Research On Synthesis Design Of Planar Lowpass Filter Based On Commensurate Line Components

In modern communication equipments,the performance of the filter is very important.Low cost,high suppression and large bandwidth are all urgent requirements of the system.Commensurate line filters have received extensive attention due to their unique structural characteristics and complete design theories such as Richard transformation and Kuroda transformation.In the commensurate line network,the electrical lengths of all transmission lines are equal,so the filter can be described with the help of a polynomial-based transfer function,and further synthesis design can be carried out,thereby greatly improving the design and research efficiency.The main contents of this paper are as follows:1.For the coupled transmission line unit,the transfer functions of the three classic structures are studied,and a self-coupling structure is proposed on this basis,that is,a uniform transmission line is connected in parallel on two non-adjacent ends of the coupled line section to form a two-port network with low-pass filter characteristics.The transfer function and the corresponding objective function are constructed.According to the preset filter index,the precise impedance value of all the commensurate line components is obtained by the synthesis method,and the amplitude-frequency response characteristics are studied and discussed.2.On the basis of the self-coupling structure low-pass filter,two open-circuit stubs are connected in parallel to form a fifth-order low-pass filter,and two two-port networks with different filtering characteristics are obtained according to the difference in parallel positions.Named as external/internal stub-loaded low-pass filter.The results of the synthesis design show that,compared with the self-coupling structure,the above two filter structures have a significant improvement in both the achievable bandwidth range and the out-of-band suppression capability.3.On the basis of the aforementioned two fifth-order low-pass filter structures,a seven-order low-pass filter loaded with internal and external branches is proposed,which has better performance.For this complex structure,the transfer function and objective function are deduced in detail.The simulation results show that the filter has both a wide bandwidth tuning range and extremely high sideband selection characteristics in performance.In summary,the low-pass filter based on the self-coupling structure has an additional dimension in tuning parameters compared with the traditional commensurate line filter.It enables the tuning of out-of-band suppression while keeping the in-band parameters such as cut-off frequency and ripple coefficient unchanged,which brings greater flexibility to filter design.