Frequency
Modulation
Operating
Modes
The 681XXA accepts a signal from an external sig-
nal generator and provides frequency modulation of
the output signal. FM deviation is proportional to
the input voltage, with sensitivity (–6 MHz/V,
+10 MHz/V, or +20 MHz/V) selectable from a menu.
During FM mode, the YIG phase-lock loop is dis-
abled to allow for greater FM deviations. The fre-
quency accuracy and stability are degraded in this
unlocked condition.
The sweep generator has two FM operation modes—
Narrow and Wide. In Narrow mode, the FM signal
is synthesized by applying the modulating signal to
the fine tuning coil of the YIG-tuned oscillator. Nar-
row FM mode allows maximum deviations of
50 MHz.
In Wide mode, the FM signal is synthesized by ap-
plying the modulating signal to the main tuning coil
of the YIG-tuned oscillator. Wide FM mode allows
maximum deviations of 100 MHz.
Providing
Frequency
Modulation
To provide frequency modulation, first set up the ex-
ternal signal generator, then connect it to either the
681XXA front or rear panel
FM IN
connector.
Next, press
MODULATION
. At the resulting menu
display, press
FM
. The External FM Status Menu
(shown below) is displayed.
LOCAL (FRONT
SIGNAL
PANEL) OPERATION
MODULATION
ERR
This error message is displayed
when the external FM modulating
signal exceeds the input voltage
range. The message “
Reduce FM
Input Level
” also appears at the bot-
tom of the FM status display. FM is
turned off until the modulating sig-
nal is within the input voltage
range.
681XXA OM
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