Design Of VHF Band DMB Frequency Shift Repeater

With the development of Digital Multimedia Broadcasting(DMB)services,new applications have placed higher demands on the quality of DMB signals.In addition,good coverage of signals is also an urgent problem to be solved in some areas with complex geographical environments.Therefore,a repeater device capable of amplifying the signal is required to ensure the coverage of the DMB signal in a designated area.At present,the same-frequency repeater equipment has great limitations in practical applications due to self-excited interference and other problems,and the existing frequency shift repeater is mainly used in the field of mobile communication,which is large in size and high in cost.Therefore,they are not compatible with small-scale,low-cost application scenarios of DMB systems.Based on the research of DMB system of Chongqing University of Posts and Telecommunications,this thesis proposes a miniaturized,low-cost design scheme for frequency shift repeater.Firstly,based on the simplex communication of DMB system and the characteristics of the receiving terminal with identity recognition technology,the system scheme of the frequency shift repeater is simplified.The DMB system is a simplex communication,so the duplex device and the uplink are not required inside the repeater;the receiving frequency of the receiving terminal of the DMB system can be set according to requirements,so after shifting frequency through one device,the other is not needed.The reason is that the DMB receiving terminal can directly receive information at the frequency point after frequency shifting.Secondly,the highly integrated AD831 mixer chip and the ADF4351 phase-locked loop chip replace the discrete components in the internal structure of the repeater,which not only improves the stability of the system,but also greatly reduces the size of the repeater.Considering the possible frequency conflict problem,this thesis designs a programmable phase-locked loop circuit with STM32 chip as the controller.Therefore,the output signal frequency of the frequency shift repeater can be set as needed.In the design of the filter module,the tap-type comb filter is used to further reduce the volume of the entire repeater system.Finally,using the DMB system being used by Chongqing University of Posts and Telecommunications,the DMB shifting frequency repeater is tested indoors and outdoors.The results show that:(1)The overall size of the frequency shift repeater does not exceed 20cm*20cm*5cm,with miniaturization features for easy indoor installation;(2)The normal working voltage of the frequency shift repeater is 12 V,the current is 135 mA,and the signal gain is 33.26 dB,the output signal adjacent channel power ratio is-36.73dBc;(3)In the field test,after using the frequency shift repeater,the signal quality of the bad area is significantly improved,which proves the engineering practicability of the design.