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A $2\times 2$ high-intensity CMUT transmit array that is capable of two-dimensional beam steering is presented. The device uses an ac drive voltage at half the ultrasound frequency without any dc bias, enabling the usage of the entire gap height. The device is designed using a large signal equivalent model approach. A fabrication method that requires a single lithographic mask has been used. The fabricated devices are operated at 76 kHz to beam steer at various angles. An equivalent element pressure of 144 dB// $20~\mu $ Pa at the transducer surface was measured. The entire half-space can be steered without any sidelobes and the beam obtained from the array is in excellent agreement with the theoretical predictions. [2020-0253]

An energy-consistent lumped-element equivalent circuit model for charged circular capacitive micromachined ultrasonic transducer (CMUT) cell is derived and presented. It is analytically shown and experimentally verified that a series dc voltage source at the electrical terminals is sufficient to model the charging in CMUT. A model-based method for determining this potential from impedance measurements at low bias voltages is presented. The model is validated experimentally using an airborne CMUT, which resonates at 103 kHz. Impedance measurements, reception measurements at resonance and off-resonance, and the transient response of the CMUT are compared with the model predictions. Manuscript received July 24, 2017; accepted November 10, 2017. Date of publication November 14, 2017; date of current version January 1, 2018. This work was supported by the Scientific and Technological Research Council of Turkey under Project 114E588. The work of A. Atalar was supported by the Turkish Academy of Sciences. (Corresponding author: Hayrettin Köymen.) H. Köymen, A. Atalar, and I. Köymen are with the Electrical and Electronics Engineering Department, Bilkent University, 06800 Ankara, Turkey (e-mail: koymen. . bilkent.edu.tr).

Date of Conference: 5-9 Nov. 2018 Conference name: 176th Meeting of Acoustical Society of America - 2018 Acoustics Week in Canada The main objective of this study is to obtain improved detection of 50 μm breast microcalcifications in ultrasound mammography. A synthetic transmit aperture imaging method combined with coded excitation and pulse compression is employed. Coded excitation by means of complementary Golay sequences and pulse compression at each transducer element improve the signal to noise ratio and enhance the visualization of the microcalcifications despite the fact that the process is computationally complex. Correlation based beamforming is also applied to the experimental data, resulting in significant improvement in image quality. In this study, pre-beamformed raw data is collected by an ultrasound research scanner, DiPhAS, where a 7.5 MHz phased array transducer with 128 element and 0.1 mm pitch was used. Benefits of coded excitation compared to tone burst excitation of equal energy are clearly observed. The ultrasound images of monofilament nylon wire targets in the phantom are presented, where the superiority of coded excitation is evident. The image reconstruction is performed by using Synthetic Transmit Aperture imaging method with single emission and 5 emissions, respectively. The experimental results show that efficient implementation of coded excitation improves signal to noise ratio, increases penetration depth and provides images with better resolution. This work was supported by the Scientific and Technological Research Council of Turkey under Project Grant 114E588. The authors also would like to express particular thanks to the acoustic group of Bilkent University for their contribution to improve this work.

Cataloged from PDF version of article. Using the small-signal electrical equivalent circuit of a capacitive micromachined ultrasonic transducer (CMUT) cell, along with the self and mutual radiation impedances of such cells, we present a computationally efficient method to predict the frequency response of a large CMUT element or array. The simulations show spurious resonances, which may degrade the performance of the array. We show that these unwanted resonances are due to dispersive Rayleigh-Bloch waves excited on the CMUT surface-liquid interface. We derive the dispersion relation of these waves for the purpose of predicting the resonance frequencies. The waves form standing waves at frequencies where the reflections from the edges of the element or the array result in a Fabry-Pérot resonator. High-order resonances are eliminated by a small loss in the individual cells, but low-order resonances remain even in the presence of significant loss. These resonances are reduced to tolerable levels when CMUT cells are built from larger and thicker lates at the expense of reduced bandwidth. © 2014 IEEE.

