Modulation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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While the studio microphone is not turned on, a current of strictly constant frequency and amplitude flows in the antenna of the broadcasting station (see the left parts of the graphs in Fig. 39). The antenna emits radio waves of constant length and power. But then the microphone was turned on in the studio, and people who were tens, hundreds and thousands of kilometers from the radio station heard the familiar voice of the announcer.
What is happening in the transmitter at this time? Sound frequency oscillations created by a microphone and amplified by a studio amplifier enter the so-called transmitter modulator and there, acting on a current with a high frequency, change the amplitude of its oscillations. This also changes the electromagnetic energy emitted by the transmitter antenna: the greater the amplitude of the current. frequency sounds, the greater the change in the amplitude of the high-frequency current and the power of electromagnetic waves radiated by the antenna (see the right parts of the graphs in Fig. 1). The higher the frequency of the audio frequency current coming from the radio studio, the more frequently the amplitudes of the current in the antenna change.
So the sound, converted by the microphone into electrical vibrations of sound frequency, receives a "ticket" to the air.
Fig.1. When sound acts on a microphone, the high-frequency current in the transmitter antenna changes in amplitude
The process of changing the amplitudes of high-frequency oscillations under the action of an audio frequency current is called amplitude modulation (AM). The high-frequency currents in the antenna that change in amplitude and the radio waves emitted by it are called modulated oscillations.
In addition to amplitude modulation, there is also the so-called frequency modulation (FM). With this type of modulation, the frequency changes, and the amplitude of high-frequency oscillations in the radio station antenna remains unchanged. Frequency modulation Used, for example, to transmit sound in television, broadcasting on VHF. In broadcasting on long, medium and short waves, only amplitude modulation is used.
Publication: N. Bolshakov, rf.atnn.ru
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