Abstract
Three optimum conditions for the tuning of NMR probes are compared: the conventional tuning optimum, which is based on radio-frequency pulse efficiency, the spin noise tuning optimum based on the line shape of the spin noise signal, and the newly introduced frequency shift tuning optimum, which minimizes the frequency pushing effect on strong signals. The latter results if the radiation damping feedback field is not in perfect quadrature to the precessing magnetization. According to the conventional RLC (resistor–inductor–capacitor) resonant circuit model, the optima should be identical, but significant deviations are found experimentally at low temperatures, in particular on cryogenically cooled probes. The existence of different optima with respect to frequency pushing and spin noise line shape has important consequences on the nonlinearity of spin dynamics at high polarization levels and the implementation of experiments on cold probes.
Highlights
The tuning of an NMR probe is an important routine procedure, often automated and invisible to users of state-ofthe-art NMR systems
One can optimize for signal reception by maximizing the received electronic noise power or, largely equivalent, by optimizing a nuclear spin noise line shape to appear as a symmetrical dip in the noise baseline, as reported previously.[6,7,8]
On all cryo-probes we investigated the frequency shift tuning optimum (FSTO) was found be- corrected by the field frequency lock, which is much less aftween the conventional tuning optimum (CTO) and the spin noise tuning optimum (SNTO)
Summary
The tuning of an NMR probe is an important routine procedure, often automated and invisible to users of state-ofthe-art NMR systems. According to the frequently used resistor–inductor–capacitor (RLC) model of NMR detection circuits[11] the SNTO, and the FSTO as well as the conventional tuning optimum (CTO) should coincide. Their discrepancy has direct implications on the implementation of NMR pulse sequences in particular on cold probes and proves the inadequacy of the RLC model in many cases.
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