The degree of signal distortion can be expressed by nonlinear distortion coefficient or distortion degree. It is defined as the square root of the ratio of total harmonic energy to fundamental energy. For purely resistive loads, it is defined as the ratio of the effective values of all harmonic voltages (or currents) to the effective values of fundamental wave voltages (or currents), that is, in the formula, U 1 is the effective value of fundamental wave voltage, U2, U3, …, and Un is the effective value of various harmonic voltages. The degree of distortion can also be expressed by its approximate value K0, and the relationship between k and K0 can be converted by the following formula: when K= 10%, the difference between k and K0 is 0.5%; When K=20%, the difference between k and K0 is 2%. The greater the k, the greater the difference.