Vibration signal obtained from a gear having local fault contains amplitude and phase modulation. Faulty teeth of a
gear often gives rise to sidebands which results from modulation of gear meshing frequency. Sidebands occur around
a central frequency and it spreads over a wide frequency range due to its short time period of impact. The central
frequency is also known as carrier frequency. The sideband is the region of importance containing local fault
features, thus a properly chosen narrow band spectrum analyser could be very useful for fault detection