Anritsu 681XXC - Manual (Page 126)

When Proceed is pressed, the date parameter
opens for data entry.
Using the key pad, enter the current date (in any
desired format). Then, press any teminator key. The
Calibration Status Menu display changes to indicate
calibration is in progress.
When the reference calibration is complete, the Cali-
bration Menu is displayed.
External Reference Not Connected
If calibration is attempted without an external
10 MHz reference signal connected to the rear panel
10 MHz REF IN connector, the Calibration Status
Menu displays the following.
681XXC OM 3-93/3-94
LOCAL (FRONT REFERENCE
PANEL) OPERATION OSCILLATOR CALIBRATION