Design And Development About Stop-band Suppression Characteristics Of High Temperature Superconducting Filter

In the radio frequency microwave band,the superconducting thin film in the superconducting state has extremely low surface resistance,HTS filter based on this feature has many advantages including low insertion loss,high band edge steepness,good restraint outside band etc… The HTS filter is applied to the front end of the mobile communication receiver,with the cryogenic low noise amplifier are composed of the front receiving system,which improves the receiving sensitivity of the base station,improves the anti-interference ability,increases the capacity of the base station,and reduces the transmission power of the mobile phone,lessens the rate of dropped calls,improves the quality of the call.Meanwhile,the HTS filter is placed in the most front end of the receiver,which not only avoids the strong signal to cause low noise amplifier saturation,effectively protect the amplifier,but does not increase the overall noise coefficient.Therefore,HTS filter has a very broad application prospect in radar receiver,police cluster communication,military communication system,deep space exploration,radio astronomy etc…Microstrip line structure is generally applied in the superconducting filter,because the structure has periodic boundary condition,so the parasitic pass-band can be generated at the frequency doubling of the fundamental frequency pass-band.When the frequency of the external interference signal is consistent with the parasitic frequency of the HTS filter,then can pass the HTS filter into the post-stage circuit of the receiver,which can interfere with the normal operation of the whole receiver system.The development of modern microwave communication system not only has a high demand for the rectangular coefficient and the insertion loss of the filter,but also requires good stop-band suppression characteristic.Three programs are designed and implemented in this paper.The first program is that primarily designs a superconducting band-pass filter using the method of combining SIR structure with interdigital capacitor structure,then the parasitic pass-band of the filter is pushed to more than 4 times,reaching the requirements of design.The other two programs are that according to the design requirements,the band-pass filter and low pass filter are designed separately,then they are cascaded with the pass-band filter,so the parasitic pass-band of the band-pass filter is further suppressed,and stop-band suppression characteristic of the HTS filter is more improved.In this paper,we design and fabricate a high temperature superconducting band-pass filter with a center frequency of 830 MHz,a low pass filter and a band-stop filter.The latter,two filters respectively cascaded with the band-pass filter.Through the low temperature test,the three programs can effectively improve the stop-band suppression characteristics of superconducting filters,a good result is obtained and achieve the goals of design.