Amplitude modulation (AM) and beat frequency are related phenomena that can be observed in certain situations. When two signals with different frequencies are combined, they can produce a resultant signal that exhibits a beat frequency pattern. Here's why:
In amplitude modulation, a high-frequency carrier signal is modulated by a lower-frequency information signal. The carrier signal's amplitude is varied in accordance with the instantaneous amplitude of the information signal. As a result, the carrier signal's amplitude is "modulated" or changed to represent the information contained in the lower-frequency signal.
When these two signals are combined, their waveforms interact, leading to the phenomenon known as beat frequency. This occurs due to the mathematical relationship between the two signals.
Consider the case where the carrier frequency is significantly higher than the information signal frequency. As the information signal varies, the amplitude of the carrier signal changes accordingly. When the information signal's amplitude is at its maximum, the carrier signal's amplitude is also at its maximum. Similarly, when the information signal's amplitude is at its minimum, the carrier signal's amplitude is also at its minimum.
The combination of these varying amplitudes creates an envelope pattern in the resultant signal. This envelope pattern appears as a slow variation in the amplitude of the high-frequency carrier signal. This variation, known as the beat frequency, is equal to the difference between the carrier frequency and the information signal frequency.
The beat frequency can be heard as a "beating" or pulsing sound if the resultant signal is an audible audio signal. The rate of this beating corresponds to the difference in frequency between the carrier and information signals.
Therefore, the appearance of beat frequency in amplitude modulation is a consequence of the interaction between the carrier and information signals, leading to the varying amplitude pattern observed in the resultant signal.