Anritsu 681XXC - Manual (Page 96)

At the Leveling menu, pressing either Internal or
Fixed Gain will turn off external leveling.
Press Previous Menu to return to the ALC Mode
Menu display.
Fixed Gain
In the fixed gain mode, the ALC is disabled. The RF
Level DAC and step attenuator (if installed) are
used to control the relative power level. Power is not
detected at any point, and the absolute power level
is uncalibrated. Fixed gain mode is selected from
the leveling menu.
Press Leveling Menu to go to the Leveling Menu.
To select fixed gain mode, press Fixed Gain .
To return to normal ALC operation, press Internal .
Press Previous Menu to return to the ALC Mode
Menu display.
681XXC OM 3-63
LOCAL (FRONT LEVELING
PANEL) OPERATION OPERATIONS