Decoupling, Tuning, Matching and Cable Length Effects on NMR Noise Spectra
Sprache des Vortragstitels:
The Third Annual East-NMR User Meeting
Sprache des Tagungstitel:
The complex tuning dependence of NMR noise spectra [1, 2] can be attributed largely to absorbed circuit noise (ACN), which exhibits a complex line shape, that depends on sample and probe properties. It is always superimposed by a purely absorptive spin noise contribution. Pure spin noise has been observed in wide line solid spectra  and in presence of static field gradients . Previously, it was shown that a symmetrical dip line shape of a strong solvent noise signal can be used as a good indicator for tuning to optimum receiving conditions: the spin noise tuning optimum (SNTO) [2, 5]. We have investigated line shape changes of NMR noise signals upon systematic variation of the tuning and matching adjustments as well as cable lengths. It turns out that on most probes more than one SNTO-dip situation can be found. On many probes one finds that the conventional tuning optimum (CTO) and SNTO are far apart in tuning frequency. Reception sensitivity has been observed to be at a maximum at the SNTO. Using the systematic tuning-matching approach and varying cable lengths one can find conditions, where both pulse excitation and reception properties of a commercial NMR probe are at optimum values . Under optimal conditions even recording of decoupled and coupled C-13 NMR noise spectra thus becomes feasible .