Zero Level

The Eye mode Zero Level is a measure of the mean value of the logical 0 of an eye diagram. Annotation labels are placed at the logical 0 level. This measurement is made in a section of the eye referred to as the eye window boundaries. The default value for the NRZ eye window is the central 20% of the symbol period.

A histogram is constructed using the sampled portion of the eye diagram within the eye window boundaries. This histogram is comprised of data points from only the lower half of the eye diagram. The histogram mean is located.

The following graphic illustrates how the histogram is constructed on an eye diagram that is pattern dependent. Pattern dependency is the behavior of a waveform as it is influenced by the preceding data pattern. For example, a logic level zero that follows a logic level one may have a different amplitude than a logic level zero that follows a previously generated logic level zero.

You will notice when there is a vertical spreading to the waveform and you are in gray scale display mode, some regions of the waveform are brighter in color than others. The brighter areas indicate a higher number of hits or sampled data points. The zero level is equal to the mean of the histogram. All data at the one level is disregarded.

Configurable Measurement Parameters

This measurement is affected by the following setting (click Measure > Configure Base Measurements):

• Eye Boundary tab

To measure

1. Select Eye/Mask Mode.
2. Click the toolbar's Eye Meas tab.
3. Click Auto Scale in the menu bar.
4. Click the More buttons to locate the Zero Level button.
5. Click the button.

SCPI Command

:MEASure:EYE:ZLEVel