_Signal_FAQ15_Carrier_in_Modulation - However, sometimes,...

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What is the meaning of carrier in AM and FM? The word “carrier” is used loosely in general to mean the use of a high frequency waveform or oscillator to shift the spectrum of a message signal to a higher value. This shifting of the spectrum is sometimes called “up conversion” and we do this so that the antenna size is reasonable for efficient transmission and reception. Sometimes, the carrier is physically there in the modulated signal. For example, in conventional AM, if we use a 200 kHz oscillator in the modulator in the transmitter, the modulated signal will indeed have a 200 kHz component.
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Unformatted text preview: However, sometimes, the carrier is physically absent in the modulated signal. Examples are AM with suppressed carrier or FM. Whether we need to regenerate the carrier in the receiver for the purpose of demodulation depends on the modulation scheme. For example, in FM, the regeneration of the carrier is not necessary. To demodulate, we just need to have some circuits to extract the instantaneous frequency of the modulated signal. For single sideband AM or suppressed carrier AM where we have to perform coherent demodulation, carrier regeneration is essential....
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This note was uploaded on 08/02/2009 for the course ECE EE2009 taught by Professor Prof.c.c.ko during the Fall '07 term at National University of Singapore.

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