Different trunked Private Mobile Radio (PMR) systems have been designed over the last several decades, all of which have symmetric downlink and uplink channel capacities. Due to this symmetry, these systems may not be spectrally efficient in case of different types of services, which are specific to PMR systems, such as group and broadcast calls. In this study, a new asymmetric trunked PMR system comprising a broadband, wide-area downlink and a narrowband cellular uplink, is proposed to achieve a higher spectral efficiency than current digital trunked PMR systems. This system is spectrally more efficient because in group and broadcast calls only a single downlink channel has to be allocated in the downlink part. However, as the number of clusters in the system increases, this advantage relative to PMR systems is lost, since the latter can employ frequency reuse. Spectral efficiency of the proposed asymmetric system (a-PMR) system and a standard TETRA system are compared using numerical case studies against different traffic loads and number of clusters. The optimum point, with respect to number of clusters, up to which the proposed a-PMR system is more efficient, is determined. It is shown that a very large PMR user population can be efficiently served using the proposed a-PMR system. The issues related to implementing such a system are discussed. © 2005 IEEE.

Date of Conference: 3-6 September 2014 Conference Name: International Ultrasonics Symposium, IEEE 2014 In this work, we study airborne CMUT cells with vacuum gap where silicon plate is operated both in elastically linear and nonlinear regimes. We report the results of a new mode of operation where the plate center swings the entire gap. The plate is kept in elastically linear region in this mode. Very large pressure levels are obtained at relatively low drive voltage levels. The operation is very efficient but the bandwidth is less than 1%. We considered operating the silicon membrane in elastically nonlinear region for larger bandwidth without sacrificing efficiency. This is achieved by employing the stiffening effect due to the atmospheric pressure. We derived the new model of the CMUT, where the membrane profile deviates from linear profile as a function of the differential static pressure on it. We present the force, the compliance models and the static analysis of stiffened CMUT cells in this work.

Date of Conference: 3-6 Sept. 2014 Conference name: 2014 IEEE International Ultrasonics Symposium We determine the response of individual cells of a CMUT array immersed in water using the small-signal equivalent circuit of a single cell and radiation impedances. Using a numerically efficient technique, we are able to simulate large arrays. The results indicate the presence of resonances at low frequencies where Rayleigh-Bloch waves are excited on the surface of the array. Reflections from the edges cause standing-wave patterns. Above the cut-off frequency of Rayleigh-Bloch waves, no standing-wave pattern exists. However, there is nonuniformity among cell velocities mainly due to unequal radiation impedance seen by the cells. Rayleigh-Bloch waves and nonuniformity in cell velocities do not cause a significant degradation in the point spread function, but the oscillations extend the duration of impulse response, limiting the dynamic range. © 2014 IEEE.

Cataloged from PDF version of article. We report a new mode of airborne operation for capacitive micromachined ultrasonic transducers (CMUT), in which the plate motion spans the entire gap without collapsing and the transducer is driven by a sinusoidal voltage without a dc bias. We present equivalent-circuit-based design fundamentals for an airborne CMUT cell and verify the design targets using fabricated CMUTs. The performance limits for silicon plates are derived. We experimentally obtain 78.9 dB//20 mu Pa@1 m source level at 73.7 kHz, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120. We also study and discuss the interaction of the nonlinear transduction force and the nonlinearity of the plate compliance.

Cataloged from PDF version of article. We present a parametric equivalent circuit model for a circular CMUT in collapsed mode. First, we calculate the collapsed membrane deflection, utilizing the exact electrical force distribution in the analytical formulation of membrane deflection. Then we develop a lumped element model of collapsed membrane operation. The radiation impedance for collapsed mode is also included in the model. The model is merged with the uncollapsed mode model to obtain a simulation tool that handles all CMUT behavior, in transmit or receive. Large- and small-signal operation of a single CMUT can be fully simulated for any excitation regime. The results are in good agreement with FEM simulations.

Date of Conference: 21-25 July 2013 In this study, an equivalent electrical circuit model for a single circular CMUT in both uncollapsed and collapsed modes is obtained. In order to model the collapsed mode mechanics, the governing differential equation is solved semi-analytically for a large number of normalized cases. Then the calculations are adapted to the equivalent electrical circuit model. The model uses the self radiation impedance of both uncollapsed and collapsed modes. The model is fully parametric in such a way that a CMUT cell of given dimensions and parameters can be simulated under an arbitrary large signal excitation. Transient simulations can be performed in less than a minute with a circuit simulator. Transient and frequency domain simulations are consistent with finite element analysis results. © 2013 IEEE.

