Avionics News December 2022 - 54

THEORY & PRACTICE
Continued from page 53
amplify the difference frequency. The heterodyne action is
occurring with the other frequencies too, but none of the
sum or difference frequencies are 10 MHz, so they will
not get amplified. Only the approach control frequency
will be heard.
When approach control hands the flight over to the tower
on 119.1 MHz, the pilot changes the channel, causing the
local oscillator to change its frequency to 109.1 MHz. The
heterodyne action now produces the 10 MHz.
Figure 2 shows more detail of a superheterodyne
receiver designed to receive amplitude modulated signals
as used in the VHF aviation communication band of
frequencies. The antenna will react to many different
frequencies, but in the context of aviation communication,
it will react best to aviation communication signals.
Shown next to the antenna is a representation of an
amplitude modulated wave form.
After the antenna is a preselector or radio frequency
amplifier. Often, this stage of the receiver will include a
filter to help keep FM broadcast, navigation signals, public
service, and other signals from interfering with reception.
In addition, many radios will include an amplifier at this
stage.
From the preselector, the signal goes to the mixer.
The mixer is where the heterodyne action occurs. Also
attached to the mixer is a stabilized master oscillator. The
SMO will produce a precise frequency with which the RF
signal will heterodyne. When the pilot changes frequency,
the output frequency of the SMO will change. The output
of the SMO will be a steady sign wave as shown.
There are many frequencies at the output of the mixer.
The job of sorting them out and amplifying the desired
frequency is the job of the intermediate frequency
amplifier. The IF amplifier is the real workhorse of the
radio receiver. The IF will include several filtering and
amplification stages. Some of these stages will be gain
controlled by the automatic gain control amplifier.
Some radios will use two mixers, two IF sections, and
a second oscillator to increase both the sensitivity and the
filtering capability of the receiver. These radios are known
as dual superheterodyne receivers.
The detector will create a varying DC voltage based
on the amplitude of the signal coming from the IF.
Schematically speaking, the circuitry is surprisingly
similar to a rectifier. The outputs of the detector will be an
audio waveform. By now, the radio frequency has done its
job and carried the information to the receiver. As a result,
it's no longer needed.
As described above, the AGC monitors the audio
coming from the detector and uses it to control the bias
voltages within the IF amplifier, thus controlling the gain
of the amplifier. You can think of the AGC as negative
feedback, which will keep the detector output at a steady
level.
The audio amplifier takes the signal provided by the
detector and amplifies it until it is powerful enough to
operate an aviation headset. This same level is adequate
for the input of the audio systems and panels used in
aircraft.
Antenna
Transmit/Receive
Switching
FIGURE 3:
A block diagram
of an AM radio
transmitter
Input from SMO
Receiver
Buffer
Pre-driver
Driver
Final
Input from microphone
Microphone
Amp
Sidetone output
54 avionics news * december 2022
Modulator
Avionics News December 2022

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