e time and decision threshold voltage. Figure 28.7(c) gives a diagrammatic view of Degradations in digital systems fall into two categories: these points. The diagram serves to illustrate how the worst case signal margin can be very much less than the ideal design value. 1. Deterministic degradations such as errors in equalising, offsets Noting the steepness of the error probability curve (Figure 28.4), it in decision timing, gain errors and possibly transmission can be seen how worst case conditions can dominate performance. echoes. It follows that reliable estimates for the worst case signal margin are 2. Stochastic degradations such as noise, interference, crosstalk crucial if the performance of a system is to be predicted accurately. and timing jitter. Some degradations, it could be argued, should appear in both categories. Transmission error performance The deterministic degradations are conveniently assessed by means of an eye diagram. This is obtained, on an oscilloscope for 28.2.1 The need for performance measures example, by writing all possible received sequences on top of each other whilst triggering the oscilloscope timebase from the data clock as illustrated in Figure 28.7(a) and (b). Transmission errors are usually the most significant impairment to The eye diagram provides a great deal of information about the be found in a digital communication system. They are also difficult performance of a digital system. In the absence of noise, the width to predict and therefore difficult to quantify. Consequently, the of the eye opening gives the time interval over which the received study of their occurrence and the means of specifying their charac- signal can be sampled without error from i.s.i. The sensitivity of the teristics has been and continues to be the subject of considerable Frequency domain Time domain Figure 28.6 Waveform shaping for zero intersymbol interference (Nyquist shown in dashed line and full raised cosine in solid line) 28/6 Digital transmission Data clock Best sample time Slope determ