Radio covers a variety of technologies by which information is imposed on electromagnetic radiation traveling without wires, through air or space. Modulation describes the set of processes of imposing the information on the unmodified carrier wave.
The frequency of the carrier wave is usually associated with a frequency band. The information may be voice, data, imagery or instrumentation signals such as radar
Radio frequency aspects
Until the recent advent of software-defined radio, all transmission, and most receiving, requires the use of an oscillator, or an electronic signal that produces a specific frequency or frequencies.
Modulation falls into two categories: those appropriate for carrying analog signals and those appropriate for digital signals. In understanding radio systems, you will have to understand how the information signal modulates the carrier wave, but also how the modulated signal is demodulated to extract the information that is being carried.
The radio signals typically received by household consumer radios are modulated in one of two ways: amplitude modulation (AM) and frequency modulation (FM).