Using Eye Diagrams

Overview

PLTS constructs measurement-based eye diagrams (or patterns) by convolving the calculated time domain impulse response (generated from frequency domain measurement data) with a synthesized pattern of bit sequences. The following is a simplified block diagram of the eye diagram creation process.

With eye diagrams you can see signal quality with one display, you can diagnose problems, such as attenuation, noise, jitter, and dispersion that arise or characterize specific parts of the system. You can then view the measurement in the Time Domain mode to help isolate the source of the problem.

2-Level (PAM2) versus 4-Level (PAM4) Eye Diagrams

NRZ (non return to zero), also known as PAM2, is a Pulse Amplitude Modulated (PAM) signal with two amplitude levels. This signaling method transmits one bit of data for every bit, unit interval, or symbol interval.

NRZ (PAM2) Signal

Instead of a 0 and 1 level as in NRZ, PAM4 has four amplitude levels (0, 1, 2, and 3). This signaling method transmits 2 bits of data for every symbol. As a result, PAM4 transmits twice as much data per unit time.

PAM4 Signal

The Eye Diagram

The eye diagrams shown below identify key eye diagram definitions.

2-Level Eye Diagram (PAM2)

4-Level Eye Diagram (PAM4)

1	Zero Level	Zero Level is a measure of the mean value of the logical 0 of an eye diagram.
2	One Level	One Level is a measure of the mean value of the logical 1 of an eye diagram.
3	Rise Time	Rise time is a measure of the transition time of the data from the 10% level to the 90% level on the upward slope of an eye diagram.
4	Fall Time	Fall time is a measure of the transition time of the data from the 90% level to the 10% level on the downward slope of an eye diagram.
5	Eye Height	Eye height is a measure of the vertical opening of an eye diagram. An ideal eye opening would be measured from the one level to the zero level. However, noise on the eye will cause the eye to close. The eye height measurement determines eye closure due to noise.
6	Eye Width	Eye width is a measure of the horizontal opening of an eye diagram. Ideally, the eye width would be measured between the crossing points of the eye. However, jitter may appear on the waveform and influence the eye opening. A preference setting determines exactly how this measurement is made. Learn more.
7	Deterministic Jitter	Deterministic jitter is the deviation of a transition from its ideal time caused by reflections relative to other transitions.
8	Eye Amplitude	Eye amplitude is the difference between the logic 1 level and the logic 0 level histogram mean values of an eye diagram.
9	Bit Rate	Bit rate (data rate) is the inverse of bit period (1 / bit period). The bit period is a measure of the horizontal opening of an eye diagram at the crossing points of the eye.
10	Two-Level	Two-Level is a measure of the mean value of the logical 2 of a PAM4 eye diagram.
11	Three-Level	Three-Level is a measure of the mean value of the logical 3 of a PAM4 eye diagram.

Time Domain Windowing and Eye Diagrams

Changing the Time Domain Window setting has the following effect on Eye Diagrams:

To select Time Domain Window Settings

Click Tools, then Time Domain Settings

Learn more about Time Domain Windowing

Changed Data Icons

During an Eye Diagram conversion, one of the following may appear in the corner of the display. Learn more about these icons.

Icon	Icon Name
	Resampled Data - This icon indicates that only the harmonically-related data points were used to generate time domain data.
	Interpolated Data - This icon indicates interpolation is performed to calculate the harmonically-related points that were not measured. Any measurements that were performed at harmonically-related points are left unchanged. The interpolated data is used, along with the measured harmonically-related data, to perform the Inverse Fast Fourier Transform (IFFT) for the calculated time domain data.
	Bad Data - This icon indicates there are less than 10 harmonically-related data points in the measured data. In this case, all of the non harmonically-related data is used to perform the Inverse Fast Fourier Transform (IFFT) to calculate time domain data. This may result in inaccurate time domain data.

Viewing Data using Eye Diagrams

Learn how to open Eye Diagram plot windows.

With Eye Diagrams, only transmission paths are displayed; no reflection paths are displayed.

1. On the Parameter Format Selection pane:

Click Parameter, then select one or more parameters to view.
Click Equation to Synthesize Eye Diagram from Equations.

2. The Choose Bit Pattern dialog box may be displayed. The dialog box is NOT displayed if the bit pattern has already been selected for the data.

The selected Eye Diagrams are plotted.