Cataloged from PDF version of article. Capacitive micromachined ultrasonic transducers (CMUTs) are usually composed of large arrays of closely packed cells. In this work, we use an equivalent circuit model to analyze CMUT arrays with multiple cells. We study the effects of mutual acoustic interactions through the immersion medium caused by the pressure field generated by each cell acting upon the others. To do this, all the cells in the array are coupled through a radiation impedance matrix at their acoustic terminals. An accurate approximation for the mutual radiation impedance is defined between two circular cells, which can be used in large arrays to reduce computational complexity. Hence, a performance analysis of CMUT arrays can be accurately done with a circuit simulator. By using the proposed model, one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with an arbitrary number of CMUT cells. We performed several finite element method (FEM) simulations for arrays with small numbers of cells and showed that the results are very similar to those obtained by the equivalent circuit model.

Date of Conference: 7-10 October 2012 An equivalent electrical circuit model valid for collapsed mode operation of CMUT is described. The across and through variables of the circuit model are chosen to be rms force and rms displacement over the surface of the CMUT membrane. The relation between rms displacement and applied voltage is obtained through analytical calculations utilizing the exact force distribution. The radiation impedance of collapsed mode CMUT is included as a load impedance in the circuit model. The resulting equivalent circuit is merged with uncollapsed mode model, to obtain a simulation tool that covers the whole operation range of CMUT. © 2012 IEEE.

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array. © 2012 IEEE.

Cataloged from PDF version of article. This paper describes a correction and an extension in the previously published large signal equivalent circuit model for a circular capacitive micromachined ultrasonic transducer (CMUT) cell. The force model is rederived so that the energy and power is preserved in the equivalent circuit model. The model is able to predict the entire behavior of CMUT until the membrane touches the substrate. Many intrinsic properties of the CMUT cell, such as the collapse condition, collapse voltage, the voltage-displacement interrelation and the force equilibrium before and after collapse voltage in the presence of external static force, are obtained as a direct consequence of the model. The small signal equivalent circuit for any bias condition is obtained from the large signal model. The model can be implemented in circuit simulation tools and model predictions are in excellent agreement with finite element method simulations.

Date of Conference: 18-21 Oct. 2011 Capacitive micromachined ultrasonic transducers (CMUTs) have a strong potential to compete piezoelectric transducers in high power applications. In a CMUT, obtaining high port pressure competes with high particle velocity: a small gap is required for high electrostatic force while particle displacement is limited by the gap height. On the other hand, it is shown in [1] that CMUT array exhibits radiation impedance maxima over a relatively narrow frequency band. In this paper, we describe a design approach in which CMUT array elements resonate at the frequency of maximum impedance and have gap heights such that the generated electrostatic force in uncollapsed mode, can sustain particle displacement peak amplitude up to the gap height. The CMUT parameters are optimized for around 3 MHz of operation, using both a SPICE model and FEM. The optimized parameters require a thick membrane and low gap heights to get maximum displacement without collapsing membrane during the operation. We used anodic bonding process to fabricate CMUT arrays. A conductive 100 μm silicon wafer is bonded to a glass wafer. Before the bonding process, the silicon wafer is thermally oxidized to create an insulating layer which prevents break down in the operation. Then, the cavities are formed on the insulating layer by a wet etch. The gap height is set to 100 nm. Meanwhile, the glass wafer is dry etched by 120 nm and the etched area is filled by gold evaporation to create the bottom electrodes. The wafers are dipped into piranha solution and bonding process is done afterwards. The fabricated CMUTs are tested in an oil tank. To eliminate the DC voltage which may cause charging problem in the operation, we tried to drive the CMUT array with large continuous wave signals at half of the operating frequency. We observed 1MPa peak to peak pressure with -23 dB second harmonic at the surface of the array (Fig. 1). The proposed design further extends the operation of CMUTs. Observing low harmonic distortions at high output pressure levels, without any charging problem, make CMUT a big candidate for high power applications. © 2011 IEEE.

The collapse mode of operation of capacitive micromachined ultrasonic transducers (CMUTs) was shown to be a very effective way to achieve high output pressures. However, no accurate analytical or equivalent circuit model exists for understanding the mechanics and limits of the collapse mode. In this work, we develop an equivalent nonlinear electrical circuit that can accurately simulate the mechanical behavior of a CMUT with given dimensions and mechanical parameters under any large or small signal electrical excitation, including the collapse mode. The static and dynamic deflections of a plate predicted from the model are compared with finite element simulations. The equivalent circuit model can estimate the static deflection and transient behavior of a CMUT plate to within 5% accuracy. The circuit model is in good agreement with experimental results of pulse excitation applied to fabricated CMUTs. The model is suitable as a powerful design and optimization tool for collapsed and uncollapsed CMUTs. The Scientific and Technological Research Council of Turkey (TUBITAK)

