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In Unlocked Wide mode, frequency modulation is
accomplished by applying the modulating signal to
the main tuning coil of the YIG-tuned oscillator. Un
-
locked Wide FM mode allows maximum deviations
of ±100 MHz for DC to 100 Hz rates.
Providing
Frequency
Modulation
To provide 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
Wide FM 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 3-75
LOCAL (FRONT SIGNAL
PANEL) OPERATION MODULATION
UNLOCKED
When Unlocked Narrow FM or Un
-
locked Wide FM is selected ON, this
warning message is displayed on all
menu displays to remind the opera
-
tor that the carrier frequency is not
phase-locked.