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Setting "0 dB" Reference

Setting "0 dB" Reference

Measuring loss requires setting a reference for "0 dB" or no loss. The method shown here is the standard method used for testing patchcords and cable plants for all networks.

There are other reference methods that are included in standards using two or three reference cables, but they will underestimate the loss of the cable under test and have higher measurement uncertainty.

We recommend you set the source power to the same level each time even if you have a meter that has a "0 dB" button to have a convenient, easily remembered reference level if you lose the reference set in the meter!

Note: The TIA 568 standard calls for using a Class 1 source and "mandrel wrap" on the launch cable to standardize test conditions.

1. Attach launch reference cable to the source

2. Attach cable to the meter

3. Turn on source and let stabilize (about a minute)

4. Turn on meter to dBm or dB scale

5. Read reference power on meter. Adjust source to convenient value like -20.0 dB for LEDs or -10.0 dB for lasers, to make calculating loss easy. If your power meter has a "zero reference" button, use it, but it's a good idea to note the reference power level in case the zero reference is lost.