Cataloged from PDF version of article. A computationally efficient method for phase aberration correction in ultrasound imaging is presented. The method is based on time delay estimation via minimization of the sum of absolute differences between radio frequency samples of adjacent array elements. Effects of averaging estimated aberration patterns over scan angles, and truncation to a single bit wordlength are examined. Phase distortions due to near-field inhomogeneities are simulated using silicone rubber aberrators. Performance of the method is tested using experimental data. Simulation studies addressing different factors affecting efficiency of the method, such as the number of iterations, window length, and the number of scan angles used for averaging, are presented. Images of a standard resolution phantom are reconstructed and used for qualitative testing.

A multilead electrocardiography (ECG) data compression method is presented. First, a linear transform is applied to the standard ECG lead signals, which are highly correlated with each other. In this way a set of uncorrelated transform domain signals is obtained. Then, the resulting transform domain signals are compressed using various coding methods, including multirate signal processing and transform domain coding techniques. >

Date of Conference: 11-14 Oct. 2010 Collapse mode of operation of the capacitive mi-cromachined ultrasonic transducers (CMUTs) was shown to be a very effective way for achieving high output pressures. However, no accurate model exists for understanding the mechanics and limits of the collapse mode. In this work, we extend the analyses made for CMUTs working in uncollapsed mode to collapsed mode. We have developed an equivalent nonlinear electrical circuit that can accurately simulate the mechanical behavior of a CMUT under any large signal electrical excitation. The static and dynamic deflections of a membrane predicted by the model are compared with the finite element simulations. The equivalent circuit model can estimate the static deflection within 1% and the transient behavior of a CMUT membrane within 3% accuracy. The circuit model is also compared to experimental results of pulse excitation applied to fabricated collapse mode CMUTs. The model is suitable as a powerful design and optimization tool for the collapsed as well as the uncollapsed case of CMUTs. © 2010 IEEE.

Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT) and is particularly useful when the membranes are driven in the nonlinear regime. One major disadvantage of FEM is the excessive time required for simulation. Harmonic balance (HB) analysis, on the other hand, provides an accurate estimate of the steady-state response of nonlinear circuits very quickly. It is common to use Mason's equivalent circuit to model the mechanical section of CMUT. However, it is not appropriate to terminate Mason's mechanical LC section by a rigid piston's radiation impedance, especially for an immersed CMUT. We studied the membrane behavior using a transient FEM analysis and found out that for a wide range of harmonics around the series resonance, the membrane displacement can be modeled as a clamped radiator. We considered the root mean square of the velocity distribution on the membrane surface as the circuit variable rather than the average velocity. With this definition, the kinetic energy of the membrane mass is the same as that in the model. We derived the force and current equations for a clamped radiator and implemented them using a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared with the conventional model. Turkish Scientific and Research Council (TUBITAK) ASELSAN

Date of Conference: 11-14 Oct. 2010 Capacitive micromachined ultrasonic transducers (CMUTs) have demonstratedvarious advantages over piezoelectric transducers. However, current CMUT designsproduce low output pressures with high harmonic distortions. Optimizing thetransducer parameters requires an iterative solution and is too time consumingusing finite element (FEM) modelling tools. In this work, we present a method ofdesigning high output pressure CMUTs with relatively low distortion. We analyzethe behavior of a membrane under high voltage continuous wave operation using anonlinear electrical circuit model. The radiation impedance of an array ofCMUTs is accurately represented using an RLC circuit in the model. The maximummembrane swing without collapse is targeted in the transmit mode. Using SPICEsimulation of the parametric circuit model, we design the CMUT cell withoptimized parameters such as the membrane radius (a), thickness (tm),insulator thickness (ti) and gap height (tg). The modelalso predicts the amount of second harmonic at the output. To verify theaccuracy of the results, we built a FEM model with the same CMUT parameters. Thedesign starts by choosing ti for the given input voltage level.First, a is selected for the maximum radiation resistance of the array at theoperating frequency. Second, tm is found for the resonance at theinput frequency. Third, tg is chosen for the maximum membrane swing.Under this condition, a frequency shift in the resonant frequency occurs. Secondand third steps are repeated until convergence. This method results in a CMUTarray with a high output power and with low distortion. © 2010 IEEE.

Date of Conference: 20-23 Sept. 2009 Conference name: 2009 IEEE International Ultrasonics Symposium In this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3″ silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured. ©2009 IEEE.

