Double-Sideband Suppressed-Carrier Amplitude Modulation and Coherent Detection

The standard AM modulated signal contains a sinusoidal component at the carrier frequency which does not convey any of the baseband message information. This component is included to create a positive envelope which allows demodulation by a simple, inexpensive envelope detector. From an information theory point of view, the power in the sinusoidal carrier component is wasted. In this experiment, you will see that it is not necessary to transmit the carrier component and that the baseband message can be recovered by a coherent demodulator. In fact, it can be shown that a coherent demodulator performs better than an envelope detector when the received signal is corrupted by additive noise. The type of modulation that will be studied in this chapter is called double-sideband suppressed-carrier amplitude modulation (DSBSC-AM). A close approximation to an ideal coherent demodulator called a Costas loop will be implemented.