AM and FM Signal Demodulation

Abstract

When a modulated signal arrives at the receiving antenna, the embedded information must somehow be extracted by the receiver and separated from the HF carrier signal. This information recovery process is known as “demodulation” or “detection”. It is based on an underlying mechanism similar to the one used in mixers, where a nonlinear element is used to multiply two waves and accomplish the frequency shifting. However, the demodulation process is centred around the carrier frequency ω0 and the signal spectrum is shifted downward to the baseband and returned to its original position in the frequency domain. Both modulation and demodulation involve a frequency-shifting process; both processes shift the frequency spectrum by a distance ω0 on the frequency axis; and both processes require a nonlinear circuit to accomplish the task. Although very similar, the two processes are different in very subtle but important details. In the modulating process the carrier wave is generated by the LO circuit, and then combined with the baseband signal inside the mixer. In the demodulating process, however, the carrier signal is already contained in the incoming modulated signal and, unless the modulated wave is of the suppressed-carrier type, it can be recovered at the receiving point.