AM
Understanding AM
AM is the simplest modulation scheme conceptually: the instantaneous amplitude of the carrier tracks the message signal. An AM signal contains the carrier, upper sideband, and lower sideband. The message is duplicated in both sidebands, making the total bandwidth twice the message bandwidth.
AM Variants
- DSB-FC (Double Sideband Full Carrier): Standard AM broadcast. Both sidebands plus carrier. Simple envelope detection.
- DSB-SC (Suppressed Carrier): Carrier removed. More power-efficient. Requires coherent detection.
- SSB (Single Sideband): One sideband removed. Most spectrum and power efficient. Used for HF communications.
- VSB (Vestigial Sideband): Partially filtered sideband. Used for analog TV.
s(t) = [1 + m(t)] x cos(2 pi fc t)
Modulation index: m = A_message / A_carrier
100% modulation: m = 1
AM bandwidth: BW = 2 x f_message_max
AM broadcast: 2 x 5 kHz = 10 kHz channel
Frequently Asked Questions
What is AM?
AM (Amplitude Modulation) varies the carrier amplitude proportional to the message signal. It is used for broadcast radio (530-1700 kHz), aviation voice, and some military communications. AM is simple but bandwidth and power inefficient.
Why is FM preferred over AM for broadcast?
FM provides better noise immunity because information is encoded in frequency variations, which are less affected by amplitude noise. AM is susceptible to noise that changes the signal amplitude. FM also provides better audio quality and stereo capability.
What is SSB?
SSB (Single Sideband) transmits only one sideband of an AM signal, eliminating the carrier and redundant sideband. This halves the bandwidth and increases power efficiency by 4x compared to conventional AM. SSB is standard for HF communications.