| 摘要 |
<p>Two half-coils (181, 182) sample a nuclear spin precession signal from two specimens (200). One capacitor of a bank (22) of capacitors forms a resonant circuit with the two half-coils. The signal at the terminals of the capacitor is amplified by a differential amplifier (121), phase shifted in a phase-shift circuit (28) introducing a constant phase shift of 90 DEG , and reintroduced into the half-coils through a potentiometer (321). The frequency of the oscillations is measured by a frequency meter (161). A circuit (26) controls the connection of the capacitors of the bank (22) in response to the value of the measured frequency so as to keep the parasitic phase shifts between the sampled signals and the reintroduced signals below a certain threshold.
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