Search

Cataloged from PDF version of article. Thesis (Ph.D.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2022. Includes bibliographical references (leaves 94-99). Diverging wave imaging is an unfocused imaging method in which a diverging beam is transmitted to insonify the entire region of interest. This diverging beam is formed by applying appropriate time delays to each transducer array element. It provides a higher data acquisition rate and thus a higher temporal resolution, quantified as a higher frame rate. Therefore, diverging wave imaging is widely used in fast ultrasound imaging applications where rates above 1000 frames per second are required. Diverging wave imaging is generally implemented with phased array transducers having a smaller aperture than their counterparts to increase the field of view. Although diverging wave imaging allows for a high frame rate, it has a decreased spatial resolution and limited SNR due to the broader unfocused beam transmission compared to conventional focused imaging techniques. Conventional focused imaging techniques employ focused narrow beam transmissions for every image line resulting in a higher spatial resolution and SNR in the focal region. However, it offers approximately 30 frames per second, and thus it is not used in fast ultrasound imaging applications. There is a trade-off between frame rate, image quality, and SNR in diverging wave imaging. Therefore, fast imaging with high SNR and resolution while maintaining a high frame rate remains a practical problem in medical ultrasound. This thesis focuses on SNR maximization of diverging waves in weakly and multiple scattering, attenuating, and diffracting media. The primary outcome is that the SNR improves at deeper regions if the transmitted burst duration or the chip signal duration in the case of coded transmission is decreased when diverging waves are used. The maximum SNR is obtained in diverging wave transmission when the transmitted burst or the chip signal is as short-duration as the array permits. This result does not comply with the expectation implying that more transmitted energy results in higher SNR. The analytical foundation for diverging wave propagation in weakly and multiple scattering media is not sufficient at the level required to derive analytical results. In order to understand this counter-intuitive result, either finite element analysis (FEA) or semi-analytical simulation tools can be utilized. FEA can predict this counter-intuitive result, but detailed modeling of the medium is quite involved and results in very long simulation times, which renders the use of FEA impossible. Unlike the other imaging modalities, the wavelength is on the order of hundred micrometers in medical ultrasound imaging; thus, the simulation of a reasonable tissue volume is impossible. Semi-analytical simulation tools based on linear spatial impulse response produce erroneous results because scatterers are modeled as monopole sources, and multiple scattering is not modeled. As there is no analytical and simulation-based solution for this problem, the experimental verification of the results is presented. The transmitted ultrasound energy spreads over a broader region in diverging wave imaging. The energy spreading further aggravates due to diffraction and multiple scattering, which may cause energy loss. Keeping the transmitted ultrasound energy within the region of interest prevents this energy loss in diverging wave imaging. Therefore, we determined the optimum diverging wave profile to confine the transmitted ultrasound energy in the imaging sector. Using this optimized profile contributes to the SNR maximization. Complementary Golay sequences and Binary Phase Shift Keying modulation are used to code the transmitted signal. We used an ultrasound research scanner, a tissue-mimicking phantom, and a 128-element phased array transducer with 70% bandwidth at 7.5 MHz center frequency for data acquisition. The SNR in speckle and pin targets is maximized with respect to chip signal length and code length. The SNR performances of the optimized coded diverging wave and conventional single-focused phased array imaging are compared on a single frame basis. The focal region in the focused scheme is used as a reference. For the 90° imaging sector, the SNR of an 8-bit coded signal is maximum when the chip signal duration is one cycle of the center frequency. The SNR of the optimized coded diverging wave is higher than that of the conventional single-focused phased array imaging at all depths and regions. One frame of diverging wave data is acquired in 200 microseconds, equivalent to 5000 frames/s, whereas the time required for single-focused phased array imaging is 181 times more. by Yasin Kumru Ph.D.

Cataloged from PDF version of article. Thesis (Master's): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2022. Includes bibliographical references (leaves 61-66). Photoacoustic Imaging (PAI) is an emerging imaging modality that provides high resolution and image contrast. PAI employs Delay-and-Sum (DAS) beamforming in which data on all array elements are processed for image reconstruction. In order to decrease system cost in PAI, it is desired to use fewer samples in image reconstruction. Reducing the number of active elements of the array without changing the aperture decreases the computational cost. However, spatial undersampling results in poor image resolution and contrast, and causes spatial artifacts called grating lobes. Compressed Sensing (CS) is a data completion scheme that alleviates the effects of undersampling using a priori knowledge of the signal of interest and the measurement scheme. We performed simulations and experiments to compare the performances of the CS image reconstruction algorithm and conventional DAS beamforming. We used Full-Width Half Maximum (FWHM) resolution and image contrast ratio (CR) as performance metrics. Simulation results offer improved image resolution and contrast when CS is used. Lateral resolution in DAS beamformed images deteriorates with depth. A lateral resolution of 150 µm is obtained regardless of depth using only a quarter of the transducer elements. However, the resolution of DAS beamformed images ranges between 255 µm to 508 µm as the depth increases. It is also shown that CS suppresses the effects of grating lobes and improves the contrast ratio up to 14 dB. We also presented the experimental verification of the results. We used an ultrasound research scanner, a tunable laser system, and an optoacoustic phantom in the experiments. We experimentally showed that the CS method mitigates the effects of spatial undersampling and outperforms the DAS beamforming method in terms of contrast by 10.81 dB on average. CS method also offers an improved lateral resolution of approximately 350 µm compared to 750 µm in DAS beamforming. by Sadık Çağatay Çelebi M.S.

Cataloged from PDF version of article. Thesis (Ph.D.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2021. Includes bibliographical references (leaves 40-43). Mechanically coupled resonators fabricated using micromachining usually require a mechanical coupling element; this introduces additional complexity to the picture. We propose a novel modeling and design approach, followed by experimental verification, for mechanically coupled clamped circular plate resonators fabricated using a commercial micromachining process in which no additional coupling element is present. In our study, the flexural mode clamped circular plate resonators overlap to an extent. Their clamps at the overlap region are removed to generate a freely moving coupling boundary between the resonators. The practical measure of the overlap is small enough to preserve the characteristics of each resonator. This result enables modeling the coupled-resonators based on the clamped circular plate resonator model. A physics-based lumped element equivalent circuit model is developed where dimensions, bias voltage, and material properties are controllable variables. Each of the model parameters is expressed as the corresponding single resonator model parameter multiplied by a function of the amount of overlap. Analytical derivations and finite element method (FEM) simulations are used to extract the dependency of the model parameters on the amount of overlap. Closed-form expressions for the coupled-resonator's center frequency, bandwidth, and termination impedance are derived using the developed model. A design procedure is introduced to determine dimensional parameters and bias voltage. The proposed coupled-resonator offers up to 5% fractional bandwidth. For a typical design using a polysilicon plate with 100 nm gap height, the ratio of the termination impedance to the center frequency is calculated to be 158 Ω/MHz. This result indicates that on-chip intermediate frequency (IF) filters can be implemented at center frequencies up to several hundred MHz using this type of coupled-resonators. A coupled-resonator is designed and realized for a proof of concept demonstration. The measurement results of the coupled-resonator show good agreement with the equivalent circuit model simulations. by Muhammed Abdulcelil Acar Ph.D.

Cataloged from PDF version of article. Thesis (Master's): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2021. Includes bibliographical references (leaves 64-67). Capacitive Micromachined Ultrasonic Transducers (CMUT) are micro-scaled electromechanical devices which are used to either transmit or receive pressure signals and applicable for various purposes such as ultrasonic sensor, medical imaging, accurate biometric sensing and parametric speakers. For transmitting CMUT transducer, different sizes and array configurations are used to intensify the transmission power depending on the application. The half-frequency driven waterborne transmitting CMUT array designed in this work is to be used for high resolution volumetric medical imaging purpose. This was accomplished by a design which prioritizes maximizing the power output, achieving a directive radiation pattern with low sidelobes which maximizes the beamformable region. In this work, the issues with steering of the focused beam are also resolved to achieve a focused steerable beam. This work is an advancement from the earlier designed half-frequency driven airborne transmit CMUT to improve power output, introduce the beamforming and focused transmission capabilities and be applicable for high resolution volumetric medical imaging purpose. To improve the power output, the design was made to compensate for the static depression. Compensating for static depression was achieved by designing to operate the CMUT without DC bias voltage which allows for full-gap swing and giving output signal of twice the input frequency. This property allows the cell to produce high power output with low voltage levels but also brings the advantage of operating the cell with very high voltages without collapsing. The CMUT was chosen to be operating at 7.5 MHz and be driven by Digital Phased Array System (DiPhAS) which allowed to have maximum of 256 channels which for volumetric transmission meant a maximum of 16 x 16 array. Since the radiation pattern and Rayleigh distance are both the functions of radius, frequency and the pitch, the design optimization was found while considering all the above preferences simultaneously. The cells’ radii were determined to be 80 µm, the plate thickness was 15 µm, the gap height was found to be 117 nm and the pitch was 192 µm. The array designing was carried out using the large-signal equivalent circuit model and the radiation impedance matrix phenomenon. The simulations showed that with this design, the maximized Rayleigh distance was 45.3 mm and the sidelobe of -17.4 dB. In simulations, very high pressure outputs were achievable with individual cells up to 425 kPa per cell with 150 VPP input while up to 1.5 MPa was emitted by the array plane wave transmission with only 10 VPP input and almost doubles when the transmitted beam was focused at zero degrees. Fabrication was done by the wafer boding and flip-chip bonding techniques where the whole process required only two lithography masks. After fabrication, the tests were performed to identify the yield of the transducer was 18.75% of the array then impedance analysis was done to characterize the functional cells and resonance frequency drift. The transducer was cased in a water-tight manner and the waterborne transmission were done with individual cells to characterize and compare the performance with the design simulations which were in the range of agreement achieving an average of 1625 Pa per cell. The functional cells were then used for plane wave transmission with 10 VPP and the output pressure of 397 kPa was achieved at resonance frequency. The measurement results showed that the design could further be improved by compensating the active area to improve the yield for better results and be able to use it for high resolution 3D medical imaging. by Yusuph Abubakar Abhoo M.S.

