Why modulation is needed
A low-frequency message signal is difficult to radiate efficiently with a practical antenna. Modulation places the information on a higher-frequency carrier. It also allows different stations to use different channels.
Analogue communication
An analogue signal varies continuously. Speech from a microphone is an example. AM and FM are common analogue modulation methods.
Digital communication
A digital system represents information with discrete symbols or bits. Digital radio can use modulation methods such as FSK, PSK and QAM. Error detection and correction can improve reliability.
Amplitude modulation
In AM, the carrier amplitude changes with the message. Conventional AM produces an upper and lower sideband and a carrier. For ordinary double-sideband AM, bandwidth is twice the highest modulating frequency.
AM bandwidth = 2 × highest modulating frequency
Frequency modulation
In FM, the instantaneous carrier frequency changes with the message. The carrier amplitude is ideally constant. FM is less affected by many forms of amplitude noise. Its occupied bandwidth depends on deviation and modulating frequency.
Phase modulation
In PM, the phase of the carrier changes with the message. FM and PM are both angle modulation methods. Their exact relationship depends on whether the message or its integral is applied.
Pulse modulation
Pulse modulation carries information by changing a pulse property. Examples include pulse amplitude modulation (PAM), pulse width modulation (PWM) and pulse position modulation (PPM). Pulse code modulation (PCM) samples and quantises a signal, then represents it as digital code.
Single sideband
SSB sends one sideband and usually suppresses the carrier. USB and LSB are the two forms. SSB saves bandwidth and transmitter power compared with conventional AM. It is widely used for HF voice communication.
Continuous wave and Morse
CW is commonly used for Morse communication. A BFO or product detector makes the received carrier audible as a tone. The transmitter keys the carrier on and off to form Morse characters.
Demodulation
Demodulation recovers the original information from a modulated carrier. An envelope detector can demodulate ordinary AM. An FM discriminator or phase-locked loop can recover FM audio. SSB needs a product detector or equivalent process.