Phase Transients in Pulsed NMR Spectrometers

When a square wave modulated rf voltage is applied to a tuned probe in an NMR spectrometer, the effective transverse magnetic field in the rotating reference frame changes direction as it grows and decays. The resulting deviations in the nutational behavior of the spins from naive expectations are typically small, but may lead to important cumulative errors in experiments employing long pulse trains. These errors can be made to vanish by very accurate tuning of the probe circuit to its free ringing frequency ωr not to ω0=(1/LC)½ and by providing suitable coherence between the rf carriers and the timing of pulses.