Vibration Time Wave form : Multiple Frequencies-Non Linear System(Frequency Modulation)

Frequency modulation is a time-varying frequency,as opposed to amplitude modulation, which is a time-varying amplitude. The lower frequency is the carrier, and the higher frequency is the modulator. The modulator is normally an excited frequency, and the source of excitation is normally the speed of the rotating unit.
Pulse Excited Natural Frequency

Frequency Modulation

Frequency modulation can be a series of high frequency bursts similar to a pulse, or the high frequency can occur periodically with a low frequency. Since the frequency response of an accelerometer is best at high frequencies, such problems may be best measured in acceleration. Frequency modulation occurs most often in impacts, such as defects on the inner race of cylindrical roller bearings, or when two shafts are rotating very close to each other. Frequency modulation can occur in screw compressors, vacuum pumps, and blowers when one shaft is bent enough to permit an impact once each revolution.

One last comparison should be noted to clarify the differences between a high frequency
riding a low frequency, amplitude modulation, and frequency modulation.

1. High frequency riding a low frequency - No looseness is present. High and low frequencies may be exact multiples of each other. No mixing of signals occurs. Changes in the phase have little or no effect.

2. Amplitude modulation - High frequency is the carrier; low frequency is the modulator. Signals go into and out of phase.

3. Frequency modulation - Low frequency is the carrier; high frequency is I the modulator.