Detection of the solute signal (typically proton concentration of 1-2 mM) in the presence of solvent signal (for water, proton concentration ~110M) presents a difficult problem since the dynamic range of the electronic components of the spectrometer is limited.
Three methods exist for suppressing unwanted signals of the solvent (dynamic range reduction):

Presaturation

of the solvent resonance during the recycle delay between the acquired scans, using a weak rf field. Presaturation of a signal excites a small region of the sample for a relatively long time, reducing the intensities of the net magnetization of signals in the region. The main disadvantages are that signals that resonate very close to the solvent may be partially saturated (the alpha protons of the protein). Also, if nuclei are exchanging between two environments that give rise to two different signals, and if one of these signals is presaturated and reduced in intensity, then the other signal will also be reduced (saturation transfer). For instance, if presaturation is used to reduce the water signal in a protein sample, the amide proton signals will also be reduced, since the amide protons exchange with the water proton. In order to use this water suppression scheme, set the pulse program to zgpr, set your carrier frequency o1 on the solvent, and irradiate for d1 of ~1-2 sec using an rf field of ~50 Hz (typically 58dB for H2O sample, and 80-100dB for samples in D₂O).

Binomial signal suppression

experiments use non-uniform excitation of the spectrum to reduce the signal intensities of small spectral regions. The 1-1 jump-return (Plateau & Gueron, JACS104, 1982) and the 1331 binomial sequence (Hore, J. Mag. Res. 55, 1983) are recommended for 1D signal suppressions. The higher-order binomial experiments better suppress peaks at the expense of a more rolling baseline. Suppression will occur at multiplets of the offset frequency. In the jump-return technique, the final read pulse in a pulse program is replaced by the sequence 90- t -90, the carrier is placed on the solvent, and the delay is set to t = 1/(2* d ) where d is the offset from the carrier to the next null. In the 1331 sequence, p1331, the delay d19 = 1/d where d is the offset from the carrier to the next null. The left half and right half of the spectrum differ in phase by 180°, and the spectrum may require lots of first-order phase correction.

Dephasing of the solvent

using spin lock and gradient pulses. There are many sequences that uses these elements to achieve solvent suppression: some uses spin-lock purge pulse in which the solute magnetization is locked while the solvent’s dephase; some use strong field gradients to dephase the solvent (WATERGATE; Piotto, Saudek & Sklenar, J. Biomol. NMR 2, 1992) while others return the water magnetization to the z-axis prior to acquisition (flip-back; Grzesiek & Bax, JACS115 , 1993). In the WATERGATE sequence, zggpwg , the gradient pulse is typically 1msec and shaped selective pulses are used. An easily set-up sequence that dephases the solvent by combining the binomial water suppression with gradient pulses is the p3919gp pulse program. It requires no special calibrations since it uses solely hard pulses and gradient pulses. As in the 1-1 jump-return sequence, d19=1/2*d where d is the offset from the carrier frequency, o1, to the next null.