| Soil moisture is one of the necessary conditions for the survival of crops.The detection of soil moisture content can assess the degree of drought and moisture of crops,and then rationally utilize water resources to ensure the sustainable development of agricultural production.On the other hand,soil moisture content can also be used to assess the extent to which crops absorb fertilizer,which can be measured properly to ensure the effective growth of crops.Therefore,the accurate measurement of soil moisture is of great significance for the transition from agricultural production to "fine agriculture".At present,the existing magnetic resonance soil moisture measuring instrument is too heavy and large,which is not conducive to the convenient and quick measurement in agricultural field.In this paper,a magnetic resonance measuring system is designed for the purpose of small and convenient,and the soil moisture measurement is finally realized.The specific work completed includes:(1)A low noise preamplifier is designed.The signal of low field magnetic resonance system is weak and easy to be interfered,so low noise amplifier should be used as the first stage amplifier.This paper takes low noise and high gain as the design goal,and selects a low noise transistor.Firstly,the static bias,feedback and input/output matching circuit are simulated by circuit simulation software.In the simulation software to get the optimal performance of the circuit diagram,and then use the circuit design software to complete the design of printed circuit board.(2)A RF power amplifier is designed.According to the characteristics of the pulse sequence of the magnetic resonance system,a power field effect tube was selected after the design objective was defined.Firstly,the basic parameters,such as static working point,stability and gain,are simulated by ADS software.After the simulation,the magnetic resonance echo signal is sensitive,and a gated circuit is designed to turn off the amplifier when no excitation is needed.Finally,the circuit design software is used to complete the whole power amplifier design.(3)Completed the circuit design of spectrum instrument.According to the required function of magnetic resonance measurement system,a core control circuit is designed.The whole system includes power supply,controller,pulse transmitting part,signal receiving part,VGA circuit,data storage and so on.The controller includes FPGA(field programmable gate array)and MCU.The former completes the function of pulse transmitting and digital demodulation and filtering of magnetic resonance echo signal,and the latter completes the data transmission and data storage.Finally,the transverse relaxation time T2 attenuation curve was displayed on the upper computer.(4)Experimental effect verification.A low noise amplifier operating at 6 MHz with a noise factor of 0.7 d B and gain of 28 d B is finally completed.An RF power amplifier with a P1 d B of 35 W,harmonic distortion of less than-30 d BC,efficiency of70% and working at 6 MHz has also been designed.The performance test of the whole circuit is completed and all normal.After the stable work,the moisture content experiment of different soil samples and different particles of sand is carried out,and the drying and weighing method is used as a comparison,and the samples with different moisture content are obtained by drying the water in a thermostat.By comparison,it is found that the larger the water content of the sample,the larger T2,and the peak area is proportional to the water content.At the same water content,T2 of large-grained sand is larger than that of small-grained sand.The results show that the larger the lateral relaxation time,the larger the pore radius.The larger the crest area,the higher the water content.The experimental results show that the portable magnetic resonance measuring instrument designed in this paper can realize the function of soil moisture measurement,which provides an idea for a simpler magnetic resonance soil moisture measuring system in the future. |
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