AM and FM Modulation

The carrier oscillates rapidly; the information signal changes more slowly. In amplitude modulation, carrier amplitude follows the information while its underlying frequency stays approximately fixed. The visible envelope therefore traces the information waveform.

In frequency modulation, carrier amplitude stays approximately constant while instantaneous frequency varies with the information. Close-packed oscillations indicate higher instantaneous frequency and wider spacing lower frequency. The rate at which that spacing pattern repeats gives the information frequency, not the maximum instantaneous carrier frequency.

For simple AM with a highest information frequency fM, sidebands extend above and below the carrier, so the required bandwidth is approximately 2fM. A carrier frequency much larger than fM does not mean the sidebands disappear.

For the approximate FM bandwidth expression use peak frequency deviation Δf and the highest modulating frequency. A larger deviation can improve resistance to amplitude noise but usually occupies more spectrum. Detailed modulation circuits and a full mathematical derivation are not required.

Random amplitude variations directly corrupt AM information. An FM receiver can limit amplitude variations because the useful information is encoded in frequency, although frequency-related noise and weak signals still degrade it. Neither technique is noise-proof.

A channel must pass the wanted sidebands without excessive distortion. Fibre and high-frequency links can offer large available bandwidths, while a crowded radio band forces compromises between channel spacing and message fidelity. Keep carrier frequency, information frequency and occupied bandwidth as three separate quantities.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.