Date of Conference: 20-23 Sept. 2009 Conference name: 2009 IEEE International Ultrasonics Symposium Capacitive micromachined ultrasonic transducers (cMUTs) offer wider bandwidth in air due to their low mechanical impedances. The impedance mismatch between the air and transducer decreases with the smaller device dimensions increasing the bandwidth at the expense of the degradation in the transmit power and the receive sensitivity. In this work, the bandwidth of cMUT is optimized by increasing its radiation resistance. This is done by properly choosing the size of cMUT membranes and their placement within an array. This selection not only brings an improvement in the transmitted power when it is used as a transmitter, but also improves the noise figure when it is used as a receiver. A further improvement in the noise figure is possible when the cells are clustered and connected to separate receivers. ©2009 IEEE.

Date of Conference: 16-19 May 2006 Conference name: OCEANS 2006 - Asia Pacific This paper is concerned with the design of an efficient, wideband and a wide-beamwidth resonant acoustic transducer for high frequency use. The general transducer structure which has two back-to-back quarter wave thick 1-3 composite ceramic elements at resonance frequency is introduced. The transducer is employed for both transmit and receive modes. Design of transmitting and receiving transducers are discussed. Several transfer functions are derived and their effective bandwidths are calculated. It is shown that the phase angle difference between two acoustic ports in receive mode can be processed at the electrical ports to maintain better throughput. The paper includes future works to be done. It is concluded that the proposed structure can be used for applications of spread spectrum schemes in underwater communications. ©2006 IEEE.

Cataloged from PDF version of article. We present a lumped element parametric model for the clamped circular membrane of a capacitive micromachined ultrasonic transducer (cMUT). The model incorporates an electrical port and two sets of acoustic ports, through which the cMUT couples to the medium. The modeling approach is based on matching a lumped element model and the mechanical impedance of the cMUT membrane at the resonance frequencies in vacuum. Very good agreement between finite element simulation results and model impedance is obtained. Equivalent circuit model parameters can be found from material properties and membrane dimensions without a need for finite element simulation.

Date of Conference: 30 June-3 July 2003 Conference Name: 8th IEEE Symposium on Computers and Communications, ISCC 2003 Several trunked private mobile radio (PMR) systems have been designed over the last decade, most of which have symmetric downlink and uplink channel capacities. These systems may not be spectrally efficient in case of group or broadcast-based voice and data calls, a common feature of PMR systems. We propose a new asymmetric PMR system comprising a wideband OFDM-based downlink and a narrowband uplink, which not only achieves a better spectral efficiency but also can support high bit rate multimedia applications. The system is shown to have high trunking efficiency since all users are assumed to use the pool of channels available in the wideband downlink. In this paper, we study the performance and capacity of a private mobile radio system using a digital audio broadcasting (DAB) downlink. In particular, we study the efficiency of such a system for voice calls using voice activity detection and statistical multiplexing. Moreover, we show that, the efficiency of the system can significantly increase, if the incoming calls, which can not find an available channel, are allowed to wait a certain amount of time before occupying a channel.

Date of Conference: 3-6 Oct. 1989 Conference name: Proceedings., IEEE Ultrasonics Symposium A conventional scanning acoustic microscope lens excites all the possible modes of acoustic waves in the solid structure under examination. The excited leaky modes contribute significantly to the high contrast obtained in the images, but because all such modes exist simultaneously, the interpretation of the images is not straightforward. A new lens geometry that can be used with acoustic microscopes to image layered solid structures is proposed. This new lens can efficiently focus the acoustic waves in only one of the Lamb wave modes of the layered solid. The images obtained are easy to interpret, and the subsurface sensitivity is high.

Date of Conference: 2-5 October 1988 Conference Name: 1988 IEEE Ultrasonics Symposium Excitation of acoustic wave modes in a layered solid is investigated when the solid is immersed in a liquid. This is done by computing the reflection coefficient of acoustic plane waves at the liquid-layered-solid interface. Then, a method to evaluate the sensitivity of various modes supported in the layered structure to subsurface defects is introduced. The method involves insonification of the layered structure with a conical wave whose axis coincides with the defect. Since all the rays in the conical wavefront hit the planar interface at the same angle, it is possible to excite a single kind of mode in the layer. By adjusting the angle of inclination of the cone, it is possible to excite the modes selectively. Since the conical waves converge to a line focus at the cone axis, the excited mode will focus on the defect. By recording the reflected signal amplitude as the cone angle is varied, a curve is obtained from which it is possible to conclude the sensitivity of various modes to the subsurface flaw. The results of such measurements indicated that the generalized Lamb wave modes are more sensitive to subsurface defects than the Rayleigh waves. An imaging system which makes use of focused Lamb waves was built and the system produced images of very small subsurface defects.

