Chapter 11 - Detector and Demodulator Circuits

Abstract

The purpose of detector or demodulator circuits is to recover the intelligence impressed on the radio carrier wave at the transmitter. The process is called demodulation, and the circuits used to accomplish this are called demodulators. They also are called second detectors in superheterodyne receivers. In a superheterodyne receiver, the detector or demodulator circuit is placed between the IF amplifier and the audio amplifier. An amplitude modulation signal consists of a slow audio signal that revolves around an average radio frequency carrier signal. Both the RF carrier and AF signals are output in essentially a multiplication or mixing process, along with the (RF – AF) and (RF + AF) signals. A factor that affects the comfort level of listening to demodulated AM transmissions is the fading out of phase with each other of the carrier and two sidebands. This can be overcome with quasi-synchronous demodulation or synchronous demodulation. Both require that the incoming carrier be eliminated. Double- and single-sideband suppressed carriers are a lot more efficient than straight AM.