Oscillators based on step-impedance and slow wave transmission lines for sensing applications

Abstract

This work investigates the capabilities of oscillators based on step-impedance and slow wave structures to sense dielectric constants. The material under test (MUT) is placed over the structure and the objective is to achieve a high sensitivity of the oscillation frequency with the advantage of low phase noise, enabled by the high quality factor of the structure. With the aid of simplified analytical models, we will initially study the variation of the resonance frequency of a istep-impedance transmission line with the dielectric constant of the MUT, paying attention to the influence of the number of line sections. The study includes the derivation of analytical expressions for the sensitivity of the resonance frequency. Next, the structure will be connected to the oscillator active core, which will be modeled with a numerical nonlinear admittance function extracted from harmonic-balance (HB) simulations. The resulting semianalytical formulation will provide insight into the variation of the oscillation frequency and amplitude with the dielectric constant of the MUT, as well as the variation of the phase-noise spectral density. It will also enable a versatile test and optimization of the various structures to achieve high sensitivity with low phase noise. The methods have been successfully applied to an field-effect transistor (FET)-based oscillator at about 2 GHz.