Research And Design Of Wideband Digital Receiver Front-end Subsystem

RF front-end system is one of the most important parts in the receiver.It has always been the focus of the study of radio.With the development of digital processing technology,digital circuit takes a great proportion in communication system.It is difficult to sample high frequency signal directly using Analog-Digital Converter without down-conversion.So the high frequency signal need to be down-converted to intermediate frequency(IF)by RF front-end system in general.Then the IF signal is sampled by Analog-Digital Converter(ADC).It is a important research topic that how to receive the useful signal effectively and depress the jam signal.In this paper,a wide band(30-1200MHz)RF front-end system used in the digital receiver is designed.Different from ordinary communication receiver,it employs the plan of wide band scanning superheterodyne structure.It is typically characterized by a wide band agile frequency local oscillator and a wide band or traceable preselector.The front-end system adopts two frequency conversion scheme,and introduces two stage mixing circuit.In order to realize the function of scanning and analysis of wideband digital receiver,the scheme of variable intermediate frequency is adopted.More specifically,it uses IF of 70 MHz in broadband scanning function,and uses IF of 21.4MHz in narrowband analysis function.In order to achieve high resolution,it adopts the first local oscillator of wide band and the second local oscillator of narrow band to combine the control scheme,and uses serial interface to achieve rapid deployment,and provide the key code to control microchips.In this paper,the design work is demonstrated in accordance with the actual project,the main design work includes the following three parts:(1)The whole front-end receive link is designed by using two stage frequency conversion model.Combined with engineering practice,the design ideas are analyzed,and the technical target of system is decomposed.The indicator requirements of the front-end system is verified by using estimation formula of the practical application of engineering.A equivalent link model of front-end circuit is build using ADS software.Each functional module gain state is budget by inserting AC simulation controller and gain control module,and the frequency synthesizer phase noise is budget by PLL module.(2)An elliptic function low-pass filter with cutoff frequency of 1250 MHz is designed using the insertion loss method.It is used to filter the input signal of the mirror image frequency and the reverse radiation.The ladder impedance microstrip filter model is adopted in the design.The electric length of each branch of the microstrip filter is calculated by querying the table of normalization parameters,and then the physical size is calculated by the AppCAD.Finally,the S11,S21 and VSWR curves are simulated by using HFSS software,and the key parameters such as insertion loss,out of band suppression and standing-wave ratio are obtained,and the optimal design parameters are obtained through optimization.(3)Two frequency synthesizers are designed by using the mixing loop model and DDS insert PLL model.A mixing loop PLL circuit in the first local oscillator is designed by using ADF4151 and HMC440 phase detector,provide the output frequency of 1560MHz-2730 MHz,and its step is 10 MHz.DDS insert PLL scheme used in the second local oscillator is designed based on a phase detector(ADF4002)and a direct digital frequency synthesizer(AD9915).The circuit realizes a single PLL loop with the output frequency of 1450MHz-1518.6MHz,step into 1 KHz.In the design process,the theoretical calculation and engineering practice are combined,and the design principle and method of frequency synthesizer are described in detail.Then,the frequency and time domain target are simulated using ADIsimPLL and ADIsimDDS software.Finally,schematic and PCB drawings are designed with Altium Designer,and the final test results are given.