385:
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27:
271:
513:
501:
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208:, a reference clock is supplied along with the signal, either at the symbol rate or at a lower (but synchronized) frequency from which a symbol clock can be reconstructed. Since the actual receiver in the system uses the reference clock to sample the data, using this clock to determine UI boundaries allows the eye pattern to faithfully display the signal as the receiver sees it: only jitter between the signal and the reference clock is displayed.
335:
469:
196:, so assuming a fixed rate will lead to the eye grossly exaggerating the actual jitter present on the signal. (While spread spectrum modulation on a clock is technically jitter in the strict sense, receivers for these systems are designed to track the modulation. The only jitter of interest to a signal integrity engineer is jitter much faster than the modulation rate, which the receiver cannot track effectively.)
240:. Since this is how the actual receiver works, the most accurate way to slice data for the eye pattern is to implement a PLL with the same characteristics in software. Correct PLL configuration allows for the eye to conceal the effects of spread spectrum clocking and other long-term variation in the symbol rate which do not contribute to errors at the receiver, while still displaying higher frequency jitter.
171:
any oscilloscope (even fully analog ones) and can provide decent visualization of noise and overall signal shape, but completely destroys the jitter content of the signal since the instrument's trigger re-synchronizes the plot to each UI. The only jitter visible with this method is that of the oscilloscope itself, as well as extremely high frequency jitter (frequencies with period less than the UI).
356:
721:
453:. The effect of this is an increase in signal rise/fall time. If the data rate is high enough or the channel is lossy enough, the signal may not even reach its full value during a fast 0-1-0 or 1-0-1 transition, and only stabilize after a run of several identical bits. This results in vertical closure of the eye.
183:
of the signal (perhaps by counting the average number of zero crossings in a known window of time) and acquiring many UIs in a single oscilloscope capture. The first zero crossing in the capture is located and declared to be the start of the first UI, and the remainder of the waveform is divided into
170:
A very simple method of slicing is to set the oscilloscope display to be slightly more than one UI wide, trigger on both rising and falling edges in the signal, and enable display persistence so that all measured waveforms "stack" into a single plot. This has the advantage of being possible on almost
508:
In the image below, an additional three inches of cable is added to the end of the same stub. The same "step" is present but is now four times as long, producing reflections at about 1280 ps or 1.6 UI. This produces extreme ISI (since the reflection of each UI arrives during the subsequent UI) which
428:
An emphasized signal will never transition from a weak state to the corresponding strong state, a weak state to another weak state, or remain in the same strong state for more than one UI. A PAM signal also normally has equally spaced levels while emphasized levels are normally closer to the nominal
330:
signal should consist of three clearly distinct levels (nominally -1, 0, +1 from bottom to top). The 0 level should be located at zero volts and the overall shape should be symmetric about the horizontal axis. The +1 and -1 states should have equal amplitude. There should be smooth transitions from
263:
Large amounts of data may be needed to provide an accurate representation of the signal; tens to hundreds of millions of UIs are frequently used for a single eye pattern. In the example below, the eye using twelve thousand UIs only shows the basic shape of the eye, while the eye using eight million
460:
The top and bottom "rails" of the eye show the final voltage the signal reaches after several consecutive bits with the same value. Since the channel has minimal loss at DC, the maximum signal amplitude is largely unaffected. Looking at the rising edge of the signal (a 0-1 pattern) we can see that
496:
In the image below, a roughly one inch (25.4 mm) open circuited stub is present in the line, causing an initial low-impedance effect (reduced amplitude) followed by a positive reflection from the end of the stub with a delay of about 320 ps or 0.4 UIs. This can be clearly seen as a "step" in the
157:
Next, the position of each sample within the UI must be determined. There are several methods for doing this depending on the characteristics of the signal and the capabilities of the oscilloscope and software in use. This step is critically important for accurate visualization of
351:
signal should consist of N clearly distinct levels (depending on the PAM order, for example PAM-4 should have four levels and PAM-3 should have three). The overall shape should be symmetric about the horizontal axis and the spacing of all levels should be uniform.
456:
The image below shows a 1.25 Gbit/s NRZ signal after passing through a lossy channel - an RG-188 coaxial cable approximately 12 feet (3.65 meters) in length. This channel has loss increasing in a fairly linear fashion from 0.1 dB at DC to 9 dB at 6 GHz.
136:
The first step of computing an eye pattern is normally to obtain the waveform being analyzed in a quantized form. This may be done by measuring an actual electrical system with an oscilloscope of sufficient bandwidth, or by creating synthetic data with a
187:
This approach can work adequately for stable signals in which the symbol rate remains exactly the same over time, however inaccuracies in the system mean that some drift is inevitable so it is rarely used in practice. In some protocols, such as
413:
The eye pattern for a signal with emphasis may be mistaken for that of a PAM signal at first glance, however closer inspection reveals some key differences. Most notably, an emphasized signal has a limited set of legal transitions:
476:
As high frequency losses increase the overall shape of the eye gradually degrades into a sinusoid (once higher frequency harmonics of the data has been eliminated, all that remains is the fundamental) and decreases in amplitude.
