Model 920 Ethernet Data Multiplexer Manual
Focal Technologies Corporation
Page 6-2
A Moog Inc. Company
920-0603-00 Rev K
6.3 Bench Test
Basic Link Operation
Basic operation of the uplink and downlink can be verified in a bench test simply by connecting fiber jumpers
between the optical access bushings. The number of jumpers and the optical attenuation required depends on
the optical configuration of the system under test. In some optical configurations, it may be required to use 5 or
10 dB optical attenuators placed between the fiber bushings on the remote modem and the bushings on the
console modem to ensure the optical receivers are not saturated. Complete a flux budget calculation for your
system, referring to the optical transmit power and receive overload level to determine whether optical
attenuation is required or not.
After power is applied to the Model 920 EDM, the power LEDs should be on. A red Link LED usually indicates
insufficient received optical power, typically due to excessive loss in the fiber link. Excessive loss may be caused
by poor fiber connections (contaminated bushings, damaged fiber, damaged ferrules) or excessive fiber bends.
It is good practice to carefully clean all fiber connections prior to mating them for testing.
If the basic optical link is present, there should be good Ethernet data continuity through the multiplexer system.
Optical Power Budget Test
1. To verify the near-to-far side flux budget (downlink), measure the transmit power of the near side
modem by connecting the optical output directly to a calibrated optical power meter using a short, low-
loss, singlemode test jumper. (Use 50 µm core multimode jumpers if it's a multimode system.)
2. Remove the test jumper and install a variable optical attenuator (VOAT) between the near and far side
modems.
3. Adjust the VOAT until the red Link LED on either one of the modules turns on, then reduce the loss to
the point where both green link LEDs are continuously on. A stable Ethernet connection is a good
reference point.
4. Measure the optical power at the far side modem by connecting that end of the VOAT to the optical
power meter. The difference between this value and the transmit power previously measured is an
estimate of optical power budget. A spool of fiber used with the VOAT can also be used to simulate
losses due to dispersion over long cable lengths.
5. Repeat steps 1-4 with connections reversed to verify the far side to near side optical budget (uplink).
