 Ideally, to ensure this symmetry property, it is necessary to have a separate equaliser for the timing path. In practice, the equaliser of Figure 28.13 will often be optimised for the decision path and Short term variations from the optimum timing of a digital decision provide little more than noise bandlimiting for the timing path. making process are known as jitter. It can be regarded as a phase Nevertheless, a Hermitian symmetry component will be present at modulation of the extracted clock relative to the original system point A even if it is diluted by the characteristics of the transmitter clock and as such is sometimes referred to as phase jitter. The effect and channel. of jitter on the decision making process within a regenerator is The non-linear network then produces, at point B, a discrete illustrated in Figure 28.14(a), where it can be seen that clock edge spectral line at the clock frequency f together with a continuous jitter can cause decisions to be made at sub-optimum times, thereby 28/14 Digital transmission Data eye Eye jitter Clock edge jitter Clock edge jitter Figure 28.14 Timing jitter and the data eye: (a) clock edge jitter; (b) effect of jitter accumulation increasing the probability of error. This jitter is caused by a combi- a single interfering component is shown by way of example in nation of: Figure 28.15. Figure 28.15(a) shows the wanted and unwanted spectral components as they might appear at the output of a clock 1. Incoming noise affecting the extraction process. selecting filter. The equivalent phasor representation is given in 2. Data dependent timing effects arising from imperfections and Figure 28.15(b) where it is seen that the resultant has both amplitude limitations in the clock extraction process. and phase modulation. This is again apparent in the corresponding waveform of Figure 28.15(c). The amplitude variations can be Both of these effects arise from the unwanted continuous spectral effectively removed by a zero-crossing detector provided that the components getting 