User guide

• Q Low Level Std.Dev

The Q Low Level Standard Deviation is the σ (Sigma), calculated from the linear

regression curve for the low level data:

= -

• Q Low Level Nr. Points

This is the number of data points used for the calculation of the Q Low Level

value. It depends on the setting of the BER Threshold and also on the setting

of the Min BER for Q parameter.

The minimum for calculating Q-factor values is two points. It is recommended

to include more than 5 points.

• Q Low Level R^2

See the definition of "Q High Level R^2".

Unavailable Values

Under certain circumstances, some numerical results are not available. This is

indicated by <invalid> or <not applicable> in the numerical results table below

the measurement graphic.

• <invalid>indicates that the value could not be calculated. This is the case, for

example, for phase margin, when the BER threshold is set to high values and

does not intersect with the bathtub curve.

• <not applicable> indicates that the value could be calculated, however is not

shown because quality criteria are not met. This is the case, for example, for

RJ/DJ results, when the r^2 value of one or both edges is <0.75. Even though

RJ/DJ values can be calculated in this case, they are not shown because

confidence in the results is too low.

Understanding the Q-Factor Results

The variable decision threshold method used by the Output Levels measurement

makes it possible to determine the Q-factor of a signal.

N O T E

The Q-factor method and the related calculations have been published under Margin

Measurements in Optical Amplifier Systems by Neal S. Bergano, F. W. Kerfoot, and

C. R. Davidson in IEEE Photonics Technology Letters, Vol. 5, No. 3, March 1993.

6 Advanced Analysis

282 Agilent J-BERT N4903B High-Performance Serial BERT