Browsing by Author "Atalar, Abdullah"

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Open Access
A 128-bit microprocessor compatible programmable correlator chip for use in synchronous communication
(IEEE, 1989) Ungan, İ. Enis; Topçu, Satılmış; Atalar, Abdullah
A single-chip microprocessor-compatible 128-b correlator is designed and implemented in a 3-μm M2CMOS process. Full-custom design techniques are applied to achieve the best tradeoff among chip size, speed, and power consumption. The chip is placed in a microprocessor-based portable data terminal using HF radio communication. It marks the beginning of a synchronous data stream received from the very noisy channel by detecing the synchronization (sync) word. The sync word can be detected for either inverted or noninverted input data streams. Two chips can be cascaded to make a 256-b correlator. The chip is fully programmable by a microprocessor to set the number of tolerable errors in detection and to select the bits of the 128-b (or 256-b) data stream to be used in the correlation.
Open Access
Accurate and process-tolerant resistive load
(IEEE, 2020) Sütbaş, Batuhan; Özbay, Ekmel; Atalar, Abdullah
Resistive terminations cannot preserve high-quality matching at high frequencies due to the parasitic effects of the nonideal resistor. Moreover, resistance values of the termination resistors in integrated circuits are subject to process variations. Therefore, it is difficult to obtain accurate and process-tolerant terminations that are crucial for high performance in microwave circuits. We propose a new resistive network that compensates for the high-frequency parasitic effects of the resistors to improve the bandwidth of the termination. In addition to maintaining accuracy, the presented network provides tolerance to variation in the resistor values. The accuracy and tolerance of the proposed structure is analytically shown and experimentally verified by three test structures at the X-band fabricated on a GaN technology. The experimental results show that a small size and wideband 50-Ω load with a return loss better than 25 dB can be obtained, while the resistor value changes ±30%.
Open Access
Accurate isolation networks in quadrature couplers and power dividers
(IEEE, 2021) Sütbaş, Batuhan; Özbay, Ekmel; Atalar, Abdullah
When quadrature couplers and power dividers are implemented in integrated circuits, accurate isolation networks can not be realized due to the nonideal resistors and the process variations. We present an isolation network design technique which cancels the resistor parasitic effects and also increases the tolerance to variations in the resistance values. A Lange coupler and a power divider are designed at Ka-band using the proposed accurate and process-tolerant isolation networks. The improvement is analytically shown and empirically verified with our in-house GaN-based microstrip MMIC process. For the coupler, the measured return losses and isolation are better than 20 dB from DC to 40 GHz. The power divider achieves 20 dB return losses and isolation in a fractional bandwidth of 50%. Both devices maintain 20 dB performance even when the variation in sheet resistance is as high as 30%.
Open Access
Acoustic microscopy with mechanical scanning—A review
(IEEE, 1979-08) Quate, C. F.; Atalar, Abdullah; Wickramasinghe, H. K.
Acoustic waves in liquids are known to have wavelengths comparable to that of visible light if the frequency is in the gigahertz range. The phenomena of Brillouin scattering in liquids is based on such waves. In helium near 2 K acoustic waves with a wavelength of 2000 Å were studied some ten years ago at UCLA. It follows from these observations that an imaging system based on acoustic radiation with a resolving power competitive with the optical microscope is within reach if an ideal lens free from aberrations could be found. Such a lens, which was so elusive at the beginning, is now a simple device and it is the basic component in the acoustic microscope that forms the basis for this review. In this article we will establish the characteristic properties of this new instrument. We will review some of the simple properties of acoustic waves and show how a single spherical surface formed at a solid liquid interface can serve as this ideal lens free from aberrations and capable of producing diffraction limited beams. When this is incorporated into a mechanical scanning system and excited with acoustic frequencies in the microwave range images can be recorded with acoustic wavelengths equal to the wavelength of visible light. We will present images that show the elastic properties of specimens selected from the fields of material science, integrated circuits, and cell biology. The information content in these images will often exceed that of the optical micrographs. In the reflection mode we illuminate the smooth surface of a crystalline material with a highly convergent acoustic beam. The reflected field is perturbed in a unique way that is determined by the elastic properties of the reflecting surface and it shows up in the phase of the reflected acoustic field. There is a distinct and characteristic response at the output when the spacing between the object and the lens is varied. This behavior in the acoustic ieflection microscope provides a rather simple and direct means for monitoring the elastic parameters of a solid surface. It is easy to distinguish between different materials, to determine the layer thickness, and to display variations in the elastic constants on a microscopic scale. These features lead us to believe there is a promising future for the field of acoustic microscopy.
