# A. AtalarBilkent University · Department of Electrical & Electronic Engineering

A. Atalar

PhD 1978 Stanford University

## About

195

Publications

32,631

Reads

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5,430

Citations

Introduction

Additional affiliations

September 1995 - August 1996

September 1986 - present

April 1980 - August 1986

Education

January 1976 - June 1978

September 1974 - January 1976

September 1970 - June 1974

## Publications

Publications (195)

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 v...

A 2×2 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 sin...

Some preferred embodiments include a microphone system for receiving sound waves, the microphone including a back plate, a radiation plate, first and second electrodes, first and second insulator layers, a power source and a microphone controller. The radiation plate is clamped to the back plate so that there is a hermetically sealed regular convex...

Some preferred embodiments include a microphone system for receiving sound waves, the microphone including a back plate, a radiation plate, first and second electrodes, first and second insulator layers, a power source and a microphone controller. The radiation plate is clamped to the back plate so that there is a hermetically sealed circular gap b...

In light of globalization in the twenty-first century, businesses are seeking employees with skills that would make them more competitive in the international arena.

An airborne Capacitive Micromachined Ultrasonic Transducer (CMUT) transmit array was designed using electro-mechanical 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 devic...

Giriş Bilimsel araştırmaların sonuçlarının ancak ciddi bir hakemlik süreci sonunda bilimsel dergilerde yayınlaması 17. Yüzyıldan beri bilimin gelişmesinde en önemli mihenk taşlarından biri olmuştur. Tipik olarak ciddi bir bilimsel bir dergiye sunulan bir çalışmayı dergi editörü ilk kalite testinden geçirdikten sonra değerlendirmeleri için konunun ü...

Some preferred embodiments include a microphone system for receiving sound waves, the microphone including a back plate, a radiation plate, first and second electrodes, first and second insulator layers, a power source and a microphone controller. The radiation plate is clamped to the back plate so that there is a hermetically sealed circular gap b...

This paper presents two types of modified CPW Wilkinson power dividers at X-band using GaN MMIC technology on a SiC substrate. Lumped element equivalents of the transmission line arms are used and they are capacitively loaded to achieve a reduced circuit size of λ/14×λ/8. A symmetrical series RLC circuit in the isolation network is used to compensa...

We propose an airborne collapse capacitive micromachined ultrasonic transducer (CMUT) as a practical viable ultrasound transducer capable of providing a stable performance at the off-resonance frequencies. Traditional practice is to bias the CMUT plate close to collapse voltage to achieve high coupling coefficient and sense the incoming ultrasound...

Noise figure expression of a balanced amplifier built with lossy divider and combiner and two imperfectly noise-matched component amplifiers is derived analytically using the method of noise waves in terms of the noise parameters of the amplifiers. We present analytical expressions for the three noise parameters of the balanced amplifier. We show t...

An energy-consistent lumped element equivalent circuit model for chargedcircular 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 measurem...

Balanced amplifiers suffer from the noise figure performance in comparison with a stand-alone amplifier even with an ideal input divider. The noise parameters degrade further with an imperfect divider. We present exact and approximate analytical results for the noise figure and noise parameters of the balanced amplifier in divider topology. Measure...

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 giv...

We introduce the large-signal and small-signal equivalent circuit models for a capacitive micromachined ultrasonic transducer (CMUT) cell, which has radiating plates on both sides. We present the diffraction coefficient of baffled and un-baffled CMUT cells. We show that the substrate can be modeled as a very thick radiating plate on one side, which...

A polynomial approximation is proposed for the mutual acoustic impedance
between uncollapsed capacitive micromachined ultrasonic transducer (CMUT) cells
with different radii in an infinite rigid baffle. The resulting approximation
is employed in simulating CMUTs with a circuit model. A very good agreement is
obtained with the corresponding finite e...

We propose an isolation network to simultaneously improve the input return loss, output return loss and isolation of the Wilkinson power divider in a wide bandwidth. The required even mode and odd mode reflection coefficients of the isolation network are calculated. Constructed even and odd mode circuits are combined to give the desired isolation n...

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...

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
targe...

This study focuses on modeling collapsed mode operation of CMUT arrays, and obtaining a small signal lumped element model for collapsed mode operation. Having the large signal model for single CMUT from previous studies, the mutual radiation impedance is presented for the collapsed mode, and a large signal model for a CMUT array is obtained for sim...

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...

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...

We analyze the bandwidth capability of a divider with a series RLC circuit at the isolation arm. Analytical expressions for optimal component values are given. Bandwidth limiting effect of the pad capacitances of the chip resistors is analyzed. These parasitic capacitors are compensated by the proposed structure. Broadband characteristic of the new...

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 col...

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 col...

We analyze the microstrip directional couplers with
a special focus on the phase difference between the coupled and
isolated ports. The analysis uses the even-odd mode decomposition
technique and network theory. The results show that the phase difference
between the signals at the coupled port and the isolated
port is close to pi in a very wide ban...

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 pat- terns over scan angles, and truncation to a...

We present Bilkent reconfigurable computer (BilRC), a new coarse-grained reconfigurable architecture
(CGRA) employing an execution-triggering mechanism. A control data flow graph language is presented for mapping the applications to BilRC. The flexibility of the architecture and the computation model are validated by mapping several real-world appl...

