A Study Of Defected Ground Structure Filters And Thin Film Bulk Acoustic Resonator Filters

The increasing demand for communication services inspires the need for filters with high performance such as miniaturization, high sensitivity and easy integration. In recent years, there have been increasing interests in Defected Ground Structure (DGS) for its merits such as simple configuration, small size and easy integration. At the same time, thin Film Bulk Acoustic Resonator (FBAR) develops rapidly. Comparing with the conventional Surface Acoustic Wave (SAW) filters and medium filters, FBAR has advantages such as high working frequency, large power capability and small dimension. It will become the leading filter for the wireless terminals in future.In this thesis, a brief introduction to the DGS and FBAR technologies is given, and the operating principles of DGS and FBAR are described. Based on the DGS theory, a novel DGS with the Hilbert fractal structure is then proposed, and its equivalent circuit is given. The effect of the dimensions to its performance is also discussed. Moreover, the parallel open stubs are loaded to the proposed DGS to enhance the loss of stopband. By cascading several modified cells, a lowpass filter with low insertion loss, high sensitivity, and small size is designed and fabricated. The measured results agree well with the simulated results. A hybrid lowpass filter with the proposed DGS and dumbbell DGS is also developed and fabricated. Simulated results show that it has a very wide stopband. Finally, MATLAB programs are written to study FBAR. With the help of these programs, some bandpass filters with good performance are designed. By introducing the equivalent permittivity method, FBAR filters are simulated with the HFSS software. The simulated results obtained by the HFSS software are compared to those by the MATLAB programs.