Design Of Short Wave Image Transmission Receiver Based On DVB-T Demodulator

Short wave image transmission receiver has an irreplaceable advantage in the complex terrain emergency rescue,fire and other areas of application,which is because of its diffraction ability,transmission distance.The COFDM modulation and demodulation technology adopted in DVB-T has high spectrum utilization efficiency,strong anti-interference ability and adjustable broadband bandwidth,which opens up a green channel for the development of short wave image transmission in mobile environment.Firstly,this paper introduces the structural characteristics and working principle of several common receiver systems in detail,and analyzes the indexes that affect the receiver performance.The determination of receiver's structure and scheme depends on the signal transmission band and the significant index of system.Taking into account the characteristics and environmental conditions of short wave image transmission,the paper puts forward two schemes to achieve short wave image transmission.After comparison,this scheme that based on DVB-T demodulator achieves image transmission is selected.The DiB9090 M DVB-T receiver module consists of two parts,the one is the tune module,its receiver structure belongs to zero-IF receiver,which is suitable for using in the mobile environment;the other is the demodulation module,the receiver integrates COFDM demodulator,The COFDM demodulator uses the existing technology to receive high quality signals in the mobile environment,it can well achieve the output of image from the base-band signal.But the frequency of the shortwave sent is 50 MHz,and the received frequency range is 218-860 MHz,in order to make the frequency coordinated,the receiver will transform the received signal frequency to 218-860 MHz range,for solving this problem,we designed the RF front-end circuit.Then,the mixer circuit of RF front end is studied and designed.This paper introduces the basic concepts of mixing frequency and the mixing principle,analyzes the key indexes which affect the work of the mixer,and analyzes several common circuit structures of the mixer.Because LO port and RF port of passive dual balanced mixer both have a wide range of operating bandwidth and better isolation,the mixer uses Mini-Circuits' ADE-25 MH chip for circuit design.Finally,in order to guarantee the quality of the received signal,this paper analyzes the circuit structure and performance of the low noise amplifier.The low noise amplifier has a higher IP3 to reduce the inter-modulation interference so that selects the SBB-2089 bipolar transistor structure to design the circuit in which good linearity and low noise figure Filter has the requirement for low insertion loss out-of-band attenuation as the circuit of controlling receiver's frequency.This paper designs a band-pass filter and simulating by RF-Filter software that circuit impedance matching and meet the 3dB bandwidth requirement.When the communication distance is uncertain,the useful signal is attenuated after passing the low noise amplifier,so that the signal of the post-stage transmission is affected.So this paper is designed an automatic gain control circuit by using Single-Ended Amplifier AD8367 for variable gain variations within 45 dB in order to meet the dynamic range requirements of the receiver and control the gain change of the transmission signal.According to the result,the 50 MHz RF signal and 168 MHz LO signal has been basically up-conversion through the low noise amplifier,mixer,band-pass filter circuit,the output signal falls in the 218MHz-860 MHz band.Then,the RF front-end circuit module and the DiB9090 M receiver module are jointly tested,the receiver mainly tests transmission distance that can transmit image information within 3~5 kilometers so that the feasibility of the design of the short wave image transmission is verified.