Research On Linear Phase Filter

With the rapid development of mobile communication technologies, user’s demand for information is growing, high-speed data transmission services has also been challenged. Filter plays an important role in the communication system. Filters with no amplitude and phase frequency distortion can reduce the error rate and improve the transmission quality of the system during transmission. So we need such filters, not only the amplitude responses meet the requirement but also have approximately constant delay characteristics within the pass-band. Therefore, the study and design of linear phase filters get more attention.In this paper, according to the theory of distributed parameter filters, we designed a eight-cavity comb-line LTCC band-pass linear phase filter by using linear phase low-pass prototype network. The dimension is 2.5mm×3.2mm×1.5mm.The pass-band is from 3.5GHz to 4.1GHz, the insertion loss is less than 3dB, the return loss is less than 16dB, the group delay is about 1.1ns, and the range of flat pass-band group delay reaches 58.3%. We analyze the coupling between each resonator according to field analytical methods and then generate transmission zeros in the imaginary axis in the complex plane in order to achieve the purpose of improving the group delay by adding a cross-coupling of the same nature. The paper also gives the other two comb-line linear phase filters. One is the linear phase wideband band-pass filter with group delay as 0.6ns and the group delay flatness range as 63.2%.The other is a high-frequency band-pass linear phase filter with group delay as 0.8ns and group delay flatness range as 50%. Using simulation software HFSS for model-building, simulation and optimization greatly reduces the filter design cycle time, which is very convenient.Furthermore we design a ring microstrip linear phase filter. The permittivity of the dielectric substrate is 40, and the thickness is 1mm. We increase the order of the resonance to improve the performance of filter. We design a four-ring filter with its pass-band as 8.82GHz-12.25GHz, the group delay as 0.258ns, the flatness as less than 0.005ns, group delay flatness range as 50%. And We design a four-ring filter with its pass-band as 10.11GHz～12.75GHz, the group delay as 0.258ns, the flatness as less than 0.002ns, group delay flatness range as 33.3%.