Fine-tuning of the resonant frequency using a hybrid coupler and fixed components in SAW oscillators for gas detection

A method for fine-tuning of the resonant frequency of a typical delay line (DL) SAW oscillator by means of a procedure simple and widely used in microwave circuits is presented. It consists of a quadrature hybrid coupler in whose coupled ports, two identical terminations are placed. Each of them is composed of a set formed by a fixed capacitor C and a fixed inductor L in series. In this way no variable components are used. Therefore the phase can be adjusted for every sensor once the polymer film is deposited and measured. The output signal has the same amplitude than the input one and its phase is changed depending on the value of C and L. The oscillator designed in this way exhibits stable frequency response and operates with a wide range of coatings in measurements with volatile organic compounds (VOCS). (C) 2004 Elsevier B.V. All rights reserved.