... Analytic theory of the integer and non-integer Shapiro steps has been developed for the so-called high-frequency limit. ... Detector response for the case of high fluctuation level has been considered as well. ... Analytical description of the Shapiro step dependence on the signal amplitude was obtained only for a high-frequency limit in the frame of resistively shunted junction (RSJ) model [1] describing an overdamped junction with McCumber parameter b ® 2pI C R2 C =U0 ( 1. ...