Amplitude Modulation (AM)
1. AM Transmitter Concept
Audio
High Frequency
RF Output
2. Experiment Controls
3. Message Signal
4. Carrier Signal
5. AM Modulated Signal
Practical Observation
- Low message amplitude: small variation in the carrier amplitude.
- Higher message amplitude: larger variation in the carrier amplitude.
- 100% modulation: message amplitude equals carrier amplitude.
- Over-modulation: message amplitude becomes greater than carrier amplitude, producing envelope distortion.
s(t) = Ac [1 + m cos(2πfmt)] cos(2πfct)
where Ac is carrier amplitude, m is modulation index, fm is message frequency, and fc is carrier frequency.
Amplitude Modulation (AM) is a technique used in communication systems to transmit information, such as voice or music, by varying the amplitude of a high-frequency carrier wave according to the information or message signal. The frequency of the carrier remains essentially constant. In a typical AM transmitter, the audio/message signal and the carrier signal are combined in a modulator to produce the AM signal, which can then be transmitted through an antenna.
The amount by which the carrier amplitude changes depends on the amplitude of the message signal. If the message amplitude is increased, the variation in the AM waveform's envelope becomes larger; if it is decreased, the envelope variation becomes smaller. When the message amplitude reaches the carrier amplitude, the system has 100% modulation. If the message amplitude becomes greater than the carrier amplitude, over-modulation occurs and the received signal can become distorted.