Demystifying Demodulation and Enveloping

Alan Friedman, Zenco
Tags: vibration analysis, maintenance and reliability
In a recent Zenco video, Alan Friedman demystifies the concepts of demodulation and enveloping vibration analysis. The video features three main graphs: a time waveform, an FFT spectrum, and a demodulated spectrum.
Alan demonstrates the relationship between the time waveform and the FFT spectrum. He modulates a 25 Hz sine wave with another sine wave, resulting in sidebands in the spectrum. This modulation, the repetitive increase and decrease in signal amplitude, is shown by peaks at 25 Hz with sidebands separated by 4.2 Hz.
Next, Alan varies the modulating frequency and amplitude, illustrating sideband spacing and amplitude changes. He explains demodulation using the bottom plot, highlighting its application in analyzing rolling element bearings. By demodulating the time waveform, the modulating frequency of 7 Hz is revealed, along with its harmonics.
Finally, Alan explores the effects of distorted modulation by clipping the waveforms, leading to additional harmonics and sidebands common in AC motors, gears, and bearings. He cautions against common misinterpretations of demodulation plots.