| The spectrum analyzer is the most widely used and most functional instrument for measuring signal frequency domain information.Compared with the desktop spectrum analyzer,the portable spectrum analyzer has the advantages of portability and high cost performance,so the research on the portable spectrum analyzer has important application value.The RF front-end is an important part of the spectrum analyzer and directly determines most of the specifications of the spectrum analyzer.Research on miniaturized,high-performance and low-cost RF front-end can promote the further development of portable spectrum analyzers,so this thesis is of great significance to the research on the RF front-end of spectrum analyzers.This thesis first introduces the development of spectrum analyzer and the research status at home and abroad,and then studies the overall structure,types and technical indicators of spectrum analyzer.The strengths and weaknesses of the superheterodyne,fast Fourier transform(FFT)and real-time spectrum analyzers are analyzed,and the key technical indicators of the RF front-end of the spectrum analyzer are studied and formulated.Secondly,a variety of portable spectrum analyzer RF front-end schemes were analyzed and compared,and finally a superheterodyne three-stage frequency conversion structure was adopted.The system noise figure,gain,and linearity are analyzed,and budget simulation,frequency sweep,and power sweep are performed for the entire link.Then the scheme is modularized,and it is determined that the scheme is composed of variable attenuator module,radio frequency switch module,low noise amplifier module,frequency conversion module,filter module and local oscillator module.Compared with other solutions,the noise figure is effectively reduced,and the linearity of the spectrum analyzer is improved.Then,the variable attenuator,amplifier and mixer chips are studied and determined,and the harmonic balance simulation is carried out.The results show that the third-order intermodulation ratio and spurious-free dynamic range of this scheme meet the requirements of the scheme.In addition,the key filters of the RF front end are studied,including low-pass filter,intermediate frequency filter and local oscillator filter.The stopband bandwidth of the traditional filter cannot meet the design requirements of the low-pass filter of this scheme.Therefore,this scheme adopts the method of cascading the elliptic function filter and the Butterworth filter,so that the advantages of the two filters are complementary,and the edge of the passband is compared.Steep and has a wide stopband bandwidth;the first IF filter uses an open-loop coupling filter,which has a smaller volume and greater stopband attenuation than traditional parallel coupling and hairpin filters;The third intermediate frequency filter is implemented by an LC filter with a bandwidth of 2 MHz,which reduces the size of the radio frequency front end compared to other design solutions.The first local oscillator filter and the second local oscillator filter are respectively designed with parallel stub filter and hairpin filter,both of which meet the requirements of the scheme design.Finally,the selection of local oscillator and the source of phase noise are studied.The phase-locked frequency synthesis technology is difficult to achieve small frequency steps and the locking time is long,and the operating frequency of direct digital frequency synthesis is low.Therefore,this thesis uses a combination of phase-locked frequency synthesis technology and direct digital frequency synthesis technology to complete the design of the local oscillator,making up for the shortcomings of both and retaining their respective advantages.Therefore,this thesis uses a combination of phase-locked frequency synthesis technology and direct digital frequency synthesis technology to complete the design of the local oscillator.,so that the advantages of the two technologies complement each other and make up for the shortcomings of the two technologies.The first local oscillator of this scheme is a frequency-sweeping local oscillator source,and the phase noise of this module is simulated.The simulation results of ADIsim PLL show that the phase noise of the first local oscillator of this scheme is lower than other schemes.In addition,a Wilkinson power divider is designed to distribute the power of the voltage-controlled oscillator(VCO),and a suitable low-noise amplifier is also selected to compensate the power of the local oscillator signal to achieve the local oscillator signal required by the mixer.power.This scheme also cleverly designs the frequency of the second and third local oscillator signals,so that the frequency of the second and third local oscillator signals is in a multiple relationship,and one local oscillator source can be directly used to provide two local oscillator signals,reducing the size of the RF front-end and reduce costs. |
