  The theoretical shapes of nuclear spin-noise spectra in NMR are derived by
considering a receiver circuit with finite, preamplifier input impedance and a
transmission line between the preamplifier and the probe. Using this model, it
becomes possible to reproduce all observed experimental features: variation of
the NMR resonance linewidth as a function of the transmission line phase,
nuclear spin-noise signals appearing as a "bump" or as a "dip" superimposed on
the average electronic noise level even for a spin system and probe at the same
temperature, pure in-phase Lorentzian spin-noise signals exhibiting
non-vanishing frequency shifts. Extensive comparison to experimental
measurements validate the model predictions, and define the conditions for
obtaining pure in-phase Lorentzian-shape nuclear spin noise with a vanishing
frequency shift, in other words, the conditions for simultaneously obtaining
the Spin-Noise and Frequency-Shift Tuning Optima.
