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Demo: sawtooth signal of 2H in acetone-d6

On this page you may watch and listen to a CW signal instead of an FID. Any NMR spectrometer employs deuterium for solution state measurements in order to achieve field stability. To find and monitor the lock signal the Z0 current of the shim system is stepped through in a "sawtooth" mode, i.e. a linear increase from a bottom to a top level and sudden drop to the bottom level again. Since the signal is thus rapidly scanned, typical "wiggles" of the signal appear. The peak-to-peak distance becomes ever smaller the more the Larmor condition becomes mismatched, along with an exponential decay of the signal intensity. After each re-scan, some magnetization is still left, leading to similar oscillations before (left to) the signal.

Operators who work or worked with older instruments constructed in the sixties or early seventies will confirm having seen such signals thousands of times on the oscilloscope.

The corresponding sound is a constant increase from low to high tone.

Sawtooth signal spectrum

Sawtooth signal sound, .aiff

Sawtooth signal sound, .wav

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August 20, 1996