The Study Of Dual-band Bandpass Filter Using Stepped Impedance Resonators

In recent years, the microwave / RF devices feature a dual-band characteristic have received widespread concern; the use of such devices can greatly simplify the design of dual-mode devices, reducing device size and lower power consumption. Due to the important role of the bandpass filters in communication systems, The dual-band bandpass filters become indispensable components in such dual-mode devices.This paper summarizes and compares the domestic and foreign dual-band bandpass filter design methods, proposes another one which is suitable for planar circuits such as microstrip circuits, and bases on traditional coupled resonator filter design method which uses external quality factor and coupling coefficient as design parameters. The method takes advantage of the inherent periodical property of RF circuits, makes use of the spurious response of RF resonators to design the second passband of dual-band bandpass filter. Since the spurious response of conventional resonators occurs only at integer multiples of the main response, but the two center frequencies in actual dual-band bandpass filter specifications are often not an integer multiple relationship, therefor only the resonators capable of configuring the location of the spurious response can meet the requirements, and the Stepped Impedance Resonator (SIR) just have this feature.After the introduction of the characteristics and design formulas of stepped impedance resonator, and the coupled resonator filter design method, as well as the extraction methods of external quality factor and coupling coefficient of couped resonators, a coupling structure of the dual-band bandpass filter was presents. In the coupling structure the SIRs are the intersections of the two coupling paths, so the external quality factor of SIR at main response and first spurious response should be analyzed. The electric field distribution at the main resonance and the first spurious response reveals that the different configurations of external quality factor are possible. The design chart of external quality factor was made when the feed line is tapped line.After drawing the design charts of coupling coefficient of coupled resonators using the specific extraction method, a dual-band bandpass filter was designed based on the design method of coupled resonator filter and previous design charts. The electromagnetic simulation of the filter reveals that there was remarkable difference with the filter specifications; the existence of cross-coupling of nonadjacent resonators is the main causation, it is necessary to optimize the design parameters. After updating the circuit design, the simulation result reveals that the characteristics of the filter have been greatly improved. The results of this paper have certain value to the application of SIR in RF circuits and the design of dual-band bandpass filters. The coupling structure in this paper may not suitable to higher order dual-band bandpass filters because of excessive insertion loss when adopt almost twice resonators compared with conventional filters, so the design method of dual-band bandpass filter using only one coupling path and all the resonators are identical SIRs will be the further research and exploration.