Cataloged from PDF version of article. Thesis (Master's): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2021. Includes bibliographical references (leaves 56-58). Capacitive Micromachined Ultrasonic Transducers (CMUTs) are MEMS devices used in ultrasound imaging, e.g. ultrasound mammography. CMUT proved to be a viable transducer solution in ultrasound mammography without the hazardous effects of conventional X-ray mammography. The CMUTs have a very high electrical impedance, where a transimpedance amplifier (TIA) is most appropriate for preamplification during reception. A TIA with 25MHz bandwidth and 120kΩ transimpedance gain is designed on Cadence Virtuoso using XFAB XC06M3 process. The CMUT small signal model is used for 50μm radius and 7.5MHz operating frequency. This model is incorporated into TIA circuit simulations. Two design options are proposed for the TIA, one with the passive feedback resistor and the other with a MOSFET feedback resistor. The latter enabled us to save space in layout design compare to the former. Transient and noise simulations are conducted and compared for schematic and layout views. The input referred current noise of the TIA is simulated to be 0.5pA/√Hz and the power consumption is simulated to be approximately 3.3mW for both designs. by Giray İlhan M.S.

Cataloged from PDF version of article. Thesis (M.S.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2019. Includes bibliographical references (leaves 116-123). Capacitive micromachined ultrasonic transducers (CMUT) are typically used as arrays consisting of separate cells or interconnected sub arrays, and these cells have several operation modes. Design procedure and measurements have been previously presented for an airborne CMUT cell without a DC bias. In unbiased mode, the plate motion spans the entire gap without collapsing. The frequency of sinusoidal electrical input signal is half of the resulting acoustical output signal. A large plate swing can be obtained at low excitation voltages using the entire remaining gap. In this work, a design procedure of arrays with unbiased operation mode is derived. Designs are validated by means of simulations and measurements on fabricated CMUTs. The problems associated with beamforming are resolved. Use of this operation mode in an array configuration enables larger acoustical power output. A better transmitted signal waveform definition is obtained when pulse width modulation (PWM) is employed. In the design process of CMUTs, large-signal equivalent circuit model is used. In order to have volumetric transmission, a 16×16 (256 elements) phased array configuration is chosen. A design procedure is presented considering the radiation pattern, Rayleigh distance and the parameters of the lumped-element model. This procedure is applied for designing two arrays, with resonance frequencies at 7.5 MHz and at 18.5 MHz. Harmonic balance and transient analyses are carried out with Gaussian enveloped tone burst, transient sinusoidal and PWM signals with different duty cycles. Outputs of these simulations are fed in beamforming toolboxes for further verification. Corresponding experimental measurements are conducted on an ultrasound measurement system. The dimensions of the CMUT elements in the array are determined as 80 μm element radius, 15 μm plate thickness, and 171 nm effective gap height at 7.5 MHz, where the center-to-center inter-element pitch is set at 192 μm. The array is designed such that it has maximum Rayleigh distance of 45.3 mm, while having a maximum sidelobe level of −17.4 dB. According to these specifications, CMUT array is manufactured using wafer scale batch compatible production. Only two lithography masks, which require conventional photolithography steps, are used in production. The vibrating plate is constructed with anodic wafer bonding and the fabricated CMUT array chip is integrated to PCB with flip-chip bonding. Measurements are conducted for this novel device by integrating the CMUT array chip manufactured with MEMS techniques and conventional macro scale manufactured PCB by using impedance analyser. by Kerem Enhoş M.S.

Bu tezde, kodlanmış işaretlerin aktif sonar sistemlerindeki performansı ile örtülü ve çevreye duyarlı sonar sistemlerine sahip olabilme olasılığı incelenmektedir. Düşük tespit edilme olasılığı ve iyi bir performans elde edebilmek için yayılı izge işaretleri kullanılmıştır. Uzunluğu 7 çip dizi uzunluğundan 127 çip dizi uzunluğuna kadar değişen, çip türü maksimal olan, doğrudan dizili yayılı izge modülasyon (DDYİ) yöntemi ile kodlanmış işaretler ile referans sinyali olarak kullanılan tek ton darbeler yayılı izge işaretleri olarak kullanılmıştır. Tek ton darbelerin uzunluğu çok yollu yayılımdan dolayı sınırsız bir şekilde artırılamaz. Hedefin tespiti, yerinin saptanması problemi ve çip türü maksimal olan kodlanmış sinyallerin birbirleriyle olan benzerlikleri de incelenmiştir. Hedefin tespiti ve yerinin saptanmasında, işaret gürültü oranı ve kodlanarak farklı sektörlere gönderilen işaretlerin birbirleriyle etkileşimi önemli paremetrelerdir. Çip dizi uzunluğu artırıldıkça, işaret gürültü oranı ve işaret etkileşim oranı artmaktadır. İşaret etkileşim oranındaki değişim, kullanılan maksimal çip türünün korelasyon özelliğinden dolayı, çok az ölçülmektedir.Anahtar sözcükler: Hedef Tespiti ve Yerinin Saptanması, Örtülü Sistem, Doğrudan Dizili Yayılı İzge (DDYİ), Maksimal Uzunluklu Diziler, Tek Ton Darbe, İşaret Gürültü Oranı, İşaret etkileşim oranı. Performance of coded signals in active Sound Navigation and Ranging (SONAR) technology is studied in this work. In this work, the possibility of having covertness and environment friendliness in SONAR systems is investigated. Spread spectrum ping signal is considered to achieve low probability of detection and interception, while maintaining good performance. Direct Sequence Spread Spectrum (DSSS) coded transmitted signals having a sequence type of maximal with lengths from 7 chips to 127 chips and tone burst pulse having single length as a reference signal are used as spread spectrum ping signal. The length of the tone burst pulse can not be increased indefinitely because of the multipath propagation. The problem of detection and localization of the targets and the cross correlation properties of the maximal length sequences are investigated as well. The Signal to Noise Ratio (SNR) and Signal to Interference and Distortion ratio (SINAD) are important parameters in detection and localization of the targets. It is found that as the number of chip length increases, the SNR and SINAD increase but the improvement of the SINAD is comparatively less because of the cross correlation properties of the maximal length sequences.Keywords: Target Detection and Localization, Covertness, Direct Sequence Spread Spectrum Modulation (DSSS), Maximal Length Sequences, Tone Burst Pulse, SNR, SINAD. 68

Ankara : The Department of Electrical and Electronics Engineering and The Graduate School of Engineering and Science of Bilkent University, 2014. Thesis (Ph. D.) -- Bilkent University, 2014. Includes bibliographical references leaves 60-64. All transducers used in airborne ultrasonic applications, including capacitive micromachined ultrasonic transducers (CMUTs), incorporate loss mechanisms to have reasonably wide frequency bandwidth. However, CMUTs can yield high efficiency in airborne applications and unlike other technologies, they offer wider bandwidth due to their low characteristic impedance, even for efficient designs. Despite these advantages, achieving the full potential is challenging due to the lack of a systematic method to design a wide bandwidth CMUTs. In this thesis, we present a method for airborne CMUT design. We use a lumped element circuit model and harmonic balance (HB) approach to optimize CMUTs for maximum transmitted power. Airborne CMUTs have narrowband characteristic at their mechanical part, due to low radiation impedance. In this work, we restrict the analysis to a single frequency and the transducer is driven by a sinusoidal voltage with half of the frequency of operation frequency, without any dc bias. We propose a new mode of airborne operation for CMUTs, where the plate motion spans the entire gap. We achieve this maximum swing at a specific frequency applying the lowest drive voltage and we call this mode of operation as Minimum Voltage Drive Mode (MVDM). We present equivalent circuit-based design fundamentals for airborne CMUT cells and verify the design targets by fabricated CMUTs. The performance limits for silicon membranes for airborne applications are derived. We experimentally obtain 78.9 dB//20Pa@1m 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. CMUTs can achieve a large bandwidth (low quality factor level) as they can be manufactured to have thin plates. Low-quality-factor airborne CMUTs experience increased ambient pressure and therefore a larger membrane deflection. This effect increases the stiffness of the plate material and can be considered by nonlinear compliance in the circuit model. We study the interaction of the compliance nonlinearity and nonlinearity of transduction force and show that transduction overwhelms the compliance nonlinearity. To match the simulation results with the admittance measurements we implement a very accurate model-based characterization approach where we modify the equivalent circuit model. In the modified circuit model, we introduced new elements to include loss mechanisms. Also, we changed the dimension parameters used in the simulation to compensate the difference in the resonance frequency and amplitude. Yılmaz, Aslı Ph.D.

