TABLE OF CONTENTS The purpose of the detector or demodulator circuits is to recover the intelligence impressed on the radio carrier wave at the transmitter. This process is called demodulation and the circuits used to accomplish this are called demodulators. They are also 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 (Fig. 27-1). This position is the same in AM, FM, pulse modulation, and digital receivers (although in digital receivers the demodulator might be in a circuit called a MODEM).
An amplitude modulation (AM) signal consists of a slow audio signal which revolves around an average radio frequency (RF) carrier signal. It is essentially a multiplication or mixing process in which the RF carrier and AF signals are both output, along with the (RF AF) and (RF + AF) signals. Because of the selectivity of the transmitter circuits, only the RF carrier and the sum and difference signals appear in the output. The AF signal is suppressed. The sum signal (RF + AF) is known as the upper sideband (USB) while the difference signal (RF AF) is known as the lower sideband (LSB). Because of this action, the bandwidth of the AM signal is determined by the highest audio frequency transmitted and is equal to twice that frequency. A total of 66.67% of the RF power in an...
