TUNNEL-DIODE OSCILLATOR CIRCUIT FOR SURFACE IMPEDANCE MEASUREMENT OF THIN-FILMS NEAR 20-MHZ

We describe and analyse a simple method for the measurement of thin fi lm surface impedance in the frequency range 10-20 MHz. This method is based on the analysis of both the frequency shift and the amplitude va riation of the signal in a tunnel diode oscillator circuit inductively coupled to the sample. The sensitivity of the method to the conductin g properties of the thin films is a function which is greatly dependen t on geometrical parameters and which can be easily optimized and incl uded in the response calculation. The analysis is illustrated by examp les with superconducting and normal metal films. The resolution of the measurements is also discussed.