Modulation and Demodulation
Modulation is a technique of imposing information (analog or digital) contained in a lower frequency signal onto a higher frequency signal. The lower frequency is called the modulating signal, the higher frequency signal is called the carrier, and the output signal is called the modulated signal. The benefits of the modulation process are many, such as enabling communication systems to transmit many baseband channels simultaneously at different carrier frequencies without their interfering with each other. One example is that many users can use the same long-distance telephone line simultaneously without creating a jumbled mess or interference. The modulation technique also allows the system to operate at a higher frequency where the antenna is smaller.
Some form of modulation is always needed in an RF system to translate a baseband signal (e.g., audio, video, data) from its original frequency bandwidth to a specified RF frequency spectrum. Some simple modulation can be achieved by direct modulation through the control of the bias to the active device. A more common method is the use of an external modulator at the output of the oscillator or amplifier. Figure 9.1 explains the concept of modulation.
There are many modulation techniques, for example, AM, FM, amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), biphase shift keying (BPSK), quadriphase shift keying (QPSK), 8-phase shift keying ...