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AM and FM

  1. Feb 27, 2007 #1
    hey everyone

    I really dont know whether this is the right stream for posting these kind of questions, but anyways here it goes..

    Can anyone please explain to me the amplitude modulation and frequency modulation, or at least any website that xplains it, bcoz i am totally confused with it... my physics teacher himself doesnt understand it and is givin me a really hard time!! :surprised

    thanx anywayz
     
  2. jcsd
  3. Feb 27, 2007 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. Here's a resource to get you started.

    http://en.wikipedia.org/wiki/Modulation

    In both AM and FM, you start with some carrier signal, typically a sine wave at some frequency with some amplitude.

    For AM, you modulate the amplitude of the carrier with the information that you want to send. So the resulting signal looks like a sine wave, with an envelope of varying amplitude.

    For FM, you modulate the carrier frequency with the information that you want to send. So you might start with a 1MH carrier sinusoid signal, and vary the frequency by +/- 1kHz to send digital information, for example.
     
  4. Feb 27, 2007 #3
    To bring it down to it's simplest form, both systems use a fixed frequency carrier wave, this is the frequency you tune your Radio into to.

    AM has a fixed carrier and the amplitude varies up and down in direct proportion to the message being sent.
    FM has a fixed carrier and the frequency varies up and down in direct proportion to the message being sent.

    The simplest AM is produced by multiplying the carrier by the message. If you take a fixed carrier of Fc and a fixed message Fm and multiply the two sine waves together, then using the half sine rules, sin(fc) x sin(fm) => {sin(fc+fm)+sin(fc-fm)}/2 [correct my maths please].
    In a real Am modulator you end up with a sin(fc) term and a sin(fm) term, you filter out the message and then you have a double sideband AM signal.

    If you use Excel or the like to plot sin(fc)+sin(fc).sin(fm) you will see that the amplitude goes up and down in line with fm (Make sure fc is at least 10 times larger than fm)

    This gives the clue to demodulating. Using a diode you can delete the negative half of the wave form, leaving a series of positive pulses of varying amplitude. (put your Excel output through the function IF({cell}<0,0,{cell})
    If you low pass filter these (get rid of fast changes in amplitude) using a capacitor and resistor, you get the average of the positive pulses, which is proportional to the message waveform. (Put a moving average Trendline on your Excel Graph)
    The average is offset by a DC level, this is proportional to the strength of the recieved carrier waveform, you use this to control your output amplifier gain (agc - automatic gain control) which keeps the amplitude constant when traveling in a car.

    FM is slightly more complicated to produce. You use a nonlinear diode called a varicap diode to shift the natural frequency of an oscillator tuned to your carrier wave.
    I would need to check up on the finer points of FM before giving more detail.
     
  5. Feb 27, 2007 #4

    Claude Bile

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    Science Advisor

    The purpose of using modulation is to take a signal with a given spectrum and shift the spectrum so it sits at a higher centre frequency. This is practical for 2 reasons, firstly the amount of data we can send per second increases with frequency, and secondly, shifting the spectrum to a higher frequency band is usually more transmission friendly.

    Take an mobile phone conversation for example, with the audio bandwidth typically between 500 Hz to 5 kHz. Transmitting this signal using an EM wave with wavelengths between 500 Hz and 5 kHz would require huge attenae and suffer from poor transmission through atmosphere. Modulating the signal to shift the spectrum into the GHz range not only negates these difficulties, it also allows us to use multiple frequency bands (or channels) to transmit multiple conversations at once.

    In terms of the spectrum though, keep in mind that all AM does is shift the spectrum into a higher frequency band. You can choose (via your receiver design) whether or not to keep one or both sidebands, and keep the carrier.

    FM is fairly similar, except the modulated signal has multiple sidebands (not just 2 in the case of AM). For Narrow-band FM (where the frequency is varied over a smaller range than standard FM), the modulated signal only possesses 2 sidebands, and is thus quite similar to AM.

    Claude.
     
    Last edited: Feb 27, 2007
  6. Feb 28, 2007 #5
    thanx berkeman, panda and claude.. that was a great help.. all 3 of u explained a different way and made my whole image clear... thanx a million..
     
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