Table of Contents
Advertisement
SDA Operator's Manual
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 subtracting the standard deviation of both the "1" and "0" levels. The measurement
basically gives an indication of the eye opening.
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 zero crossings
in the signal. The standard deviation of each distribution is subtracted from the difference
between these two means.
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 fully shut off
because a relatively long period of time is required to turn the laser back on, thus 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 accuracy is greatly affected by any offset in the input of the measurement system.
Optical signals are measured 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 (i.e., dark), and this value is subtracted from all subsequent measurements.
Eye Crossing
Eye crossing is the point at which 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 displacement of this slice is varied.
Average Power
The average power is a measure of the mean vale of all levels that the data stream contains. 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 we separate the ones and
zeroes histograms, the average power is the mean of both histograms. Depending on the data
coding that is used, the average power can be affected by the data pattern. A higher density of
ones, for example, will result in a higher average power. Most coding schemes are designed to
maintain an even ones density resulting in an average power that is 50% of the overall eye
amplitude.
371
SDA-OM-E Rev H
Advertisement
Table of Contents
