Calibration of STUD decoupling to achieve selected sideband amplitudes

Experimental calibration of the amplitude of sidebands resulting from STUD decoupling over a range of decoupled bandwidths from 2 to 120 kHz shows that sideband amplitude depends on the length of each single sech/tanh pulse, T-p, and the bandwidth divided by the square of the maximum RF amplitude during the pulse, bwdth\(RF(max))(2). Plots of sideband amplitude versus bwdth/(RF(max))(2) are independent of bandwidth and so provide convenient calibration curves enabling choice of T-p. Such calibration curves will vary modestly between NMR probes because of the differing homogeneity of the RF field produced by the probe across the sample, The dependence on bwdth/(RF(max))(2) arises from the inversion efficiency of a single sech/tanh pulse, This dependence corresponds to the principle that the optimal RF power required for STUD decoupling is linearly proportional to the chosen decoupled bandwidth, This behavior, which is common to adiabatic decoupling methods, is different from composite-pulse decoupling schemes where chosen bandwidths and corresponding optimal RF power settings are more limited, and effective bandwidths are proportional to the applied RF amplitude rather than RF power, The STUD method achieves the widest effective decoupled bandwidths of any broadband technique to date when decoupling schemes are compared at the same average power. This is particularly relevant to applications where sample heating must be minimized. Additional criteria for good decoupling performance are considered in further detail. The accurate measurement of sideband amplitudes is shown to provide a convenient means for evaluating the efficiency of different decoupling schemes, Adiabatic decoupling methods are especially interesting since, in contrast to composite-pulse methods, the RF amplitude and pulse length can be varied separately, providing additional opportunity for comparison with theoretical models of decoupling. In particular, exceptions to a frequently cited condition for minimizing sideband intensity, T(c)J much less than 1, where T-c is the cycle time to return the irradiated spins to their initial orientation, are discussed. (C) 1996 Academic Press, Inc.