## Carrier recovery for ASK

Clearly it is beneficial to use coherent detection of ASK if possible, but first the problem of how to derive a frequency and phase-coherent carrier reference in the receiver must be addressed. Obtaining sufficiently stable free-running oscillators in both TX and RX units is not possible, and some means of recovering the carrier frequency and phase from the incoming data signal is needed.

One method that we have already encountered is to send a reference signal alongside the data signal from which the source carrier frequency and phase can be measured. Here we shall consider an alternative technique that recovers the carrier from the modulated data signal itself. By locking an oscillator to the phase of the incoming carrier when a carrier-on symbol is sent, and holding this oscillator phase when the carrier is off, it is possible to produce the required coherent reference. A technique that is well suited to this task is the phase-locked loop (PLL). Unfortunately the carrier reference is not perfect, as the carrier-on symbols are corrupted by incoming noise. However, by averaging the carrier over several on-symbols (equivalent to a narrow bandwidth in the PLL system), a cleaner reference can be obtained. |