EXFO IQS-12002B Calibration System - Manual (Page 73)

Calibration Methods
Calibration System 69
FTB-7000D and FTB-7000B-B OTDR Modules
Dynamic Range Test
The dynamic range is verified using various combinations of pulse duration
and distance range with the averaging time set at 45 s. Only one pulse
duration is included in the calibration report, usually 10 μs for singlemode
and 1 μs for multimode. The dynamic range is the difference (in dB)
between the launch level (backscatter trace extrapolated at distance zero)
and the noise floor. The noise floor has many definitions, but the one
mostly accepted in the fiber-optic industry is when the signal-to-noise ratio
equals 1 (SNR = 1). For EXFO OTDRs, we use custom software that
measures the noise floor on the raw data (before conversion in dB), which
is the only way to properly evaluate the standard deviation at the noise
floor level.
When trying to measure the dynamic range, the main difficulty is to
determine the proper noise floor. In general, alternative noise floor
definitions are used (e.g. peak noise level or 98 % noise level) and are then
converted to SNR = 1. Because the interpretation of noise is difficult and
the correct evaluation of the noise floor, when the raw data is not available,
is hard to make, and also because noise has different definitions, it is not
uncommon to find deviations of up to 2 dB between measurements done
with EXFO’s IQS-12002B software and done manually without raw data.
The software indicator of the injection level at each pulse is also verified
when measuring the dynamic range. The results might indicate that an
adjustment is required for one or more pulses. Note that a failure on an
injection level test does not imply a failure on the dynamic range; these are
separate parameters. Moreover, the injection level being only a visual
indicator, its result is not shown on the calibration certificate.
Note: There is no uncertainty value associated with this measurement.