Research On Broadband Notch Filter Based On Multi-layer Octagonal Defect-ground Coupling Structure

The increasing requirement for large capacity,high speed,small volume,and low power consumption make wideband communication systems necessary.As an important part of the wideband communication system,the filter needs enough working bandwidth,but also suppresses the interference of the spurious frequency signal within the band,which raises higher performance requirements of the wideband filter.In response to the above problems,this paper has carried out a variety of wideband notch filters based on the octagonal defect ground coupling structure.The main work and innovations of the paper are as follows:(1)A wideband notch filter coupled with octagonal defects based on the interdigital structure is designed.The filter uses a multi-layer octagonal defect coupling structure to achieve wideband response,and uses an interdigital coupling structure as the input/output unit for feeding,achieving a relative bandwidth of 62%.Further using the asymmetry of the input/output coupling structure to generate a zero point,a notch with suppression exceeding10 d B is formed at 6.8 GHz in the passband.(2)Two octagonal defect coupled wideband notch filters with adjustable zero point are designed.In order to improve the out-of-band suppression,two adjustable zero points are introduced in the stop band by paralleling short-circuit and open-circuit stubs on the input/output ports of the octagonal defective coupling structure.On this basis,the first filter additionally embeds a quarter-wavelength open stub in the input port to generate a transmission path difference achieving a notch.Tests show that the filter has a relative bandwidth of 146%,and the notch frequency is 4.98 GHz,suppression exceeds 20 d B.The second filter moves the transmission zero point into the passband by adjusting the length of the open-circuit branch in parallel,and further loads the varactor diode to make the notch frequency move within the passband 0.92-6.23 GHz as the capacitance changes.The test shows that notch suppression exceeds 35 d B under different capacitance values.(3)A two-stage cascaded octagonal defect coupled wideband filter with adjustable notch is designed.Aiming at the problem that the input/output ports of the single-stage coupling structure are not in the same plane,a multi-stage cascaded octagonal defect ground coupling structure is proposed and its scattering parameter matrix is derived.Based on the above theory,the two-stage cascaded octagonal defect coupling structure is used to realize the adjustable notch in the passband by loading parallel open branches and series varactor diodes.The 3 d B bandwidth of the filter is 1.42-6.5 GHz.The bandwidth is 128%,and the test shows that the notch suppression of different capacitance values exceeds 30 d B.This paper studies four wideband notch filters based on the coupling structure of multilayer octagonal defects,and gradually realizes the conversion from a single-stage coupling structure to a multi-stage coupling structure,and a fixed-frequency notch to an adjustable frequency notch in the passband.The design method and circuit structure involved in it provide a reference for the design of increasingly complex radio frequency circuit devices.