Amplitude modulation (AM) is a modulation technique used in radio frequency (RF) communication systems to transmit information. When an RF carrier wave is modulated by a sinusoidal tone, it results in a complex wave that contains multiple frequency components. Let's understand how this process works step by step.
Carrier Wave: The carrier wave is a high-frequency sinusoidal signal that serves as a carrier for the information to be transmitted. It typically has a much higher frequency than the modulating signal.
Modulating Signal: The modulating signal, which is a lower-frequency sinusoidal tone, carries the actual information to be transmitted. It could be an audio signal, for example, generated by a microphone.
Modulation Process: In amplitude modulation, the amplitude of the carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. The modulating signal acts as a control signal that determines the changes in the amplitude of the carrier wave.
Multiplication of Signals: The modulating signal and the carrier wave are multiplied together. Mathematically, this is achieved by multiplying their instantaneous values at each point in time. This multiplication process results in the creation of sidebands.
Sidebands: Sidebands are additional frequency components that are created around the original carrier frequency due to the modulation process. The sidebands appear at frequencies that are offset from the carrier frequency by the frequency of the modulating signal.
Frequency Components: The sidebands carry the information contained in the modulating signal. The upper sideband (USB) is located above the carrier frequency, and the lower sideband (LSB) is located below the carrier frequency. The frequency separation between the carrier and the sidebands is equal to the frequency of the modulating signal.
Spectrum of the Complex Wave: When the complex wave resulting from the modulation process is analyzed using a spectrum analyzer or Fourier analysis, it reveals the presence of multiple frequency components. The spectrum shows the carrier wave, the upper and lower sidebands, and their associated frequencies.
By modulating an RF carrier wave with a sinusoidal tone, amplitude modulation generates a complex wave with multiple frequency components, including the original carrier frequency and the sidebands. This allows the transmission of information in the form of variations in the carrier wave's amplitude, which can be demodulated at the receiver to retrieve the original modulating signal.