Bi-Directional Testing - DTX Compact OTDR Module

If your launch lead has a different backscatter coefficient than the cable you are testing, you may well end up with a suspiciously low loss value for your launch event (first connector). You may even observe a negative loss. An example of this from the DTX Compact OTDR is shown below:

Bi-directional Testing with DTX Compact OTDR

You could make a launch lead using the same fiber as you are testing. Alternatively, you could make the measurement in the opposite direction and take the average. This is defined in IEC 14763-3 and is referred to as bi-directional testing.

Bi-directional testing

Set the launch + receive fiber compensation - click here for details.
Rotate the dial on the DTX CableAnalyzer to SETUP.
Highlight Fiber OTDR and press ENTER.
Move to Tab 2 and make sure Testing From says End 1.
Rotate the dial to AUTOTEST.
Go ahead and test the link.
Save it as 001 (or your own Cable ID scheme).
Continue to test the rest of your links whilst saving them to the DTX CableAnalyzer.
Rotate the dial on the DTX CableAnalyzer to SETUP.
Highlight Fiber OTDR and press ENTER.
Move to Tab 2 and make sure Testing From says End 2.
Rotate the dial to AUTOTEST.
Swap ends with the DTX CableAnalyzer (leave the launch/recieve fibers in place).
Go ahead and test the first link again.
Save it as 001 (or whatever you chose previously)
Continue to test the rest of your links whilst saving them to the DTX Cableanalyzer.
The bi-directional calculation is done within the LinkWare Software. Go ahead and download your OTDR results into LinkWare.

The two measurements made in different directions are recorded as one result.
Click on Utilities > Generate Bidirectional OTDR Results; the following window will open:
Select the Cable IDs you wish to implement bi-directional averaging on and click OK.
You will now see your test result calculated using bi-directional averaging:
Looking at the result, we see:
The loss at each connector is 0.20 dB using bi-directional averaging in accordance with IEC 14763-3. Note the 0.02 dB reported when the measurement was made in one direction only.