| Along with the development of wireless communication, the existing analog trunking systemscan not play a full role in the functions of command and scheduling. It is imperative that trunkingcommunication should apply digital technology. DMR (Digital Mobile Radio) digital trunkingsystem, which usually consists of portable interphone, vehicle radio and relay station so on, has amuch more open protocol standard and wide market prospect. This paper mainly focus on the studyand design of radio frequency receiving front-end circuits in the DMR portable interphone.The radio frequency receiving front-end circuit applies superheterodyne structure withtwo-time frequency down-conversion. That circuit mainly achieves the process of filtering,amplifing and down-conversion mixing for the radilo frequency weak signal. The topology networkhas advantages of excellent performance and reliable structure. And the key points of related circuitdesign are to restrain the image frequency interference effectively and determine the frequency ofintermediate frequency reasonably. The portable interphone operates in the frequency range of350-390 MHz in the UHF band. The intermediate frequency is designed to define 73.35 MHz as thefirst one and 450 KHz as the second one. The key techniques and functional circuits ofsuperheterodyne receiving link are studied and designed in this paper.Radio frequency filter circuits include the design of the pre-selected low-pass filter and imagefrequency rejection filter. Pre-selected low-pass filter is composed of lumped LC components. Itspassband insertion loss is -0.3 dB or so and attenuation level is -33 dB or even better out of thepassband through the software simulation, which can select the useful frequency from the cluttersignal and prevent the output of secondary harmonics. In order to complete the tunable performanceof image frequency rejection filter, the electrically tunable resonance network is designed to applyvaractor diode HVC350B and inductance. The equivalent capacitance of varactor diode varies asthe change of reverse bias voltage between anode an cathode. The attenuation capability of imagefrequency is -60 dB or above out of the useful signal’s passband, which can suppress the imagefrequency interference effectively.Then the low noise amplifier is designed by the bipolar junction transistor AT-41511 with lownoise. According to simulation results, the noise figure of low noise amplifier is about 1.3 dB with21 dB power gain, which shows that the radio frequency small signal can be amplified properly onthe condition that a little noise is introduced into the circuit.The down-conversion mixer is belong to double balance diode mixing circuit. It consists ofschottky barrier diodes HSMS-2829 and balun transformers 4BLH. The frequency conversion loss of mixer is -5.6 dB or so after the software simulation, which satisfies the demand ofdown-conversion mixing process for the signals.The related circuit mentioned above is manufactured to hardware physical for test andverification in the paper. There are definitely exsisting some differences between the real circuitparameters and software simulation, however, the designed circuits satisfy the applicationrequirements of the portable interphone. That is to say, the whole radio frequency receivingfront-end circuits has a performance of good sensitivity and selectivity. Finally, some ideasproposed from the process of debugging and test are discussed to improve and optimize the circuitperformance during the subsequent study. |
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