Birçok ultrasonik teknoloji uygulamasında, güçlü, geniş frekans bantlı ve hüzmeleme açısından iyi performansa sahip akustik sinyallerin yayınlanabilmesi için geniş kapasitif mikroişlenmiş ultrasonik çevirici (CMUT) dizinlerinin kullanılması gerekmektedir. CMUT'ların karakteristik özelliklerinden en iyi şekilde faydalanabilmek için çok sayıda CMUT hücresini beraber analiz edebilen araçlar kullanılmalıdır. Bu sebeple, sonlu eleman analizi (FEA) araçları sayısal olarak ağır işledikleri için çok sayıda CMUT hücresi içeren dizinlerde tercih edilmezler ve genellikle de kullanımları pratik olarak mümkün değildir. Bazı FEA simülasyonlarında uzun 1-D CMUT dizin elemanlarının sonsuz uzunlukta olduğu varsayılarak basitleştirilmiş modeller yapılmış olsa da sonuçları yanıltıcı olmuştur. Bu tür modellerde elemanın sadece tek bir periyodik kısmı modellenerek simetri yüzeylerine katı sınır koşulları uygulanmaktadır. Böylece dizindeki herbir hücrenin elektriksel olarak aynı fazda sürüldüğü varsayılır ve periyodik kısım için elde edilen sonuçlar bütün eleman boyunca aynı kabul edilir. Fakat, bu basitleştirilmiş FEA modelleri CMUT hücreleri arasındaki karşılıklı akustik etkileşimlerin önemli etkilerini göz ardı ettiklerinden dolayı doğru sonuçlar verememektedir.Bu çalışmamızda, çökmemiş dairesel CMUT hücreleri için doğrusal olmayan bir eşdeğer devre modeli geliştirdik. CMUT dizinlerindeki karşılıklı akustik etkileşimleri inceledik ve bu olgunun dizin performansı üzerinde oldukça etkili olduğunu gösterdik. Bu etkileşimler, dizindeki her hücrenin bulunduğu ortamda yarattığı basınç alanının diğer hücrelerin üzerineetkimesi ile gerçekleşmektedir. Bu etkiyi araştırmak için dizindeki herbir hücrenin eşdeğer devresini, bütün hücre çiftleri arasındaki karşılıklı radyasyon empedanslarına ek olarak, öz radyasyon empedanslarını da içeren bir matrise bağladık. Böylece geniş dizinlerin performans analizi devre simülatörleri ile kolaylıkla çözülebilen bir devre teorisi problemi haline dönüşmüş oldu.Yüzey mikroişleme teknolojisi sayesinde maliyet ve birçok fiziksel sınırlama sorunu çözülerek geniş CMUT dizinlerinin yığınlar halinde üretilmesi mümkün kılınmıştır. Tasarımcılar olası birçok farklı dizin konfigürasyonunu dikkate almak durumundadır. Yaklaşık son yirmi yıldır uygun tasarım ve analiz araçlarının eksikliği yüzünden dizin performansının tam olarak incelenebilmesi mümkün olamamıştır. önerilen bu model ile çok sayıda çökmemiş CMUT hücresinden oluşan dizinlerin doğrusal frekans ve doğrusal olmayan geçici rejim analizleri hızlı bir şekilde yapılabilmektedir.Çok geniş CMUT dizinleri yüzlerce hücre içeren onlarca elemandan oluşabidiği için hızlı sonuç veren devre teorisi teknikleri ile dahi analiz süreleri oldukça uzayabilmektedir. Problemi ufak parçalara ayırmak için dizinde bulunan elemanlardan sadece birkaçını elektriksel olarak sürdük ve geri kalanları bir öngerilime maruz bırakarak elektrikselbağlantılarını bir yük ile sonlandırdık. Daha sonra sürülmeyen elemanların sürülenlerüzerindeki etkisini hesaba katarak bütün elemanların sürüldüğü duruma göre çok daha ufakboyutlu bir indirgenmiş radyasyon empedans matrisi elde ettik. Böylece sadece sürülen elemanlar bu indirgenmiş radyasyon empedansı matrisi ile hücreleri bağlaşım halindeyken simüle edilebilmektedir. Bu sayede küçük işaret koşullarında az sayıdaki eleman gruplarının tepkileri ayrı ayrı hesaplanabilir ve üstdüşüm prensibi kullanılarak bütün elemanların aynı anda sürüldüğü durumdaki tüm tepkiyi bulmak için toplanabilir. Bu yöntem, büyük bir karmaşık simetrik matrisin tersinin alınması kaydıyla, simülasyondaki hücre sayısını ve dolayısıyla da radyasyon empedans matrisinin boyutunu önemli ölçüde azaltmaktadır.Anahtar sözcükler: CMUT, toplu öğeli doğrusal olmayan eşdeğer devre modeli, büyük işaret modeli, küçük işaret modeli, çökmemiş çalışma modu, dizin, karşılıklı akustik bağlaşım, öz ve karşılıklı radyasyon empedansları, indirgenmiş radyasyon empedans matrisi Many ultrasonic technology applications require capacitive micromachined ultrasonic transducers (CMUTs) to be used in the form of large arrays to attain better performance in terms of powerful, broadband and beam-formed radiated acoustic signals. To entirely benefit from its important characteristics, it is necessary to use analysis tools that are capable of handling multiple CMUT cells. In this regard, finite element analysis (FEA) tools become unfit for use because in arrays with large number of cells it is computationally very cumbersome and often practically impossible. Although, some simplification had been done by assuming long 1-D CMUT array elements as infinitely long, the results of these FEA simulations are misleading. In these models only a single periodic portion is modeled and rigid boundary conditions are applied at the symmetry planes. All the cells are assumed to be electrically driven in phase with the rest of the cells and the solution obtained for this portion is extended over the entire element. However, these simple models are not exact, because they exclude the important effects of mutual acoustic interactions between the cells.In this work, we developed an accurate nonlinear equivalent circuit model for circular uncollapsed CMUT cells. We investigated the effects of mutual acoustic interactions in uncollapsed CMUT arrays and showed that the performance of the array is highly influenced with this phenomenon. These mutual acoustic interactions rise through the immersion medium caused by the pressure field generated by each cell acting upon the others. To study its effects, we connected 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 radiation impedances. Hence, analysis of the performance of a large array became a circuit theory problem and can be scrutinized with circuit simulators.Surface micromachining technology enables batch fabrication of large CMUT arrays, which resolves cost issues and many physical limitations. Designers have to consider a great number of different array configurations. For nearly two decades, the lack of appropriatedesign and analysis tools prevented the investigation of array performance. By using the proposed model, one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with a large number of uncollapsed CMUT cells.Although, we use rapid circuit theory techniques, efficient analysis of very large arrays is still challenging, since a typical CMUT array may contain many tens of elements with hundreds of cells in each, which makes it computationally cumbersome. To partition the problem, we electrically drive a small number of elements in the array and keep the rest undriven but biased and with their electrical ports terminated with a load. The radiation impedance matrix can be partitioned and rearranged to represent these loads in a reduced form. In this way, only the driven elements can be simulated by coupling their cells through this reduced radiation impedance matrix. Under small signal regime, the separately calculated responses of element clusters can be added by using the superposition principle to find the total response. This method considerably reduces the number of cells and the size of the actual radiation impedance matrix, at the expense of calculating the inverse of a large complex symmetric matrix.Keywords: CMUT, lumped element nonlinear equivalent circuit model, largesignal model, small signal model, uncollapsed mode of operation, array, mutual acoustic coupling, self and mutual radiation impedances, reduced radiationimpedance matrix. 90

