Figure 4: The top graph: volt-ampere characteristic—the dependence of sample voltage on operating current (on inset conventional VCC of sample with regular arrangement of reference axes ) is shown. The middle graph: averaged component of sample response to periodical variation of microwave irradiation frequency of backward-wave tube, registered on modulation frequency . The bottom graph: averaged component of sample response on amplitude modulation with a depth reduced to deviation factor of frequency modulation (6%). The appreciable exceeding of response on FM relative to response on AM is the evidence for synchronicity of low frequency oscillations of Shapiro voltage steps, corresponding to different Josephson junctions. These junctions are formed below  K due to appearance of spatial inhomogeneity of superconducting phase distribution in the sample.