Tolerable Q-Factor Variations in PCM Lines

This paper makes a theoretical assessment of the performance impairments that may be caused on PCM lines by the use of different timing extraction bandwidths in tandem. Two basic situations are examined: 1) random transmitted pattern, and 2) transition between two repetitive patterns. In both cases, the analysis of jitter propagation and acceptance indicates that there is a limited degree of bandwidth inconsistency that may be tolerated. The amount of tolerable inconsistency is derived in both cases, according to different criteria that are established for convenience in each case. The effect of systematic line jitter on the correlation between a repeater performance in the field and in bench tests is also discussed. The study is based on a well-known model of jitter propagation introduced by Byrne et al. [1]. The general conclusion of the paper is that -factor variations in a PCM line should be kept lower than a factor of two in order to avoid errors during pattern transitions and lower than 1.5 in order to avoid noise immunity loss.