2-12
Figure 2-9.—AM superheterodyne receiver and waveforms.
The IF carrier is applied to the IF amplifier. The amplified IF carrier is then sent to the detector. The
output of the detector is the audio component of the input signal. This audio component is then passed
through an audio frequency amplifier. The amplified audio component is sent to a speaker for
reproduction. This allows you to hear the signal.
You should note that a superheterodyne receiver may have more than one frequency-converting
stage and as many amplifiers as needed to obtain the desired power output. (Additional amplifiers are not
shown.)
HETERODYNING.—As you know the intermediate frequency is developed by a process called
heterodyning. This action takes place in the mixer stage (sometimes called a converter or first detector).
Heterodyning is the combining of the incoming signal with the local oscillator signal. When heterodyning
the incoming signal and the local oscillator signal in the mixer stage, four frequencies are produced. They
are the two basic input frequencies and the sum and the difference of those two frequencies. The amplifier
that follows (IF amplifier) will be tuned to the difference frequency. This difference frequency is known
as the intermediate frequency (IF). A typical value of IF for an AM communications receiver is 455
kilohertz. The difference frequency is a lower frequency than either the rf input or oscillator frequencies.
This lower frequency gives slightly better gain but does increase the chances of image frequency
interference. Image frequencies will be discussed later in this chapter.
DETECTION.—Once the IF stages have amplified the intermediate frequency to a sufficient level,
it is fed to the detector. When the mixer is referred to as the first detector, this stage would be called the
second detector. The detector extracts the modulating audio signal. The detector stage consists of a
rectifying device and filter, which respond only to the amplitude variations of the IF signal. This develops
an output voltage varying at an audio-frequency rate. The output from the detector is further amplified in
the audio amplifier and is used to drive a speaker or earphones.
Frequency Modulated Receiver
The function of a frequency-modulated receiver is the same as that of an AM superheterodyne
receiver. You will find some important differences in component construction and circuit design caused
by differences in the modulating technique. Figure 2-10 is a block diagram showing waveforms of a
typical fm superheterodyne receiver. Comparison of block diagrams in figures 2-9 and 2-10 shows that in
both AM and fm receivers, the amplitude of the incoming signal is increased in the rf stages. The mixer
