Microelectronics Engineering

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http://etd.aau.edu.et/handle/12345678/194

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Browsing Microelectronics Engineering by Author "Mohammed, Abdo (Prof.)"

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    FPGA Based Optimum-Settling Automatic Gain Control Circuit Design for Multistage Amplification
    (Addis Ababa University, 2018-03) Abreha, Teklu; Mohammed, Abdo (Prof.)
    Many electronic applications require their input signal to be of constant amplitude. Especially, in communication systems, the transmitted signal may be faded and the amplitude of the received signal may be below the desired level due to multi-path fading and different positions. This thesis work solves problems of inconsistent amplitude signal arrival at receiving sides of electronic systems. Automatic Gain Control (AGC) circuits are mainly used in such applications to adjust the gain of a Variable Gain Amplifier (VGA) and to provide relatively constant amplitude signal to the receivers. This work makes use of multi-stage amplification circuits to increase the dynamic range of the AGC. Three amplification stages are designed which increased the gain from 21 dB in single stage to 63 dB in three stages. Due to their flexibility better than ASICs as well as high performance speed and better power efficiency than microcontrollers, FPGAs are preferred in modern high speed technologies. Consequently, this thesis work used FPGAs to generate control voltages to adjust the gain of VGAs in the multistage amplification circuit. The designed AGC employs both feedback and feedforward loops to optimize the settling time. Feedback loop is used in input stage and feedforward loop is used in intermediate and output stages. When the amplitude level of the input signal diverges from the desired level, the FPGA generates a control voltage that adjusts the gain of the VGA and the result is an output signal with consistent amplitude level. A reference input of 25 mVpeak is considered and the AGC circuit is tested with input signals of amplitude below and above the chosen reference value. For all tested inputs a constant amplitude signal of 87 mVpeak is achieved.
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    Study and Evaluation of Multilayer PCB Manufacturing Inaccuracies on Malfunctioning of Circui
    (Addis Ababa University, 2018-06) Hgigat, Aregawi; Mohammed, Abdo (Prof.)
    The PCB is a mechanical base used for interconnection of electronic components. In PCB manufacturing industry, the quality and reliability of circuits is highly dependent upon the accuracy of the manufacturing processes. Because of the different factors affecting the processes to function imperfectly, manufacturing processes inaccuracies like photolithography inaccuracy, etching inaccuracy, drilling inaccuracies and inaccuracies in other processes may be encountered. These inaccuracies cause severe problems that may result in degradation of circuit performance and malfunctioning of circuits. In this research the levels of effects of the manufacturing inaccuracies on the malfunctioning of PCB circuits is investigated and analyzed. The effects of each process inaccuracies on circuits as well as how the processes are related with the affecting parameters are addressed in this thesis. A four-layer multilayer board is considered and the software selected to study the effects of manufacturing inaccuracies is OrCAD PCB designer. The major PCB manufacturing processes considered are Photolithography, Etching, and Drilling. Schematic development, PSpice simulation and layout design are made for selected circuits to study the effects of inaccuracies. According to the results obtained, drum speed, resolution and ratio of track to track spacing are the affecting parameters in Photo plotting while depth of focus, dose and mask bias variations are the factors affecting photolithography. In addition, inaccurate etching results in either larger or smaller width of traces, leaves unwanted conductive materials, creates unnecessary holes or open circuits and short circuits. In the case of drilling, when there is wrongly or inappropriately drilled, the top and bottom layers may not be connected and creates open circuit. In the over etching case simulation, 64 % of the output voltage reduction occurred for a 69% increase in resistance. This simply considering only one simple scenario the effect will be even worsened when similar individual effects are added to it. For capacitive effects, the output becomes unstable and oscillating.