The Design And Realization Of Tunable Bandstop Filter

With the rapid development of wireless communication systems,radio frequency receiver is developing towards the miniaturized,highly integrated and high frequency selectivity direction.There are some problems about the filter bank such as its bulky volume,complex structure and low frequency selectivity.So the filter band as the key device of the radio frequency receiver is gradually being replaced by the tunable filter.This paper has designed two types of tunable bandstop filters,one is the continuously tunable bandstop filter and the other is the digital tunable bandstop filter.The specific contents are summarized as follow.Firstly,This paper introduces the resonance conditions of the multi-mode resonator(MMR)and proposed the design steps about how to design the UWB filter with the multi-mode resonator structure.Then this paper presents the structure of the RF-MEMS capacitive shunt switches and analysis its equivalent and RF performance.The contents described above is necessary for later tunable filter design.Sencondly,A continuously tunable bandstop filter is designed.The design is divided into two parts: The first part is the design of the bimodal microstrip line structure,which can realize the controllable bandwidth and this paper has presented the detailed theoretical analysis about controllable bandwidth;the second part is the design of the bias circuit in which the varactor as the tunable device is be modeled and simulated,and the relations of the capacitance and voltage characteristics about the varactor is analyzed.Then the bias circuit can be achieved through reasonably loading varactor,DC power,choke and other devices.Finally,the performance simulation,physical production and test results demonstrates the effectiveness of the design method.The tunable range is from 1.31GHz~1.55 GHz and the-10 dB bandwidth maintains constant with the value of 35 MHz.Finally,a digital tunable UWB filter is designed.The design is also divided into two parts: the first part is the design of UWB filter with notch band,This paper first chooses the multi-mode resonator structure for UWB filter design,and then analysis the uniform impedance resonator(UIR)with the cascaded network analysis methods through which the resonance condition of the UIR and the variation of notch band center frequency can be achieved;the second part is the design of the RF-MEMS switch network design.Theequivalent circuit of the RF-MEMS switch network has been analyzed and the S-parameter of the RF-MEMS switch network has been simulated.Finally,RF-MEMS switch network is loaded on the UWB filter and then the structure has been simulated.The simulated results demonstrates that the filter achieves the tunable range from 3.1GHz to 10.6GHz and four tunable states which is 6.4GHz,6.8GHz,7.0GHz and 7.8GHz.