When we modulate a carrier signal with amplitude modulation, there will be four frequency components as the result. The first is the modulating signal itself, the second is the frequency carrier, the the latest two are the difference and the sum of the carrier and the modulating signal. The spectrum of these frequencies can be seen using frequency analyzer. In radio transmission, the modulating frequency components is completely filtered out since the frequency is far lower than the other three. To increase the power efficiency, double sideband suppressed carrier modulator (DSB-SC) remove the carrier frequency part, so the transmitted frequencies consist only the side bands: the sum and the difference. In the schematic diagram below, a double-sideband suppressed-carrier modulator circuit is presented. There is no carrier appear in the output because of the basic current. The Carrier amount appear in at the output can be controlled by adding offsets to the carrier differential pairs. The function of the modulation signal-AM modulation is the amplitude.
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