In Eye/Mask Mode, you can make eye diagram measurements and eye mask tests on NRZ (Non-Return to Zero) and PAM4 coded signals. Measurements are made using a database for multi-valued waveforms to which histograms and statistically-based measurements are applied. To enter Eye/Mask mode, use the mode select button, or click Setup > Mode > Eye/Mask.
Eye/Mask Mode Overview
Eye/Mask Mode makes eye diagram measurements and eye mask tests on NRZ (Non-Return to Zero) and PAM4 coded signals. Eye diagrams are typically produced by triggering the instrument with a synchronous clock signal. With the exception of average power, all measurements in Eye/Mask Mode are based on statistical data collected from incoming signals. These measurements use all the data of the eye diagram, a multiple-valued waveform , as opposed to the single-valued waveform displayed in Oscilloscope Mode.
The eye measurement (Eye Meas) and mask test (Msk Test) toolbars include typical eye measurements such as rise time, fall time, jitter, and extinction ratio, as well as the mask test start and stop controls.
You'll find a table of Eye measurement and mask test results located below the display graticule. A maximum of 32 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/Mask Mode measurements only work when an eye diagram, and not a pulse waveform (single-valued waveform), is present on the screen. Measurements made on a pulse waveform while in Eye/Mask Mode will fail.
Making signals appear as single-valued waveforms
In Eye/Mask mode, you can make a multi-valued waveform appear as a single-valued waveform instead of an eye diagram in order to view individual pattern symbols. The waveform's data is taken from the measurement database and is multi-valued.
- Turn on pattern lock. Pattern lock requires N1000A Option PLK, 86100D Option ETR, or N109X-series DCM-M.
- Open the Acquisition Setup dialog and select the Waveform tab.
- Unselect Acquire Entire Pattern. Or,select Acquire Entire Pattern and unselect Wrap Waveform.
|Without Pattern Lock
|With Pattern Lock
Saving waveforms to a file
To save a displayed waveform to a file, click File > Save > Waveform. You can save the waveform to a file in either internal FlexDCA format or a text/csv format.
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. When in Eye/Mask mode, 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 differenct 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.
The configurations you set for the channel, trigger, or timebase are not modified when you switch between operating modes. Any measurement configuration changes you make to the instrument are recalled after you exit and return to each operating mode.
Measurement Acquisition and Database
Selecting Eye/Mask Mode immediately starts waveform acquisition as FlexDCA begins building the measurement database, 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.
When Eye/Mask Mode is selected, the gray scale display persistence is automatically turned on. You can change it to color grade display persistence by clicking Setup > Display. When in the gray scale display mode, the highest density of data points are represented by the brightest regions of the waveform. 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.
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.