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
Abstract
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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