How It Works
At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency such as a sine wave, that serves to "carry" information. The carrier is much higher in frequency than the data signal. The carrier signal and the incoming data signal (called the baseband signal) are applied to a modulator circuit. The modulator alters some aspect of the carrier signal, such as its amplitude, frequency, or phase, with the data signal, "piggybacking" the data on the carrier. Multiple modulated carriers at different frequencies are sent through the transmission medium, such as a cable or optical fiber.
Each modulated carrier consists of a narrow band of frequencies, centered on the carrier frequency. The information from the data signal is carried in sidebands on either side of the carrier frequency. This band of frequencies is called the passband for the channel. As long as the carrier frequencies of separate channels are spaced far enough apart so that their passbands do not overlap, the separate signals will not interfere with one another. Thus the available bandwidth is divided into "slots" or channels, each of which can carry a data signal.
At the destination end of the cable or fiber, for each channel, an electronic filter extracts the channel's signal from all the other channels. A local oscillator generates a signal at the channel's carrier frequency. The incoming signal and the local oscillator signal are applied to a demodulator circuit. This translates the data signal in the sidebands back to its original baseband frequency. An electronic filter removes the carrier frequency, and the data signal is output for use.
Modern FDM systems often use sophisticated modulation methods that allow several data signals to be transmitted through each frequency channel.
Read more about this topic: Frequency-division Multiplexing
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