Aperiodic artificial substrates for multipole bandpass filters

Abstract

Though periodic structures have been the focus of intense research for the past decade, in practice, a truly periodic structure is rarely the proper topology to give a desired or specified transfer function. Traditionally, filtering in periodic substrates was shown by creating defects in an otherwise periodic lattice. However, this approach limits the transfer function of the resulting filter. The use of perturbations in the periodicity in order to create multipole filters with arbitrary transfer functions that are not available inside simple periodic substrates is demonstrated. A methodology is presented to design the aperiodic substrates. Measured and simulated four-pole filter responses are shown for both all-dielectric substrates with high-dielectric-constant titania inclusions and metallodielectric substrates with metallic inclusions inside low-temperature cofired ceramics (LTCC). The measured all-dielectric substrate demonstrates a filter at 21.6 GHz with an insertion loss of 1.69 dB and a narrow bandwidth of only 0.89%, indicating quality factors >1000. Filters with specified Chebyshev transfer functions inside LTCC metallodielectric substrates show consistency within < 0.5% at 10.89 GHz due to the mature fabrication process.