Considering the prevailing existence of sideband in the vibration signals in gear system fault
the distribution characters of sidebands of modulation signals under the condition of single amplitude modulation
single frequency modulation and co-effect of amplitude modulation and frequency modulation are analyzed in theory
and the cause of formation of the sideband in the gear system is discussed.Cases are adopted to interpret and analyze the character of sideband of vibration signals in details.Results show that analyzing the cause of sideband of vibration signals
the reference can be provided for fault diagnosis in the gear system.