3. To view a single plot, double-click on the plot. Double-click again to view all plots.

Note: Beginning with PLTS 4.5, the vertical axis of Eye Diagrams can be scaled using the scaling toolbar. Reset the vertical scale by right-clicking the Eye diagram, then click Reset.

4. When a Multi-channel Eye Diagram is active, the buttons (red circle in above image) are displayed.

The left button, selected by default, displays an Eye Diagram.
The right button, displays the simulated, enhanced bit pattern.

Eye Diagram Markers

To add markers to an Eye Diagram, right-click on the plot, then click Insert.

Each eye diagram marker has two components:

The X-axis component is displayed as a line that extends from the top to the bottom of the eye diagram.
The Y-axis component is displayed as a line that extends from the left side to the right side of the eye diagram.

These markers can be moved ONLY from Marker Bar slider control.

Automated Eye Diagram Measurement Results

To see Eye Diagram measurement results, statistics, and the following Eye Diagram options:

On the Parameter toolbar click the Parameter Meas Tab

The following buttons line the top of the Parameter Meas tab (shown in red circle below - from left to right):

1. Hide Histogram and re-measure Eye Parameters - Click to refresh the results.

2. Draw Eye Color Grades - Click to show the densest patterns as red, then yellow, then light green, then dark blue (as least dense).

3 through 8 (not available for PAM4 signal) Show histograms and dimensions of how the measurements are made:

Amplitude, Height, Width, Jitter, Rise Time, Fall Time.

9. Launch the Configure Eye Measurements dialog.

2-Level Signal (PAM2)

Eye Amplitude plotted on selected eye diagram.

Name	Definition
Eye Level Zero (mV)	Mean value of logical zeros
Eye Level One (mV)	Mean value of logical ones
Eye Level Mean (mV)	Mean value of logical ones and zeros
Eye Amplitude (mV)	Difference between logical 1 and logical 0 histogram mean values.
Eye Height (mV)	Vertical opening of the eye diagram
Eye Height (dB)	Vertical opening of the eye diagram
Eye Width	Horizontal opening of the eye diagram
Eye Opening Factor	Ratio of eye height to eye amplitude
Eye Signal to Noise	Ratio of the signal difference in logical 1 to logical 0 relative to the noise present at both levels.
Eye Duty Cycle Distortion	Time separation between the rising edge and falling edge at the 50% (middle) threshold of the eye diagram.
Eye Duty Cycle Distortion (in %)	Percent of duty cycle distortion
Eye Rise Time (20-80)	Mean transition time of the data on the upward slope (rising edge).
Eye Fall Time (80-20)	Mean transition time of the data on the downward slope (falling edge).
Eye Jitter (PP)	Time variances between the rising and falling edges. These edges affect the crossing point of the eye. PP= full width of the histogram at the eye diagram crossing point.
Eye Jitter (RMS)	Time variances between the rising and falling edges. These edges affect the crossing point of the eye. RMS= standard deviation of the histogram at the eye diagram crossing point.

4-Level Signal (PAM4)

Click on a parameter to view the result on the plot

Name	Definition
Levels
Level 0	Mean value of logical zeros
Level 1	Mean value of logical ones
Level 2	Mean value of logical twos
Level 3	Mean value of logical threes
Levels RMS
Level 0 RMS	Jitter amplitude in RMS of level zeros
Level 1 RMS	Jitter amplitude in RMS of level ones
Level 2 RMS	Jitter amplitude in RMS of level twos
Level 3 RMS	Jitter amplitude in RMS of level threes
Levels Peak-Peak
Level 0 Peak-Peak	Peak-peak jitter amplitude of level zeros
Level 1 Peak-Peak	Peak-peak jitter amplitude of level ones
Level 2 Peak-Peak	Peak-peak jitter amplitude of level twos
Level 3 Peak-Peak	Peak-peak jitter amplitude of level threes
Levels Skew
Level 0 Skew	Skew time of level zeros
Level 1 Skew	Skew time of level ones
Level 2 Skew	Skew time of level twos
Level 3 Skew	Skew time of level threes
Eye Levels
Eye 0/1 Level	Skew time between level zeros and level ones
Eye 1/2 Level	Skew time between level ones and level twos
Eye 2/3 Level	Skew time between level twos and level threes
Eye Skews
Eye 0/1 Skew	Eye skew time between level zeros and level ones
Eye 1/2 Skew	Eye skew time between level ones and level twos
Eye 2/3 Skew	Eye skew time between level twos and level threes
Eye Heights
Eye 0/1 Height	Vertical opening of level zeros and level ones
Eye 1/2 Height	Vertical opening of level ones and level twos
Eye 2/3 Height	Vertical opening of level twos and level threes
Eye Widths
Eye 0/1 Width	Horizontal opening of level zeros and level ones
Eye 1/2 Width	Horizontal opening of level ones and level twos
Eye 2/3 Width	Horizontal opening of level twos and level threes
Linearity
Linearity	Linearity RLM (Ratio Level Mismatch) between all four amplitude levels (0, 1, 2, 3) of a PAM4 eye diagram. Linearity is a measure of the variance in amplitude separation (distribution) between the different PAM4 levels.
Random Noise
RN 0 RMS	Random noise level of level zeros
RN 1 RMS	Random noise level of level ones
RN 2 RMS	Random noise level of level twos
RN 3 RMS	Random noise level of level threes
Jitter
TJ 0 RMS	Total jitter of level zeros
TJ 1 RMS	Total jitter of level ones
TJ 2 RMS	Total jitter of level twos

