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Originally Posted by cool Dear all expects,
In AM, is it the carrier signal must be with the higher freq? and the lower freq must be the modulation signal?? |
Generally the carrier frequency is high, and the modulation frequency is low.
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I used a 555 timer to generate a square wave pulse to modulate an infrared signal. then, which is the carrier?? and which is the modulation signal??
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The IR is the carrier (at an extremely high frequency), the 555 generates the modulation.
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Also, does this modulating signal contain the upper-side freq and lower-side freq?? and what is the envelope of it???
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On a normal AM transmission, you get upper and lower sidebands. So for a 1,000KHz transmitter AM modulated by a 1KHz tone you would get three outputs - the carrier at 1,000KHz, the upper sideband at 1,001KHz, and the lower sideband at 999KHz. The complete information is contained in either one of the two sidebands, the carrier and other sideband are only wasting power - so SSB (Single Side Band) transmissions only transmit one sideband, putting all the transmitter power into that (which gives the much greater range).
However, your IR beam isn't modulated in the same way, it's simply pulsed on and off - so you will get 'carrier' then 'no carrier', more like morse code than AM. Morse code is commonly known as CW, for Carrier Wave.
10th April 2004, 05:05 PM
amplitude modulation?? 
Linear Mode
