Research And Implementation Of Microwave Active Filter

Filter is widely used in microwave circuits and system. Owing to the improvement of MMIC circuit technology, plenty of developments have been built on the research of miniaturized and integrated microwave circuits and system. However, tranditional passive filters can hardly contribute to this trend, generally with a fairly large insertion loss. Fortunately, novel active filters show great potentials to those urgent demands. This thesis focuses on the reseach and implementation of active filter in microwave frequencies. Three typical kinds of active bandpass filters are fabricated, and the details are as follows:Firstly, active Q-enhanced filter is studied and synthesis approach of bandpass filter is reviewed. A second-order bandpass filter using active capacitance circuit is designed. Its center frequency is 1.OGHz and its insertion loss is less than 2dB in the passband. In addition, its noise figure is 4.Secondly, gyrator-C active filter is analyzed. It has been confirmed that active inductor can be realized by one gyrator, which consists of two operational transconductance amplifiers, loaded by a capacitance. According to this fundamental principle, a new structure of active inductor is presented, with the insertion of a suitable variable passive compensation network. Based on the particular active inductor, a one-order bandpass filter is fabricated. Its center frequency is 2.92GHz and the 3dB bandwidth is only 140MHz. Therefore, the quality factor is about 21.Thirdly, both the active Q-enhanced filter and the gyrator-C active filter show great performance at lower microwave frequencies. In higher frequencies, it's much better that active filters are realized by distributed circuit form. The active device, in the distributed active filter presented here, is designed to be coupled to the tank circuit. It makes the characteristics of the structure easy to predict. The fabricated fixed frequency filter has a good quality factor of 38. Moreover, a varactor diode is used in series with the former structure, helping the resonant frequency become tunable. The measured tunable filter has a wide tuning range from 9.25GHz to 9.55GHz, and its quality factors are about 50.Compared with passive filter, active filter takes on plenty of advantages while many unsolved difficulties should not be ignored. Finally, this work is summarized and the suggestion about the future study is discussed.