Date

2016

Author

Arash, Ebrahimi Jarihani

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Digital Step attenuators are used in many communication systems to regulate the signal level to achieve maximum system performance. The figure of merit of these attenuators are linearity, accuracy, and their power handling capabilities. Attenuators currently offered in the industry performs well to meet stringent requirements of the modern communications systems. However, all step attenuators currently offered introduces a phase-shift with respect to the attenuation setting and frequency. This phase-shift creates problems in communications systems, where multiple antennas and phased array systems are employed. In phased array systems, in order to create desired beam formations, the amplitude excitation and the phase excitation of each element in the array should be controlled accurately. In most applications, the amplitude excitation value is controlled with a digital step attenuator, whereas the phase excitation is controlled with a digital phase shifter in the transmitter path. In such systems, it is very important for the amplitude and the phase excitation values to be controlled independent of each other to simplify the design of a phased array system. Therefore, the development of phase coherent digital step attenuators is crucial for communication systems. This thesis presents novel digital step attenuators (DSAs) with very low phase variation under attenuation state and frequency changes. Thanks to their phase coherency, the presented attenuators are very useful in phased array systems, where attenuation and phase of each element should be adjusted accurately to achieve desired beam formation and null points. Several methods to address the phase shift introduced by the low-pass, high-pass filter response of the attenuator core under insertion loss and attenuation states respectively are investigated, while keeping all other specifications (linearity, accuracy, etc.) on par with the state of the art. Compensation circuit include inductive and capacitive switchable blocks. Firstly, an ideal attenuator is designed and implemented for the proof of concept. Then, 3D EM simulations are utilized to analysis the effects of the wirebonds inside a commercial package. With these data, a mathematical model is implemented for an attenuation block. Based on the mathematical model, an automated MATLAB program with a graphical user interface (GUI) is developed which is used to determine the initial values for compensation elements. With the initial values gathered from the MATLAB program, the attenuators are implemented in Cadence Design System and compensation elements are further optimized. The attenuators are integrated with the digital control circuitry on the same chip. Finally, the fabricated chips (encapsulated in commercial packages) are soldered on an printed circuit evaluation board for measurements. Unlike previous studies on phase coherent DSAs, the presented work achieves very low phase variations in a quad-flat no-leads (QFN) package, where wirebond effects are significant. The presented attenuators have 6-bit and 7-bit control range and achieves accurate attenuation settings in 0.5 dB and 0.25 dB steps respectively, in a very wide frequency range covering LTE (Long-Term Evaluation) 2600 and all GSM bands (100 MHz to 2.7 GHz). The attenuators are fabricated on a commercial RF Silicon-On-Insulator (SOI) process. These attenuators demonstrate an performance far better than the any attenuator developed to date (both commercial and research). The measurements results show that the attenuators exhibit amplitude and phase errors of less than 1.5 dB and ±3o , respectively up to 2.5 GHz, where the insertion loss is less than 3 dB. Worst case return loss is -14 dB across the complete frequency band. Moreover, very low phase shift is achieved without sacrificing linearity. The input 1 dB compression point and the input third order intermodulation point are measured at 33 dBm and 49 dBm respectively. Total chip size, including pads, is 1.95 mm × 0.95 mm.

Subject Keywords

Wave guides., Radio attenuators., Digital Step Attenuators.

URI

http://etd.lib.metu.edu.tr/upload/12620421/index.pdf 
https://hdl.handle.net/11511/25945

Collections

Graduate School of Natural and Applied Sciences, Thesis