Coupling-of-Modes Studies of Surface Acoustic Wave Oscillators and Devices

Abstract

<p>Recent advances in the area of communications has inspired the concept of ubiquitous coverage onto the market place. A pocket-size mobile unit will be able to provide two-way messaging on a global perspective. In order to realize this "radio on a chip" philosophy small, rugged, low power and inexpensive components must be designed. In this thesis, new circuits and devices that meet this communication challenge are examined. These involve the use of surface acoustic wave (SAW) components and devices.</p> <p>The coupling-of-modes theory is used to study the effects of injecting a very small stimulus signal into a SAW oscillator loop. This diminutive injected signal may force the loop, if the conditions prevail, to follow and track it. These injection-locked oscillators (ILOs) follow the theory of phase locked loops (PLLs) to provide circuits suitable for amplification and demodulation of digital modulation signals. The locking phase angle of an ILO follows a similar sigmoidal shape found in neurons. An equivalent SAW ILO neuron circuit is presented and further developed into a self organized neural network. The SAW-based ILOs are then modified by controlling the quality factor of the SAW feedback element to produce carrier recovery, analog to digital converters and data multiplexer circuits. A very non-linear, near-chaotic ILO is introduced and examined with the aid of the Duffing equation. The Duffing model accurately verifies a periodic pulse generator employing a driven unlocked SAW ILO.</p> <p>A crucial component in any communication receiver is a "front-end" filter that eliminates any interfering frequencies that are close-in to the receiver channel. A study of such a component reveals that a dual-mode leaky-SAW resonator filter is ideal for the task. Antenna theory is applied to model the radiation conductance of thick-film interdigital transducers (IDTs) in dual mode wideband leaky-SAW resonator-filters on 64° Y-X LiNbO₃ employing a three-IDT structure. As well, SAW coupling-of-modes reflection-grating equations and IDT S-parameters are modified to cater for increased leaky-SAW attenuation above the Bragg frequency. Theoretical frequency response computations are in excellent agreement with experimental results obtained for several low-loss 85-MHz structures fabricated with metallization film thickness ratios (h/λ) of up to 3%.</p>