# Eye Measurements

Subtopics

- Bit Rate
- Crossing Percentage
- Crossing Time
- Database Peak Hits
- Delta Time
- Duty Cycle Distortion
- Eye Amplitude
- Eye Height
- Eye Width
- Fall Time
- Jitter
- One Level
- Rise Time
- Signal-to-Noise Ratio
- Zero Level

The following NRZ Eye measurements are available on the toolbar. To measure PAM signals, use the PAM toolbar. The only measurement on the Eye Meas toolbar that can be applied to NRZ waveforms is the Database Peak Hits measurements.

None of the measurements listed on the Eye Meas toolbar are compatible with PAM waveforms except Database Peak Hits.

## Toolbar 1 of 3

Jitter
Eye Rise Time
Eye Fall Time
One Level
Zero Level
Eye Height
Eye Width

## Toolbar 2 of 3

Crossing %
Signal-to-Noise Ratio
Duty Cycle Distortion
Bit Rate
Eye Amplitude
∆ Time
Crossing Time

## Toolbar 3 of 3

Database Peak Hits

## Eye Measurements Overview

You can make eye diagram measurements and eye mask tests on NRZ (Non-Return to Zero) signals.

Eye measurements are made on an overlaid waveform database (as are histograms and other statistically-based measurements).

Eye diagrams are produced by overlaying (sometimes called folding) UIs (unit intervals) of waveform data about recovered clock edges. Overlaid data is saved in a database that records "pixel" hits where waveforms cross small boxes of vertical range and time range. Overlaid waveform databases are displayed so that pixels with a higher numbers of hits use more "intense" colors.

With the exception of average power, all eye measurements are based on the statistical data saved in the overlaid waveform database. These measurements use all the data of the eye diagram (whereas non-eye measurements are made on waveform data that has not been overlaid).

Overlaid waveform databases are sometimes referred to as "multiple-valued waveform data".

Non-overlaid waveform data is sometimes referred to as "single-valued waveform data".

The eye measurement (Eye Meas) toolbar includes typical eye measurements such as rise time, fall time, and jitter.

Like eye measurements, mask testing is also performed using overlaid waveform databases.

The mask test (Mask Test) toolbar includes start and stop controls.

You'll find a table of Eye measurement and mask test results located below the display graticule. A maximum of 64 eye measurement test results are displayed and include any failures or instrument messages that may occur. Measurement results are not reported until a significant sample size of data is acquired.

Eye measurements are made on eye diagrams only.

## Auto Scale

Click Auto Scale on the menu bar to ensure that an optimum eye diagram is displayed on the graticule. An optimum NRZ eye diagram consists of a full display of the eye in addition to portions of the waveform preceding and following the eye.

Perform an Auto Scale before performing eye measurements. This will clear the measurement database and ensures that an optimum eye diagram is displayed on the graticule. An optimum eye diagram consists of two full crossing points and four to six divisions of vertical scale. The Auto Scale algorithm is optimized to display an eye diagram.

The measurement database and the graticule display will clear upon the following events:

- switch to different operating mode,
- timebase change,
- vertical scale/offset change,
- trigger level change,
- or different source.

If this happens, press the front-panel Clear Display or Autoscale button or cycle the power.

Consequently, any configuration change clears the database and the instrument begins building a new database.

## Measurement Acquisition and Database

When an eye diagram is enabled, Infiniium immediately starts building the measurement database from waveform acquisitions, which consists of all of the data that is displayed on the screen. The measurement database provides the data for the construction of histograms, generation of mask tests, and a visual representation of the eye via the gray scale (or color grade) persistence display mode. Every time a display sample point is acquired on a display coordinate, the counter for that coordinate is incremented. As the total count increases, the range of hits also increases. The maximum count for each counter is 232.

Color grade persistence is automatically turned on for eye diagrams. When in color grade display mode, the density of data points is represented by a color spectrum.

Data acquisition from all enabled input channels does not stop until you perform any of the following events:

- Press the Clear Display button on the front panel,
- Switch between operating modes,
- Change the timebase,
- Change the vertical scale and offset,
- Change trigger level or source,
- Remove or insert a plug-in module, or
- Cycle the power.

As the instrument continues to acquire data, the measurement database continues to build. However, measurement results will not be reported until there is a full histogram to constitute a significant sample size. The measurements continue to update as more data is acquired.

A full histogram is made up of at least 16 sampled points of data for each time acquisition point across the horizontal display. This requires acquisition of a minimum of 16 waveforms. There will be a slight delay as the instrument gathers enough data for the first measurement. Once the minimum amount of data has been acquired, subsequent measurements simply add to the existing database and result values are updated.

Once an eye diagram is displayed on the screen, you can perform amplitude and timing measurements. The instrument builds the appropriate histograms for the measurements selected.

- Vertical axis histograms are used for amplitude measurements to determine the mean logic one and zero levels
- Horizontal axis histograms are used for timing measurements to determine mean time location when waveform transitions intersect and pass through defined threshold levels

For amplitude measurements, the histogram relies on the data sampled within the eye-window boundaries. The instrument default for NRZ eye window boundaries is the central 20% of the symbol period (a recommendation from OFSTP-4A, TIA/EIA). However, if you would like to open the eye window boundaries beyond 20% (for example, to include the rising and falling edges) you can change the boundaries in the Configure Measurement dialog box.

Widening the eye window boundaries changes the histograms and the measurement results.

Timing measurements are based on data sampled in a thin measurement window along the horizontal axis of the eye diagram. Unlike the eye window boundaries and the threshold levels, this timing measurement window is not user definable.