Basics 06 Mod

  
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The very first form of radio communication was radio telegraphy, as opposed to wired telegraphy. The telegraph key turns the transmitter on when it is closed and off when open. The operator taps out the message in a long and short, dashes and dots, code and the person on the other end who knows the code reads the dots and dashes and translates the
Basics 06 Mod - schematic

m into letters and then to words. I don`t know what year radios began to talk but the method was amplitude modulation (AM). Those early transmitters probably produced as much frequency modulation (FM) as they did AM but FM hadn`t been invented yet and there was no known way to detect it. So we will begin with AM. An unmodulated radio transmitter puts out a steady single frequency of constant amplitude. When it is amplitude modulated the voltage of the transmitter output is varied according to the modulating voltage. The unmodulated transmitter output is called the carrier. As we will see when we study Single Sideband (SSB) this is a total misnomer. But never mind that, it`s universally called the carrier and that`s what we will call it. It was named that because the early radio engineers thought the steady RF signal carried the audio through the ether. Suppose we want to transmit a steady sine wave. That`s not very interesting to anyone who might be listening but who cares. We`re going to transmit it anyhow. The top wave in Figure 6. 1 shows an unmodulated carrier wave at the top, an audio sine wave second, and at the bottom, the carrier modulated by the audio wave. This looks pretty straight forward. The audio wave is added to 1 to make it all above the axis and then it multiplies the carrier wave. If I didn`t need to go on to explain single sideband I could just leave it at that. But I am going on to SSB so we need to dig...



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