66:). It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. It is a tool for the evaluation of the combined effects of channel noise, dispersion and
260:, logarithmic scaling, or other mathematical transformations may be applied in order to emphasize different portions of the distribution, and a color gradient is applied to the final eye for display.
725:
149:
may also be applied at this time in order to increase the number of samples per unit interval (UI) and produce a smooth, gap-free plot which is more visually appealing and easier to understand.
465:, but continues to rise slowly over the duration of the UI. At around +300 ps, the signal either begins falling again (a 0-1-0 pattern) or continues rising slowly (an 0-1-1 pattern).
497:
rising edge in which the signal rises to a fraction of the full value, levels off for the round trip delay of the stub, then rises to its full value when the reflection arrives.
103:
performance measurements can be derived by analyzing the display. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too
141:
in order to evaluate the signal integrity of a proposed design. A combination of the two approaches may be used as well: simulating the effects of an arbitrary circuit or
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331:
the 0 state to the +1 and -1 states, however there should be no direct transitions from the -1 to +1 state (which would indicate the signal is PAM-3 rather than MLT-3).
384:
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applied to a signal produces an additional level for each value of the signal which is higher (for pre-emphasis) or lower (for de-emphasis) than the nominal value.
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visible as defects in the edges of the signal. Reflections with a delay greater than one UI often render the eye completely unreadable due to
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92:(UI). In other words, it shows the probability of the signal being at each possible voltage across the duration of the UI. Typically a
34:
modulation scheme. Constant binary 1 and 0 levels are shown, as well as transitions from 0 to 1, 1 to 0, 0 to 1 to 0, and 1 to 0 to 1.
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on a measured signal, perhaps to determine whether a signal will still be intelligible after passing through a long cable.
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55:
860:
348:
770:"Matlab's help file description of how to use the Eye Diagram Functions in the Communications Toolbox"
698:
Christopher M. Miller "High-Speed
Digital Transmitter Characterization Using Eye Diagram Analysis".
252:, with the X axis representing time within the UI and the Y axis representing voltage. This is then
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120:
67:
70:
on the performance of a baseband pulse-transmission system. The technique was first used with the
805:
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111:, this can be observed from the eye diagram. An open eye pattern corresponds to minimal signal
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signal should consist of two clearly distinct levels with smooth transitions between them.
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8:
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Strong state to opposite strong state (second transition of a 1-0-1 or 0-1-0 bit pattern)
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is applied to the PDF in order to make small brightness differences easier to visualize.
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Weak state to opposite strong state (second transition of a 1-1-0 or 0-0-1 bit pattern)
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A simple way to have the eye pattern display jitter in the signal is to estimate the
93:
846:
Understanding Data Eye
Diagram Methodology for Analyzing High Speed Digital Signals
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679:
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Each form of baseband modulation produces an eye pattern with a unique appearance.
20:
706:
493:, however those with a shorter delay can be easily seen in the shape of the eye.
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Stubs, impedance mismatches, and other defects in a transmission line can cause
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Loss of printed circuit board traces and cables increases with frequency due to
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by dividing the value in each histogram bin by the value in the largest bin.
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31:
26:
788:"HP E4543A Q Factor and Eye Contours Application Software Operating Manual"
257:
71:
58:
from a receiver is repetitively sampled and applied to the vertical input (
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From a mathematical perspective, an eye pattern is a visualization of the
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Eye pattern of eight million UIs (unit intervals) of a 1.25 Gbit/s signal
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There are many measurements that can be obtained from an eye diagram:
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Eye pattern of a 1.25 Gbit/s NRZ signal through a lossy channel
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Eye pattern of a 1.25 Gbps NRZ signal with 6 dB of pre-emphasis
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30:
Graphical eye pattern showing an example of two power levels in an
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Eye pattern of a 1.25 Gbit/s NRZ signal with a four-inch stub
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Eye pattern of a 1.25 Gbit/s NRZ signal with a one-inch stub
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Strong state to corresponding weak state (1-1 or 0-0 bit pattern)
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62:), while the data rate is used to trigger the horizontal sweep (
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159:
100:
618:
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UIs shows far more nuance on the rising and falling edges.
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Eye pattern of twelve thousand UIs of a 1.25 Gbit/s signal
842:
Gives an example video of construction of an eye pattern
248:
The samples are then accumulated into a two-dimensional
107:, or too slow to change, or have too much undershoot or
192:, the symbol rate is intentionally varied by use of
759:
806:"Agilent 71501D Eye-Diagram Analysis User's Guide"
123:and noise appears as closure of the eye pattern.
