In Unlocked Widemode, frequency modulation is
accomplished by applying the modulating signal to
the main tuning coil of the YIG-tuned oscillator.Un-
locked WideFM mode allows maximum deviations
of ±100 MHz for DC to 100 Hz rates.
Providing
Frequency
Modulation
Toprovide frequency modulation, first set up the ex-
ternal signal generator,then connect it to either the
681XXC front or rear panel FM IN connector.
Next, press MODULATION. At the resulting menu
display,press FM . The right-most External FM
Status Menu (below) is displayed. Pressing More
brings up the left-most menu.
Now,press the menu soft-key FM Modes . The third
External FM Status Menu (below) is displayed.
This menu lets you select the FM operating mode.
Press Locked LN to select the Locked Low-Noise
FM operating mode; Press Locked to select the
Locked FM operating mode; press Unlocked Narrow
to select the Unlocked Narrow FM operating mode;
or press Unlocked Wide to select the Unlocked
WideFM operating mode. The FM status display
will reflect your selection.
Press Previous Menu to return to the main Exter-
nal FM Status Menu display (next page).
681XXC OM 3-75LOCAL (FRONT SIGNAL
PANEL) OPERATION MODULATION
UNLOCKED
When Unlocked Narrow FM or Un-
lockedWide FM is selected ON, this
warningmessage is displayed on all
menudisplays to remind the opera-
tor that the carrier frequency is not
phase-locked.