Radio wave modulation – AM and FM

What it shows

Audio signals have frequencies of only hundreds to thousands of hertz, far too low to be radiated efficiently by a practical antenna, so radio transmits them on a high-frequency carrier wave. In amplitude modulation the carrier's amplitude follows the audio signal, so the envelope of the wave has the shape of the signal; the modulation depth m sets how strongly the amplitude varies. In frequency modulation the amplitude stays constant while the instantaneous frequency rises and falls with the signal. Because noise mainly disturbs amplitude, an FM receiver that reads only frequency recovers a cleaner signal.

How to use

Start in AM and vary Depth m from small values up to 1 while watching the dashed envelope of the output wave. Change the Signal frequency and the Carrier ratio to see how many carrier cycles fit inside one signal cycle. Then switch Type to FM and note how the cycles bunch together and spread out. Finally tick Amplitude noise and compare the signal recovered by the receiver for AM and for FM.

Parameters you can change

• Modulation type Amplitude modulation (AM), Frequency modulation (FM)
• Audio-frequency signal 200–2000 Hz
• Carrier-to-signal frequency ratio 4–20 times
• Modulation depth m 0.1–1
• Add amplitude noise in transmission

Questions to explore

1. In an AM wave, which feature of the wave carries the sound information, and which feature carries it in an FM wave?
2. What happens to the shape of the AM wave as the modulation depth m approaches 1?
3. Why does amplitude noise distort the recovered AM signal but hardly affect the FM signal?