Chapter
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Modulation
Introduction |
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Keyed Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Amplitude Modulation (AM) |
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Single Sideband (SSB) |
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Frequency Modulation (FM) . . . . . . . . . . . . . . . . . . . . . . . . |
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Phase Modulation |
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Pulse Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Emission Designators . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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3 Modulation
Modulation 3
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Modulation 3
Introduction
Modulation is the name given to the process of adding information to a radio wave or the formatting of radio waves for other purposes. Of the main forms of modulation, five have application in aviation:
Keyed Modulation
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation
Pulse Modulation
The modulation of a radio frequency is generally associated with the transmission of audio information, although the transmission of data, including that in satellite navigation systems, and the determination of bearing in VOR, for example, require modulation for other purposes.
Before an audio signal can be added to a radio wave it must be converted to an electrical signal. This will be achieved by the use of a microphone, which is quite simply a device that converts sound waves to an electrical current.
It will be assumed for AM and FM that this conversion has already been accomplished.
Keyed Modulation
The simplest way to put information onto a carrier wave is to quite simply interrupt the wave to give short and long bursts of energy.
Modulation 3
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‘K’
Figure 3.1 Morse ‘K’ in keyed modulation
By arranging the transmissions into short and long periods of carrier wave transmission we can send information using the Morse code. This is known as telegraphy and until the development of other forms of modulation was the only means of passing information. Keyed modulation is still used by some non-directional beacons (NDBs) for identification and will be discussed further in Chapter 7.
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3 Modulation
Amplitude Modulation (AM)
In AM the amplitude of the audio frequency (AF) modifies the amplitude of the radio frequency (RF).
Modulation 3
Figure 3.2 Amplitude modulation
As can be seen from the diagram above, positive amplitude in the AF gives an increase in amplitude in the RF and negative amplitude in the AF gives a decrease in amplitude in the RF.
The process of combining a radio frequency with a current at audio frequencies is known as heterodyning. Looking in more detail at the process; the heterodyning process combines the two frequencies, leaving the RF unchanged but producing new frequencies at the sum and difference of the RF and AF. For example an audio frequency of 3 kHz is used to amplitude modulate a radio frequency of 2182 kHz. The RF remains unchanged but the AF is now split into 2 sidebands extending upwards from 2182.001 kHz to 2185 kHz – the upper sideband (USB) and a lower sideband (LSB) extending downwards from 2181.999 kHz to 2179 kHz. The spread of frequencies is from 2179 kHz to 2185 kHz giving a bandwidth of 6 kHz, i.e. double the audio frequency used.
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2185 kHz |
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Upper Sideband |
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(100 W) |
RF |
2182 kHz |
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2182.001 kHz |
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(100 W) |
2182 kHz |
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(50 W) |
AF |
3 kHz |
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2181.999 kHz |
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Lower Sideband |
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2179 kHz |
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Figure 3.3 AM sideband production |
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