The Design And Research Of Fractal Structure Microstrip Bandpass Filter

RF filter is usually located in the receiver and transmitter terminal of the wireless communication system,which has the selection function of the frequency band and channel,and can play the role of eliminating harmonic.So the RF filter performance will directly affect the performance of the whole communication system.At present,the development of wireless communication technology is very rapid,which raises more and more requirements for RF filters performance,such as bandpass loss,stopband loss,small volume.Microstrip line structure of the filter,with its small size,light weight,wide frequency band,easy processing and matching,easy integration,has become a research hot spot in the field of microwave integrated circuits,and has been widely applied.The microstrip line theory and microstrip filter principle was introduced in this paper.The traditional microstrip coupling bandpass filter was designed.Then the two kinds of Koch fractal curve was used into the traditional microstrip coupling unit structure of the narrow gaps,and the design was adjusted and optimized to change the current path.It reduced the difference of the phase velocity of mode and even mode,which suppress the second harmonic,while achieve good pass band performance.The simulation results show that:a center frequency of 2 GHz microstrip ladder coupled bandpass filter of the traditional structure was designed in this paper.Compared to the traditional coupling unit structure,fractal coupling unit improves the second harmonic suppression filter by 25dB,and the passband insertion loss is less than-0.55dB.Through optimizing the fractal coupling unit,the second harmonic of filters can further decrease by 37dB,while the passband performance is not affected.The measured results of the filter show consistent match with its theoretical ones.A cross coupling dual-band micro-strip band-pass filter was proposed based on the quarter wavelength stepped impedance resonator(SIR)in this paper.The K inverse converter was realized by loading metal short-circuit via at the end port of the SIR,and J inverse converter was achieved by internal couple capacitor between the two resonators.The 4th-order dual-band band-pass filter was realized by designing the cascaded coupling parameters of the adjacent SIR and the cross coupling parameters between nonadjacent SIR.The simulated results indicate that the central frequency of the dual-band micro-strip band-pass filter is 3.76GHz and 8.67GHz,the central frequencies ratio is above 2.For the first pass-band,the measured results show that the minimum insertion loss is-0.43dB,and the return loss is-22.09dB.For the second pass-band,the minimum insertion loss is-1.13dB and the return loss is-35.61 dB.The measured results match with the simulated ones.This filter has the simple structure and low loss.Finally,through the fractal optimization to the coupling unit of narrow gap,the stop band becomes wider,transition zone much steeper.The result of this research has important reference value for the fractal structure design in research and design of the small filter.