Date of Conference: 4-7 December 1990 Conference Name: 1990 IEEE Ultrasonics Symposium The Lamb wave lens is a type of lens that can replace the conventional lens of an acoustic microscope for some applications. When it insonifies a layered solid structure, it excites a single mode in the structure, provided that the frequency is properly adjusted. Measuring the return signal as a function of frequency results in a characteristic curve showing the excited modes. Since the number of excited modes, the frequencies at which they are excited, and the efficiency of excitation are highly dependent on the elastic and physical parameters of the layered solid, a characterization method emerges. The authors describe this characterization method for the layered structures using the Lamb wave lens. Theoretical and experimental results are presented.

Date of Conference: 7-10 Sept. 1997 Conference name: Computers in Cardiology 1997 In this paper, the morphological information redundancy in standard 12 lead ECG channels is studied. Study is based on decomposing the ECG channels into orthogonal channels by an SVD based algorithm and then reconstructing them. Then 7 of 8 independently recorded ECG channels are decomposed and the missing channel is reconstructed from these orthogonal channels. Thus the unique morphological information content of each ECG channel is assessed through the loss of clinical information in the reconstructed signal. A comparison of the clinical parameters measured from the reconstructed and original ECG is reported.

Chapter 8 Radio provides an important technology for emergency preparedness and emergency response. It not only offers rapid communication for emergency workers within the disaster area, but also affords the means of disseminating early warning of disasters and advice on how to respond to the public, and a means of monitoring of events and follow-up work in the wake of a disaster. Effective telecommunications are thus a vital part of emergency planning and response (Cate 1994, DHA 1995, Mulilis 1995, Zimmerman 1997). How well radio-based systems work, however, depends on a range of factors, including their geographic coverage, their population coverage, their field strength and their received power level. All these factors are highly dependent upon terrain. Moreover, due to the growing demands on the radio-frequency spectrum, there is a need to improve spectrum management techniques. The increase in the shared use of spectrum among administrations requires the use of more complex analysis methods. The efficient solution of spectrum managementproblems depends upon data storage and analysis capabilities, andconsequently requires the application of computer-aided techniques for data management and frequency assignment.

Cataloged from PDF version of article. The frequency planning of the VHF and UHF broadcasts in Turkey is described. This planning is done with the aid of computer databases and digital terrain map. The frequency offset is applied whenever applicable to increase the channel capacity. The offset assignment is done through Simulated Annealing algorithm. The international rules and regulations concerning Turkey are also considered.

Chapter 4 The purpose of the first Lemons-Quate acoustic microscope(1) was to image the surfaces of materials or biological cells with a high resolution. Unfortunately, competition with the optical microscope was only partially successful due to the high degree of absorption in the liquid-coupling medium at high frequencies. Increasing the resolution beyond optical limits was possible with the use of hot water(2) or cryogenic liquids,(3) at the cost of operational difficulty and system complexity. Meanwhile it was shown that the acoustic microscope can generate information that has no counterpart in the optical world.(4) The presence of leaky waves resulted in an interference mechanism known as V(z) curves. The V(z) method involves recording the reflected signal amplitude from an acoustic lens as a function of distance between the lens and the object. This recorded signal is shown to depend on elastic parameters of the object material. After underlying processes are well understood, new lens geometries or signal-processing electronics are designed to emphasize the advantage of the acoustic lens. In any case, the aim has been to increase the quantitative characterization ability of the microscope.

Date of Conference: 31 Oct.-3 Nov. 1994 Conference name: 1994 Proceedings of IEEE Ultrasonics Symposium Lamb wave lenses with conical refracting surfaces are fabricated for use at 400 MHz and 1 GHz. The conical surfaces are ground and polished with mechanical means and they are sufficiently smooth for the frequencies of interest. The wide bandwidth of transducers allow frequency tuning necessary for Lamb wave lenses. The fabricated lenses show the expected V(Z) performance. At high frequencies the attenuation in the coupling medium can be very high, but due to the smaller wavelength the resolution is better and defocus distance can be reduced. Inherently higher leaky wave sensitivity of Lamb wave lens enables a good V(Z) characterization ability at higher frequencies as compared to the conventional spherical lens. Subsurface imaging with these Lamb wave lenses gives satisfactory results for layered structures. Chosen object has leaky wave modes within the angular coverage of the lens. The images exhibit a resolution close to the diffraction limit. Experimental V(Z) curves obtained with these lenses along with images are presented.

