Yazar "Demirtas, Mehmet" seçeneğine göre listele

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    Analysis and design of a transimpedance amplifier based front-end circuit for capacitance measurements
    (Springer Int Publ Ag, 2020) Demirtas, Mehmet; Erismis, Mehmet Akif; Gunes, Salih
    In this study, transimpedance amplifier based front-end circuits which can be employed to measure small capacitances were designed, analyzed and simulated using analog electronic circuit simulator. The front-end circuit converts the current flowing through the measured capacitance into a modulated voltage value which contains information regarding the desired capacitance. The frequency-domain, time-domain, stability and noise analyzes were carried out numerically and in simulation environment using a circuit simulator. The analytical, numerical and simulation results can be used to design optimized, precise and stable transimpedance amplifiers with low-noise value. The measured capacitance value was 10 pF which is low enough to simulate various real-world applications. Three commercially available, off-the-shelf operational amplifiers with different peripheral passive components were employed for computer based analysis. The designed transimpedance amplifiers are suitable to connect with capacitance extraction circuits which use analog or digital demodulation techniques.
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    A Lossy Capacitance Measurement Circuit Based on Analog Lock-in Detection
    (Kaunas Univ Technology, 2020) Demirtas, Mehmet; Erismis, Mehmet Akif; Gunes, Salih
    This paper presents a lossy capacitance measuring circuit which is based on analog lock-in detection technique. Lossy capacitance can be modelled as a pure capacitor connected in parallel with a resistor. The measurement circuit mechanism consists of an excitation signal to drive the lossy capacitance, a transimpedance amplifier to produce a voltage, and a lock-in detection circuit to extract lossy values of capacitance. The lock-in detector multiplies its input with a square wave using switches and filters out high frequencies to give a DC output that is actually in proportional to the measured values. A field programmable gate array is employed to generate direct digital synthesis based sinusoidal excitation signal to generate reference signals required for demodulation and to measure the output of lock-in detection. The phase shift between the excitation signal and reference signals is controlled accurately in digital domain. Thus, due to the phase mismatch, errors are properly reduced. Also, analog phase shifter and analog switch-driving circuits are no longer required. Three different lossy capacitors realized using discrete components are simulated and tested. The maximum relative error is 1.62 % for the resistance measurement and 6.38 % for the capacitance measurement.
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    A new RGB color image encryption scheme based on cross-channel pixel and bit scrambling using chaos
    (Elsevier Gmbh, 2022) Demirtas, Mehmet
    A new RGB color image encryption algorithm is presented in this paper. The hyperchaotic two-dimensional logistic-sine-coupling map (2D-LSCM), which was previously suggested in the literature, is used in this study to generate the necessary parameters for the cross-channel pixel and bit scrambling, and the diffusion stages. The SHA-512 hash value of the plain color image is used to calculate the initial values of the map. In the pixel-level scrambling stage, a novel cross-channel scrambling method is implemented. The pixels in each channel are transformed by exchanging rows and columns between different channels according to the random sequences generated by 2D-LSCM. The four most significant bits of each channel's pixels are concatenated and circularly shifted horizontally and vertically in the cross-channel bit scrambling stage. Finally, in the diffusion stage, each channel's scrambled pixels are diffused using the diffusion sequences that are controlled by the control sequences. To assess the security performance of the presented method, several analyses such as key sensitivity analysis, statistical analysis, information entropy analysis, differential attack analysis, and noise & data loss analysis are performed. The experimental analyses and comparisons with other methods show that the proposed method can be used to efficiently encrypt color images.
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    A novel multiple grayscale image encryption method based on 3D bit-scrambling and diffusion
    (Elsevier Gmbh, 2022) Demirtas, Mehmet
    Since a large number of digital images are shared widely in our daily life, the demand and need for multiple-image encryption algorithms are increasing to ensure the security of the shared images. To present a new perspective to this request, a secure multiple-image encryption method based on 3D bit-scrambling and diffusion operations is proposed for grayscale images in this paper. A three-dimensional bit array is created by extending the eight bit planes of each grayscale image to a third dimension. This 3D bit array is pseudo-randomly scrambled by changing the order of the rows, columns, and layers. The scrambled bit planes are then used to reconstruct the shuffled images. A controlled diffusion operation is also applied to the shuffled and merged images. Both scrambling and diffusion parameters are obtained separately by iterating the chaotic Henon map twice. The initial values of the Henon map are plain images dependent. Several performance analyses including key space and sensitivity, histogram, correlation, entropy, dif-ferential, data loss, noise, and running time are carried out to test the proposed method's resis-tance against various cryptanalytic attacks. Simulation results show that the proposed method can encrypt multiple images effectively and securely.
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    Tendon Interposition and Ligament Reconstruction with ECRL Tendon in the Late Stages of Kienbock's Disease: A Cadaver Study
    (Hindawi Publishing Corporation, 2013) Karalezli, Nazim; Uz, Aysun; Esmer, Ali Firat; Demirtas, Mehmet; Tasci, Arzu Gul; Kutahya, Harun; Ulusoy, Gurhan
    Background. The optimal surgical treatment for Kienbock's disease with stages IIIB and IV remains controversial. A cadaver study was carried out to evaluate the use of coiled extensor carpi radialis longus tendon for tendon interposition and a strip obtained from the same tendon for ligament reconstruction in the late stages of Kienbock's disease. Methods. Coiled extensor carpi radialis longus tendon was used to fill the cavity of the excised lunate, and a strip obtained from this tendon was sutured onto itself after passing through the scaphoid and the triquetrum acting as a ligament to preserve proximal row integrity. Biomechanical tests were carried out in order to evaluate this new ligamentous reconstruction. Results. It was biomechanically confirmed that the procedure was effective against axial compression and distributed the upcoming mechanical stress to the distal row. Conclusion. Extensor carpi radialis longus tendon has not been used for tendon interposition and ligament reconstruction in the treatment of this disease before. In view of the biomechanical data, the procedure seems to be effective for the stabilization of scaphoid and carpal bones.