Signal modulation is the process of modifying the characteristics of a carrier wave signal using an information signal (such as voice or data). The characteristics that can be changed include amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM). A pure electrical, radio, or optical carrier signal carries no information aside from either being in on or off state. When the carrier signal is modified from a normalized state, it is called a modulated signal. This modulated signal is the carrier of the information that is used to modify the carrier signal. When the carrier signal is received, its signal is compared to an unmodulated signal to reverse the process (called demodulation). This allows the extraction of the original information signal. A carrier wave signal can be carried by wire, fiber, or electromagnetic waves transmitted through the air (radio).
When a carrier signal is modulated, the frequency changes above and below its reference frequency. The difference between the upper and lower maximum frequency changes is called the bandwidth. The relationship between the amount of frequency bandwidth of an information signal (the baseband) and the channel bandwidth of the modulated carrier determines if the system is a narrowband or wideband system. Narrowband systems have a relatively small communications channel bandwidth, typically below 100 kbps. When the bandwidth of the broadband carrier is much higher than the bandwidth of the information source, it is called a wideband system.
The device that modifies the carrier signal with the information source (baseband signal) is called a modulator. An assembly or device that combines the function of modulating and demodulating signals is called a Modulator/DEModulator (MODEM).
Combined types of Modulation
Today’s sophisticated modulation systems can use all three variable parameters: frequency, amplitude, or timing (phase) at the same time to transfer analog or digital information. One of the more popular forms of combined modulation technologies is Quadrature Amplitude Modulation (QAM). QAM is a modulation technique that enables two information signals to modulate a single carrier frequency. The two different signals’ amplitude modulates two samples of the carrier that are of the same frequency, but differ in phase by 90 degrees. The resultant two signals can be added together and both signals recovered at a decoder where they are then demodulated 90 degrees apart.
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