Research On Synthesis Of Wideband Filter And Its Reconfigurable Application

As a key component of the radio frequency transceiver front-end,microwave filters play an important role in suppressing interference inside and outside the system and improving the system signal-to-noise ratio.With the rapid development of radio systems such as communications and radar,broadband RF front-end systems have attracted much attention due to their wide-range signal detection capabilities,flexible applicability,and versatility.However,due to the complexity and variability of the wireless communication environment,the broadband RF front-end receiving system faces the threat of interference signals outside the passband and interference signals appearing within the passband.It requires higher anti-interference capabilities and more flexible reconfigurability,which puts new requirements on the filters in the front end.The wideband filter can effectively suppress interference outside the passband while receiving signals in the wide frequency domain,improving the signal-to-noise ratio of the system.The wideband filter with a reconfigurable notch function can further flexibly suppress interference at different frequencies and powers within the passband,thereby minimizing the impact of interference signals on system performance.Filter synthesis is the process of using mathematical calculation methods to create the desired filter circuit topology and design parameters based on given filter requirements and design constraints.However,current filter synthesis methods are unable to meet the demand for direct synthesis of circuit parameters for multiple coupling topologies of wideband filters,and cannot provide guidance for designing wideband filters with in-band notches.To address the aforementioned needs and issues,a method for synthesizing cross-coupled and in-line lumped wideband filters is proposed,and the relationship between the complex zeros within the filter band and the frequency and amplitude of the notches in the band is examined.A synthesis method of wideband filter with in-band complex zeros is further proposed and applied to design wideband filters with in-band notch reconfigurability.Specifically,the main work and innovations of this dissertation are as follows:1.A synthesis method for a lumped multi-coupled wideband filter admittance matrix that can characterize the circuit response in the full frequency domain is proposed.By using matrix similarity transformation,all negative inductive coupling elements in the traditional wideband filter synthesis method can be converted into positive capacitive coupling elements.Therefore,the obtained wideband filter admittance matrix can be directly implemented with lumped elements,and the parameters of all lumped elements can be calculated without optimization,achieving the goal of directly synthesizing crosscoupled wideband filters with transmission zeros at arbitrary positions in the bandpass domain.It solves the problem that current filter synthesis methods are only effective for narrowband(fractional bandwidth <5%)or medium bandwidth(fractional bandwidth20%-30%)filters,while wideband(fractional bandwidth >30%)filter design strongly relies on approximation and optimization.A fourth-order filter synthesis example with a fractional bandwidth of 50% is fabricated and verified,and the measured results,synthesis matrix response,and schematic response fit well.2.A synthesis method for any order cascaded and in-line coupled wideband filters with multiple zeros is proposed.Firstly,arbitrary-order filters can be converted into cascade triplet or cascade quadruple topology.Then,the method of converting the negative inductors in the triplet and quadruple topology into physically achievable positive capacitors or mixed-coupling is used to realize the synthesis of cascaded and inline wideband filters of any order,any bandwidth,and any out-of-band transmission zero.It solves the problem that existing wideband filter methods result in single circuit topology,and they are difficult to directly synthesize the lumped circuit parameters of higher-order,multi-transmission zero wideband filters.Two fifth-order wideband filters with three transmission zeros and different topologies are fabricated,achieving a center frequency of 1.2 GHz and fractional bandwidth of 70.5% and 68.3% respectively.The measured results show that both fabricated filters can generate three transmission zeros at specified locations,and their measured responses are well fitted with the synthesized filter responses in the range of 0-5 GHz,which validate the effectiveness of the proposed synthesis method.3.A design method for wideband filters with in-band reconfigurable notch is proposed.Two notch resonance units are loaded into the synthesized embedded transversal fifth-order wideband filter.One of the notch units is mounted on the main path of the wideband filter to form a signal channel,and the other notch unit is connected in parallel with the wideband filter to form another signal channel.The reconfigurability of the notch amplitude is achieved through the principle of the two channels canceling or complementing each other at the notch frequency.This feature can be achieved by adjusting the resonant frequency difference of the loaded notch units.In addition,the inband notch frequency can be tuned through tuning the resonant frequencies of the load notch units.It solves the problem that existing wideband filters cannot reconfigure the inband notch amplitude.A microstrip multimode resonator is used to implement the wideband filter,and an evanescent mode resonator is used to implement the notch units.The circuit is fabricated and measured.The measured results show that the proposed wideband filter can achieve reconfiguration of the in-band notch frequency and amplitude.Its in-band notch frequency tuning range is greater than 29%,and the notch attenuation can be tuned in the range of 2 d B to 45 d B.4.A synthesis method for quasi-Chebyshev wideband filters with specific in-band complex zeros is proposed.These in-band complex zeros can be used to control the inband notch frequency/amplitude of the wideband filter.It can be found that the imaginary part of the in-band complex zeros determines the frequency of the in-band notch,while the real part determines the attenuation of the in-band notch;the larger the real part of the in-band complex zeros,the smaller the attenuation of the notch.Using the idea of superimposing two passbands,a composite transfer function calculation method is proposed to synthesize the polynomials and transversal matrices of quasi-Chebyshev wideband filter with in-band complex zeros,and then the method of matrix rotation is applied to initial matrix to obtain wideband filter topologies that can realize the in-band notch with reconfigurable amplitude and frequency.When specifying parameter requirements such as the passband range of wideband filter,location of transmission zeros,and notch frequency,a variety of circuit topologies that meet the requirements can be obtained by using the proposed synthesis method.This solves the problem that the design of a wideband filter with reconfigurable in-band notches strongly relies on electromagnetic optimization and cannot quickly obtain the best topology.Based on the proposed synthesis method,one of the topologies is experimentally verified using rectangular waveguide resonators.The measured results show that the passband of the designed wideband filter with in-band complex zeros is 8-12 GHz,the insertion loss is only 1.1 d B in the minimum notch attenuation state,and the in-band return loss is greater than 17 d B,showing a synthesized quasi-Chebyshev response.By only tuning the resonant frequencies of two resonators,the notch attenuation can be tuned from 1.1 to 60 d B,and the notch frequency can be tuned from 9.3 to 10.4 GHz,which verifies the advancement and effectiveness of the proposed synthesis method.