This simulator shows the core idea of radio: a high-frequency carrier that carries information because something about it is made to follow an audio signal. Switch between AM, where the amplitude envelope follows the audio, and FM, where the frequency follows it, then change depth, index and frequencies to see how each waveform and its bandwidth respond.
• An AM view with the modulated carrier in cyan and its dashed amber envelope, and an FM view with the modulated carrier and a dashed purple audio reference. • AM controls: modulation depth, carrier frequency and audio frequency. FM controls: modulation index β and audio frequency. • An information card showing depth and occupied bandwidth for AM, or modulation index and Carson bandwidth for FM. • Notes, formulas and a worked example below the controls.
In AM the carrier frequency is fixed and its amplitude is multiplied by 1 plus the depth times the audio signal. At 0 percent depth you see a plain carrier; at 100 percent the envelope just touches zero. Above 100 percent the envelope would cross zero and distort, which the simulator flags as overmodulation (the slider stops at 100 percent). AM occupies roughly twice the audio frequency in bandwidth because of its upper and lower sidebands.
In FM the amplitude stays constant and the instantaneous frequency moves with the audio, so the waves bunch up and spread out. The modulation index β is the peak frequency deviation divided by the audio frequency. Carson's rule estimates the bandwidth as 2(β + 1) times the audio frequency, so a larger β takes more spectrum but resists amplitude noise. The waveforms here are conceptual illustrations, not spectrum-accurate signals.
AM varies the carrier's amplitude in step with the audio while the frequency stays fixed. FM varies the carrier's frequency in step with the audio while the amplitude stays constant. FM needs more bandwidth but is less affected by amplitude noise.
Modulation depth m is how far the AM envelope swings relative to the unmodulated carrier amplitude. At 100 percent the envelope reaches zero at its minimum; beyond that the signal is overmodulated and distorts.
Carson's rule estimates the bandwidth containing nearly all of an FM signal's power as 2(β + 1) times the highest audio frequency. With β = 3 and an audio frequency of 1, the bandwidth is about 8 times that frequency.
No. Carrier and audio frequencies are shown as multiples of a reference frequency so the shapes are easy to see. The lab illustrates the concepts of AM and FM rather than simulating a real transmitter or receiver.