Modulation
Modulation is the process of imposing the information to be carried onto the amplitude, frequency or phase property of a fixed-frequency carrier wave. Which modulation is chosen on a control link directly determines the range obtained at the same output power and the resistance to noise; the difference between ASK and FSK is a choice worth tens of dB in the field.
A carrier on its own carries no information; it is a sine wave of constant amplitude and constant frequency. To add information, one of three quantities is varied:
- Amplitude: ASK and its simplest form, OOK. The circuit is cheap, but amplitude is also the quantity noise affects directly.
- Frequency: FSK and its derivative GFSK. Because it is unaffected by amplitude fluctuations, it is preferred in industrial environments.
- Phase: the PSK families. They give a higher data rate in the same bandwidth, at the cost of increased receiver complexity.
In a remote control application the payload carried is a few bytes; button state, address and checksum. So the choice is made not on data rate but on decodability at low levels. Receiver sensitivity on sub-GHz FSK receivers is typically reported between -125 and -130 dBm; OOK receivers in the same band need a markedly higher level. In the field you feel this difference while an inverter or a welding machine is running: when the noise floor rises, amplitude-based links drop first.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety