Communication Systems: Amplitude modulation — Carrier, Sidebands and Modulation Index
How AM puts a message on a carrier: the modulation index, sidebands and bandwidth, and why so much of its power carries no information.
Try 3 questions
1. In amplitude modulation, which property of the carrier stays constant while the message changes?
- Its peak amplitude
- The total transmitted power
- Its frequency
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Answer: Its frequency
AM varies the carrier's amplitude in step with the message; the carrier frequency stays fixed, which is what a receiver tunes to.
2. Why is audio modulated onto a high-frequency carrier instead of being radiated directly?
- Audio-frequency antennas would be impractically huge
- Sound waves can't travel through air
- High frequencies make voices louder
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Answer: Audio-frequency antennas would be impractically huge
Efficient antennas are a sizable fraction of a wavelength, and a 1 kHz wave is 300 km long; a 1 MHz carrier needs an antenna of tens of meters instead.
3. Besides antenna size, why do radio stations use different carrier frequencies?
- So voices sound clearer at night
- So many stations can share the air
- So signals don't fade over distance
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Answer: So many stations can share the air
Placing each station's signal around its own carrier is frequency-division multiplexing: a receiver picks one by tuning to its frequency.
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