Bu tezde doğrudan dizili yayılı izge modülasyon (DDYİ) yönteminin su altı akustiği mesafe ölçümü konusunda sergileyeceği başarım incelenmektedir. Düşük güçlü DDYİ kodlarını kullanarak yaplan mesafe ölçümü hem teorik açıdan, hem de 400 kHz merkez frekansı kullanarak deneysel açıdan tahlil edilmekte olup kullanılan çip süresi 20 us, çip türü ise maksimaldir. Dizi uzunluğunun gerekli iletim gücüne etkisi 7 çip dizi uzunluğundan 127 çip dizi uzunluğuna kadar ölçülmektedir. Bu yöntemin başarımı yaygınca kullanılan tek ton darbeler ile mesafe ölçümü tekniği ile karşılaştırılmaktadır. 127 çip 2,54 ms uzunluğunda ve 132,8 dB re 1 uPa rms @ 1 m kaynak seviyesinde, DDYİ darbesi kullanıldığında, 4.5 m'lik bir test mesafesi 1,5 cm derinlik çözünürlüğü ile, aynı uzunlukta ve büyüklükte tek ton darbe kullanıldığında ise 190,5 cm çözünürlük ile ölçülmektedir. Ayrıca, DDYİ almaç işaretinin izgesel yüksekliğinin ortam gürültüsünün bir hayli altında kaldığı görülmekte olup işaret gürültü oranı -14,8 dB bulunmakta, diğer yandan bu oran tek ton almaç darbesi için ise 12,2 dB olmaktadır.Anahtar sozcukler: Dogrudan Dizili Yayl _Izge, Modulasyon, Mesafe Olcumu, Tek Ton Darbe.. Performance of direct sequence spread spectrum modulation (DSSS) in underwater acoustical range finding is investigated in this thesis. Range estimation using low power DSSS codes is both analyzed theoretically and implemented at 400 kHz for experimental assessment, where the chip duration is 20 us and sequence type is maximal. The effect of sequence length on the required transmit power is measured for sequence lengths from 7 chips to 127 chips. The performance of this method is compared to the widely used tone burst pulse range estimation technique. It is found that at a sequence of 127 chips and a pulse sequence length of 2,54 ms, range is estimated with 1,5 cm resolution using source level of 132,8 dB re 1 uPa rms @ 1 m source level , while it is 190,5 cm for the same length and magnitude tone burst modulation, at a reference test range of 4.5 m. Moreover, spectral height of received DSSS signal is well below the ambient noise level so that signal to noise ratio (SNR) for received DSSS signal is -14,8 dB, while it is 12,2 dB for received tone burst pulse.Keywords: Echo Sounding, Direct Sequence Spread Spectrum, Modulation,Range Estimation, Tone Burst Pulse 92

Bu tezde dikey frekans bölmeli çoklama (OFDM) kullanan bir sualtı akustik modemin tasarımı, simülasyonu ve testi anlatılmıştır.Tez, hedeflenen kanalın özelliklerine uygun teknikleri anlatan teorik kısmı, simülasyonları ve sistem kurulumu ve testlerianlatan pratik kısmı birleştirir. Gerçek zamanlı veri ve video göndermeye ilgi artmıştır. Askeri ve bilimsel uygulamalar içininsansız sualtı araçlar(UUVs) önemli hale gelmiştir. Sualtı uygulamalarına önemli faydalar getirecek bilimsel keşifler içinokyanus gözlemi, ticari araştırmalar, kıyı şeridi koruma ve hedef tespiti kapsamında dağıtık ve ölçeklenebilir kablosuzsualtı sensör ağları (UWSN) geliştirmek araştırmacıların hedefindedir. Buna dayanarak, yüksek veri hızı olan somut birsistem tasarımından birçok sualtı akustik uygulamasında yararlanılacaktır. Literatürde mevcut sistemler , tek taşıyıcılı iletişimve semboller arası girişimi(ISI) bastırmak amacıyla doğrusal veya doğrusal olmayan denkleştirme teknikleri kullanmaktadır.Bu karmaşık denkleştiriciler gerektirir ve düşük veri aktarım hızına neden olur. Bu nedenle, çok taşıyıcılı modülasyon kullandık.Bu tezde, Sıfır ile Dolgulanmış-OFDM alıcısı geliştirilmiş ve taşıyıcı frekans ofseti, pilot-sembol destekli kanal kestirimi, ve veridemodülasyonu her OFDM blok bazında yapılmıştır. OFDM sistemi MATLAB ortamında geliştirilmiştir. MATLAB kodları tarafındanoluşturulan ve OFDM bloklarıiçeren veri paketleri kullanılan veri edinme (DAQ) kartı ile bir dizüstü bilgisayardan donanıma aktarılır. Sistemin diğerucunda ise alıcı dizüstü bilgisayar bir veri edinme kartı sayesinde veriyi alır. Verinin alınması ile beraber, MATLAB kodları iledemodülasyon işlemi gerçekleştirilir. Simülasyonlar, algoritmaların doğru uygulandığını görmek amacıyla, yaratılan OFDM sinyaliniBellhop sualtı kanal modeli ve gürültü kaynağındadan geçirerekyapılmıştır. Sistem, Bilkent Üniversitesi Gölet Tesisinde, sığ suda test edilmiştir. 12 kHz bant genişliği içinde, alttaşıyıcı sayısı1024 iken dördün faz kaydırmalı modülasyonu(QPSK) ile veri aktarım hızı 13.92 kb/s olmuştur. Bit hata oranı (BER), herhangibir kodlama olmadan 9x10^-2'den daha azdır. This thesis is about design, simulation and testing of an underwater acoustic modemusing OFDM. The thesis work combines a theoretical part, whose objective is to understandthe appropriate techniques to deal with the characteristics of the targeted channel, simulationsand a practical part regarding the system deployment and experimental tests. Therehas been a great growing interest in transmitting real-time data and video. Unmanned underwatervehicles (UUVs) for military and scientific applications have become important.Building distributed and scalable underwater wireless sensor networks (UWSN) that willbring significant advantages and benefits to underwater applications, such as ocean observationfor scientific exploration, commercial exploitation, coastline protection and targetdetection in military events has been in the scope of researchers. Based on these, designinga concrete system with high data rate will benefit many underwater acoustic (UWA) applications.The existing systems in literature use single carrier transmission and rely on linearor non-linear equalization techniques to suppress inter-symbol interference (ISI), howeverthis requires complex equalizers and results in low data rates. Therefore we concentrate onmulticarrier modulation. In this thesis ZP-OFDM (Zero Padded-Orthogonal Frequency DivisionMultiplexing) receiver is built, where CFO (Carrier Frequency Offset) compensation,pilot-tone based channel estimation, and data demodulation are carried out on the basisof each OFDM block. The implemented OFDM system has been developed in MATLAB.MATLAB scripts generate a data burst that contained OFDM blocks, and then they aretransmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. Atthe other side of the system, the receiver laptop gets the data by using a DAQ Card. As thedata is received, MATLAB scripts are demodulated and data is detected. Simulations aimto provide correct implementation of all the algorithms by coupling the generated OFDMsignal to a channel using Bellhop underwater channel model and noise addition algorithm,that artificially introduces some of the real channel effects into the signal. The method istested in a shallow-water experiment at Bilkent Lake. Over a bandwidth of 12 kHz, thedata rate is 13.92 kb/s with QPSK modulation, when the number of subcarriers was 1024.Bit-error-rate (BER) is less than 9x10^-2 without any coding. 67

