Shrinking microwave filters

Abstract

An adaptive predistortion technique has been presented and verified through the design and fabrication of practical filters in both the C and Ku bands. The method allows the realization of microwave filters at a lower cost, lighter mass, smaller volume, and better performance with minimum insertion loss penalties.The concept of lossy filters has been presented from a practical perspective. A simple lossy synthesis technique using any synthesized lossless (nontransversal) filter was shown, which can be used with hyperbolic rotations for loss distribution. Moreover, the limitation on the minimum Q of lossy resonators has been studied using a one-pole filter as a fundamental building block. Lossy four-pole Chebyshev and quasi-elliptic synthesis examples were presented. A four- pole Chebyshev lossy filter in the Ku band has been synthesized, modeled, and fabricated successfully using mixed combline and microstrip technologies. The design has the advantage of having all input-output paths going through more than one resonator, which minimizes unwanted source-to-load coupling, especially at high frequencies. The lossy approach is still at its early stages of development and needs more research and development effort to become as mature as the predistorted filters.