IMPROVEMENTS IN ELECTRONIC DECOUPLING OF TRANSMITTER AND RECEIVER COILS

This paper focuses on the problem of decoupling transmitter and receiv er coils when noise arises predominantly from the coil resistor, the g eneral case in high-resolution NMR as well as in NMR of small biologic al samples, Test procedures for assessing the efficiency of decoupling schemes are proposed. It is shown that the residual current flowing i n the decoupled coil, which distorts the magnetic field profile of the working coil, may be easily calculated from comparison of the resonan ce frequencies of the isolated coils and of the combined coils, It is also pointed out that the measurement of the modification of the quali ty factor of the receiver coil due to the introduction of the decoupli ng component inaccurately reflects the increased losses, These princip les are demonstrated in the case of coaxial surface transmitter and re ceiver coils, The severe problems caused at high frequency by earth lo ops and capacitive coupling between components have been circumvented by designing all circuits and coils in an electrically balanced manner , Particular attention has been paid to the receiver coil, and a solut ion based on the use of twin lines is described which preserves the se nsitivity of the coil even with lossless samples, (C) 1995 Academic Pr ess, Inc.