The influence of missing array elements on aberration correction based on time delay estimation using radio frequency signals of neighboring elements is experimentally inves-tigated. Normalized cross correlation and sum of absolute differences are employed as the cost functions in aberration estimation. Their performance is tested through various measurements using radio frequency data acquired with a 3.3 MHz, 64-element phased array. Variation of cost functions and phase estimation error are obtained for different combinations of number of missing elements, amount of aberration, and noise level. For a particular combination of these parameters, a set of B-scan images is reconstructed and presented to examine the effects of residual phase errors on image quality. © 1994 IEEE

Cataloged from PDF version of article. For phased-array ultrasound imaging, alternative beamforming techniques and their VLSI circuits are studied to form a fully digital receive frontad hardware. In order to increase the timiig accuracy in beamforming, a computationally efficient interpolation scheme to increase the sampling rate is examined. For adaptive beamforming, a phase aberration correction method with very low computational complexity is described. Image quality performance of the method is examined by processing the non-aberrated and aberrated phased-array experimental data sets of an ultrasound resolution phantom. A digital beamforming scheme based on receive focusing at the raster focal points is examined. The sector images of the resolution phantom, reconstructed from the phased-array experimental data by beamforming at the radial and raster focal points, are presented for comparison of the image resolution performances of the two beamforming schemes. VLSI circuits and their implementations for the proposed techniques are presented.

Date of Conference: 31 October-3 November 1993 Conference Name: Ultrasonics Symposium, IEEE 1993 A new directional acoustic lens is introduced. The geometry is very similar to the line-focus lens except the lens cavity, which is shaped as a groove with flat-bottom V cross section. The slanted planar edges of the groove are inclined in order to generate waves incident on the object surface at a critical angle. Hence, the edges of the groove act like two wedge transducers facing each other. The cross section of the lens is the same as that of the Lamb Wave Lens. Therefore, it enjoys the same sensitivity to surface wave excitations. On the other hand, since the cross section remains the same along one of the lateral directions, it has directional properties very similar to that of the Line Focus Beam Lens. The waves normally incident on the object surface generated from the flat-bottom, interfere with those at the critical angle, giving rise to a V(Z) effect. Calculated responses of the lens are presented for silicon (001) surface as a function of crystal orientation. The calculated curves are compared with measurement results. The leaky wave velocities are extracted from the measurement results using the conventional FFT algorithm. A new model based algorithm is proposed for extracting the velocity information from V(Z) data.

In a conventional scanning acoustic microscope the excited leaky modes contributes significantly to the high contrast obtained in the images. However, all such modes exist simultaneously, and the interpretation of the images is not straightforward, especially in layered media. A new lens geometry is proposed that can be used with acoustic microscopes to image layered solid structures. This new lens can focus the acoustic waves in only one of the Lamb wave modes of the layered solid with a high efficiency. V(Z) curves obtained from this lens are more sensitive to material properties compared to that obtained from conventional lens. Measuring the return signal as a function of frequency results in another characteristic curve, V(f). The Lamb wave lens and the associated characterization methods for the layered structures are described. The results presented show that the Lamb wave lens is at least an order of magnitude more sensitive than the conventional lens and can differentiate between a good bond and a disbond in a layered structure easily. © 1992 IEEE

Ultrasonic axicons generate waves which focus on a line. They are used in various imaging applications as hulk wave focusing devices with a very long depth of focus. A new type of conical axicon is introduced. It consists of a concave parabolic surface immersed in a liquid medium and insonified obliquely by wavefronts generated by a plane transducer. The parabolic cylinder can be approximated by a portion of a circular cylinder without losing significantly in the focus- ing performance of the axicon. It is also shown that conical axicons can be used to excite surface waves provided that the cone angle of the axicon coincides with the Rayleigh critical angle of the liquid-solid in- terface. The generated surface waves focus into a diffraction-limited spot. This new surface wave focusing scheme is easy to use and has a conversion efficiency and sensitivity far better than other existing tech- niques.

