urther processing is Clock extraction 28/13 Received data signal Equaliser Frequency Non-linear network Clock selecting filter Zero crossing detector Extracted clock pulses Figure 28.13 Clock extraction using a non-linearity required before a clock component can be extracted. This can be spectrum arising from the random data pattern. The purpose of the achieved by subjecting the received signal to a non-linearity such as clock selecting filter is to select the wanted clock whilst rejecting as rectification, clipping or squaring or, often in practice, a combina- much as possible of the pattern related spectrum together with any tion of these. The Hermitian symmetry within the received signal, associated noise. The selection filter may be a narrow bandpass when subjected to the non-linearity results in pairs of spectral filter or a phase locked loop with effective Q factors ranging in components, at frequency fc, with equal amplitude and complemen- practice from about 80 to in excess of 200. The choice of Q will tary phase. These components combine in a constructive manner to depend critically upon the stability of the incoming data rate and the produce a non-zero spectral line at frequency fc (Blackman and tuning accuracy of the filter. A high Q will minimise the unwanted Mousavinezhad, 1990). spectral components but will give rise to large phase shifts in the extracted clock if the tuned frequency deviates from the incoming 28.4.3 Clock extraction circuits clock frequency f Finally, the signal at point C is applied to a zero crossing detector A commonly used method of clock extraction for data signals which or hard limiter which removes the amplitude variations and give have little or no spectral energy at the clock frequency is shown in clock pulses for use in the decision making and regenerating cir- cuits. Figure 28.13. The previous section has discussed how a signal spectrum with Hermitian symmetry about fd/2 will, after non-linear processing, produce a spectral component at the clock frequency. 28.4.4 Jitter