Measuring system for time-variant impedances

This paper describes a measuring system developed to investigate time-variant impedances. The initial points for the development of the measuring system were investigations on coaxial λ/4-resonators. At a frequency close to the fundamental resonant frequency of the resonator, plasma can be generated at the tip of the inner conductor, operating the coaxial resonator on a high power level. First investigations had shown a significant change of the resonator input impedance at the feeding point during the process of plasma generation. To optimize the geometry of the resonator with regard to power consumption, time-, frequency- and power-dependent network models of the resonator including the plasma effects were to be found. In the following, a low-cost measuring system is described that facilitates accurate measurements of complex, time-variant input impedances in a defined frequency and power range. The measurement principle-like in commercially available network analyzers-is the determination of the magnitude and phase of the reflection coefficient with the subsequent calculation of the input impedance. To determine the time-variant behavior of the reflection coefficient, a modified detector circuitry is used. The measurement setup consists mainly of a directional coupler, a combined magnitude and phase detector, and an oscilloscope for sampling. Due to the application, the setup requires high-frequency and high-power supply.