Surface Acoustic Wave Convolver Research Articles

Applications of SAW Convolvers to Spread-Spectrum Communication and Wideband Radar

Surface acoustic wave (SAW) convolvers and support circuitry have been developed to provide matched filtering of wideband waveforms having lime-bandwidth products exceeding IO3 and continu- ously changing spreading codes for both spread-spectrum communication and wideband radar applications. For spread-spectrum Communication, additional signal processing techniques have been developed to provide correlation of waveforms having time-bandwidth products of IO6 or more with a search window of microseconds for a 100-MHz signal as well as to perform antimultipath processing for data demodulation and for ranging. For wideband radar, high-speed optoelectronic track-and-hold circuits for range gating as well as buffering and charge-coupled-device matrix- matrix-product chips for Doppler processing are being incorporated along with a SAW convolver to provide 0.75-m range resolution and 32 Doppler bins for each of 1280 range bins.

SAW Convolver Using Multi-Channel Waveguide

Theoretical and experimental results are reported for anew type of surface acoustic wave (SAW) convolver using a multi-channel waveguide fabricated on YZ-LiNbO3 substrate. The convolution signal is obtained from an output inter-digital transducer which picks up the SAW propagating in the direction normal to the waveguide. The F-factor, i.e., the convolution efficiency, is about 15 dBm higher than that of the conventional SAW convolver.

On the Design of High Performance Planar Surface Acoustic Wave Convolvers

The design of planar surface acoustic wave convolvers is discussed, specifically with respect to beam compressor, waveguide, and output circuitry. It is shown that wide-band beam compression with a few tenths of a decibel in lossesand low excitation of undesired guide& modes may be implemented by multistrip structures. Further it is demonstrated how metal strip acoustic waveguides can be designed with only a few degrees of phase distortion over a 30 percent relative band- width in convolvers with timebandwidth products exceeding 3000. The convolver output configuration is modeled as a lossy coplanar electromagnetic waveguide, and it is shown how the output circuit should be designed to maintain the required bandwidth with aminimum of circuit losses. Measured data for several convolvers are presented.

A new ZnO-on-Si convolver structure

A new surface acoustic wave (SAW) convolver has been fabricated on a silicon substrate. The device employs a planar-processed, high-speed DMOS transistor as a mixer to implement the convolution cross terms, utilizing the nonlinear dependence of drain current on gate voltage. The input waveforms are SAW's at 45 MHz, and the interaction path length is 0.9 mm. A value of -29 dbm is obtained for the internal bilinear conversion efficiency F int .

Surface acoustic wave parametric oscillation in Si/LiNbo3 substrate

AbstractSurface acoustic wave parametric oscillation due to the backward nonlinear process in a surface acoustic wave convolver is an interesting phenomenon and various applications have been considered. This paper discusses the theory and experimental results with a SAW reflector design with a low oscillation threshold, using a Si/LiNbO3 substrate, which yields the highest convolution efficiency. The results show that the oscillation threshold power can be greatly reduced by using reflectors (threshold power is reduced to 1/30 for a reflection coefficient of 0.75, as compared with the reflectionless case). In the experiment oscillation could be achieved with such low input power that the oscillation threshold was only 14.4 dBm, and thus SAW parametric oscillation is possible. Also, the experimental oscillation characteristics agree well with theory.

High frequency ZnO-SiO 2 -Si surface acoustic wave convolvers

The separate comb grating transducer configuration is used in the fabrication of high frequency convolvers in the metal-ZnO-SiO2-Si surface acoustic wave device structure. A Rayleigh convolver with an operating frequency of 285 MHz and a Sezawa convolver with an operating frequency of 355 MHz are reported.

Design criteria for surface-acoustic-wave convolvers

The effect of acoustic dispersion and attenuation on the performance of surface acoustic wave convolvers is shown to be equivalent to an ideal convolver in cascade with a linear time-invariant filter. Using this result calculations of the implementation loss due to acoustic dispersion and attenuation, transducer bandpass response, and input/output matching networks are given showing the effects of phase and amplitude responses which are representative of typical convolver designs.

(Invited) Surface-Acoustic-Wave Devices for Spread-Spectrum Communication

The technique of spreading the transmitted bandwidth of a message far in excess of the information bandwidth has long been used in military communication systems for interference rejection. Spread-spectrum systems have also become of increasing interest for domestic applications such as police communication. Additional benefits of spread-spectrum communication include the prevention of eavesdropping and unauthorized use of the channel. There are a variety of surface-acoustic-wave (SAW) devices which are of current interest to the spread-spectrum community because of their powerful ability to demodulate received spread-spectrum signals. One of these devices is the SAW convolver. The operation of SAW convolvers and convolver-based communication systems will be described. The advantages of spread-spectrum techniques, in terms of their interference rejection properties will be reviewed. The time-frequency-plane description of large-time-bandwidth-product wave forms will be used to indicate typical spread-spectrum signals such as pseudonoise and frequency hopping. Correlation reception is convenient when the timing of a message is relatively well known. In circumstances where timing is not so well known, the technique of matched filtering is preferred. Until recently, available technology for matched filtering has dictated the use of a fixed spreading code in spread-spectrum systems. The development of the SAW convolver has allowed the use of continuously changing wave forms while still providing matched-filter operation. The operation of an acoustoelectric, separated-media convolver will be described. Other convolver structures of current interest will will also be reviewed.

