# amplitude modulation -- why is ωc >> ωm necessary?

by karakov
Tags: ωc, ωm, >>, amplitude, amplitude modulation, modulation
 P: 6 i just want to ask why is ωc >> ωm is necessary for a good amplitude modulation
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P: 39,723
 Quote by karakov i just want to ask why is ωc >> ωm is necessary for a good amplitude modulation
Welcome to the PF.

Where have you seen that condition referred to (textbook/web page/lecture notes/etc.)? Is it discussed there?
 P: 6 we admit that condition . but why??
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## amplitude modulation -- why is ωc >> ωm necessary?

 Quote by karakov we admit that condition . but why??
What do you mean? You use that condition in some calculations? Can you show us the calculations?
 P: 6 i have in a lesson about amplitude modulation : technically to have a good modulation .two conditions should be considered : M< 1 and ωc >> ωm (at least ten times )
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P: 39,723
 Quote by karakov i have in a lesson about amplitude modulation : technically to have a good modulation .two conditions should be considered : M< 1 and ωc >> ωm (at least ten times )
Correct. Can you see why M < 1 is necessary to avoid distortion of the modulating waveform that rides on the carrier waveform? There are good pictures that you can find with a Google Images search on Amplitude Modulation.

The requirement that ωc >> ωm is a little more subtle. Think about what the modulated carrier looks like when ωm gets close to ωc. What happens if they are equal?
 P: 6 it is going to lead to distortion of the modulating waveform ?!
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 Quote by karakov it is going to lead to distortion of the modulating waveform ?!
Yes, when you try to demodulate it with a simple rectifier, the recovered signal will be distorted compared to the original modulating signal. With a little Google searching, you may be able to find a graphical waveform calculator to show the distortion as the two frequencies get too close...
 P: 6 I really can't find wavesform calculator .so tell me what happens ?
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P: 39,723
 Quote by karakov I really can't find wavesform calculator .so tell me what happens ?
You didn't look for very long...
 P: 6 ok whatever just tell me what happens ?? please.
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P: 39,723
 Quote by karakov ok whatever just tell me what happens ?? please.
No, this is your schoolwork, and you should be able to figure this out. Especially if it is part of an assignment, we can give you hints, but we are not allowed to do your work for you.