Many modern NMR pulse sequences (e.g. the 1D gradient selective NOESY experiment) depend on shaped pulses for selective excitation or inversion of specific resonances. Since the width of the excitation profile of a shaped pulse is determined by its duration, the pulse duration is chosen by the user for the selectivity needed. The longer the pulse, the higher the degree of selectivity. Some spectrometer software will calculate the pulse duration based on a selected region in a spectrum containing the desired resonance for excitation. The calculation depends on an initial pulse calibration. The standard calibration method for hard pulses involves incrementing the pulse duration at a fixed power level. The 90° pulse is at the first maximum and the 180° pulse is at the first null. Since the duration of a the selective pulse is fixed by the desired selectivity, the 90° and 180° pulses must be found by varying the pulse power rather than the pulse duration. The figure below shows the calibration for three different 50 msec shaped pulses on a Bruker AVANCE spectrometer.
An on-resonance water signal was observed as a function of pulse power using a selective one-pulse sequence. The scale is in units of decibels of attenuation. Maximum power is at -6 dB so the scale goes from low power on the left to higher power on the right. The 90° and 180° pulses are indicated with arrows. The intensity profiles are not sinusoidal due to the logarithmic dB scale. The 90° and 180° pulses are separated by 6 dB of attenuation as expected.