The Study Of Microstrip Dual-band Filter For Wireless Communication

With the development of wireless communication, the study for filters which function as an important part in communication system is making progress constantly. Now almost all the work that has been done for filters focuses on new structures for the single band-pass filter, however, recently, microwave diplexer has played an important role in wireless communication, to realize the dual-band characteristic, designing two independent circuits that work in different frequencies is one method while we can cont-rol the whole circuit with a switch. But that will make the whole circuit more complex, in the meantime it will need a really large room to layout the circuit and increase the cost. To overcome these drawbacks, the other way is to mix the circuit together with one circuit. Thus, this paper studies the theory of dual-band filters and designs several filters that work at the frequency of wireless communication.Firstly, the paper discusses the basic theory for filter, it includes three attenuation types of filters, the main technical specification, design methods and how to use inverter to divert the filter circuit.Secondly, the paper introduces a theory that transforming frequency and circuit twice to get dual-band filter characteristic. And the first dual-band filter is designed with this theory. The filter works for GPS, while the two center frequencies are 1.227GHz and 1.575GHz. After calculating the components parameters in software Mathcad, these results are transformed to schematic circuit in ADS. The simulation results tell us the theory is correct. To get a good performance, the filter structure is optimized in ADS. Then optimization result is imported to HFSS, and the simulation result is good. Another filter which works for GSM is also designed (the passband are 890MHz-915-MHz and 1710MHz-1785MHz) with the same way, but the result shows us that this kind of structure can't realize dual-band characteristic which has such a high frequency ratio. Thus paper proposes another kind of design theory. The filter is realized by stepped impedance transmission line and it can realize dual-band characteristic at GSM pass-band. Also filter parameters are calculated in Mathcad, and then it is simulated in ADS and HFSS. Finally, the GSM dual-band filter is fabricated, and the measurement result matches well with the simulation result. To validate the new theory, paper designs and fabricates another filter which works in WLAN dual-passband, and it works out very well.