Calculating the Fiber-Optic Cable Power Margin for EX Series Devices
Before calculating the power margin, calculate the power budget (see
Calculating the Fiber-Optic Cable
Power Budget for EX Series Devices
).
Calculate the link's power margin when planning fiber-optic cable layout and distances to ensure that
fiber-optic connections have sufficient signal power to overcome system loss and still satisfy the
minimum input requirements of the receiver for the required performance level. The power margin (
P
M
)
is the amount of power available after you subtract attenuation or link loss (
LL) from the power budget
(
P
B
).
When you calculate the power margin, you use a worst-case analysis to provide a margin of error, even
though not all parts of an actual system operate at worst-case levels. A power margin (
P
M
) greater than
zero indicates that the power budget is sufficient to operate the receiver and that it does not exceed the
maximum receiver input power. This means that the link will work. A (
P
M
) that is zero or negative
indicates insufficient power to operate the receiver. See the specification for your receiver to find the
maximum receiver input power.
To calculate the worst-case estimate for the power margin (
P
M
) for the link:
1. Determine the maximum value for link loss (
LL) by adding estimated values for applicable link-loss
factors—for example, use the sample values for various factors as provided in
(here, the link is 2 km long and multimode, and the (
P
B
) is 13 dBm):
Table 24: Estimated Values for Factors Causing Link Loss
Link-Loss Factor
Estimated Link-Loss Value
Sample (LL) Calculation Values
Higher-order mode losses
(HOL)
•
Multimode—0.5 dBm
•
Single mode—None
•
0.5 dBm
•
0 dBm
Modal and chromatic
dispersion
•
Multimode—None, if product of
bandwidth and distance is less
than 500 MHz/km
•
Single mode—None
•
0 dBm
•
0 dBm
Connector
0.5 dBm
This example assumes 5 connectors.
Loss for 5 connectors:
(5) * (0.5 dBm) = 2.5 dBm
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