Recent Progress in Miniature Microwave Filters

Abstract

This paper aims to review some miniature microwave filters developed recently, including dual-mode microstrip openloop resonator filters, folded waveguide resonator filters, multilayer filters and superconducting filters. The concepts and techniques explored to achieve the miniaturization are described with some demonstrators.This very interesting and unique property can be exploited for further development of higher order quasi elliptic-function filters without using any cross coupling. For example, a fourpole filter can be designed to produce two finite-frequency transmission zeros using the coupling structure of Fig. 3. I. DUAL-MODE OPEN-LOOP RESONATOR FILTERSDual-mode microstrip resonators are attractive because each of dual-mode resonators can be used as a doubly tuned resonant circuit, and therefore the number of resonators required for a given degree of filter is reduced by half, resulting in a compact filter configuration [1]. Recently, a dual-mode microstrip triangular patch has been investigated [2], showing at the first time that the two degenerate modes, which are referred to as the even-and odd-modes, do not couple even after the perturbation introduced. This leads to the development of a new class of miniature filters including those using dual-mode triangle loop resonators [3]-[4] or dual-mode open-loop resonators [5].In particular, the new dual-mode open-loop resonator is developed from a single-mode (operated) open-loop resonator [6]. The open-loop resonator is well known for its flexibility to design cross-coupled resonator filters as well as its compact size, which amounts to A/8 by 2/8, where i is the guided wavelength at the fundamental resonant frequency. It has been shown that the dual-mode open-loop resonator has a size that is the same as the single-mode open-loop resonator, which, however, is much smaller than the conventional dual-mode loop resonator. The size of dual-mode open-loop resonator is only about one quarter of that of dual-mode square loop resonator, which is a significant size reduction. Fig. 1 demonstrates two dual-mode open-loop resonator filters, which have a coupling structure of Fig. 2. The two filters exhibit very distinct frequency responses. The one has a finitefrequency zero on the low side of the passband, and the other has a finite-frequency zero on the high side of the passband. It can be shown that the transmission zero is closely associated with the even mode. The transmission zero is allocated on the low side of the passband when the even-mode frequency is lower than the odd-mode frequency. On the other hand, the transmission zero is on the high side of the passband when the even-mode frequency is higher than the odd-mode frequency.-10

Compact interdigital filters with transmission zeros in LTCC technology

“…Various geometries for the resonators and their couplings have been proposed in order to compact the distributed parameter filters . These geometries also allow the introduction of transmission zeros by means of the cross‐couplings.…”