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Carrier wave

  1. Feb 1, 2004 #1
    CAn someone explain how sound waves is modulated into Am and FM carrier waves? I only have a poor illustrarion i dont understand.
     
  2. jcsd
  3. Feb 1, 2004 #2

    drag

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    Science Advisor

    Greetings !

    Well, in an FM transmission, after the sound wave
    frequency is the transmitted (via a microphone) into
    electromagnetic oscillations the frequency of the sound
    waves can be "added" or "substracted" in a set proportion
    from the set FM transmission frequency. The amplitude of the
    FM waves is determined by the amplitude of the sound waves.

    In an AM transmission the reverse happens. The amplitude of
    the AM waves is determined by the frequency of the sound waves
    and the frequency of the AM waves is determined by the amplitude
    of the sound waves - with some proportionate addition/substraction
    from the set AM transmiossion frequency.

    In both cases it's also possible to make a digital date
    transmission. In the FM case you would need 1 or two
    frequencies and for an AM transmission you'll require
    1 or 2 amplitudes (1 because the 1 could just be represented
    by a stop in the transmission/reception).

    Live long and prosper.
     
  4. Feb 1, 2004 #3
    Hello, I have two descriptions:

    1. As quoted from one of my old military scrap books:

    In AM transmitters, the instantaneous amplitude of the rf output signal is varied in proportion to the modulating signal. The modulating signal may consist of many frequencies of various amplitudes and phases, such as the signals making up your own speech pattern. The microphone converts the audio frequency input (a person’s voice) into corresponding electrical energy. The driver amplifies the audio, and the modulator further amplifies the audio signal to the amplitude necessary to fully modulate the carrier. The output of the modulator is applied to the power amplifier. The power amplifier combines the rf carrier to the modulating signal in the power amplifier to produce the amplitude-modulated (AM) signal output for transmission. In the absence of a modulating signal, a continuous rf carrier is radiated by the antenna.

    In frequency modulation (FR) the modulating signal combines the carrier to cause the frequency of the resultant wave to vary with the instantaneous amplitude of the modulating signal. The modulating signal applied to a variable capacitor (varicap) causes the reactance to vary. The varicap is connected across the tank circuit of the oscillator. With no modulation, the oscillator generates a steady center frequency. With modulation applied, the varicap causes the frequency of the oscillator to vary around the center frequency in accordance with the modulating signal. The oscillator output is then fed to a frequency multiplier to increase the frequency and then to a power amplifier to increase the amplitude to the desired level for transmission.

    2. There is usually more to it than this, but to keep it basic, basically what the above states is:

    AM radio waves are oscillated energy waves who’s amplitude (current and/or voltage levels) are increased and decreased, even though its frequency stays the same for a given AM radio channel. For AM radio’s, when someone speaks into the microphone, the microphone converts the sound vibrations into electricity, which is then amplified in power levels so that it can effectively influence the power levels of the wave form produced by the frequency generator that generates the carrier wave (the oscillator tank circuit) when it is applied to the carrier wave (usually in the form of biasing) on the oscillator tanks circuits output to produce a varying voltage and/or current levels of the carrier wave, and then the whole is amplified again and then sent to a transmitter.

    FM radio waves are oscillated energy waves who’s frequency (oscillation pulse rate in Hertz) are increased and decreased, even though its amplitude (current and/or voltage levels) stays pretty much the same for a given FM radio channel. For FM radio’s, when someone speaks into the microphone, the microphone converts the sound vibrations into electricity, which is then amplified in power levels, which is then introduced to the variable capacitor of the oscillator tank circuit (the frequency generator that generates the carrier wave), which causes the tank circuit to produce a varying frequency directly proportional to the microphones signal influence, which is then sent to an amplifier and then to a transmitter.

    Hope this helps.
     
    Last edited: Feb 6, 2004
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