  We report the experimental study of linear and nonlinear ac properties of two
types of samples of MgB2 prepared by different technologies. A commercial
sample (Alfa Aesar) demonstrates two stages of a broad phase transition,
whereas home-made metallic sample have one sharp transition. The temperature
dependence of the rectified signal shows two peaks for the commercial and one
peak for the metallic samples. Two types of weak links (intergrain and
intragrain) are responsible for the nonlinear response of these samples. The
properties of these weak links are discussed.
