Modulation Technique

Modulation is the process of varying some characteristic of a periodic wave with an external signals. Modulation is utilized to send an information bearing signal over long distances. Radio communication superimposes this information bearing signal onto a carrier signal. These high frequency carrier signals can be transmitted over the air easily and are capable of traveling long distances.The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal.In the field of communication engineering, the information bearing signal is also known as the modulating signal.The modulating signal is a slowly varying signal - as opposed to the rapidly varying carrier frequency

Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance with the information bearing signal. The envelope, or boundary, of the amplitude modulated signal embeds the information bearing signal. The total power of the transmitted signal varies with the modulating signal, whereas the carrier power remains constant. A nonlinear device is used to combine the carrier and the modulating signal to generate an amplitude modulated signal. The output of the nonlinear device consists of discrete upper and lower sidebands.The output of a nonlinear device does not vary in direct proportion with the input.

Frequency modulation is a type of modulation where the frequency of the carrier is varied in accordance with the modulating signal. The amplitude of the carrier remains constant. The information-bearing signal (the modulating signal) changes the instantaneous frequency of the

carrier. Since the amplitude is kept constant, FM modulation is a low-noise process and provides a high quality modulation technique which is used for music and speech in hifidelity broadcasts. In addition to hi-fidelity radio transmission, FM techniques are used for other important consumer applications such as audio synthesis and recording the luminance portion of a video signal with less distortion. There are several devices that are capable of generating FM signals, such as a VCO or a reactance modulator.