11
2270 Fiber Optic Modem
1.7 Loss Budget
The maximum possible distances with either standard or long distance versions is dependent on
the overall power loss over the fiber optic link. This is called the link loss. The launch power for the
modem is compared with receiver sensitivity. The determination of the difference is the loss budget
(refer to Table 1-B). To insure normal operation over a long term, the link loss should be at least 3
dB less than the loss budget for the modem.
For example, consider a data link 7 kilometers long using 50/125 optical fiber that has a measured
power loss of 9 dB, and a patch panel connector loss of 3 dB, for a total link loss of 12 dB.
The minimum launch power of the 2270 High Power (HP) laser version is -6 dBm in high-power
mode, and the worst-case receiver sensitivity is -21 dBm. The guaranteed loss budget is 15 dB.
Subtracting the link loss (12 dB) from the loss budget (15 dB) leaves a margin of 3 dB -- is within
the suggested limit.
NOTE: Optical power measurements for the 2270 are sensitive to the data rate. All mea-
surements quoted are for a 20 Mbps data rate.
1.8 Optical Bandwidth
The 2270 requires nearly 70 MHz of optical bandwidth. As a result, the specified fiber optic cable
must provide this bandwidth at the required distance.
For example, if 500 MHz/km fiber is used, this bandwidth requirement will limit the end-to-end
distance to 7 km (70 MHz multiplied by 7 km = 490 MHz).
1.9 General Installation
1.9.1 Fiber Optic Cable and Connectors
The 2270 can be used with virtually any size of multimode or single mode fiber optic cable includ-
ing 50/125, 62.5/125 and 8-10/125.
The proposed fiber optic cable must provide adequate bandwidth and power loss characteristics
for the intended modem link.
Fiber optic connectors terminated on the cable must match those terminated on the modem.
CAUTION:
It is important to keep all connectors free of water, dust, dirt, etc. When not in use, the
connectors should be covered with protective plastic caps.
1.9.2 Initial Unit Testing
The following is a basic list of check points to consider:
•
Have you set your optical power and receiver sensitivity switches correctly for the
loss of the fiber optic link?
•
Are the fiber optic cables marked correctly? Connect the Tx cable to the Tx connec-
tor, the Rx cable to the Rx connector. If System Test fails, try swapping cables at one
end of the link.