Amplitude and frequency modulation

Modulation means variation of the amplitude or the phase (or both) of an otherwise constant sinusoidal RF carrier wave in order that the signal carry information: digital data or analog waveforms such as audio or video. In this chapter we look at pure amplitude modulation (AM) and pure frequency modulation (FM). Historically, these were the first methods to be used for communications and broadcasting. While still used extensively, they are giving way to modulation schemes, mostly digital, some of which amount to simultaneous AM and FM. The simplest form of AM is on/off keying . This binary digital AM (full on is a data “1” and full off is a data “0”) can be produced with a simple switch, originally a telegraph key in series with the power source or the antenna. The first voice transmissions used a carbon microphone as a variable resistor in series with the antenna to provide a continuous range of amplitudes. With AM, the frequency of the carrier wave is constant, so the zero crossings of the RF signal are equally spaced, just as they are for an unmodulated carrier. The simplest FM uses just two frequencies; the carrier has frequency f 0 for data “zero” and f 0 + Δ f for data “one.” FM is usually generated by a VCO (voltage-controlled oscillator). For binary FSK (frequency shift keying), the control voltage has only two values: one produces f 0 and the other produces f 0 + Δ f .