Research On Ring Type Ultra-Wideband Bandpass Filter Based On Parallel-Coupled Line

According to Shannon’s channel capacity formula,increasing the channel bandwidth helps to improve the channel capacity of noisy channels.However,Various communication standards have been proposed to meet the communication needs of different scenarios,resulting in the increasing scarcity in available spectrum resources.Ultra-wideband(UWB)signals with extremely narrow pulses provide an extremely wide frequency spectrum,enabling the receiver to demodulate information at extremely low signal-to-noise ratios and maintain a high transmission rate even at close distances,moreover,the low power spectral density allows the signal to be submerged in noise without causing interference to existing communication systems,which effectively alleviates the contradiction between the requirement for high-speed communication rates and the allocation of frequency resource.Filter is a critical component of ultra-wideband RF transceiver systems,the technology specifications of which greatly affects the performance of communication systems.In order to meet the requirements of system miniaturization and system signal integrity,this thesis proposes a ring type ultra-wideband bandpass filter based on parallel-coupled line structure,discusses the optimization of its circuit topology,and seeks to offer a synthesis approach of design to realize the control of the filter bandwidth,return loss,and roll-off characteristics.The proposed filter consists of two signal path in parallel.Each signal path consists of three coupled lines in cascade,where all the coupled lines are of the same electrical length,and the coupled lines on both sides of each path come with the same characteristic impedance.Mathematical analysis of this filter and the coupled-linecascaded Chebyshev filter reveals the potential mechanism within the superior performance of this filter compared to traditional filters,as well as the inherent logic of its topological structure.Moreover,the transfer function is mathematically constrained and the design equation is derived to generate the synthesis method.This synthesis method can realize the control of filter bandwidth,return loss and roll-off characteristics,and ensure the flatness of group delay in the band while satisfying the equal-ripple performance.At the same time,in order to solve the problem that the mathematical solution of the design equation may lead to difficulties in implementing the filter due to processing accuracy,this thesis discusses the replacement of some coupled line of the prototype topology from the perspective of model equivalence,and proposes five simplified topologies with the same transfer function form,which could broaden the implementation and application range of the filter.Lastly,based on the summarized constraints and design equations,the synthesis approach of design for the ring type ultra-wideband bandpass filter is proposed,which is described in detail,along with the design examples of this approach under multiple conditions regarding different return loss,band bandwidth and group delay.The above examples prove the effectiveness of the synthesis approach,and also provide data support for the necessity of simplifying topology.On this basis,a design example with specific parameters is electromagnetic simulated,circuit fabricated and measured,implementing an ultra-wideband filter with a 3d B bandwidth of 90.6%,and verifying the correctness of the theory.The proposed ring type ultra-wideband bandpass filter based on parallel coupled line structure with this synthesis approach of design can realize controllable group delay while providing 8 reflection zeros with equal-ripple.Compared with the coupled-linecascaded Chebyshev filter,with the same number of coupling lines,the filter proposed in this thesis can creatively generate more reflection zeros by coupling signals between different paths.Moreover,the proposed synthesis approach is not constrained by the inherent equal-ripple formula,and can achieve the filtering characteristics steeper than the Chebyshev response through the degrees of freedom of the model,thus completing the optimization of the coupled-line-cascaded Chebyshev filter.