APPARATUS FOR MEASURING IMPEDANCE OF A RESONANT STRUCTURE

摘要

Apparatus for measuring the value of a physical quantity which affects the impedance of an electrically resonant structure, said apparatus comprising an electrically resonant structure, an RF electrical energy source, a bidirectional RF transmission line connecting said source to said resonant structure, a directional coupler associated with said transmission line, said source providing an excitation signal to said structure, said directional coupler detecting the voltage or phase of a reflected signal returned from said resonant structure characterised in that the resonant structure is substantially non energy radiating and, at a given frequency of said source, has an impedance which varies continuously as a function of the value of said physical quantity. Typically the electrically resonant structure is at least partially composed of piezoelectric material.