| 摘要 |
In applications where a tuned coil is used as a receiving antenna, the coil's resistance can produce excessive noise. If a low-loss coil is used, such as with cooled or supercoductive wire, the resulting Q results in inadequate bandwidth. To provide the desired bandwidth without additional losses the coil and its tuning capacitor are periodically de-energized using one or two switches. This sets the signal to substantially zero where it starts again to build up, effectively widening the bandwidth. The resulting signal is synchronously detected to provide the in-phase and quadrature signals. In an NMR system this enables the performance to be limited by sample or object noise. In MRI systems using an oscillating readout bias, the shorting is timed with the transition of the bias waveform.
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