| The rapid development of wireless communication technology promotes the innovation of design scheme and technical application in the field of spectrum measurement.Due to the limitation of its weight and volume,the traditional spectrum analyzer is difficult to play a role in the field real-time measurement.Miniaturized and low-cost portable microwave spectrum analyzer is more and more welcomed by the market and valued by RF engineers.RF front-end,as the core component of spectrum measurement equipment,directly determines the overall performance of spectrum analyzer.It is the key point to develop RF front-end components suitable for miniaturized portable spectrum analyzer.This paper aims to complete the design and implementation of a miniaturized RF front-end component of microwave real-time spectrum analyzer connected to computer through USB interface.This design adopts two-stage down conversion structure,RF input 300 MHz ~ 6.0GHz signal,the first stage down conversion is 1250 MHz and 2650 MHz dual IF,the second stage down conversion is 140 MHz IF signal output.The main work of this paper includes:Firstly,the working principle and technical points of the commonly used spectrum analyzer are analyzed,the indexes of the RF front-end of the spectrum analyzer and the factors restricting the performance indexes are clarified,and the advantages and disadvantages of various receiver schemes are compared.According to the needs,purposes and indexes of this subject,the two-stage and three medium frequency superheterodyne receiver scheme is selected,and the overall scheme of the RF front-end is designed.Secondly,according to the previous receiver scheme and the design requirements of this project,the technical indicators are allocated to each module and specific devices to complete the RF link design and device selection of the whole machine,mainly including pre attenuation and pre amplifier module,signal pre selection and first mixing module,IF filtering and amplification module and second mixing module.The whole link simulation is carried out in ADS,and the simulation results are excellent According to the design requirements.Next,the frequency conversion principle of superheterodyne receiver is studied.Based on the requirements of high performance and low cost of this project,the frequency channel preselection scheme combined with RF switch is proposed to solve the problem of image frequency interference and spurious suppression of each independent channel,thus abandoning the expensive YTO components widely used in the RF front-end of broadband receiver and the design difficulties For the wide-band tunable filter with large degree,from the point of view of suppressing the image frequency interference,harmonic interference inside the mixer and signal leakage at the mixer port,the frequency band division and the determination of the pre selected filter bank index are completed,so that the harmonic interference can be suppressed up to 70 dbc in typical cases.Then,the design of each pre selected filter in the whole machine is completed,which mainly includes two low-pass filters and three band-pass filters.According to the actual needs,various considerations in the selection of filter structure are analyzed,and the microstrip structure which is easy to be processed and molded with the circuit board of the whole machine is selected.Then,the design process of each microstrip filter is introduced in detail,and the test results after the design and processing are as follows: Compared with the filter that is easy to buy in the market,it performs better in relevant indexes.Finally,the whole PCB drawing,processing,component welding and related index test are completed,and the related results are compared and the existing problems are analyzed.Thus,the research and design of RF front-end components of USB spectrum analyzer is realized. |
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