Kapasitif Mikroislenmis Ultrasonik Çeviriciler (CMUT), yüzey üzerinde askıda duran,mikro-elektromekanik (MEMS) zar yapılarıdır. CMUT'ların mekanik zara vealttasa entegre olmak üzere iki adet elektrotları bulunur. Elektrotlara uygulananelektriksel sinyaller sayesinde uyarılan zar, hareket eder ve ortama akustik dalgalaryayar. CMUT'ların, piezoelektrik çeviricilerle karsılastırıldıklarında, genis bantlıolmaları gibi ve üretim teknolojisinin getirdigi (çok elemanlı diziler, CMOS uyumlulugu,seri üretim) avantajları vardır. Üretim ve elektronik entegrasyon ile ilgiliteknolojik zorluklar ve problemler geçtigimiz 15 yıl içerisinde çözülmüs ve birçokuygulama CMUT'lar kullanılarak basarı ile sonuçlandırılmıstır. Fakat CMUT'larınticarilesmesi henüz gerçeklesmemistir. Her nekadar zar yapılarının özelligi sayesindegenis bantlı olsalar da, piezoelektriklere göre üstün bir sinyal-gürültü oranı veçalısma derinligi elde edilebilmesi için çıkıs güçlerinin artırılması gerekmektedir.Bu tezde, CMUT'ların fizigi ve fonksyonları ile ilgili daha genis kapsamlı biranaliz sunuyoruz. Bu analizi kullanarak CMUT çıkıs güçlerinin önemli ölçüde arttırılabileceginindeneyini yapıyoruz. Literatürde, CMUT'ların bükülme profillerininanalitik bagıntıları sadece çökmemis zarlar için verilmektedir. Fakat, çökmüs bir zarkullanılarak daha fazla bir toplam bükülme elde edilebilir. Zarın çökmüs haldekihareketi kontrol edilerek, çeviricinin çıkıs gücü arttırılabilir. Bu tezde, yuvarlakzarlar için bükülme profilinin analitik bagıntıları çökmüs ve çökmemis halde ikentüretilmistir. Bu profiller kullanılarak, zar üzerine etki eden kuvvetler bulunmus vebir elektriksel darbe uygulandıgında, zarın ortama yayacagı enerji hesaplanmıstır.Sonuç olarak, çökme geriliminin çok üzerinde olusan dogrusal olmayan kuvvetlerkullanılarak, zarın yayacagı akustik enerjinin önemli ölçüde arttırılabilecegi belirlenmistir.Elde edilen analitik bagıntılar kullanılarak dogrusal olmayan bir esdeger dev-remodeli gelistirilmistir. Bu model kullanılarak yürütülen benzetimler ile herhangi birelektriksel uyarı ile yayın yapan bir CMUT'ın mekanik davranısı belirlenebilmektedir.Öte yandan, model, farklı zar boyutlarını ve malzeme özelliklerini de kullanabilmekteve zar hareketini, zarın çökmüs ve çökmemis iken belirleyebilmektedir.Ayrıca, CMUT'ların hizterez özelligi de model tarafından öngörülebilmektedir.Dogrusal olmayan bu model, bir SPICE devre benzetimi kullanılarak uygulanmısve modelin dogrulugu sonlu eleman metodu (FEM) kullanılarak sınanmıstır. Bunagöre, zarın DC gerilimler altında duragan bükülmesi %3'ten daha iyi bir hassasiyetile belirlenebilmistir. Diger taraftan, zara yüksek gerilimli bir darbe uygulandıgındaolusan basınç dalga sekli, %5'ten daha iyi bir hassasiyet ile hesaplanabilmektedir.Gelistirilen esdeger devre modeli kullanılarak, CMUT zarlarının dinamiklerinianlayabiliyoruz. Olusturulan fiziksel anlayıs temel alınarak, CMUT'lardan istenenfrekansta genis bantlı ve yüksek güçte bir basınç darbesi elde etmek amacıyla derinçökme modunu öneriyoruz. Bu yeni mod ile CMUT'ların çıkıs güçlerinin önemliölçüde arttırılabilecegi, benzetim modelleri (esdeger devre ve FEM) kullanılarak vedeneysel olarak gösterilmistir. Deneyler, Bilkent Üniversitesi'nde üretilen CMUT'larkullanılarak gerçeklestirilimistir. 160V genliginde bir elektriksel darbe kullanılarak,yaklasık 7 MHz etrafında 100% oransal bantlı ve 3.5 MPa'dan daha büyük basınçdarbeleri ölçülmüstür. Öte yandan derin çökme modunu eniyilestiren CMUT boyutlarıve elektriksel darbe özellikleri belirlenmistir. Deneysel sonuçlar ve benzetimsonuçları karsılastırılmıs ve sonuçların uyumlu oldugu anlasılmıstır. Bu tez sonucundaanlasılmıstır ki, derin çökme modu kullanılarak CMUT'ların çıkıs güçlerikontrollü bir sekilde ve önemli miktarlarda arttırılabilir ve dolayısıyla piezoelektrikçeviricilere karsı daha güçlü bir rakip olabilirler.Anahtar sözcükler: Kapasitif Mikroislenmis Ultrasonik Çeviriciler, CMUT, MEMS,Esdeger Devre, Çökme Modu, Derin Çökme, Mikro-fabrikasyon, Dairesel PlakalarınBükülmesi, Sualtı CMUT. Capacitive micromachined ultrasonic transducers (CMUTs) are suspended microelectromechanicalmembrane structures with a moving top electrode and a rigid substrateelectrode. The membrane is actuated by electrical signals applied between theelectrodes, resulting in radiated pressure waves. CMUTs have several advantagesover traditional piezoelectric transducers such as their wider bandwidth and microfabricationmethodology. CMUTs as microelectromechanical systems (MEMS), arefabricated using CMOS compatible processes and suitable for batch fabrication. Lowcost production of large amount of CMUTs can be fabricated using already establishedintegrated circuit (IC) technology infrastructure. Contrary to piezoelectrics,fabricating large 2-D arrays populated with transducer elements using CMUTs islow-cost. The technological challenges of CMUTs regarding the fabrication andintegration issues were solved during the past 15 years, and their successful operationhas been demonstrated in many applications. However, commercialization ofCMUTs is still an overdue passion for CMUT community. The bandwidth of theCMUTs are inherently superior to their piezoelectric rivals due to the nature ofthe suspended membrane structure, however, their power output capability must beimproved to achieve superior signal-to-noise ratio and penetration depth.In this thesis, we gave a comprehensive discussion about the physics and functionalityof CMUTs and showed both theoretically and experimentally that theirpower outputs can be increased substantially. Using the conventional uncollapsedmode of CMUTs, where the suspended membrane vibrates freely, the lumped displacementof the membrane is limited. Limited displacement, unfortunately, limitsthe power output of the CMUT. However, a larger lumped displacement is possiblein the collapsed state, where the membrane gets in contact with the substrate. Bycontrolling the movement of the membrane in this state, the power output of theviviiCMUTs can be increased. We derived the analytical expressions for the profile ofa circular CMUT membrane in both uncollapsed and collapsed states. Using theprofiles, we calculated the forces acting on the membrane and the energy radiated tothe medium during an applied electrical pulse. We showed that the radiated energycan be increased drastically by utilizing the nonlinear forces on the membrane, wellbeyond the collapse voltage.Using the analytical expressions, we developed a nonlinear electrical equivalentcircuit model that can be used to simulate the mechanical behavior of a transmittingCMUT under any electrical excitation. Furthermore, the model can handle differentmembrane dimensions and material properties. It can predict the membrane movementin the collapsed state as well as in the uncollapsed state. In addition, it predictsthe hysteretic snap-back behavior of CMUTs, when the electric potential across acollapsed membrane is decreased. The nonlinear equivalent circuit was simulatedusing SPICE circuit simulator, and the accuracy of the model was tested using finiteelement method (FEM) simulations. Better than 3% accuracy is achieved for thestatic deflection of a membrane as a function of applied DC voltage. On the otherhand, the pressure output of a CMUT under large signal excitation is predictedwithin 5% accuracy.Using the developed model, we explained the dynamics of a CMUT membrane.Based on our physical understanding, we proposed a new mode of operation, thedeep collapse mode, in order to generate high power acoustic pulses with largebandwidth (>100% fractional) at a desired center frequency. We showed both bysimulation (FEM and equivalent circuit) and by experiments that the deep collapsemode increases the output pressure of a CMUT, substantially. The experimentswere performed on CMUTs fabricated at Bilkent University by a sacrificial releaseprocess. Larger than 3.5MPa peak-to-peak acoustic pulses were measured on CMUTsurface with more than 100% fractional bandwidth around 7 MHz using an electricalpulse amplitude of 160 Volts. Furthermore, we optimized the deep collapse modein terms of CMUT dimensions and parameters of the applied electrical pulse, i.e.,amplitude, rise and fall times, pulse width and polarity. The experimental resultswere compared to dynamic FEM and equivalent circuit simulations. We concludedthat the experimental results are in good agreement with the simulations. We believethat CMUTs, with their high transmit power capability in the deep collapse modecan become a strong competitor to piezoelectrics.Keywords: Capacitive Micromachined Ultrasonic Transducers, CMUT, MEMS,Equivalent circuit, Collapse mode, Deep collapse, Microfabrication, Sacrificial releaseprocess, Deflection of circular plates, Underwater CMUT. 144

Sonlu eleman methodu (SEM) kapasitif mikroişlenmiş ultrasonik çeviricilerin (KMUÇ) geçici dinamik rejim analizinde kullanılmakta ve özellikle de membranlar doğrusal olmayan şekilde sürüldüğünde faydalıdır. Geçici rejim SEM analizi, DC gerilim altında çok düşük AC uyarımda dahi KMUÇ'un şiddetli bir şekilde doğrusal olmayan davranış sergilediğini göstermektedir. SEM'in başlıca dezavantajlarından bir tanesi, simulasyon için aşırı zaman gerektirmesidir. Diğer taraftan, Harmonik Denge (HD) analizi, doğrusal olmayan devrelerin denge durumundaki tepkisinin doğru tahminini çok hızlı sağlamaktadır. KMUÇ'un mekanik kısmını modellemek için Mason'ın eşdeğer devresi yaygın olarak kullanılmaktadır. Fakat, özellikle, daldırılmış bir KMUÇ için Mason'ın mekanik LC kısmını katı bir pistonun radyasyon empedansı ile sonlandırmak uygun değildir. Geçici rejim SEM analizini kullanarak membran davranışını araştırdık ve membran hareketinin seri rezonans civarındaki birçok harmoniklerinin yanlarından tutturulmuş bir ışıyıcı olarak modellenebildiğini bulduk. Devre elemanı olarak ortalama hız yerine, membran yüzeyindeki hız dağılımının etkin değerini dikkate aldık. Bu tanım ile membran ağırlığının kinetik enejisi modeldeki ile aynıdır. Yanlarından tutturulmuş bir ışıyıcı için kuvvet ve akım denklemlerini türettik ve ticari bir HD simülatörü kullanarak bunları modelde uyguladık. Olağan modele göre, SEM ile önerilen eşdeğer model arasında çok daha iyi uyuşma gözlemledik. Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT), which is particularly useful when the membranes are driven in the nonlinear regime. A transient FEM analysis shows that CMUT exhibits strong nonlinear behavior even at very low AC excitation under DC bias. 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 themechanical 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 in a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared to the conventional model. 70