Date of Conference: 17-19 Nov. 1986 Conference name: Ultrasonics, IEEE Symposium (IUS) 1986 The resolving power and detection ability of the focused surface acoustic wave (SAW) imaging modality is investigated in this paper. In this mode of imaging, conical bulk acoustic waves are used to generate and focus leaky surface acoustic waves on smooth surfaces of materials. Imaging systems built using this technique has diffraction limited focusing property. An imaging system using this focusing principle has been built, operating at 1.5 and 20 MHz. A slow mechanical scanning system controlled by a personal computer scans the surface of the object, and the data is acquired by the computer to generate a color or a black and white image on its graphic screen. The results of the initial experiments show that the inaging system is very sensitive to the grain structure and possible residual stresses on the surface of the object. It can resolve subsurface gratings of spacing less than a SAW wavelength very close to surface. The imaging system is inherently zero background, providing a high sensitivity not found in similar systems.

Date of Conference: 14-16 October 1987 Conference Name: 1987 IEEE Ultrasonics Symposium Focusing of surface acoustic waves on material surfaces was achieved by axicons in liquid immersion. The system consisted of a plane acoustic wave generator obliquely insonifying a cylindrical mirror placed perpendicularly on the surface to be examined. A similar axicon can be implemented with a solid cylindrical structure which does not require an immersion liquid. The lower cap of a solid cylinder is placed on the surface of the solid object, while the upper cap, where a shear-wave transducer is placed, is inclined with respect to the axis of the cylinder. Shear waves generated by the transducer are reflected from the free cylindrical surface of the solid. The reflected waves are incident on the bottom surface at a critical angle and couple to the object surface as surface waves to converge to a diffraction limited focus. The cylindrical solid material must be chosen from low-shear-wave-velocity materials. The critical angle determines the inclination of the top surface; it is found from the solution of reflection problem for shear waves at a solid-solid interface with a slippery boundary condition. The focusing system is implemented successfully with lucite as the solid and aluminum as the object material.

Date of Conference: 17-19 Nov. 1986 Conference name: IEEE 1986 Ultrasonics Symposium It is possible to define a V(Z) function for the surface acoustic wave (SAW) focusing system for a class of reflectors similar to that defined in acoustic microscopy. In this case V(Z) is a function which relates the transducer output voltage to the distance between the focal point and the reflection line. It is possible to express V(Z) in an analytical expression using angular spectrum techniques. V(Z) is measured for straight edge reflectors forming various angles with the surface. V(2) of straight step reflectors are investigated. These curves show a dependence on the type of reflector. The experiments are performed at 1.5 MHz in conjunction with a computerized data acquisition system. The phase information is also recorded in all these measurements in addition to the amplitude. It is possible to get an inversion of V(Z) to get the SAW reflection coefficient at a straight discontinuity.

A new type of imaging technique is presented which incorporates focussed surface acoustic waves. Surface acoustic waves are generated on the surface to be imaged by use of conical wavefronts. The conical wavefronts are obtained through the reflection of planar wavefronts from a parabolic cylindrical mirror. An imaging system is built which uses focussed surface acoustic waves in a mechanical scanning arrangement controlled by a computer. The resulting images show subsurface features with diffraction limited resolution.

Axicons are generators of waves which focus on a line. They are used in various imaging and nondestructive testing applications as bulk wave focusing devices with a very long depth of focus. In this letter, a new type of conical axicon is introduced and it is shown that this axicon, immersed in a liquid, insonifying a plane solid surface can be used to excite surface waves on the solid surface provided that the cone angle of the axicon coincides with the Rayleigh critical angle of the liquid‐solid interface. The generated surface waves focus into a diffraction‐limited spot. This new surface wave focusing scheme is easy to use, has a conversion efficiency and sensitivity far better than other existing techniques.

Date of Conference: 31 October-3 November 1994 Conference Name: 1994 IEEE Ultrasonics Symposium In this paper a new lens design is proposed for characterization of layered materials. Lamb wave lens employs Lamb waves for this purpose since these waves propagate along interfaces. However, below cut-off angle, the critical angles of Lamb wave modes are low and the generated V(z) curves have small number of oscillations, which in turn causes measurement difficulties and accuracy degradation. Bi-angular lens described in this paper, generates an extra obliquely incident wave, instead of normally incident beam, in order to provide the reference specular reflection. Simulation results as well as experimental results are presented and it is shown that a high sensitivity can be obtained by using this new lens.