Agilent Technologies 83400 Series User Manual page 46

General Information
Theory of Operation
Active bias circuitry is used in the first amplifier stage to hold its DC bias
fixed, while the DC photocurrent from the PIN diode varies. A combination
threshold/hysteresis function on the DC board monitors the output of the
active bias current and uses the signal to light the OVER ILLUMINATION LED if
the average optical input power exceeds 3.5 mW. If the input power continues
to increase, the receiver goes into an unspecified state and measurement data
will be invalid. If average power exceeds 5 mW, permanent damage could
result.
Figure 2-12. Agilent 83411C/D block diagram
2-23