Bu tezde düşük frekanslarda yüksek güç gerektiren uygulamalara uygun 1. Sınıf fıçı tipi, gerilim ile bükülen (Barrel-stave flextensional) akustik dönüştürücü tasarımı tartışılmaktadır. Fıçı tipi dönüştürücünün iç kısmında halka tipi piezoelektrik malzeme bulunmaktadır. Bu piezoelektrik malzemeye elektriksel gerilim uygulanmakta ve piezoelektrik malzemede boylamsal hareket oluşturulmaktadır. Fıçının orta kısmında oluşan boylamsal hareketler kabuk kısmında bükülmelere neden olmaktadır. Bu şekilde fıçı mekanik bir transformatör etkisi göstererek eksenindeki küçük genlikli titreşimleri geniş fıçı yüzeyi marifeti ile ortama yükselterek aktarmaktadır.Fıçı dönüştürücü eşdeğer devre çözümlemesi MATLAB programı ile gerçekleştirilmiş ve fıçı parametrelerinin rezonans frekansı üzerindeki etkileri incelenmiştir. Fıçının kritik analiz çözümlemeleri sonlu eleman modelleme (Finite Element Model: FEM) yöntemi kullanılarak gerçekleştirilmiştir. ANSYS programında, fıçı, su içerisinde üç boyutlu modellenerek akustik performansı incelenmiştir. Akustik analizler piezoelektrik malzemeye gerilim uygulayarak, boşlukta ve suda dışbükey fıçı tipi dönüştürücü için gerçekleştirilmiştir. Fıçı boyutları, kabuk kalınlığı ve eğriliği, kabuk malzemesi gibi fıçının yapısal parametrelerinin, dönüştürücünün elektriksel giriş empedansı, elektro-akustik transfer fonksiyonu, rezonans frekansı, kalite faktörü üzerindeki etkileri incelenmiştir. Tasarlanan dönüştürücü ısıl analizleri sonlu eleman modellemesi ile gerçekleştirilmiştir. Dönüştürücü ölçüm sonuçları teorik sonuçlar ile karşılaştırılmıştır.Anahtar Kelimeler: Sualtı Akustik Dönüştürücü, Barrel-Stave, Flextensional Transducer, Düşük Frekans, Yüksek Güç, Piezoelektrik This thesis describes the design of low frequency, high power capability class-I flextensional, otherwise known as the barrel-stave, flextensional transducer. Piezoelectric ceramic rings are inserted inside the shell. Under an electric drive, ceramic rings vibrate in the thickness mode in the longitudinal axis. The longitudinal vibration of the rings is transmitted to the shell and converted into a flexural motion. Low amplitude displacements on its axis create high total displacement on the shell, acting as a mechanical transformer.Equivalent circuit analysis of transducer is performed in MATLAB and the effects of structural variables on the resonance frequency are investigated. Critical analysis of the transducer is performed using finite element modeling (FEM). Three dimensional transducer structure is modeled in ANSYS, and underwater acoustical performance is investigated. Acoustical analysis is performed by applying a voltage on piezoelectric material both in vacuum and in water for the convex shape barrel-stave transducer. Effects of transducer structural variables, such as transducer dimensions, shell thickness, shell curvature and shell material, on the electrical input impedance, electro-acoustical transfer function, resonance frequency and quality factor are investigated. Thermal analysis of designed transducer is performed in finite element analysis. Measured results of the transducer are compared with the theoretical results.Keywords: Underwater Acoustic Transducer, Barrel-Stave, Flextensional Transducer, Low Frequency, High Power, Piezoelectric. 119

ÖZETÇOĞUL FM RADYO KANALI KULLANILARAK PASİFTUTARLI KONUMLAMA RADAR SİSTEMLERİNDEUZAKLIK ÇÖZÜNÜRLÜĞÜ ARTIRMAAkif Sinan TaşdelenElektrik ve Elektronik Mühendisliği Bölümü Yüksek LisansTez Yöneticisi: Prof. Dr. Hayrettin KöymenOcak 2007Kooperatif olmayan aydınlatıcı olarak tek FM radyo kanalı kullanan pasif tutarlıkonumlama (PTK) radar sistemleri, makul Doppler çözünürlüğüne sahipolmalarına karşı, dar modülasyon bant genişliğinden, ve o an yayınlanın yayınıniçeriğine olan yüksek bağımlılıklarından dolayı sınırlı uzaklık çözünürlüğünesahiptirler. FM radyo kanal dağılımının gevşek şekilde denetlendiği bazıülkelerde bulanabileceği şekilde aynı vericilerden yayınlanan birden fazla FMradyo istasyonu kullanıldığı zaman uzaklık çözünürlüğünde bir gelişme eldeedilebilir. Önerilen sistem, aynı yerde bulunan FM vericisinden alınan direksinyalin öz ilinti fonksiyonunu hesaplar ve olası hedeflerden yansıyarak gelensinyalin direk sinyalle olan çapraz ilinti fonksiyonuyla karşılaştırır. Problemingeometrisi monostatik radar sistemlerindeki gibidir. Uzaklık bilgisi çapraz ve özilinti fonksiyonlarının arasındaki zaman farkından elde edilir. Modülasyon bantgenişliği 25 kHz olan tek bir FM istasyonu kullanıldığında 6 km uzaklıkçözünürlüğü elde edilmektedir. Bu tezde gösterilmektedir ki, yanyana bulunan 3FM radyo kanalının sinyallari kullanıldığında sinyal gürültü oranı (SGO)â33dB'ye inene kadar (ki bu 110 km uzaklığa tekamül etmektedir) aralarındakiuzaklık 3 km olan iki hedef başarıyla bulunabilir. 7 FM radyo kanalı ilebirbirlerinden 100 m uzakta bulunan iki hedefi tespit etmek mümkündür.Anahtar Kelimeler:.Pasif tutarlı konumlama, uzaklık çözünürlüğü, modülasyonbant genişliği, FM radyo, çapraz ilinti ABSTRACTRANGE RESOLUTION IMPROVEMENT IN PASSIVECOHERENT LOCATION RADAR SYSTEMS USINGMULTIPLE FM RADIO CHANNELSAkif Sinan TaşdelenM.S. in Electrical and Electronics EngineeringSupervisor: Prof. Dr. Hayrettin KöymenJanuary 2007Passive coherent location (PCL) radar systems that use single FM radio channelsignal as illuminator of opportunity have reasonable Doppler resolution, butsuffer from limited range resolution due to low modulation bandwidth and highdependence on the content that is being broadcasted from the FM station. Animprovement in range resolution is obtained by using multiple adjacent FMchannels, emitted from co-sited transmitters, which is often the case in largetowns. The proposed scheme computes the autocorrelation function of the signaldirectly received from the co-located FM transmitter, and compares it to thecross-ambiguity function, obtained from direct and target scattered signals. Thegeometry of the problem is like in the case of monostatic radar. The rangeinformation is obtained by the delay between the cross-ambiguity function andthe autocorrelation function. When a single FM channel that has a modulationbandwidth of 25 kHz is employed the range resolution is 6 km. It is shown thatdown to â33dB signal to noise ratio (SNR), which corresponds to a distance of110 km from the receiver, targets that are 3 km separated from each other canbe detected with 3 adjacent FM channels. It is possible to detect targets that are100 m separated from each other with 7 FM channels. The detection of the timedelays is a linear estimation problem.Keywords: Passive coherent location (PCL), range resolution, modulationbandwidth, FM radio, cross-ambiguity. 65

ÖZETSUALTI İLETİŞİMİ İÇİN GENİŞ BANDLI VEÇİFT-YÖNLÜ AKUSTİK ÇEVİRİCİ MODELLEMESİIşıl Ceren ElmaslıElektrik ve Elektronik Mühendisliği Bölümü Yüksek LisansTez Yöneticisi: Prof. Dr. Hayrettin KöymenNisan 2007Bu tezde yüksek frekanslı kısa mesafe sualtı iletişimi için iki seramik katmanlıçevirici yapısı irdelendi. Yapının bir oktav genişliğinde çalışma bandıbulunmaktadır ve merkez çalışma frekansı 350 kHz'dir. Çevirici yapısında ikielektriksel, iki akustik port vardır. Seramik katmanlar her bir ışıma yapanyüzeyden su yüküne çeyrek dalga kalınlığındaki eşleştirici katmanlar aracılığıylaeşlenmişlerdir. Yapının sudaki uzun akustik boyunu dengelemek için elektrikselportların birer birer kullanılması incelenmiştir. Bu şekilde geniş bir bant aralığındaçalışma sağlanmıştır. Yapının hüzme genişliği arka arkaya ışıyan elemanlardandolayı dardır. ABSTRACTDESIGN OF A WIDEBAND AND BI-DIRECTIONALTRANSDUCER FOR UNDERWATERCOMMUNICATIONSIşıl Ceren ElmaslıM.S. in Electrical and Electronics EngineeringSupervisor: Prof. Dr. Hayrettin KöymenApril 2007A two ceramic layer stacked transducer structure for short range underwatercommunications at high frequencies is studied in this work. The structure has awide bandwidth of one octave and operates at 350 kHz center frequency.Transducer structure inherently has two electrical and two acoustic ports. Ceramiclayers are matched to water load through quarter wavelength thick matching layerson each radiating face. Using electrical ports separately to compensate for the largeacoustic length of the structure in water is also investigated. It is shown that thewide bandwidth operation can be maintained. The beamwidth of the structure isnarrow due to end - fire effect of two back-to-back radiating elements. 104

