Designs Of VLF Weak Signal Preamplifier And Acquisition Device

With the continuous progress of science and technology,both VHF and VLF communications have been developed by leaps and bounds.Among them,VLF communication is widely used in submarine communication,underwater communication due to the advantages of low frequency,small attenuation(in water)and long propagation distance.Fields such as medium distance communication and air communication.However,in submerged communication,the depth of water nonlinearly attenuates the VLF signal,which makes the received VLF signal extremely weak.In order to ensure the communication quality,it is bound to put forward higher requirements on the gain and noise floor of the VLF weak signal acquisition device.Based on the current high requirements for very low frequency weak signal acquisition devices,this thesis proposes a very low frequency weak signal preamplifier and acquisition system.The system is mainly composed of low-noise amplifier circuit,band-pass filter and acquisition and display module.In the amplifier circuit part,firstly,the input matching network and bias network of transistor MMIC(Monolithic Microwave Integrated Circuit)are studied.The chip noise figure,voltage standing wave ratio,gain and 1d B compression point and other data are analyzed,and the low noise amplifier circuit is designed.The actual measurement shows that the low-noise amplifier circuit meets the design requirements.In the filter part,firstly,by analyzing the performance of low-pass,high-pass,band-pass and band-stop filters,the filter type suitable for this topic is selected as Chebyshev band-pass filter.By analyzing the design principle and order of the filter,the topology of the filter is designed.After normalization and de-normalization,the corresponding component values are obtained,and the topology design is completed.After simulation and actual measurement,the filter meets the design requirements.In the acquisition and display module part,a very low frequency weak signal acquisition device based on FPGA control is constructed through the WM8978 module,which not only realizes the high-speed acquisition of 32 MSPS,but also meets the requirements of equipment portability.After testing,the acquisition and display system can achieve the same function as the spectrum analyzer,and it can demodulate and display the spectrum of the received signal.In order to provide a better debugging environment for the VLF weak signal preamplifier and acquisition system,this thesis builds a low-noise environment test platform conforming to the YB/T86 d national standard through a three-layer shielding cylinder.The permeability is 300000H/m,and the coercivity is 0.439A/m.The main research work of this thesis has three aspects,the first is the design of the low-noise amplifier circuit;the second is the design of the band-pass filter;the third is the design of the acquisition system.The innovation of the VLF weak signal preamplifier and acquisition system developed in this subject has two aspects.First,it has made certain breakthroughs in system gain;secondly,it has also made certain breakthroughs in system noise floor suppression.The system not only has strong acquisition and processing capabilities for weak signals in the very low frequency band,but also has the characteristics of modular structure,high acquisition rate,debuggability and strong anti-interference ability,and has strong practical value.