Teledyne LeCroy SDA II - Manual (Page 24)

Serial Data Analysis II Software
Eye Height
The eye height is a measure of the signal-to-noise ratio of a signal. The mean of the 0 level is subtracted
from the mean of the 1 level as in the eye amplitude measurement. This number is modified by sub-
tracting three times the standard deviation of both the 1 and 0 levels. The measurement provides an indi-
cation of the eye opening and is made on the central region (normally 20%, user changeable) of the UI
(bit period). This measurement algorithm is best suited to eye diagrams that are rendered from optical
rather than electrical signals. In the presence of inter-symbol interference and/or equalization,it may not
give reasonable results.
Eye Width
This measurement gives an indication of the total jitter in the signal. The time between the crossing
points is computed by measuring the mean of the histograms at the two 0 crossings in the signal. Three
times the standard deviation of each distribution is subtracted from the difference between these two
means. This measurement algorithm is best suited to eye diagrams that are rendered from optical rather
than electrical signals. In the presence of inter-symbol interference and/or equalization, it may not give
reasonable results.
Extinction Ratio
This measurement, defined only for optical signals, is the ratio of the optical power when the laser is in
the ON state to that of the laser in the OFF state. Laser transmitters are never completely shut off since a
relatively long period of time is required to turn the laser back on (therefore limiting the rate at which the
laser can operate). The extinction ratio is the ratio of two power levels (one very near zero) and its accu-
racy is greatly affected by any offset in the input of the measurement system. Optical signals are meas-
ured using optical-to-electrical converters on the front end of the SDA. Any DC offset in the O/E must be
removed prior to measurement of the extinction ratio. This procedure is known as dark calibration. The
output of the O/E is measured with no signal attached (also referred to as being dark) and this value is
subtracted from all subsequent measurements.
Eye Crossing
Eye crossing is the point where the transitions from 0 to 1 and from 1 to 0 reach the same amplitude. This
is the point on the eye diagram where the rising and falling edges intersect. The eye crossing is expressed
as a percentage of the total eye amplitude. The eye crossing level is measured by finding the minimum
histogram width of a slice taken across the eye diagram in the horizontal direction (as the vertical dis-
placement of this slice is varied).
Average Power
The average power is a measure of the mean value of the signal, derived from all levels contained within
the eye diagram. It can be viewed as the mean of a histogram of a vertical slice through the waveform
covering an entire bit interval. Unlike the eye amplitude measurement where the 1 and 0 histograms are
separated, the average power is the mean of both histograms combined. Depending on the data coding
used, the average power can be affected by the data pattern. A higher density of 1s, for example, results
in a higher average power. Most coding schemes are designed to maintain an even 1s density resulting in
an average power that is 50% of the overall eye amplitude.
Eye BER
Eye BER is an estimate of the bit error ratio is made from the eye diagram. It is derived from a meas-
urement of the Q-factor, as described below.
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