A planetary gear set introduces complicated gear vibration frequency structures, especially in sequentially phased planetary gear sets. Frequency components from a fixed sensor can be enhanced and/or canceled with different planets and ring gear configurations. In this study, an amplitude modulation based formula and simple trigonometric function manipulations are used to reveal the major physical phenomena in a planetary gearbox. The formulation used in this study permits unlimited modulation indices as well as unrestricted planet passing signal responses sensed at fixed transducer locations on the gearbox. Numerical simulations and field examples are given in the end of the paper to validate the analysis. Copyright © 2012 John Wiley & Sons, Ltd.