Tunable And Switchable Filters Based On Defected Ground Structures

With the rapid progress and wide application of wireless techniques, the RF filters play more and more important roles in the communication systems. Their performances are the key point of the systems. Traditional filters fail to fully utilize the limited frequency resource, so the reconstruction filters are introduced especially the tunable and switchable filter with good performance This paper is focused on the application of the Defected Ground Structure (DGS) and Stepped-Impedance Resonator (SIR) in filters in order to design compact tunable and switchable filters.The work involved in this thesis includes two main parts, as followsFirst,a tunable lowpass filter based DGS with varactor and SIR has been designed and implemented. This lowpass filter has a tuanable range from 0.94GHz to 1.2GHz; the range is about 24%. The measurement results indicate that the proposed compact filter has a return loss in the passband above 20dB but has a narrow stopband. SIR, in place of mircrostrip, is introduced to produce a new resonatant frequency in the stropband, due to its characteristic of controllable harmonic resonating frequency. According to the simulated results, it is shown that the performance in the passband does not change, but the stopband is widened much more than the previous filter.Second, a switchable dual-band bandpass filter based on DGS resonator and SIR loaded with PIN diode has been designed and implemented. The two resonators respectively achieve the bands of 900MHz and 1800MHz. From the measured results, it can be observed that when two bands are both open, the insertion losses of the bands are 1.9dB and 2.2dB respectively; when the first band is open but the second band is close, the insertion loss is 1.9dB and the isolation in the second band is about 34dB. Otherwise, the insertion loss of the second passband is about 2.3dB and the isolation of the first one is about 23dB. When the two bands are both close, the isolation of the bands are 21dB and 35dB respectively. The advantages of the filter proposed in this thesis are shown below: the first one is that the two switchable passbands are achieved by two resonators independently; the second one is that the proposed filter is compact because of implementing the DGS resonator techniques.