Amplitude Modulation (AM)

AM varies a carrier's amplitude with the message. Learn the AM equation, modulation index, spectrum (carrier + sidebands), bandwidth, power distribution and efficiency, DSB-SC and SSB, envelope and coherent detection, over-modulation, and the superheterodyne receiver.

AM broadcasting began in the 1920s and is still used for medium-wave radio and aviation voice. Its variants (SSB, QAM's amplitude component) remain central in modern communication.

An analogy

AM is like a lighthouse whose brightness pulses with a message: the light (carrier) is always there, and its intensity carries the information.

Spectrum

Multiplying the message onto the carrier creates the carrier at f_c plus upper and lower sidebands at f_c ± f_m. All information is in the sidebands; bandwidth is twice the highest message frequency.

Power and efficiency

Even at μ = 1 only 33 % of power is in the sidebands — conventional AM wastes power on the carrier. μ > 1 (over-modulation) distorts the envelope.

Variants

DSB-SC suppresses the carrier (needs coherent detection). SSB transmits one sideband — half bandwidth, efficient (amateur and marine radio). VSB is used in analog TV.

Receivers

Envelope detectors (diode + RC) demodulate AM cheaply. Superheterodyne receivers mix the RF signal to a fixed intermediate frequency (455 kHz for AM) for easy filtering and gain; image frequency rejection is a key design issue.

Formulas

• AM signal: s(t) = A_c [1 + μ cos(2πf_m t)] cos(2πf_c t)
• Modulation index: μ = (V_max − V_min)/(V_max + V_min)
• Bandwidth: B = 2 f_m
• Total power: P_t = P_c (1 + μ²/2)
• Efficiency: η = μ² / (2 + μ²)

Worked example: Power and efficiency

Given: P_c = 100 W, μ = 0.8

Find: P_t and η.

• P_t = 100 × (1 + 0.32) = 132 W
• η = 0.64/2.64 = 24 %

Answer: 132 W total; only 32 W carries information

Worked example: Index from a scope

Given: V_max = 1.6 V, V_min = 0.4 V

Find: μ.

• μ = 1.2/2.0 = 0.6

Answer: μ = 0.6

Common mistakes

• Wrong: μ > 1. — Right: Over-modulation distorts and splatters spectrum.
• Wrong: Thinking the carrier carries information. — Right: Only sidebands carry the message.

Applications

• MW/SW broadcasting: Long-range voice radio.
• Aviation: VHF AM air-traffic voice.
• SSB radio: Amateur, marine HF.

Key takeaways

• s(t) = A_c[1 + μm(t)]cos ω_c t.
• B = 2f_m.
• P_t = P_c(1 + μ²/2); η ≤ 33 %.
• DSB-SC, SSB improve efficiency.

Going further

• Image rejection: Choosing IF and front-end filters.
• Coherent detection: Carrier recovery with PLLs.
• Quadrature AM: Foundation of QAM.