Reserch Of Harmonic Suppression In Microstrip Filters

With the current rapid development of mobile communication technologies, multi-frequency working mode becomes very common, so the demand of isolating frequencies is becoming more strict and microwave filters are playing more and more important rules in the communication systems. Microstrip filters, with its smaller size,easy manufacturing process by light-etch,easy integration with other active components and wide frequency applications using variable medium materials, becomes the most common used component in RF and microwave circuits.However, due to the periodicity of transmission line's frequency response, spurious responses would appear at the integral times of the main passband, which is to cause the asymmetry between the upper and lower stopbands and strongly restrict its applications. Spurious responses are caused by unequal phase velocities between the odd and even mode microwave in each coupled section and the odd mode phase velocity is usually faster. Dielectric layer with higher dielectric constant tends to have more serious spurious outputs because of the larger phase velocity difference.In this paper, we cleaned up and analyzed many other papers in this field, summed up the developing courses,design theories,research focus and representative techniques to suppress the harmonic passband. Based on parallel coupled microstrip filters, we proceeded from the upper microstrip structure, decuced the resonant characteristics and design parameters of SIR filters. After that, an SIR filter working in X-frequencyband is designed and optimized, the simulation results show that this designed filter meets the scheduled targets and the attenuation at 20GHz is higher than-30dB, hence the goal of harmonic suppression is achieved.Then, from the idea of developing the bottom plate of microstrip filters, some representative DGS structures are simulated and analyzed, the effects of various size parameters on its performance are summarized. On this basis, the traditional structure of dumbbell-shaped DGS is appropriately improved, by extracting its equivalent circuit and three-dimensional full wave simulation we analyzed its performance, the results show that the improved structure has lower resonant frequency and more steep attenuation characteristics, while the radiation intensity also decreased which makes its electromagnetic effects to the peripheral circuits smaller. Finally, a parallel coupled microstrip filter working at 2GHz is designed using DGS structures, the optimized results show that the filter meets the scheduled performances without size increase, the second harmonic suppression is up to more than 40dB with improved stopband attenuation performance. Finally, we try to apply DGS structure to the coupled sections of the filter, simulation results show that DGS structure is still able to play the role of low-pass function, while reducing the length of each coupled section significantly, size reduction of the final filter can be up to 21%, which verifies the correctness and effectiveness of the improved structure.