Agilent Technologies J-BERT N4903, User Manual

Page: 127 / 434

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Adjust the location of the sampling point.
What is an Eye Diagram?
An eye diagram provides a way to view all possible transitions between
adjacent bits of a data stream by overlaying them on the display of a
high-speed oscilloscope. It is typically produced by triggering the
oscilloscope with a synchronous clock signal.
When the oscilloscope is triggered by a clock signal, data patterns will
not be examined. Instead, if persistence is set high enough, the shape of
an "eye" will be observed. The eye is bounded by overlaid logic 1 and 0
voltages (top and bottom) and multiple 0 to 1 and 1 to 0 transitions (left
and right).
Different portions of the bit pattern
Eye diagram
Overlay of all
portions of the bit pattern
The sampling point of the error detector must be set within the data eye.
The error detector uses the eye diagram to graphically display the
location of the sampling point. Additionally, the appearance of the eye
is useful for determining the quality of the data signal.
The height and width of the eye in the eye diagram are also displayed
in the diagram. Rise time, overshoot, and jitter are not displayed in the
diagram. Note that the diagram does not reflect the different shapes of
signals in RZ or R1 pulse format.
What is an Eye Edge?
In the eye diagram, the eye has 4 edges. An eye edge is defined as a data
input delay value or 0/1 threshold voltage value in the data eye that
meets the following conditions:
•
The instantaneous bit error ratio (BER) is greater than or equal to
the alignment BER threshold.
•
An adjacent point has an instantaneous BER that is less than the
alignment BER threshold.
Setting up the Error Detector
4
Agilent J-BERT N4903 High-Performance Serial BERT 127