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Thread: sibedand frequencies

  1. #1 sibedand frequencies 
    aj
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    I'm just learining the basics of amplitude modulation and i had a question about sideband frequecies.... Why is there a need for sideband frequecies? if amplitude modulation is just changing the amplitude of a carrier with respect to the instantaneous amplitude of a modulating signal...why must the frequency of the carrier wave be affected at all? i dont understand the connection at all. why cant i just add or subtract to the voltage level of the carrier....why go for nonlinear mixing?

    and my other question is in the armstrong indirect fm transmitter we use multipliers to increase the frequency by a factor of 72....then why not just use a bunch of multipliers on the original information signal in the transmitter stage to increase the frequecy to radio frequency...why go for modulation at all?


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  3. #2  
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    It not so much a need as it is a side effect of the modulation.

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


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  4. #3  
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    Not so much as a side effect as essential. The sidebands carry the information. The carrier in fact can be removed and one set of sidebands can be removed. This is Single Side Band.. SSB. A very efficient means of communication.

    http://en.wikipedia.org/wiki/Single-sideband_modulation
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    I had posted a reply to this, but it got lost in the transfer to the new server.

    What it basically said was, the only way a signal has just one frequency is if it is a pure sine wave. When you modulate the carrier, then transform that signal from the time domain to the frequency domain using Fourier analysis, you get the additional frequencies.
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  6. #5  
    aj
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    thank you for your replies.

    i understand much better now.

    but what about my question of the multipliers? instead of a complex AM circuit cant i just use frequency multipliers to increase frequency of the original information signal?
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    Quote Originally Posted by aj
    thank you for your replies.

    i understand much better now.

    but what about my question of the multipliers? instead of a complex AM circuit cant i just use frequency multipliers to increase frequency of the original information signal?
    I'm not sure how that would work for a real time broadcast. If a sound track is 1 minute long, then you compress the frequency to double that, it would run in half the time, wouldn't it?
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  8. #7  
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    If you took audio, 30 - 15000 Hz say, and multiplied it 72 times you would get a very wide signal from 2100 Hz to 1.08 MHz. It would be impossible to do because a multiplier would produce many x N multiples falling within that band. e.g 60 x 36 is also = 2100. It would be distorted mush and carry no useful info.

    Starting with a low frequency crystal oscillator which can be deviated a small amount is one way of producing an FM signal around 100 MHz. The deviation is multiplied by the multiplication factor of the transmitter...72 or whatever.

    Modern transmitters use free running but stabilised oscillators at 10.7 MHz and mix that up to the final frequency.. Like a superhet receiver in reverse.
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