Research On Miniaturized Semi-lumped Tuning Receiver Front-end In UHF Band

With the continuous development of modern science and technology,wireless communication technology has permeated various fields,leading to an increasing scarcity of spectral resources in the medium and low-frequency bands.Furthermore,due to the coexistence of various communication signals,the presence of interference signals has become a growing concern.In order to address these issues,this thesis conducts research on tunable bandpass filters based on suspended microstrip line structures.A compact UHF frequency range continuously tunable front-end system is designed,which exhibits excellent out-of-band suppression capabilities and maintains a nearly constant bandwidth,among other advantages.Regarding the tunable bandpass filter based on suspended microstrip line structures,given the operating frequency range of 300-500 MHz,which corresponds to longer wavelengths,size reduction becomes a primary concern.Initially,an analysis and comparison of various resonator structures are performed to identify the most suitable semi-lumped resonator unit for this study.Subsequently,aλ/2 center-symmetric dual-spiral structure filter is designed,demonstrating superior performance compared to traditionalλ/4 spiral structure filters.By selecting appropriate varactor diodes,the varactor diode tuning circuit and filter circuit are jointly simulated,optimized,fabricated,and tested.The results indicate that the operating frequency can be tuned within the range of 303 MHz to 472 MHz,providing a tuning range of approximately 44%.The insertion loss(IL)is below 9 d B,return loss(RL)exceeds 14.1d B,and the bandwidth varies between 12 MHz to 14 MHz with an intra-band fluctuation of less than 0.5 d B.The out-of-band suppression is greater than 65 d B.The dimensions of the tuning filter circuit are approximately 0.09λ_g×0.11λ_g.For the low-noise amplifier,a suitable MMIC(Monolithic Microwave Integrated Circuit)chip is selected based on the desired frequency range,noise figure,and gain requirements.The amplifier circuit is then modeled and analyzed using electromagnetic simulation software to evaluate its S-parameters,noise figure,stability factor,and other characteristics.Through simulation optimization,the optimal operating point is determined.Finally,the amplifier is fabricated and tested to validate its performance.The test results demonstrate that the amplifier achieves a gain of 20 d B±1 d B within the operating frequency range.The noise figure is below 0.6 d B,and the 1 d B compression point output power is measured at 19.25 d Bm.Based on the designed system architecture,the low-noise amplifier and tunable bandpass filter are cascaded and assembled to form a compact semi-lumped parameter front-end for UHF frequency range reception.The system is subjected to testing,and the results indicate that the operating frequency of the front-end system can be tuned within the range of 303 MHz to 472 MHz,providing a tuning range of approximately 44%.The overall in-band gain is measured at 13±0.4 d B,and the return loss(RL)exceeds 15.2d B.The bandwidth varies between 12 MHz to 14 MHz with an intra-band fluctuation of less than 0.5 d B.The out-of-band suppression is greater than 60 d B.The noise figure is below 0.80 d B,and the 1 d B compression point output power is measured at 8.2 d Bm.These results confirm that the front-end system meets the design requirements.