Microwave microstrip resonators tuning without quality factor deterioration

Abstract

Stub and ring resonators with resonance frequency micromechanical tuning are presented. Benefits and main differences of micromechanical resonance frequency tuning method from other methods are shown. Normalized dependences of effective permittivity on normalized air gap values for various microstrip line electrode width to substrate height ratios are obtained. Effective permittivity analytical formulas for the case of infinitely wide electrodes are derived. Calculated and experimental dependences of resonance frequency on air gap value and stub experimental unloaded quality factor dependences are given. Air gap influence on resonance frequency value depending on substrate permittivity is shown in terms of the resonance frequency sensitivity. Error estimation for measured experimental data is presented.
Adding tunable heterogeneity between the microstrip resonator signal electrode and the substrate provides not only the resonance frequency tuning but preserves unloaded quality factor. The preservation of the unloaded quality factor during the resonance frequency tuning achieved due to the metal and dielectric loss reduction. Air gap doesn’t have dissipative losses and has permittivity of one, which makes it the best solution for unloaded quality factor preservation. Another important conclusion is that insertion of the air heterogeneity reduces values of dielectric and metal losses arising when substrates with high permittivity are used. For dielectric loss reduction, it is important to maintain low ratio of microstrip line width to substrate height. In contrast to dielectric losses for metal loss reduction the ratio of microstrip line width to substrate height should be high. However, that ratio is limited by impedance permissible range.