Open Access
Acoustic microscopy: resolution of subcellular detail
(National Academy of Sciences, 1979-07) Johnston, R. N.; Atalar, Abdullah; Heiserman, J.; Jipson, V.; Quate, C. F.
Recent advances now permit the use of scanning acoustic microscopy for the analysis of subcellular components. By sequential viewing of identified fixed cells with acoustic, light, and electron microscopy, we have established that the acoustic microscope can readily detect such features as nuclei and nucleoli, mitochondria, and actin cables. Under optimal conditions, images can even be obtained of filopodia, slender projections of the cell surface that are approximately 0.1-0.2 micron in diameter. Small objects separated by as little as 0.5-0.7 micron can successfully be resolved. Three aspects of the acoustic micrographs prepared in this preliminary survey seem especially prominent. These are, first, the extraordinary level of acoustic contrast that can differentiate the various cytoplasmic organelles, even in regions of very thin cytoplasm; second, the reversals in acoustic contrast that occur when altering the plane of focus; and third, the sensitivity of the acoustic response to overall cytoplasmic thickness. The acoustic microscope uses a novel source of contrast that is based on local mechanical properties. In addition, it can provide a degree of resolution that is comparable to that of the light microscope.
Open Access
Acoustical tuning of CMUT receiver arrays
(IEEE, 2016) Taşdelen, Akif Sinan; Atalar, Abdullah; Enhoş, Kerem; Köymen, Hayrettin
Cell placement in an element and structural modifications on the array baffle significantly change the bandwidth, band shape and signal to noise ratio of a CMUT receiver array. In this paper, optimum receiver performance tailoring by means of cell placement, cell size variation and use of dummy cells in the array elements is discussed. The performance of the array is modified acoustically at the acoustic port of the elements.
Open Access
Analysis and design of an interdigital cantilever as a displacement sensor
(A I P Publishing LLC, 1998-06-15) Yaralioglu, G. G.; Atalar, Abdullah; Manalis, S. R.; Quate, C. F.
The interdigital (ID) cantilever with two sets of interleaving fingers is an alternative to the conventional cantilever used in the atomic force microscope (AFM). In this paper we present a detailed analysis of the interdigital cantilever and its use as a sensor for the AFM. In this study, we combine finite element analysis with diffraction theory to simulate the mechanically induced optical response of the ID. This model is used to compare this system with the optical lever detector as used in conventional instruments by analyzing the ratio of signal to noise and overall performance. We find that optical detection of the cantilever motion with interdigital fingers has two advantages. When used in conjunction with arrays of cantilevers it is far easier to align. More importantly, it is immune to laser pointing noise and thermally excited mechanical vibrations and this improves the sensitivity as compared to the optical lever.
Open Access
Analysis of mutual acoustic coupling in CMUT arrays using an accurate lumped element nonlinear equivalent circuit model
(2012) Oğuz, H.Kağan; Atalar, Abdullah; Köymen, Hayrettin
We use an accurate nonlinear equivalent circuit model to analyze CMUT arrays with multiple cells, where every cell in the array is coupled to other cells at their acoustic terminals through a mutual radiation impedance matrix. We get results comparable to finite element analysis accuracy. Hence, the analysis of a large array becomes a circuit theory problem and can be scrutinized with circuit simulators. We study the mutual acoustic interactions that arise through the immersion medium due to the influence of the generated pressure field by each cell on the others. We compare the performance of different 1D cMUT arrays, where each element is half-wavelength wide and 10 and 20 wavelengths long at the resonance frequency of a single cell. © 2012 IEEE.
Open Access
Analysis of tip-sample interaction in tapping-mode atomic force microscope using an electrical circuit simulator
(AIP Publishing, 2001-05-07) Sahin, O.; Atalar, Abdullah
We present a mechanical model for the atomic force microscope tip tapping on a sample. The model treats the tip as a forced oscillator and the sample as an elasticmaterial with adhesiveproperties. It is possible to transform the model into an electrical circuit, which offers a way of simulating the problem with an electrical circuit simulator. Also, the model predicts the energy dissipation during the tip–sample interaction. We briefly discuss the model and give some simulation results to promote an understanding of energy dissipation in a tapping mode.
