, the techniques for enhanced stopband rejection and low insertion loss can be both obtained. For experimental verification, two 27 GHz LTCC bandpass filters are designed, fabricated, and measured. Very low insertion losses of only 0.73 dB and 0.84 dB, wide stopband, and multiple transmission zeroes are realized, indicating the proposed LTCC filters good candidates for 5G system-in-package applications.