Design Of Receiver Used In Nuclear Magnetic Resonance Spectrometer

The technology of Nuclear Magnetic Resonance(NMR) is based on the theory that the nuclear magnetic moments in the magnetic field will absorb specific frequencies transmitted by the RF field. So the object of its research is microscopic nuclei and nuclear systems. Nuclear magnetic resonance spectrometer is an instrument used to observe NMR signals. Compared with the traditional observation, the NMR detection has no harm to observed objects, as well as to internal structure of object with higher accuracy. Therefore it is widely used in industry, agriculture, energy, medical fields and any other relative fields, and research on nuclear magnetic resonance spectrometer is very significant.Probe is the most important section in NMR spectrometer with main function to transmit input signal and make samples resonance. The resonance signal is amplitude modulated sine signal with sample information. Meanwhile, the probe receives NMR signal to analyze internal structure of samples.This thesis designed a Double-conversion superheterodyne receiver probe used for receiving NMR signal. To reduce its image frequency as well as improve SNR, the Double-conversion structure was used. The receive frequency range of receiver is between 10 MHz to 50 MHz. To satisfy performance of transmitter worked in NMR spectrometer, the sensitivity achieves-90 d Bm, dynamic range is between-90 d Bmand-30 d Bm. In the process of designing receiver, the main works are shown as below:1.Collected relative data concerning NMR spectrometer, and researched the fundamental principles of NMR. Proposed the test methods of NMR signal and selected one of them as final method.2. Studied receiver technology deeply, and focused on PLL which is used as local oscillator. Analyzed various receivers and compared the advantages and disadvantages of them. And hence this design used superheterodyne scheme. Finished the schematics of receiver, PCB layout, and entire module.3.Tested on sub-circuits and entire system of the receiver. Analyzed the measured results, and based on the results, modified this design. The final performance of this design included: Dynamic Range:-80 d Bm~-30 d Bm; receiver sensitivity:-80 d Bm; IF rejection: greater than 60 d B.