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UH ECE 6340 - Analyzing Signals Using the Eye Diagram

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50 High Frequency ElectronicsHigh Frequency DesignEYE DIAGRAM TUTORIALAnalyzing Signals Using the Eye DiagramBy Gary BreedEditorial DirectorThe eye diagram is auseful tool for thequalitative analy-sis of signal used in digi-tal transmission. It pro-vides at-a-glance evalua-tion of system perfor-mance and can offer insight into the nature ofchannel imperfections. Careful analysis of thisvisual display can give the user a first-orderapproximation of signal-to-noise, clock timingjitter and skew.The eye diagram is an oscilloscope displayof a digital signal, repetitively sampled to geta good representation of its behavior. In aradio system, the point of measurement maybe prior to the modulator in a transmitter, orfollowing the demodulator in a receiver,depending on which portion of the systemrequires examination. The eye diagram canalso be used to examine signal integrity in apurely digital system—such as fiber optictransmission, network cables or on a circuitboard. Figure 1 shows a simple eye diagramthat is undistorted, and another that includesnoise and timing errors.Radio CommunicationsThe transmission of digital signals byradio requires modulation of the RF signal bya train of digital pulses. At its simplest, thedata will be a single sequence of logical zerosand ones that are either referenced to zerovolts (RZ, or return-to-zero) or with no voltagereference (NRZ, or non-return-to-zero). NRZdata signals are most common, and are thebasis for the illustrations in this tutorial.The simplest 2-state modulation types areon-off keying (OOK), frequency-shift keying(FSK) and phase-shift keying (FSK). They allrely on a transition between two states to con-vey the binary digital data.In our beginning communications theorycourses, we learned that pulses contain con-siderable amounts of energy at multiples ofthe repetition rate (harmonics). The amount ofenergy in the harmonics, relative to the fun-damental, is related to rise and fall time andpulse duration. Fast rise and fall times(“square” transitions) and narrow pulse dura-tions create the most harmonic energy. This isunlike a purely digital system, where these“clean” transitions are highly desirable.In order to reduce interference, radiotransmission channels are not permitted tohave unlimited bandwidth. Otherwise, theharmonic energy in the data signal would cre-ate corresponding modulation sidebands thatThe eye diagram providesvisual information that canbe useful in the evaluationand troubleshooting of digital transmission systemsFigure 1 · At top is an undistorted eye dia-gram of a band-limited digital signal. Thebottom eye pattern includes amplitude(noise) and phase (timing) errors. The vari-ous transition points can provide insight intothe nature of the impairments.From November 2005 High Frequency ElectronicsCopyright © 2005 Summit Technical Media52 High Frequency ElectronicsHigh Frequency DesignEYE DIAGRAM TUTORIALextend well beyond the intended bandwidth of the allo-cated communications channel.To reduce these unwanted sidebands, the data signalmust be band-limited (filtered) in a manner that reducesthe harmonic energy while maintaining the integrity ofthe transmitted data. The eye diagram can be used to lookat the signal before transmission, to assure that the filteris behaving properly (Figure 2). A more obvious use of theeye diagram is to evaluate the received signal quality. Thediagram in Figure 3 illustrates the type of informationthat can be determined from the eye diagram.Impairments to the signal can occur in many places,from the pre-filtering in the transmitter, through the fre-quency conversion and amplifier chain, propagation path,receiver front-end, IF circuits, and baseband signal pro-cessing. Information from the eye diagram can help great-ly with troubleshooting. Noise problems will most often beexternal to the equipment and timing issues can be iso-lated to the receiver or transmitter with tests on each. Itis also important to record the eye diagram so it is avail-able for comparison if new problems arise in the future.Digital Signal IntegrityThe eye diagram is also a common indicator of perfor-mance in digital transmission systems. Makers of digitalcommunications hardware often include eye diagrams intheir literature to demonstrate the signal integrity per-formance of their products.When evaluating digital signals, the eye diagram willbe nearly square, since filtering is not usually required forthese contained systems. Figure 4 shows the idealizedundistorted eye diagram of a digital signal. As expected,the eye opening is wide and high.Some common impairments of a high-speed digitalsignal are illustrated in Figure 5. At (a) we see the effectsof time errors. These may be due to clock jitter or poorsynchronization of the phase-locking circuitry thatextracts timing data from the received signal in a trans-mission system (wireline or optical). In (b) note that as bitrate increases, the absolute time error represents anincreasing portion of the cycle, reducing the size of the eyeopening, which increases the potential for data errors.Figure 6 shows a digital signal with reflections from apoorly terminated signal line [1]. If these reflected wavesare of significant amplitude, they can reduce the size ofthe eye opening and, therefore, increase the potential forerrors. Additional errors not shown include amplitudedistortion due to losses in the transmission system andproblems such as crosstalk with other signal lines.Crosstalk can be elusive, since it may involve signalsother than the desired one. The non-synchronizedcrosstalk effects may not be clearly visible on the display.Figure 2 · At the top is the eye diagram of a raisedcosine filtered signal (αα= 0.6) as it might be applied toa modulator. The two lower waveforms represent thesquare-wave bitstream before and after filtering (theseare not aligned with the eye diagram display).Best time to sample (decision point)most open part of eye = best signal-to- noiseSignal-to-noise atthe sampling pointAmount of distortion(set by signal-tonoise ratio)Time variationof zero crossingFigure 3 · Basic information contained in the eye dia-gram. The most important are size of the eye opening(signal-to-noise during sampling), plus the magnitudeof the amplitude and timing errors.Figure 4 · Idealized digital signal eye diagram (withfinite rise and fall times).High Frequency DesignEYE DIAGRAM TUTORIALA final note that applies primarily to high-speed digi-tal signal


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