Teledyne LeCroy SDA III-CompleteLinQ - Manual (Page 36)

SDAIII-CompleteLinQ Software
Eye Analysis Theory
Eye diagrams are persistence maps, where each pixel in the map takes on a color that indicates how
frequently a signal has passed through the time (within a UI) and voltage specified for that pixel. Theeye
diagram shows all values a digital signal takes on during a bit period. A bit period (or UI) is defined by the
data clock, so some sort of data clock is needed to measure the eye pattern.
Eye Diagram
Eye Diagramming in Older Oscilloscopes
The "traditional" method of generating an eye pattern that was used before oscilloscopes had the ability
to determine an eye from a single sweep involved building a persistance map over many sweeps. Signals
were acquired using the data clock as a trigger. One or more samples taken on each trigger were stored
in a persistence map with the vertical dimension equal to the signal level, with the horizontal position
equal to the sample position relative to the trigger. The eye pattern filled in after a large number of mul-
tiple occurrences of time and amplitude values (counted by incrementing counters in each x,y bin) for all
the data points collected. Users would typically put the trigger point "off screen", and build the per-
sistance map until it showed sufficient data.
Timing jitter is indicated by the horizontal distribution of the points around the data crossings. The his-
togram of the bins around the crossing points provides the distribution of jitter amplitude. This tra-
ditional method, unfortunately, includes the oscilloscope's trigger jitter, with the consequence that the
jitter seen in the eye diagram was not just the jitter in the signal itself.
A recovered clock is used if there is no access to a data clock. In the traditional method, the recovered
clock is normally a hardware PLL designed to operate at specific data rates and with a cutoff frequency of
Fd/1667. A drawback of a hardware clock recovery circuit is that jitter associated with its trigger circuit
adds to the measured jitter by creating uncertainty in the horizontal positioning of the eye pattern sam-
ples. Another drawback is that the clock recovery PLL is not likely to be controllable, and may not meet
the exact requirements for a specific standard.
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