Rapid design of SAW oscillator electronics for sensor applications

Abstract

This paper presents electronic techniques and a general methodology for the rapid design of two-port surface acoustic wave oscillators. By using a methodical approach, the stability problems and much of the time-consuming experimentation common to radio frequency (RF) oscillator design was avoided. Several oscillators in the range of 200 MHz to 1 GHz were designed using variations of the same basic circuit. Circuit designs included both two-port resonator and delay line SAW devices. The electronics were small in size ( 1.75 cm×1.75 cm ) and inexpensive (<US$ 10 in part cost). The active circuit element of the oscillators was a radio frequency integrated circuit (RFIC) wide-band amplifier. The first step of this systematic method was to identify a readily available RFIC amplifier that met the specifications of the oscillator. The specifications included amplifier gain, bandwidth and maximum input power. Choice of the proper amplifier allowed the same circuit to be used for several SAW oscillators spanning a large frequency range. Next a passive LC filter was designed to limit the open loop gain to a small frequency region around the SAW device’s fundamental frequency. This filter eliminates the problems of mode hopping and oscillation at harmonics. The S-parameters of the oscillator were then measured in an open-loop configuration to determine the phase shift requisite for the closed loop oscillation condition of 0° phase shift. To achieve stable oscillation, a passive LC phase-shifting filter was designed. Using the properties of Butterworth filters and a simple computer program, filters of exact phase shift were designed. The last step of design was the use of microstrip layout techniques to reduce wave reflections and susceptibility to electromagnetic interference.