Cataloged from PDF version of article. Acoustic waves are being used in several underwater applications, such as SONARs, underwater communication systems. Most of already developed and deployed underwater communication systems use narrow band communication and lacks layered communication approach. In this thesis, we propose a spread spectrum, layered architecture for underwater communication system, such as for SCUBA divers. The communication device shall be designed such that divers can communicate with each other in shallow water, short range in a multi-user fashion and provide not only voice communication but also data transmission as well. The device shall use Orthogonal Frequency Division Multiplexing (OFDM) as a spread spectrum technique. The OFDM technique is selected from other spread spectrum techniques due to it’s inherent ability to combat the channel impairments and flexibility of implementing the communication system using software defined radio (SDR). The spread spectrum system shall operate in 100 kHz to 300 kHz frequency band using wideband acoustic transducers. In this work, we studied a layered architecture for the communication device. We mainly studied the application layer, data link layer and physical layer in order to analyze the achievable data rate and performance. In this work, we tried to find the optimal communication parameters to achieve guaranteed communication performance for possible scenarios. The communication parameters are set in order to achieve best performance for the worst condition. Using the optimal parameters, the system shall occupy 5 users voice and data communication at the same time using the entire frequency band at the same time, however with certain Grade of Service (GOS) the capacity shall be increased. The capacity of the system shall further be increased if the system uses adaptive communication parameters that are adapted to changing channel and user conditions. The system using adaptive communication parameters shall provide at most 16 users’ voice and data communication using the entire frequency band at the same time. Öktem, Kemalettin Kerem M.S.

Cataloged from PDF version of article. The aim of this work is to design a complete PMR (Professional Mobile Radio) network based on CDMA 450 in Istanbul metropolitan region. Coverage and capacity analysis are performed by the developed simulation software tool. Trade off between several system parameters such as quality of service, user mobility, traffic distribution, data rate and bandwidth are considered to optimize the capacity and coverage of the network. Since, managing system interference by precise power control is a very critical concept for CDMA; forward and reverse link interference analysis, and their effects are also studied. Coverage and handoff areas are shown on the map by the aid of GIS (Geographical Information Systems) tool. System capacity and coverage are presented as the main results of the proposed network. Kızılırmak, Refik Çağlar M.S.

ÖZET PMR SİSTEMLERİNDE TRAFİK ANALİZİ VE MODELLENMESİ Başak Can Elektrik ve Elektronik Mühendisliği Bölümü Yüksek Lisans Tez Yöneticisi: Prof. Dr. Hayrettin Köymen Temmuz 2003 Analog PMR (Private Mobile Radio) istemlerinden dijital ve demetlenmiş PMR sistemlerine geçiş kriterlerini belirlemek için güvenilir trafik bilgisine ihtiyaç vardır. Bu çalışmada iki konu incelenmiştir. İlk olarak, analog PMR ağlanndaki kanal doluluk dağılımı analog bir PMR ağma ait olan teletrafik verisi kullanılarak modellenmiştir. Servis zamanlarının yoğunluğu 0.7 saniye gecikmiş bir üstel fonksiyon olarak bulunmuştur. Ortalama servis zamanı yaklaşık olarak 2.5 saniyedir. İletim demetli bir PMR sistemine gelen ses arama varışlarının dağılımının Poisson olmadığı gösterilmiştir. Bu sebeple, M/G/C (Poisson girişli ve genel servis dağılımına sahip C adet kanalı bulunan kuyruk) modeline dayalı olarak yapılan analitik ve simülasyon metodlan, sistemi G/G/C (genel giriş ve servis dağılımına sahip C adet kanalı bulunan kuyruk) modeli kadar iyi modelleyemez. Son on yıl içerisinde, bitakım demetlenmiş PMR sistemleri dizayn edilmiştir. Bunların birçoğu simetrik yer-uydu bağı ve uydu-yer bağı kanalı kapasitesine sahiptir. Bu sistemler, grup veya yayın konuşmaları ve veri iletişimi göz önüne alındığında spektrum verimliliği açısından etkili olamayabilirler. İkinci bir çalışma olarak, geniş bantlı OFDM (Orthogonal Frequency Division Multiplexing)'e dayalı bir uydu-yer bağından oluşan bir asimetrik PMR sistemi inceledik. OFDM' e dayanan uydu-yer bağı olarak Dijital Ses Yayım sistemi göz önüne alınmıştır. 2% hizmet niteliği ve 2.86 saniyelik bir ortalama konuşma süresi için sistemin destekleyeceği kullanıcı sayısı 315000 olarak saptanmıştır.Anahtar Kelimeler: PMR, ses trafiğinin modellenmesi, asimetrik PMR sistemi, ses arama varışları, kuyrukta bekleme süresi. vı ABSTRACT TRAFFIC ANALYSIS AND MODELING IN PMR SYSTEMS Başak Can M.S. in Electrical and Electronics Engineering Supervisor: Prof. Dr. Hayrettin Köymen July 2003 Reliable knowledge of traffic in PMR (Private Mobile Radio) systems is essential for assessing the issues in migration from analog to digital and trunked PMR systems. In this work, we investigated two concepts. First, we modeled the service time distribution of conventional PMR networks by using teletraffic data of a conventional PMR network. It is found that the density of the service time is a shifted exponential which is delayed by 0.7 second. The mean service time is about 2.5 seconds. We showed that voice call arrivals to a transmission trunked PMR network are not Poisson distributed. Analytical and simulation methods based on M/G/C (a C server queue with Poisson input and general service) models may not model the system as well as G/G/C (a C server queue with general input and general service) models. Several trunked 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. Second, we studied a new asymmetric PMR system comprising of a wideband OFDM (Orthogonal Frequency Division Multiplexing)-based downlink, such as Digital Audio Broadcasting (DAB) system. We found that for 2 % GoS and a mean service time of 2.86 second, PMR users that can be supported by the proposed system is 315000. mKeywords: Private Mobile Radio (PMR), voice traffic modeling, asymmetric PMR system, voice call arrivals to a PMR system, waiting time in the queue. IV 172

ÖZET şehir İçinde yayılım simülasyonu için işin İzleme algoritması geliştirilmesi ve uygulanması Gökhan Moral Elektrik ve Elektronik Mühendisliği Bölümü Yüksek Lisans Tez Yöneticisi: Prof. Dr. Hayrettin Köymen Ağustos 2002 Kullanımı gün geçtikçe artan radyo tabanlı iletişim yöntemleri, fiziksel olarak kısıtlı bir kaynak olan frekans spektrumunun daha verimli kullanılması gereğini doğurmuştur. Bunu sağlamak için son zamanlarda iç ve dış mekanlardaki alan şiddeti ve gecikme dağılımı gibi parametreleri hesaplamak için çok popüler olan ışın izleme yöntemi geliştirilmiştir. Bu yöntem, içinde bulunulan ve kanal olarak kullanılan ortamdaki yolların, caddelerin ve cisimlerin etkilerini ayrı ayrı in celediğinden ve maddelerin elektromanyetik özelliklerine bağlı olarak yansıma ve saçınımları hesapladığından dolayı diğer empirik yöntemlere göre daha doğru ve mantıksal sonuçlar vermektedir. Mobil ağ planlamasında kullanılabilecek değerli bir bilgi olan dürtü yanıtı bu yöntem ile bulunabilmektedir. Bu tezde ışın izleme algoritması geliştirilmiş, bir takım örnek şehir verisi kullanılarak test edilmiş, sonuçlar diğer empirik modellerin ve benzer ışın izleme yöntemlerinin sonuçları ile karşılaştırılmıştır. Anahtar kelimeler: yayılım tahmini, ışın izleme, radyo tabanlı iletişim, şehir içinde yayılım modeli. iv ABSTRACT DEVELOPMENT AND APPLICATION OF A RAY TRACING ALGORITHM FOR PROPAGATION SIMULATION IN URBAN AREAS Gökhan Moral M.S. in Electrical and Electronics Engineering Supervisor: Prof. Dr. Hayrettin Köymen August 2002 With the increasing demand on radio communication systems, the natural spec trum resource should be used more efficiently. In this thesis, a ray-tracing method is implemented in order to calculate signal strength values for outdoor wireless communication in an urban environment. The ray tracing method considers the electromagnetic properties of the communication environment and it gives more accurate results and more information about the channel in comparison to available empirical models. In this thesis, a 2D ray-tracing algorithm has been developed with some assumptions to optimize the speed while conserving the accuracy. The computer program developed is then used to simulate radio propagation in several example environments and the results obtained from the simulations are compared with the results obtained by using another ray-tracing computer software package. Keywords: propagation prediction, ray-tracing, wireless radio communications, urban propagation models in 81

