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Eye diagram is a means of evaluating the quality of a received “digital waveform” • By quality is meant the ability to correctly recover symbols and timing • The received signal could be examined at the input to a digital receiver or at some stage within the receiver before the decision stage Eye diagrams reveal the impact of ISI and noise Two major issues are 1) sample value variation, and 2) jitter and sensitivity of sampling instant Eye diagram reveals issues of both Eye diagram can also give an estimate of achievable BER Check eye diagrams at the end of class for participation EE 541/451 Fall 2006
The eye diagram is created by taking the time domain signal and overlapping the traces for a certain number of symbols. The open part of the signal represents the time that we can safely sample the signal with fidelity Figure 7.22 EE 541/451 Fall 2006
Eye diagram is a retrace display of data waveform • Data waveform is applied to input channel • Scope is triggered by data clock • Horizontal span is set to cover 2-3 symbol intervals Measurement of eye opening is performed to estimate BER • BER is reduced because of additive interference and noise • Sampling also impacted by jitter EE 541/451 Fall 2006
The vertical eye opening or noise margin is related to the SNR, and thus the BER • A large eye opening corresponds to a low BER The horizontal eye opening relates the jitter and the sensitivity of the sampling instant to jitter • The red brace indicates the range of sample instants with good eye opening • At other sample instants, the eye opening is greatly reduced, as governed by the indicated slope EE 541/451 Fall 2006
The larger , the wider the opening. The larger , the larger bandwidth (1+ )/Tb But smaller will lead to larger errors if not sampled at the best sampling time which occurs at the center of the eye. EE 541/451 Fall 2006