Research And Design Of Cavity Filters At RF Transceiver End Of S-band Harmonic Radar

The S-band harmonic radar is a detector that can effectively search and identify nonlinear junctions.It can determine whether the detection target contains semiconductor junctions or metal junctions by detecting the second and third harmonic signals reflected by the detection target.It is mainly used for confidentiality and privacy protection.Due to the non-linearity of the transceiver channel of the harmonic radar,the generated harmonic signal will cause serious interference to the performance of the whole machine,so improving the harmonic suppression capability has always been the focus of research.Due to the need for portability,the miniaturization of harmonic radar is also one of the research focuses.The filter is a key frequency selection device for the radio frequency transceiver of the harmonic radar.The high-performance,miniaturized filter plays a very important role in improving its own harmonic suppression capability and reducing the volume of the whole machine.The main work of this paper is to study the high-performance,miniaturization of the cavity filter at the RF transceiver of harmonic radar.(1)According to the design requirements of RF transmitter filter for harmonic radar,a structure with a disc on the open end of the resonator is designed,the effect of the equivalent capacitance in the resonator on the resonant frequency and parasitic passband frequency is analyzed.It is a miniaturized,wide stopband and high rejection cavity bandpass filter with a center frequency of 2.45 GHz and a bandwidth of 100 MHz was optimized using HFSS software,and the influences of the radius of the disc,the length of the resonant rod,the length and the radius of the tuning screw on the resonance frequency of the cavity filter are studied.The test results show that the insertion loss in filter passband is less than1 dB,the attenuation in the stopband from 3 GHz to 9 GHz is better than 90 dB,the volume is only 56.1 mm ×11 mm×26 mm.(2)By analyzing the filter design index of harmonic radar RF receiver,the interfinger cavity bandpass filter with center frequency of 4.9 GHz and 7.35 GHz and bandwidth of 5%is designed by using HFSS software.On this basis,diaphragm structure is added adjacent resonators to reduce the size of the filter,and the influences of the width of coupling window,the length of resonant rod,the length of tuning screw and the height of excitation port on the coupling coefficient and resonant frequency are studied.In order to further reduce the volume,the integrated 4.9 GHz and 7.35 GHz cavity bandpass filters are obtained by optimizing and combining the interfinger cavity bandpass filters with diaphragm structure.Compared with the cross-fingered cavity bandpass filter,the length is reduced by more than9 mm and the width is reduced by 3 mm.HFSS simulation results show that the insertion loss in filter passband is less than 0.5 dB,and the attenuation at 2.45 GHz,7.35 GHz,2.45 GHz and 4.9 GHz,all of them are better than 120 dB.