ÖZET EGZERSİZ TESTİ SIRASINDA ÖLÇÜLEN QT DAĞILIMININ KALP HASTALIKLARININ TEŞHİSİNDEKİ ÖNEMİ Nabil Ben Ahmed Jemel Elektrik ve Elektronik Mühendisliği Bölümü Yüksek Lisans Tez Yöneticisi: Dr. Hayrettin Köymen Şubat 2000 Elektrokardiyogram(EKG), kalp tarafından bir kalp atımı sırasında oluşturulan elektriksel potansiyallerin kaydedilmiş halidir. Kalpteki anormallik ler sıradışı elektrik akımlarının akmasına neder olurlar ki, bu da kaydedilen EKG'de olağandışı morfolojiler olarak gözlenir. Bu morfolojilerin bazıları sadece kalp stres altındayken gözlenebilir. Egzersiz (Stres) EKG testi bu amaçla uygulanır. Çalışmamıza konu olan iskemik bozukluklar en önemli kardiyolojik bozuk luklardandır. Bu durum kalp dokusunun bir bölümü öldüğünde veya zarar gördüğünde ortaya çıkar. QT dağılımı (QTd) iskemik bozuklukları teşhis et mek için önerilen bir yöntemdir. QTd, en bilinen şekliyle, standart 12 EKG kanalı içinde en uzun ve en kısa QT aralıkları arasındaki fark olarak tanımlanır. Araştırmamızda egzersiz EKG testinin QTd üzerine etkisini incelenmiş ve bundan iskemik bozuklukların teşhisinde kullanılabilecek önemli bir bilgi eldeedilip edilemediğine bakılmıştır. Bu çalışma sırasında QT aralığını ölçmek için yeni bir yöntem geliştirilmiş ve eski yöntemlerle karşılaştınlmıştır. QTd her 10 saniyede bir hesaplanan averaj EKG sinyali (bir kalp atımı) üzerinde bu yöntem kullanılarak ölçülmüştür. Bu ölçümlerden zamana bağlı bir QTd eğrisi (QTd trend) çizilmiştir. QTd ve kalp hızı eğrileri kullanılarak, iskemik bozuklukların teşhisi için değişik yöntemler önerilmiştir. Bu çalışmamız göstermiştir ki, QT aralığı ölçümlerindeki iyileşmeye karşın önerilen yöntemlerden hiçbiriyle, QTd eğrilerinin kardiyolojik bozuklukların teşhisinde klinik değer taşıdığı gösterilememiştir. Anahtar Kelimeler. QT aralığı, EKG, QT dağılımı, Egzersiz EKG testi, Stres EKG testi, İskemik bozukluklar vı ABSTRACT SIGNIFICANCE OF QT DISPERSION AS A DIAGNOSIS TOOL FOR CARDIAC PATIENTS Nabil Ben Ahmed Jemel M.S. in Electrical and Electronics Engineering Supervisor: Prof. Dr. Hayrettin Köymen February 2000 Electrocardiogram (ECG) is the recorded electrical potentials generated by the heart during a cardiac cycle. Cardiac abnormalities cause unknown current flows leading to strange waveform morphologies in the recorded ECG. Some of these abnormalities are only visible when the heart is under stress. Exercise ECG is conducted for this reason. Ischemia is one of the important cardiac abnormalities and is the focus of our study. It occurs when a part of the heart tissue dies or is injured. QT dispersion (QTd) is a proposed method for diagnosing Ischemia. The classical definition for QTd is the difference between the maximum and the minimum QT intervals within the 12 leads. In this study the effect of exercise on QT dispersion (QTd) is studied and whether QTd could give significant information for diagnosing patients with ischemic heart disease is investigated. A new method for measuring QT interval mis developed and is compared with previous methods. QTd is measured on average beats calculated for 10 seconds intervals throughout the exercise ECG test and a trend curve which we call QTd, is generated. Several decision rules for the diagnosis of cardiac patients are proposed by analyzing these QTd trend curves and the accompanying heart rate curves. It is shown that despite the improvement in QT interval measurements, none of the decision rules proved to be a clinically useful discriminate of cardiac patients, with sufficient confidence. Keywords: Exercise ECG, QT interval, QT dispersion, Ischemia. IV 93

ÖZET AKUSTİK MİKROSKOPİ İÇİN VERİ EDİNME SİSTEMİ TASARIMI Nadir Zafer Baştürkmen Elektrik ve Elektronik Mühendisliği Bölümü Yüksek Lisans Tez Yöneticisi: Prof. Dr. Hayrettin Köymen Ağustos 1996 Geleneksel akustik mikroskopların, tarama mekanizmalarının karmaşıklık ve düşük hız sorunları vardır. Bu aletlerin fiyatları yüksekken, çerçeve hızları düşüktür. Bu çalışmada, yüksek hızlı, basit, dolayısıyla ucuz bir tarama mekanizmasına sahip bir akustik mikroskop için kesin konumlarda satır imge verisi edinen bir veri ed inme sistemi tasarlanmıştır. Tasarım, bu sistemde kullanılmak üzere tasarlanmış bir veri edinme tümdevresi üzerine kurulmuştur. Gerçekleştirme, sistemin standart bir kişisel bilgisayara monte edilmesine olanak sağlayacak biçimde yapılmıştır. Bu yoEa bilgisayar ekranında saniyede 50 satır hızla akustik imge elde etmek mümkündür. Anahtar Kelimeler : Akustik mikroskopi, akustik imgeleme, X-Y tarayıcısı, imge veri edinme. iv ABSTRACT DATA ACQUISITION SYSTEM DESIGN FOR ACOUSTIC MICROSCOPY Nadir Zafer Baştürkmen M.S. in Electrical and Electronics Engineering Supervisor: Prof. Dr. Hayrettin Köymen August 1996 Conventional acoustic microscopes suffer from the complexity and low speed of their scanning mechanisms. The frame rates of these instruments are low, while their cost are high. In this work, a data acquisition system for acquiring raster image data at precise positions was designed for an acoustic microscope which uses a high speed, simple, and hence low cost scanning mechanism. The design is based on a data acquisition integrated circuit which is specifically designed to be used in this system. The implementation was done in sucha way that the system can be mounted to a standard personal computer. It is possible to obtain acoustical images on the monitor of the personal computer at rates as high as 50 lines per second, using this system. Keywords : Acoustic microscopy, acousting imaging, X-Y scanner, image data ac quisition. m 75

Ankara : The Department of Electrical and Electronics Engineering and Institute of Engineering and Sciences of Bilkent University, 1996. Thesis (Master's) -- Bilkent University, 1996. Includes bibliographical references leaves 44-46 Exercise electrocardiography (the exercise ECG test or the stress EGG test) is one of the most popular and the most important non-invasive diagnostic tests in the field of cardiovascular disease. Arrhythmia analysis is an important part of the exercise ECG. A new approach to arrhythmia analysis is proposed in this thesis. 12 lead ECG signal is first reduced into three orthogonal channels which contain all the power of ECG. The orthogonalization process, an online Singular Value Decomposition (SVD) algorithm, maintains that these channels are free from both baseline wander and EMG noise. The third output channel has very low power with respect to first two. Making use of the orthogonality of these new channels. Total Power Signal (TPS) is calculated by summing the squares of orthogonalized channels. Employing the first two channels in TPS yields 92-99% of ECG power contained in all channels. Any arrhythmic behaviour during exercise test effects the TPS. In order to obtain the fiducial points of QRS complex first derivative of TPS is calculated. The method is compared with an algorithm proposed previously. The method and the algorithm are tested on 22 complete stress ECG test each with a duration between 9.5 to 26.5 minutes. Çağlar, B Kerem M.S.

Ankara : Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 1996. Thesis (Master's) -- Bilkent University, 1996. Includes bibliographical references leaves 69-70. Electrocardiogram (ECG) is the measurement of potential differences occurring on the body due to the currents that flow on the heart during diastole and systole. Cardiac abnormalities cause uncommon current flows, leading to strange waveform morphologies in the recorded ECG. Since some abnormalities become visible in ECG only during activity, exercise ECG tests are conducted. The sources of noise during an exercise test are electro myogram (EMG) due to increased muscle activity and baseline wander (BW) due to mechanical motion. Frequency band filtering, used to eliminate noise, is not an efficient method for filtering noise because usually frequency spectra of the interference and the ECG overlap. Rather, a fast morphological filter is required. This thesis is focused on an online filtering approach which separates noise and ECG signals without changing the morphology. The redundancy present in standard 12 lead ECG records is made operational by a Singular Value Decomposition based orthogonalization of the input signals. ECG is represented in a minimum dimensional space whose orthogonal complement takes on noise. The signals in this low dimensional space are used to reconstruct the input signals without noise. Noise elimination also improves data compression. A comparative study of the ST analysis of original and reconstructed signals is presented at the end. Acar, Burak M.S.

Ankara : Department of Electrical and Electronics Engineering and Institute of Engineering and Science, Bilkent Univ., 1992. Thesis (Ph. D.) -- Bilkent University, 1992. Includes bibliographical references. For phased array medical ultrasound imaging, digital beamforming techniques and VLSI circuits are studied to form a fully digital receive front-end hardware. To increase the timing accuracy in beamforming, a computationally efficient upsampling schme is examined. A digital beamforming method based on receive focusing at the raster focal points is proposed. 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 performance of the two beamforming schemes. For adaptive beamforming, a phase aberration correction technique with very low computational complexity is described. Image quality performance of the technique is examined by digitally processing the non-aberrated and aberrated phased array experimental data sets of an ultrasound resolution phantom. VLSI circuits and their implementations for the proposed techniques are presented. Karaman, Mustafa Ph.D.