HP 54750A, User Manual, Page: 168 / 478

The Digital Communications
Analysis Menus
Measure Eye Menu
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Extinction ratio...
This function is only available with the HP 83480A, or if the HP 83480K
option is installed in an HP 54750A.
Extinction ratio for an eye diagram is simply the ratio of the logic \high"
level (V
top
or P
top
) to the logic \low" level (V
base
or P
base
). A very high
extinction ratio typically implies that the logic low level is very small. In the
case of an optical transmitter , the logic low level would approach a condition
where the laser is nearly turned o. The accuracy of the extinction ratio
can be dominated by the accuracy with which the low level was measured.
This in turn can be signicantly aected by any noise generated by the
measurement system such as O/E converter , DC osets , or electrical osets in
the instrument electronics . Thus the extinction ratio measurement is dened
as:
V
top
0 V
dar k
V
base
0 V
dar k
V
top
(or P
top
) and V
base
(or P
base
) are determined from vertical histograms
of the eye window . The eye window is the center of the eye and is dened
in the Dene Measure menu. The default window boundaries are 40% and
60%. Thus , the middle 20% of the eye is analyzed to determine V
top
and
V
base
. The window can be adjusted to analyze any portion of the eye (100%
window) by adjusting the window boundaries accordingly .
The vertical histogram of the eye window is used to determine the most
prevalent high level and the most prevalent low value of the eye . The
histogram is typically bi-modal, and V
top
and V
base
correspond to the means
of the histogram about the most prevalent high and low values .
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