Signal Processing with Nonlinear Integrated Optics

We present here the integrated optics analog to the well-known surface acoustic wave convolver. Picosecond signal processing capabilities are achieved by the non-degenerate four wave mixing in a Ti in-diffused LiNbO3 waveguide. This second order interaction between two oppositely propagating guided modes was observed to generate a radiation field at the sum frequency into a direction normal to the waveguide surface during the pulse overlap. The potential applications of this phenomenon to processing functions such as convolution, time inversion etc., to precision timing and picosecond metrology, and to ultra-fast transient digitizers are discussed.

Enhancement of convolution voltage due to transverse drift of carriers in piezoelectric semiconductor SAW convolvers

The enhancement of the convolution voltage of piezoelectric surface acoustic wave (SAW) convolvers due to the transverse drift of carriers is predicted theoretically. The theoretical prediction is qualitatively in agreement with the result of experiments using photoconductive CdS samples.

Contactless semiconductor surface characterization using surface acoustic waves

The electric field which accompanies a surface acoustic wave (SAW) propagating on a piezoelectric substrate can interact with the free charge carriers near the surface of an adjacent semi-conductor. This interaction produces a measurable DC acoustoelectric voltage across the semi-conductor and attenuates the SAW. The magnitude of these effects are dependent on the semi-conductor conductivity. Monitoring the acoustoelectric voltage or SAW attenuation while varying the conductivity by external means can yield information about the density and the dynamics of the surface and impurity states in the semiconductor. By observing the optical wavelength dependence of the acoustoelectric voltage developed across the semiconductor, the energy profile of the states in the bandgap can also be determined. Experimental results will be presented for several semiconductor samples. These include GaAs and GaP with or without added impurities and silicon. Tlie results show that the use of the SAW convolver provides a simple sensitive contactless and nondestructive means of evaluating the electrical properties of the semiconductor surface.

Photoconductivity study of semiconductors using the surface acoustic wave convolver

Photoconductivity study of a semiconductor using the SAW convolver is presented. The semiconductor is placed either on or a small distance above the SAW delay line. A fast rise time pulse of light is applied to the semiconductor surface through the piezoelectric substrate, and the resulting change in the semiconductor conductance is observed by the relaxation of the SAW propagation loss. This technique has the important advantage that no ohmic contact to the semiconductor is needed.

Surface wave recirculation loops for signal processing

A repetitive signal can be correlated in a surface acoustic wave (SAW) recirculation loop, thus increasing its SNR. Various imperfections which limit the practical performance of the loop are examined here, particularly the delay line frequency response and spurious levels, and nonlinearity. The conclusions are related to a practical system in which a long pseudonoise signal is correlated by means of a SAW convolver followed by a SAW recirculation loop. The convolver correlates segments of the waveform, giving a repetitive sequence of correlation peaks which can then be summed in the loop. For the latter, a novel SAW delay line was developed using an ST-X quartz substrate, giving a 3-dB bandwidth of 22 percent at 140 MHz with 35-us delay. The system was used to correlate 2.1 ms of a 10 Mbit/s input signal, using 70 circulations in the loop. With an input SNR of -41 dB, an output SNR of +4 dB was obtained.

The Effects of Radiation-Induced Permanent Displacement Damage on the Operation of Surface Acoustic Wave Convolvers

Results are reported of an experimental study of the effects of neutron radiation on five different types of surface acoustic wave (SAW) convolver. A general model for the operation of a SAW convolver is described, and the varying effects of the neutron radiation on the five convolver types are explained in terms of this model. The diode-tapped delay line was found to be the most radiation-hard SAW convolver.

The Effects of Pulsed Ionizing Radiation on the Operation of Surface Acoustic Wave Amplifiers and Convolvers

In a previous paper (IEEE Trans. Nucl. Sci.; NS-21: No. 6, 141 (1974)) we described the effects of transient ionizing radiation on passive surface acoustic wave (SAW) devices and also the permanent displacement damage resulting from high-energy nuclear radiation on the operation of a SAW monolithic amplifier. This paper will conclude the study of the effects of transient ionizing radiation on SAW devices by presenting the effects of a short-duration, high-intensity ionizing radiation pulse on the SAW amplifier, various SAW convolver structures and reflective-array pulse compression (RAC) filter device. By way of brief summary, the operation of the amplifier was found to be relatively unaffected by short-duration radiation pulses, whereas many of the convolver structures were affected by the radiation. A long-term change in phase as a result of radiation was found in the RAC device, and this change is postulated to result from a radiation-induced temperature change. (auth)

Pulse compression using nonlinear interaction in a surface acoustic wave convolver

Nonlinear interaction can be achieved over extended signal path lengths in a surface acoustic wave device. Using such a device variable pulse compression was achieved through the mixing of a chirped acoustic signal and its time inversion.