Avionics News December 2022 - 53

Radio
Local
Sum
Difference
. MHz . MHz . MHz . MHz
. MHz . MHz . MHz . MHz
.. MHz .. MHz .. MHz . MHz
... MHz
.. MHz
.. MHz . MHz
Tower on . MHz
Local Osc . MHz
Difference . MHz
FIGURE 1:
There are many radio
frequencies around the
airport. A superheterodyne
receiver uses a local (inside the
radio) frequency to help sort
them out. The pilot will hear
communications on the radio
frequency resulting in a 10
MHz difference frequency. The
channel selector chooses the
local frequency based on the
channel selection by the pilot.
The fourth type of receiver deserves to be mentioned
at length, for it is by far the most used in all radio
industries. The superheterodyne receiver uses a process
known as heterodyning to create a super audio (higher
than audio) frequency known as the intermediate
frequency. The IF will be amplified and filtered before
it is detected. The superheterodyne receiver can detect
weak radio signals and can be adjusted easily to different
frequencies.
Understanding the heterodyning action is the key
to understanding superheterodyne receivers. You may
want to refer to the October issue of Avionics News
where we discussed what happens when two signals are
mixed together. In the case of amplitude modulation,
the mixed signals are audio and radio. In any frequency
mixing action (heterodyne action), the result is the
original two frequencies along with sum and difference
frequencies - four total.
In a superheterodyne receiver, we mix two radio
frequencies together. In most cases, we take the difference
frequency, filter it, amplify it, and send it to a detector.
Changing channels is as easy as changing the frequencies
we choose to mix.
Refer to Figure 1 for an example of heterodyne
frequency selection. At this airport, there are many radio
signals in the air, of which four are listed. Three of them
are common airport frequencies, and another (96.9) is an
FM stereo broadcast frequency. As the airplane approaches
the airport, the pilots wish to talk to controllers on 124.6
MHz. When they set the channel selector for 124.6 MHz,
a local oscillator inside the radio produces a signal on
114.6 MHz. This signal mixes, or heterodynes, with all of
the frequencies; however, the amplifiers within the radio
are designed to amplify and filter 10 MHz. They will
Continued on following page
Antenna
AGC Amplifier
FIGURE 2:
A block diagram of
a superheterodyne
AM receiver
Preselector
or
RF amplifier
Stabilized Master Oscillator
avionics news * december 2022 53
Mixer
IF Amplifier
Detector
Audio Amplifier
Headphone speaker

Avionics News December 2022

Table of Contents for the Digital Edition of Avionics News December 2022

Avionics News December 2022 - Intro
Avionics News December 2022 - Cover1
Avionics News December 2022 - Cover2
Avionics News December 2022 - 1
Avionics News December 2022 - 2
Avionics News December 2022 - 3
Avionics News December 2022 - 4
Avionics News December 2022 - 5
Avionics News December 2022 - 6
Avionics News December 2022 - 7
Avionics News December 2022 - 8
Avionics News December 2022 - 9
Avionics News December 2022 - 10
Avionics News December 2022 - 11
Avionics News December 2022 - 12
Avionics News December 2022 - 13
Avionics News December 2022 - 14
Avionics News December 2022 - 15
Avionics News December 2022 - 16
Avionics News December 2022 - 17
Avionics News December 2022 - 18
Avionics News December 2022 - 19
Avionics News December 2022 - 20
Avionics News December 2022 - 21
Avionics News December 2022 - 22
Avionics News December 2022 - 23
Avionics News December 2022 - 24
Avionics News December 2022 - 25
Avionics News December 2022 - 26
Avionics News December 2022 - 27
Avionics News December 2022 - 28
Avionics News December 2022 - 29
Avionics News December 2022 - 30
Avionics News December 2022 - 31
Avionics News December 2022 - 32
Avionics News December 2022 - 33
Avionics News December 2022 - 34
Avionics News December 2022 - 35
Avionics News December 2022 - 36
Avionics News December 2022 - 37
Avionics News December 2022 - 38
Avionics News December 2022 - 39
Avionics News December 2022 - 40
Avionics News December 2022 - 41
Avionics News December 2022 - 42
Avionics News December 2022 - 43
Avionics News December 2022 - 44
Avionics News December 2022 - 45
Avionics News December 2022 - 46
Avionics News December 2022 - 47
Avionics News December 2022 - 48
Avionics News December 2022 - 49
Avionics News December 2022 - 50
Avionics News December 2022 - 51
Avionics News December 2022 - 52
Avionics News December 2022 - 53
Avionics News December 2022 - 54
Avionics News December 2022 - 55
Avionics News December 2022 - 56
Avionics News December 2022 - 57
Avionics News December 2022 - 58
Avionics News December 2022 - 59
Avionics News December 2022 - 60
Avionics News December 2022 - 61
Avionics News December 2022 - 62
Avionics News December 2022 - 63
Avionics News December 2022 - 64
Avionics News December 2022 - Cover3
Avionics News December 2022 - Cover4
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