Low-field Magnetic Resonance Fid Signal Amplification Circuit Analysis And Research

Since the low field NMR signal is quite weak and is of poor SNR(Signal to Noise Ratio), the design and test of a signal reception circuit used for detecting such a signal is always difficult. The function blocks of the low field NMR circuit system are analyzed at first. Then the signal amplitude is calculated. The antenna interface and the preamplifier are studied in detail at last.Antenna interface is the interface circuit for the RF transmitting circuit, the signal reception circuit and the probe. It not only has to realize the switch of the three-states(transmitting, dump, receiving), but also serve as the passive amplifying section of the signal reception circuit. After analysis of the function of the antenna interface circuit, the simplified circuit model is built. Then the calculations are done based on it. The test guidance parameters fit for the highest Q value and the bandwidth expected are gotten. The calibration circuit is designed based on it.Preamplifier is used for the active amplifying of the NMR signal. In view of the weak signal ant its poor SNR, a preamplifier circuit is given in detail after discussions and comparisons of several schemes. Then the bias point is calculated and dynamic simulations are made for the analysis of the states. Its gain coefficient is calculated and the amplitude-frequency character of every stage is analyzed. Also the noise analyses are given.By imitating of the design characters of the high frequency circuit, the origin and the classification of the noise interferences of the low field NMR circuit system are analyzed. The items that need to be focused on the device chosen and the circuit layout are discussed. Some measures are taken to low the effects of the noise interferences on the electronic circuit.The experimental results are compared with the theoretical analyses and calculations, which prove the conclusions of analyses.