EXFO FVA-3150 Variable Attenuator - Manual (Page 31)

Setting Up Your Variable Attenuator
Variable Attenuator 25
Selecting and Using an Attenuation Display Mode
Note: The active display mode is always indicated on the display.
Offset (X + B) The displayed attenuation is the sum of two components:

X = physical attenuation introduced by the unit.

B = correction factor or offset; it may be used to
compensate for losses along the connecting fibers or
for losses displaying the attenuation as a power value.
Offset mode can be best explained by the following
example:

An optical light source with a power output of -14 dBm
at 1310 nm is connected to a variable attenuator.
Because of internal losses, the minimum insertion
loss at 1310 nm is approximately 2 dB. This means that
the maximum optical power available at the output
port of the attenuator is -16 dBm (-14 + -2). If we now
set an offset value of -14 dBm, the (large digits) will
display the optical power level at the output port of the
attenuator. Using the above setup (that is, light source,
variable attenuator, and an accurately determined
offset value), EXFO has created a precise variable light
source with a dynamic range of nearly 100 dB.
Note: You must define the appropriate B value for
the operating wavelength before enabling
X+B mode. A different B value may be
assigned to each wavelength in the
wavelength box.
Display Mode Description