852:
700:1266 Hewlett-Packard Journal 45(1994) Aug., No,4
338:Eye pattern of a 125 Mbit/s MLT-3 signal
16:Oscilloscope display of a digital data signal
598:
317:Eye pattern of a 1.25 Gbit/s NRZ signal
461:the signal starts to level off around -300
449:, which causes the channel to behave as a
652:Intersymbol interference, additive noise
632:due to interruptions in the signal path
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216:Most high speed serial signals, such as
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77:secure speech transmission system.
13:
614:Eye opening (height, peak to peak)
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14:
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818:from the original on 2022-10-09.
800:from the original on 2022-10-09.
724: This article incorporates
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399:can be seen in the eye pattern.
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378:
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232:which is intended to allow easy
737:General Services Administration
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491:inter-symbol interference (ISI)
359:Eye pattern of a PAM-3 signal (
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243:
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126:
1:
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640:Timing synchronization &
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204:With some protocols, such as
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82:probability density function
7:
658:
509:completely closes the eye.
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115:. Distortion of the signal
10:
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371:
152:
18:
599:Interpreting measurements
547:Eye signal-to-noise ratio
685:
626:Eye overshoot/undershoot
321:
194:spread spectrum clocking
121:intersymbol interference
68:intersymbol interference
19:Not to be confused with
538:Eye crossing percentage
528:Amplitude measurements
224:, and most variants of
830:Ruckerbauer, Hermann.
732:Federal Standard 1037C
726:public domain material
579:Horizontal eye opening
535:Eye crossing amplitude
517:
505:
473:
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367:
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35:
745: (in support of
665:Constellation diagram
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395:Many properties of a
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347:The eye pattern of a
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326:The eye pattern of a
316:
306:The eye pattern of a
84:(PDF) of the signal,
29:
675:Raised-cosine filter
561:Deterministic jitter
553:Vertical eye opening
481:Impedance Mismatches
363:automotive Ethernet)
184:chunks one UI long.
606:Eye-diagram feature
582:Peak-to-peak jitter
441:High-Frequency Loss
54:display in which a
46:, also known as an
705:2021-01-26 at the
557:Time measurements
518:
506:
474:
438:
374:Phase-shift keying
365:
340:
319:
40:telecommunications
36:
861:Data transmission
656:
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609:What it measures
564:Eye crossing time
143:transmission line
139:circuit simulator
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832:"An Eye is Born"
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739:. Archived from
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680:Extinction ratio
670:Signal integrity
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21:Optical illusion
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707:Wayback Machine
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451:low-pass filter
447:dielectric loss
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391:Channel effects
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200:Reference clock
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824:External links
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743:on 2022-01-22.
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550:Quality factor
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429:signal level.
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372:Main article:
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236:by means of a
234:clock recovery
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212:Clock recovery
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56:digital signal
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585:Random jitter
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573:Eye rise time
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570:Eye fall time
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532:Eye amplitude
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147:Interpolation
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90:unit interval
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28:
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741:the original
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709:, pp. 29-37.
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594:Total jitter
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527:
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521:Measurements
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262:
258:Tone mapping
247:
215:
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186:
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162:in the eye.
156:
135:
98:
79:
63:
59:
52:oscilloscope
47:
43:
37:
747:MIL-STD-188
649:Eye closure
487:reflections
244:Integration
222:DisplayPort
181:symbol rate
132:Source data
127:Calculation
48:eye diagram
44:eye pattern
781:References
630:distortion
591:CRC jitter
588:RMS jitter
541:Eye height
361:100BASE-T1
294:Modulation
254:normalized
175:Fixed rate
166:Triggering
113:distortion
94:color ramp
637:Eye width
617:Additive
576:Eye width
567:Eye delay
544:Eye level
250:histogram
230:line code
109:overshoot
855:Category
813:Archived
795:Archived
793:. 1999.
703:Archived
659:See also
644:effects
408:Emphasis
403:Emphasis
228:, use a
226:Ethernet
117:waveform
99:Several
50:, is an
837:YouTube
397:channel
153:Slicing
119:due to
75:SIGSALY
642:jitter
160:jitter
101:system
86:modulo
64:x-axis
60:y-axis
816:(PDF)
809:(PDF)
798:(PDF)
791:(PDF)
728:from
686:Notes
619:noise
328:MLT-3
322:MLT-3
105:noisy
42:, an
218:PCIe
206:HDMI
190:SATA
88:the
72:WWII
368:PSK
349:PAM
343:PAM
308:NRZ
302:NRZ
238:PLL
38:In
32:OOK
857::
834:.
811:.
749:).
735:.
463:ps
220:,
840:.
772:.
23:.
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