Optimized Design and Cryogenic Characterization of Flip-Chip InP-Based Balanced UWB BPF

In this article, a flip-chip InP-based balanced ultrawideband (UWB) bandpass filter (BPF) is optimally designed and characterized under cryogenic conditions. The filter chip is designed using spiral inductors and metal–insulator–metal (MIM) capacitors. An electromagnetic (EM)-circuit co-optimization method is introduced to assist in the design process, and the gray wolf algorithm is adopted to iteratively refine variables during each optimization stage. The filter die is then flip-chip bonded to a microstrip-line (MSL) structure on an alumina ceramic substrate. The final filter module is experimentally characterized at 300, 70, and 5 K. Measurements show minimal variations in differential-mode (DM) center frequency (15.45 GHz), bandwidth (101.9%), out-of-band suppression, and common-mode rejection ratio (CMRR) (20 dB in the range of 0–24.5 GHz) across different temperatures. Notably, the minimum DM in-band insertion loss (IL) decreases from 1.02 dB at 300 K to 0.72 dB at 70 K, with improvements of 0.3–0.8 dB across the passband. Analysis of the temperature distribution and deformation shows that the reduction in IL is primarily attributed to the increased conductivity of gold at low temperature, which significantly outweighs the effects of dielectric loss and structural deformation.