Interplay among Recovery Time, Signal, and Noise: Series- and Parallel-tuned circuits are not always the same."

J. B. Miller, B. H. Suits, A. N. Garroway, and M. A. Hepp
The quality factor, Q, of the magnetic resonance probe and the manner in which the probe is coupled to the spectrometer play important roles in the quality of the obtained experimental data. These parameters have a strong effect on the signal-to-noise ratio (SNR), the probe recover time after a pulse, radiation-damping of strong signals, and the coupling between multiple coils in a probe. However, it may not be possible to adjust the probe Q or coupling to the spectrometer to simultaneously optimize SNR, recovery time, etc. For example, while the highest SNR may be attained with a high-Q probe, the long recovery time of such probes can preclude the observation of signals with short decay times. One means of effecting a compromose between SNR and recovery time is to adjust the coupling of the probe and preamplifier by mismatching their impedances. Here, the optimum SNR in the presence of an impedance-mismatched low-noise preamplifier with different tuning schemes is calculated and compared to measurements for the particular case of a low frequency, high-Q NQR probe. Unlike the more common impedance-matched case, the conditions for optimum performance for parallel- and series-tuned probes are found to differ significantly.

Concepts in Magnetic Resonance 12, 125-136 (2000).

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