Research On Low Cost Spectrum Analyzer For Teaching

Spectrum analyzer is widely used in aerospace,military,communications and other related fields.It is a very important instrument in frequency domain measurement and has the reputation of "frequency domain oscilloscope".However,in the current teaching of colleges and universities,spectrum analyzer has not been popularized.The main reason is that the high-end spectrum analyzer is too expensive,complicated to operate and bulky.Meanwhile,there is not low-cost spectrum analyzer that meets its frequency band requirements in the current market.Therefore,the development of low-cost,high-performance spectrum analyzer in low-end field has broad market prospects.At present,the frequency of the signal generally does not exceed 10MHz in the experimental teaching of electronic technology in colleges and universities.In order to analyze the higher harmonic components,a spectrum analyzer with higher frequency range is needed.According to the situation of the National Undergraduate Electronic Design Contest in recent years,the frequency requirement of the signal is basically within 300 MHz.Therefore,the purpose of this thesis is to design a low-cost 300MHz spectrum analyzer for teaching.Compared with FFT analyzer,superheterodyne spectrum analyzer has the advantages of low design cost,high resolution and wide measurement range.Therefore,this system finally adopts the scheme of combining high-IF superheterodyne structure and digital IF processing.STM32F429 is used as the controller and Altera Cyclone V series FPGA is used as the signal processor in the system.The signal under test is processed by RF front-end and the spectrum is shifted to the intermediate frequency of 10.7MHz.Sampled by A/D,the signal is processed in FPGA,then the spectrum of the signal is displayed on the TFT screen.This system is mainly composed of three parts:RF front-end hardware circuit,digital IF signal processing firmware and display controlling software.The RF front-end hardware circuit includes the RF front-end signal conditioning module,the synthetic local oscillator module,the key analog filter design module and the AD module.Among them,the RF front-end signal conditioning module mainly includes the program-controlled attenuator,the low-noise amplifier,the three mixers and the image rejection filter.The synthetic local oscillator module mainly realizes the design of the 433 MHz to 733 MHz sweep frequency oscillator and the design of the second and third stage fixed local oscillators.The key analog filters include the three stages of band-pass filters and the design and simulation of filter circuits for three-stage local oscillators.AD module mainly includes AD front-end conditioning and peripheral circuit design.The digital IF signal processing firmware adopts a digital down-conversion method to realize the processing of IF signal.After digital orthogonal frequency mixing,multi-sampling data processing and digital detecting,the amplitude of the signal is obtained.The control and display software includes the communication control between STM32 and FPGA,the control of local oscillator and other programmable chips,the display control of signal spectrum and the design of GUI.Finally,the system and the key indexes were tested.The phase noise index of the three local oscillators is better than-95dBc/Hz@1OkHz,the spurious level is lower than 60dBc,and the spurious level of the third intermediate frequency signal is lower than 60dBc.The test results show that this system can correctly analyze the spectrum of signals within 300MHz and achieve the design goal of this thesis.