#### Question 5.10

## Carrier recovery for coherent PSK (cont.)

For practical filtered PSK, the squared signal contains an envelope modulation component which manifests itself as additional symmetrical frequency components around the twice carrier term. Fortunately a PLL-based filter is insensitive to envelope modulation and this will not significantly affect the performance of the carrier recovery circuit.

The filtering that is applied to the twice carrier term to reduce the effects of channel noise is, however, very important in reducing the amount of phase jitter on the recovered carrier reference. There are some applications of digital communications, such as cellular radio, where it is not possible to employ a very narrow filter in the carrier recovery circuit. This is owing to the frequency uncertainty in the location of the twice carrier term caused by local oscillator error and, more directly, variable amounts of Doppler shift resulting from the user's motion. In these cases, the carrier recovery process can often have significant residual 'phase jitter' which degrades the performance for the expected ideal coherent PSK detector. |