Various methods developed to design phase gratings with specified diffraction patterns are systematized. In addition to differences with respect to the numerical technique applied, they differ in their structure modulation schemes. They are described and some of their characteristics, e.g., feature size in their structures, are compared. The finite precision of a fabrication device of phase elements is taken into account. The validity of the diffraction theory usually used for the calculation techniques of phase gratings is considered, dependent on the typical feature size, which results from a modulation scheme.