Configure Eye Measurements

Click on the Parameter Measurement Results tab.

Configure EYE Measurements dialog box help

2-Level Signal (PAM2)

In the Sample Window Boundaries, you can increase or decrease the size of three measurement areas. This will change the results of the corresponding measurements.

Sample Window Boundaries

Click one of the Measurements Affected selections.
Increase or decrease the sample window value. Click Default Value to reset the value to its default setting.
Click OK to recalculate the results on the display and in the Results table.

Eye Diagram User Preferences tab

This tab contains shortcuts to set the following Eye Diagram User Preferences. These preferences also appear in the main User Preferences dialog.

Disable the use of the 3 sigma buffers for eye measurement calculations.

Check to use the following equation:

eye width = (tmin of right cross – t max of left cross)

Clear (uncheck) to use the following equation:

Do not update plot when calculating eye data

When checked, do NOT show progress dialog and do NOT draw eye until all data calculated. When plotting long bit pattern eye diagrams, this setting can save a LOT of time.

Clear to show progress and draw eye diagram as data is being calculated.

Eye Mask - Use measured amplitude values for eye mask scaling.

This preference applies ONLY for eye masks defined in ratios.

When cleared (default setting), use the amplitude of selected bit pattern dialog to scale mask when importing.
When checked, use measured amplitude as was done in previous PLTS revisions.

4-Level Signal (PAM4)

Receiver Sample Timing - This setting configures the timing for sampling the data. Use this setting to match the timing method used by a receiver. For signals with skewed eyes, measurements results will vary depending on this setting.

Base On Center Eye - Sampling timing is based on the location of the center eye (eye 1/2). This is the default setting.

Independent Per Eye - Sampling timing is independently based on the location of each of the three eyes.

Eye Center Location - This setting selects the basis for determining the location of an eye's center on the waveform.

Maximum Eye Width - The eye's center is located at the eye's maximum width midway between the eye's inside left and right edges. This is the default setting.

Maximum Eye Height - The eye's center is located at the eye's maximum height midway between the eye's inside top and bottom edges.

Eye Level Width - This setting defines the timespan over which an eye's amplitude level is measured. The timespan is specified as a percentage (from 1% to 25%) of the symbol period. The default setting is 10%.

Time of Level - This setting specifies the method used to locate the time at which to measure an eye's level.

Eye Center - The time of a level is the average of the centers of the adjacent eyes. This is the default setting.

Minimum RMS - The time is located within the eye's level width at the minimum level thickness (RMS values).

Time Units - This setting specifies the time units.

Second - Width and skew measurements are reported in seconds. This is the default setting.

Unit Interval - Width and skew measurements are reported in unit intervals.

Amplitude Units - This setting specifies amplitude units.

Amplitude - Level and height measurements are reported in the current amplitude units: volts or Watts. This is the default setting.

Percent - Level and height measurements are reported in percent.

Last Modified:

8-Dec-2016	Added PAM4 information
13-Nov-2012	New features for 2013
21-Oct-2011	Broke out topics. Added results buttons
5-Oct-2009	Added Eye Mask create (5.2)
16-Dec-2008	Added Import Bit Pattern (5.0)
3-Apr-2008	Added scaling note
26-Feb-2008	Added Eye Mask Test