Open Access
Analytic modeling of loss and cross-coupling in capacitive micromachined ultrasonic transducers
(IEEE, Piscataway, NJ, United States, 1998) Bozkurt, A.; Degertekin, F. L.; Atalar, Abdullah; Khuri-Yakub, B. T.
The structural loss mechanism of capacitive micromachined ultrasonic transducer (cMUT) is investigated using finite element analysis and the normal mode theory. A single micromachined transducer membrane on an infinite silicon substrate is simulated by incorporating absorbing boundary conditions in the finite element method. This enables direct evaluation of the mechanical impedance of the membrane. Furthermore, the field distribution along the thickness of the silicon substrate due to outward radiating wave modes is obtained. The normal mode theory is applied to extract the contributions of different wave modes to the complicated field distributions. It is found that, the lowest order Lamb wave modes are responsible for the loss. Evaluation of absolute and relative power losses due to individual modes indicate that the lowest order anti-symmetric (A0) mode is the dominant radial mode in agreement with experimental measurements. The results of the analysis are used to derive a detailed equivalent circuit model of a cMUT with structural loss.
Open Access
An analytical approach to the design of multiple mode rectangular cavity waveguide filters
(Institute of Electrical and Electronics Engineers Inc., 2017) Kelleci, C.; Atalar, Abdullah
The multiple mode rectangular cavity structure with square corner cuts is revisited. An attempt to predict the physical dimensions of the cavity for dual mode second-order and triple mode third-order filters is made. Analytic expressions are formed to be used in the design process. The classical triple mode cavity filter structure is altered to give a finite frequency transmission zero either in the lower or upper sideband of the center frequency. The concept is illustrated with example designs. A novel additive manufacturing technique is used to fabricate a selected filter structure. The experimental results are in agreement with the expectations.
Open Access
An angular‐spectrum approach to contrast in reflection acoustic microscopy
(A I P Publishing, 1978-11) Atalar, Abdullah
The scanning acoustic microscope in the reflection mode has proved to be a rather simple and direct means for monitoring the elastic properties of a solid surface. When smooth surfaces of crystalline material are examined in a liquid with a highly convergent sound beam they exhibit a distinct response. This characteristic response, which can be treated as a ’’signature’’, is obtained by recording the output of the microscope as the spacing between the acoustic lens and the object is varied. An angular‐spectrum approach is used to derive an expression for this output in terms of the reflectance function. This function has an angular dependence determined by the bulk constants of the material itself. The expression resulting from this treatment can be used to explain the source of contrast in acoustic images.
Open Access
Anisotropy sensitivity of an acoustic lens with slit aperture
(IEEE, 1993) Atalar, Abdullah; Ishikawa, I.; Ogura, Y.; Tomita, K.
A conventional spherical acoustic lens is modified by restricting its aperture in the form of a slit to provide directional sensitivity. The spacing between the two parallel absorbing sheets forming the slit is adjustable to obtain varying slit widths. The resulting lens can be used in conjunction with V(Z) method to obtain leaky wave velocities of the sample under investigation as a function of direction. The theoretical V(Z) analysis of the lens involves a two-dimensional integral rather than one-dimensional integral of the conventional lens. Single crystal anisotropic materials are chosen as test samples. Reflection coefficients for anisotropic single crystals of given surface cut and orientation are calculated. Numerically evaluated V(Z) curves are used to deduce the surface wave velocity of the object for the given orientation. This is compared with the surface wave velocity directly calculated from the elastic parameters of the object. Results show the compromise between signal-to-noise ratio and angular resolution as the slit width is varied. V(Z) measurement results of a slitted lens are presented to be compared with calculated curves. The new lens is used to measure the acoustic velocity on the (001) surface of GaAs along varying directions with differing slit widths.
Open Access
Application of asymptotic waveform evaluation for time-domain analysis of nonlinear circuits
(Taylor & Francis, 1997) Topçu, S.; Atalar, Abdullah; Tan, M. A.
A method is described to exploit asymptotic waveform evaluation (AWE) in the time-domain analysis of nonlinear circuits by using SPICE models for nonlinear devices such as diodes, transistors, etc. Although AWE has been used for linearized circuits only, the aim is to enhance the accuracy of the simulation while preserving the computational efficiency obtained with AWE and to eliminate the piecewise-linear modelling problem. Practical examples are given to illustrate significant improvements in accuracy. For circuits containing weakly nonlinear devices, it is demonstrated that this method is typically at least one order of magnitude faster than SPICE.
