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Eye diagram


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Eye diagram

  1. 1.  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
  2. 2.  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
  3. 3.  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
  4. 4.  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
  5. 5.  10 points in the final EE 541/451 Fall 2006
  6. 6.  Circuit design EE 541/451 Fall 2006
  7. 7.  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