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 c...

In this work, we propose a new approach to obtain wider bandwidth for an airborne capacitive micromachined ultrasonic transducer (CMUT) cell without sacrificing sensitivity. This is done through employing the stiffening effect created by the atmospheric pressure. We use an equivalent lumped element circuit to model both electrical and mechanical pr...

We have recently developed a circuit theory based method to analyze large CMUT arrays and shown that mutual acoustic interactions significantly influence the transducer performance. We connect each cell in the array to a radiation impedance matrix that contains the mutual radiation impedance between every pair of cells, in addition to their self ra...

In the original publication of the paper [1], the funding source was inadvertently omitted from the footnote on page 1016. The footnote should have read "Manuscript received November 20, 2012; accepted February 14, 2013. This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under project grant 110E216. A....

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 actin...

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 theor...

We present a design method for airborne capacitive micromachined ultrasonic transducers (CMVT). We use an equivalent lumped element circuit to model both electrical and mechanical properties of CMUT and analyze it in frequency domain using harmonic balance approach. We use this method to design CMUTs for large transmitted power generation at low dr...

A very accurate equivalent circuit model for circular CMUT cell is recently proposed. As a consequence of this model, the operating characteristics of the cell is derived and presented as a biasing chart for CMUTs. We discuss these operating characteristics and show how the coupling coefficient can be derived from the model. We present an analysis...

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 CM...

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...

The radiation impedance of a capacitive micromachined ultrasonic transducer (CMUT) array is a critical parameter to achieve high performance. In this paper, we present a calculation of the radiation impedance of collapsed, clamped, circular CMUTs both analytically and using finite element method (FEM) simulations. First, we model the radiation impe...

Capacitive micromachined ultrasonic transducers (CMUTs) have been introduced as a promising technology for ultrasound imaging and therapeutic ultrasound applications which require high transmitted pressures for increased penetration, high signal-to-noise ratio, and fast heating. However, output power limitation of CMUTs compared with piezoelectrics...

Radiation impedance is one of the important parameters in designing efficient and wideband capacitive micro-machined ultrasonic transducer (CMUT) arrays. It determines how much acoustical power is generated in the surrounding medium given the membrane motion. Recently, considerable effort has been put to characterize the radiation impedance of CMUT...

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 han...

Collapse and deep-collapse mode of operations have boosted the pressure outputs of capacitive micromachined ultrasonic transducers (CMUTs) considerably. In this work, we demonstrate a CMUT element operating in the deep-collapse mode with 25 V pulse excitation and without the effects of charge trapping. The fabricated CMUT element consists of 4 by 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...

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 electric...

We propose a force-spectroscopy technique where a higher order mode of a cantilever is excited simultaneously with the first. Resonance tracking of both vibration modes through a frequency modulation scheme provides a way to extract topographical information and the gradient of the tip-sample interaction within a single surface scan. We provide an...

Capacitive micromachined ultrasonic transducers (CMUTs) have demonstrated various advantages over piezoelec- tric transducers. However, current CMUT designs produce low output pressures with high harmonic distortions. Optimizing the transducer parameters requires an iterative solution and is too time consuming using finite element (FEM) modelling t...

We propose capacitive micromachined ultrasonic transducers (CMUTs) as a candidate for underwater acoustic applications, in this paper. We have designed and fabricated CMUT membranes by utilizing anodic bonding technology and tested them underwater. We developed a nonlinear electrical equivalent circuit for immersed CMUTs and compared the model with...

The mapping of high level applications onto the coarse grained reconfigurable architectures (CGRA) are usually performed manually by using graphical tools or when automatic compilation is used, some restrictions are imposed to the high level code. Since high level applications do not contain parallelism explicitly, mapping the application directly...

In this work, we propose a novel method to increase the quality factor of extensional mode micromechanical resonators. The proposed resonator topology is suitable for integration in a silicon-based process to fabricate micromechanical filters and oscillators. It is a half-wavelength-long strip excited longitudinally by electrostatic forces, and it...

A cantilever with a grating coupler engraved on its tip is used for measuring displacement. The coupled light in the cantilever is guided to a single mode optical waveguide defined at the base of the cantilever. The grating period is 550 nm and is fabricated on a SOI wafer using nanoimprint lithography. The waveguide and the cantilever are defined...

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 acc...

A nonlinear equivalent circuit for immersed transmitting capacitive micromachined ultrasonic transducers (CMUTs) is presented. The velocity profile across the CMUT surface maintains the same form over a wide frequency range. This property and the profile are used to model both the electromechanical conversion and the mechanical section. The model p...

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 el...

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, th...

In this study, we demonstrate that periods of metallic gratings on elastomeric substrates can be tuned with external strain and hence are found to control the resonance condition of surface plasmon polaritons. We have excited the plasmon resonance on the elastomeric grating coated with gold and silver. The grating period is increased up to 25% by a...

In this paper, we have fabricated an optical cantilever with an integrated grating coupler. We have used an inexpensive and repeatable method for integrating the grating to the silicon cantilever with a microfabrication compatible process. The sensitivity of the method can be further increased by integrating the detection circuitry onto the cantile...