Open Access
Automated parallel high-speed atomic force microscopy
(A I P Publishing LLC, 1998-05-04) Minne, S. C.; Yaralioglu, G.; Manalis, S. R.; Adams, J. D.; Zesch, J.; Atalar, Abdullah; Quate, C. F.
An expandable system has been developed to operate multiple probes for the atomic force microscope in parallel at high speeds. The combined improvements from parallelism and enhanced tip speed in this system represent an increase in throughput by over two orders of magnitude. A modular cantilever design has been replicated to produce an array of 50 cantilevers with a 200 μm pitch. This design contains a dedicated integrated sensor and integrated actuator where the cells can be repeated indefinitely. Electrical shielding within the array virtually eliminates coupling between the actuators and sensors. The reduced coupling simplifies the control electronics, facilitating the design of a computer system to automate the parallel high-speed arrays. This automated system has been applied to four cantilevers within the array of 50 cantilevers, with a 20 kHz bandwidth and a noise level of less than 50 Å. For typical samples, this bandwidth allows us to scan the probes at 4 mm/s.
Open Access
A backscattering formula for acoustic transducers
(A I P Publishing, 1980-06) Atalar, Abdullah
A backscattering formula for a single transducer geometry is derived. It expresses the output voltage of the transducer in terms of the angular spectra of scalar and vector acoustic potentials on a plane. The formulation is suitable for acoustic problems involving wide angular spectrum beams. The derived formula gives a simple expression for the problem of a circular transducer facing a plane reflector. The output voltage of a transducer receiving the backscattered waves from an arbitrary size spherical flaw is also presented.
Open Access
Bandwidth improvement in a cMUT array with mixed sized elements
(IEEE, 2005-09) Bayram, Can; Olcum, Selim; Şenlik, Muhammed N.; Atalar, Abdullah
A capacitive micromachined ultrasonic transducer (cMUT) is typically fabricated by concatenation of several cMUT cells with identical physical dimensions. If the membrane thickness is kept fixed, the radius of the cMUT determines the center frequency of operation. A smaller radius implies a greater center frequency. Therefore, it should be possible to put cMUTs with different sizes in parallel to get a larger bandwidth at the expense of gain. In this study, we investigate the optimization of the bandwidth characteristics of a cMUT by using mixed size cells. We designed two mixed size cMUT arrays with a predicted optimized fractional bandwidth value of about 155% at 5.4 MHz, and 146% at 8.8 MHz. These values are about 55% and 58% better than what can be achieved with a uniform size array at the corresponding center frequencies. There is almost no loss in the gain bandwidth product when two different sized cMUTs are used in parallel. There is about 9% increase in gain bandwidth product when three different sized cMUTs are used in parallel. It is shown, in this study, that gain bandwidth product and bandwidth can be enhanced by use of mixed size cMUT cells. © 2005 IEEE.
Open Access
Bandwidth, power and noise considerations in airborne cMUTs
(IEEE, 2009-09) Şenlik, Muhammed N.; Olcum, Selim; Köymen, Hayrettin; Atalar, Abdullah
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.
Open Access
Beam steering in a half-frequency driven airborne CMUT transmitter array
(IEEE Computer Society, 2019) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
An airborne Capacitive Micromachined Ultrasonic Transducer (CMUT) transmit array was designed using electromechanical modelling for unbiased airborne operation. The array elements are designed for maximum swing at 10V p-p unbiased drive, whereas conventional practice is to bias CMUT close to the collapsed voltage to achieve higher swing. The devices were fabricated using a customized single photolithographic process with a combination of wet and dry etching. The wafer level fabrication enabled the usage of 2x2 and 3x3 arrays. Driving CMUTs in an unbiased mode at half frequency drives the ‘static pressure’ depressed silicon membrane at a larger swing without letting it collapse. The 2x2 array displays 3.375 kHz bandwidth when characterized in air. The phase and amplitude differences due to the dispersion of resonance frequencies of the elements are compensated for beamformed and beamsteered airborne operation.
Open Access
Bi-angular lens for material characterization
(IEEE, 1994) Yaralıoğlu, Göksen Göksenin; Atalar, Abdullah; Köymen, Hayrettin
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.