upon the detail of the clock extraction process and in particular on ring the phasor diagram of Figure 28.15(b). If the unwanted compo- the parameters of the clock selecting filter. For a simple tuned circuit nents always had Hermitian symmetry centred on f each pair would the jitter power transfer characteristic (Byrne et al., 1963) will produce a vector of equal amplitude but opposite phase with respect depend upon the Q of the filter as shown in Figure 28.16. Thus, low to the unwanted clock. These would rotate about the f vector in opposite directions and SO would produce a resultant which had amplitude modulation but no phase modulation, that is, no jitter. It has already been noted that any data sequence (in impulse form) will have Hermitian symmetry properties which, after squaring, will produce the requisite Hermitian symmetry about f It thus remains to provide equalisation to compensate for finite width and channel characteristics to ensure that the spectral symmetry condition is preserved at the output of the clock selecting filter. Unfortunately this requirement is not consistent with the skew symmetry spectral requirements for a good received eye and SO must be provided by separate spectral shaping within the clock path. Although this dem- onstrates a theoretical requirement, in practice separate clock path equalisation is not always found to be necessary. Acceptably low jitter performance can often be obtained by a judicious combination of: careful transmission coding, to ensure an adequate clock com- ponent relative to the continuous spectrum after non-linear process- ing (Jones and Zhu, 1987), received noise limiting, and an Figure 28.16 Jitter transfer characteristic for a tuned appropriate choice of Q for the clock selecting filter. circuit 28.4.5 Jitter accumulation frequency jitter will be transferred through a regenerator with very Jitter arising from the clock extraction and retiming process in a little attenuation whilst higher frequency jitter will be reduced. It regenerator will be transmit