LeCroy DDA 7 Zi series, Operator'S Manual

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Operator's Manual
WP700Zi-OM-E-RevA
420
clock timing points and the segments delimited by the clock samples are overlayed by aligning the clock samples
for each segment. A monochrome or color persistence display is used to show the distribution of the eye pattern
data. The jitter added by the measurement system in this case is from the sampling clock which, for the SDA is
very low: on the order of 1 ps rms.
Eye Violation Locator (ASDA Option)
The eye pattern is measured by overlaying segments of a continuous acquisition. Since the complete data record
is available, the location of individual bits can be determined by comparing each bit interval in the original
waveform to the selected mask. The mask is aligned horizontally along the mean bit interval, and vertically along
the mean one and zero level in the case of a relative mask. Absolute masks exist for some standards and are
defined in the vertical dimension by specific voltage values.
Figure 4 below shows this alignment. When mask
testing is turned on, the entire waveform is scanned bit-by-bit and compared to the mask. When a mask hit is
detected, the bit number is stored and a table of bit values is generated. The table is numbered starting with the
first bit in the waveform. This table can be used to index back to the original waveform to display the waveform of
the failed bit.
Figure 5-34. Eye Mask Alignment for Violation Locator
Eye Pattern Measurements
There are several important measurements that are made on eye patterns. These are specified as required tests
for many standards. Eye measurements mainly deal with amplitude and timing, which are outlined below.
Eye Amplitude
Eye amplitude is a measure of the amplitude of the data signal. The measurement is made using the distribution
of amplitude values in a region near the center of the eye (normally 20% of the distance between the zero
crossing times). The simple mean of the distribution around the “0” level is subtracted from the mean of the
distribution around the “1” level. This difference is expressed in units of the signal amplitude (normally voltage).
Figure 5-35. Eye Measurements.