HP 54750A, User Manual, Page: 94 / 478

Eye , Mask and Eyeline Mode Measurement T utorials
Making Eye Diagram Measurements
Measuring extinction ratio
F or HP 83480A only
Extinction ratio is the ratio of the most prevalent high logic level to the
most prevalent low logic level over one bit interval. As the logic low level
approaches 0 volts or watts , the extinction ratio can become very high. Any
residual oset signals within the instrument can become signicant if their
magnitude approaches the logic 0 level. A more accurate measurement of
extinction ration would be:
E xtinction r atio per cent = 100
P
base
0 P
dar k
P
top
0 P
dar k
Because of this relationship , part of the procedure for performing an
extinction ratio measurement is to do a dark calibration. Y ou are instructed
to turn o the laser so that the instrument's internal oset can be measured.
After the calibration is performed, data for the database will begin to acquire .
Some of the data will be measured points acquired while the laser is still
o, therefore , the laser should be turned on, allowed to stabilize , and then
the instrument's display should be cleared to have a valid database . The
extinction ratio measurement is dened using the measurement database as
the mean high level over a windowed portion of the eye . This window is
nominally the middle 20% of the eye .
F or more information on dening your measurements , refer to the Dene
Measure menu in Chapter 10 of this manual.
Marker lines The marker lines delineate the measurement boundaries . The X markers
dene the measurement window . Only data taken inside this window is
used to determine the measurement results . The Y markers dene vertical
boundaries of the measurement.
The extinction ratio measurement is made by rst determining a window in
the center 20% of the eye . All data points in that measurement window are
used to determine the logic level 1 and 0.
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