Signal processing in acoustic imaging

A review of signal processing methods which can be used to improve the effectiveness of systems designed for acoustic imaging and bearing estimation is presented. Topics covered include a) signal processing for increased resolution, b) the processing of stochastic acoustic signals, c) image processing, enhancement, and pattern recognition. The discussion of resolution processing includes lateral resolution improvement by both superresolution techniques and aperture synthesis, and improvement of both range and Doppler resolution. The stochastic signal-processing section addresses adaptive processing, as well as methods of imaging in the case of incoherent, noisy signals.     

Signal processing with charge-coupled devices

The purpose of this paper is to discuss applications of charge-coupled devices in signal processing systems. Both analog and digital CCD concepts are considered. Recent developments in high-speed (∼100 MHz) CCD's are discussed, and the uses of high-speed CCD and surface-acoustic wave (SAW) devices together are considered. Examples of the applications of CCD's in electro-optical systems, secure voice communication systems, sonar systems and radar systems are given. Finally, projections for future uses of CCD's in signal processing systems are presented.     

Crystal growth and properties of TI3BX4 crystals for acoustic surface-wave and bulk acoustic devices

Crystals of Tl₃TaS₄ and T1₃Tase₄ possess properties which indicate that they could be extremely useful for acoustic devices. They are piezoelectric, have low acoustic velocities (6.89 to 8.53 x 10₄ cm/sec), high effective values of electromagnetic coupling (k² up to ~ 3%) and contain directions in which the thermal coefficients of delay are zero. Materials preparation and crystal growth and properties are described.     

Signal processing in the reflective acoustic microscope

We describe a new technique to enhance the image in the scanning acoustic microscope in the reflection mode. We use a 20 ns pulse modulated r.f. signal and a sampling oscilloscope to separate the incident signal from the reflected signal. The resulting image shows up details that are of the order of one micrometre with a good signal/noise ratio. We can further improve this resolution by phase measurements.     

Signal processing for underwater acoustic communications

The performance and complexity of signal processing systems for underwater acoustic communications has dramatically increased over the last two decades. With its origins in noncoherent modulation and detection for communication at rates under 100 b/s, phase-coherent digital communication systems employing multichannel adaptive equalization with explicit symbol-timing and phase tracking are being deployed in commercial and military systems, enabling rates in excess of 10 kb/s. Research systems have been shown to further dramatically increase performance through the use of spatial multiplexing. Iterative equalization and decoding has also proven to be an enabling technology for dramatically enhancing the robustness of such systems. This article provides a brief overview of signal processing methods and advances in underwater acoustic communications, discussing both singlecarrier and emerging multicarrier methods, along with iterative decoding and spatial multiplexing methods.     

Electrode interactions in acoustic surface-wave transducers

Results are given which show the importance of acoustic interactions in interdigital surface-wave transducers on piezoelectric materials.     

Ambiguity-function generator using acoustic surface-wave convolvers

It is shown that ambisuity function of a time-limited signal can be obtained using ultrasonic convolvers. Experimental results are presented using a separate media space charge coupled acoustic surface-wave convolver.     

Immittance characterization of acoustic surface-wave transducers

An analytical technique for finding an immittance function, given its real part, for simple equivalent circuit of an interdigital surface-wave transducer is utilized. The effectiveness of this technique is demonstrated using LiNbO3surface-wave delay lines.     

Stabilising surface-wave devices against temperature variations

Surface-elastic-wave devices can be temperature-stabilised with a feedback circuit which includes a phase comparator for a reference signal and a deposited thin-film heater. In a 50 MHz yz lithium-niobate test delay line, the phase variation corresponded to a maximum temperature change of ±0.017°C, while the ambient temperature varied from 25 to 35°C.     

Matching optimisation to interdigital surface-wave devices

Computer optimisation of simple matching networks to interdigital transducers using an equivalent-circuit model has been carried out. Low insertion loss and phase dispersion with wide bandwidth can be obtained even with seriously mismatched devices. The theory is verified experimentally for a 3-pair interdigital device at 30 MHz on lithium niobate.     

Temperature-stabilized piezoelectric ceramics for acoustic surface-wave filters

Suppression of temperature variation and improvement of surface roughness on filter materials were accomplished by using hot-pressed piezoelectric ceramics consisting of ternary complex ceramics, PbTiO3and PbZrO3, and a compound Pb(Mn1/3Nb2/3)O3. The electromechanical coupling factor kswas 0.239, temperature variation of wave velocity TVs, was 17 ppm/°C, and average pore size was 1.5 µm. The ceramics obtained are suitable for such practical devices as surface-wave filters.     

Bonded planar structures for efficient surface-wave devices

Acoustic surface waves have been transferred from yz lithium niobate to s.t. cut quartz surfaces across thin epoxy-resin bonds in a planar geometry. In the best case to date, only 7 dB transmission loss was measured through two bonds at 50 MHz in a lithium-niobate-quartz-lithium-niobate structure.     

Multiple UHF frequency generation using acoustic surface-wave filters

A new technique of using acoustic surface-wave filters for multiple UHF frequency generation has been demonstated. The 21 contiguous surface-wave filters, which are spaced 5.3 MHz apart from 520 to 650 MHz, occupy an area of 2 cm × 0.9 cm, and no multipliers or mixers are required.     

16-bit switchable acoustic surface-wave sequence generator/correlator

A code variable acoustic surface-wave signal processing device has been developed utilizing silicon-on-sapphire (SOS) diodes as the switching elements. The SOS diodes provide rugged devices which are simple to fabricate and require relatively low power levels to operate.     

Spurious acoustic responses in SAW devices

Substantial documentation of the character of spurious responses in SAW devices has shown that they can be completely characterized as families of plate modes when the substrates have parallel top and bottom surfaces. Presented here are theoretical plate mode calculations and the corresponding experimental results for commonly used substrate orientations. It is shown that quantitative terminal response predictions are possible when all the plate modes are included in the analysis. Results for a new substrate cut, 46° (ZYW) LiNbO3, show that it can have lower than -60-dB spurious modes relative to the Rayleigh mode.