Compact, Coupled Strip-line Broad-Band Bandpass Filters

A miniature broad-band bandpass filter with size less than lambda/8 times lambda/8 times h (lambda is wavelength at the midband frequency; h is substrate height, much smaller than lambda/8), employing broadside-coupled strip-line resonators is introduced. Such resonators can be used to design combline and interdigital filters of very wide relative bandwidth. The synthesis procedure follows the classical coupling matrix approach, which generates very good initial responses. Optimization by mode matching method (MMM) and fine-tuning in HFSS are combined to improve the filter performance. Two design examples are given to illustrate the design procedures and a 45% bandwidth interdigital filter in LTCC is being built for testing