Loading…
Amplitude modulation (AM)
Varying carrier strength: sidebands, bandwidth and efficiency.
Varying carrier strength: sidebands, bandwidth and efficiency.
In amplitude modulation (AM) the carrier's strength follows the audio: louder sound, taller carrier. It is the oldest voice mode and the simplest to receive, which is why it survives in broadcasting and on the aircraft band.
AM multiplies the carrier by (1 + m × audio), where the modulation depth m runs from 0 (no audio) to 1 (100%). Multiplying two sine waves produces their sum and difference frequencies. So one audio tone at fm makes two new signals, at fc + fm and fc − fm. The result looks like a carrier whose height wobbles, but it is really three steady sine waves added together.
Real speech is a spread of tones, so each sideband becomes a band: an upper sideband (USB) and a mirror-image lower sideband (LSB). Both carry the same information.
Each sideband has amplitude m/2 of the carrier, so power m²/4 of the carrier power:
P = Pc × (1 + m²/2), where Pc is the unmodulated carrier powerAt m = 1 that is 1.5 times the carrier power, and only 1/3 of it is in the sidebands. The carrier, two thirds of the power, carries no information at all. That inefficiency is why SSB replaced AM for HF.
Push m above 100% and the carrier is cut off in the troughs of the audio. The envelope no longer matches the audio, so the signal is distorted and new sidebands appear far from the carrier: splatter, which interferes with neighbouring stations.