SurfaceAcousticWave Delay Lines and Transversal Filters
Novel, simple and compact electronic devices can be realized by exciting and detecting acoustic waves electrically on the surface of a solid. Technological advances in low-loss delay lines and bandpass filters are discussed.
by Waguih S. fshak, H. Edward Karrer and William R. Shreve
SURFACE WAVES are familiar to anyone who has thrown a pebble into a pond and watched the concentric ripples emanate from the splash. These waves carry energy outward from 1 lie splash at a velocity dependent on the properties of the medium and the magnitude of the restoring force, in this case water and gravity. An observant person on the shore would notice that a cork floating on the surface would follow an elliptical path In a vertical plane as the wave passed [Fig, la). An observer, perhaps a fish, at the bottom of a deep pond would not feel these waves since they are confined to a surface layer about one wavelength thick.
Surface waves also can exist on solids. These waves were first analyzed by Lord Ravleigh in 1B85.1 His work explained one of the classic problems of geology in the last century, that is. how to locate an earthquake by interpreting seismogram recordings from several locations. Rayleigh showed that an earthquake not only generates longitudinal (compressional) and transverse (shear) waves which tend to follow chorda! paths through the earth, hut also surface waves which travel at a slower velocity around the earth's circumference. The surface waves are guided by the stressfree boundary between the earth and its atmosphere. These waves cause the largest ground motion in an earthquake.
During this century, surface waves were extensively studied bv the Russian scientist Viktoruv.- He showed that
- Water
- Solid
Fig, 1. The amplitude of the motion ol panicles m a medium disturbed by the passage ol a surface wave decreases with the depth ol the particle below the surface In a liquid (a) the motion at the surface is in the opposite direction from motion in a solid (b).
surface waves on a solid have a surface particle motion that is retrograde elliptical and opposite to the motion of a particle caused by a surface wave on water (Fig, 1). He also showed that the amount of particle motion falls off exponentially with depth below the surface.
A real breakthrough came in 1965 when White and Volt-mer discovered an easy way to generate and detect surface acoustic waves (hereafter abbreviated SAWs) in the laboratory.J They used metallic interdigital transducers (fDTs) deposited in a vacuum on a piezoelectric substrate (Fig. 2). In an IDT the fingers are spaced by AJ2 where A=v/f. Here f is the excitation frequency, v is the surface wave velocity, and K is the wavelength,
When an alternating voltage of frequency f is applied to the IDT. an electric: field is created between adjacent fingers. A strain field is also generated because of the electromechanical interaction in the piezoelectric substrate. This strain field is an imprint of the IDT and propagates away from the IDT in both directions as a surface wave. The surface wave can be detected by a second remote IDT as the SAW passes through it because the surface wave carries with it an electric field via the piezoelectric effect which generates a voltage between adjacent IDT fingers.
This advance in SAW excitation initiated a new wave of interest in SAW devices to capitalize on their novel properties. Such devices are small because surface-wave velocities are typically 300U metres per second, five orders of magnitude slower than electromagnetic waves. The IDT is a planar structure that can be made with a single metallization step. Advances in microelectronic fabrication and photolithography brought about by the booming semiconductor industry were immediately applicable to the emerging SAW technology. The growing research led to new piezoelectric materials and material cuts optimized for particular requirements such as temperature stability nr large fractional bandwidth.
The advance of SAW technology can be traced in more detail by the interested reader who examines some review articles, special issues of technical publications, and books published in the field.4"1'
Delay-Line Filters
Delay-line fillers were among the first SAW components to be used in circuits. The impulse response of these filters can be accurately controlled in both amplitude and phase, and their nondispersive nature leads to a linear change of
Absorber flF Output -O
Absorber flF Output -O
- Surface Acoustic Wave (a)
Fig. 2, (a) Basic configuration for a surface-acoustic-wave delay line The absorbing layer at each end reduces reflections from the edges of the substrate, (b) Scanning electron microscope photograph of a section of a typical interdigital transducer a! 2300 x The varying lengths !apodization) of the electrodes are chosen for a desired response
Fig. 2, (a) Basic configuration for a surface-acoustic-wave delay line The absorbing layer at each end reduces reflections from the edges of the substrate, (b) Scanning electron microscope photograph of a section of a typical interdigital transducer a! 2300 x The varying lengths !apodization) of the electrodes are chosen for a desired response
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