Operation
Model 8642A/B
Special Functions (cont'd)
DETAILED DESCRIPTIONS OF ALL SPECIAL FUNCTIONS
0. Special Functions 4 through 9 Off. This special function turns off Special Functions 4 through 9.
(Sometimes all these special functions will not be turned off if the instrument state is dependent on a
special function. A message will be displayed to notify you if a special function remained on after you
selected Special Function 0.) Special functions can also be turned off individually. (Refer to page 3-169.)
3.
Service Mode. See Manual. This special function causes the Signal Generator to enter its service
mode. Refer to page 3-201 for information about writing messages to the display, or refer to the Service
Manual or the On-Site Manual for information about servicing the instrument. Select any function to
exit this mode if this key was accidentally selected.
4.
ALC Off. This special function opens the output leveling loop to provide (typically) a 5 dB
improvement in third order intermodulation at a 2 kHz offset. Typical output level accuracy is ±2 dB
when ALC Off is selected.
5.
External Low Rate FM On
112.
Internal + External Low Rate FM On
These special functions typically provide 0.4 Hz to 200 kHz external, ac-coupled FM with the Signal
Generator remaining in a phase locked state. Input coupling required for these functions makes the
Signal Generator especially sensitive to dc voltage fluctuations. Therefore, relatively small dc voltage
changes can cause the Signal Generator to lose phase lock, requiring one to two seconds to regain the
locked condition. Special function 5 or
11
2 is turned off when another FM modulation source is selected.
6. FM Pre-emphasis On. Special Function 6 can be selected to pre-emphasize internal or external FM
modulating signals with a 7 50 us time constant. (Pre-emphasis boosts high frequencies in the modulating
signal prior to modulating the carrier.) Refer to Modulation, FM, page 3-113.
7.
Low Distortion FM/'i'M On. This special function switches shaper circuitry into the FM/fcM loop,
reducing total harmonic distortion. Refer to either Modulation, FM, page 3-115, or Modulation, <J>M,
page 3-
1
24.
8. Prefer Heterodyne Band. This special function causes the Signal Generator to operate in the
heterodyne band (at RF frequencies below 132.187501 MHz) regardless of FM or 4>M deviation or Phase
Continuous Frequency Sweep Span. Refer to RF Frequency, page 3-39.
9. Attenuator Range Hold On. When Attenuator Range Hold is selected, the Signal Generator's
output attenuators are fixed and the output ALC loop controls signal amplitude. This produces a 30 dB
range with monotonic, step-to-step amplitude increments. Refer to Amplitude, page 3-74.
100.
All Special Function Codes Less than 200: Off. This special function turns off special functions 4
through 9,
1
11 through 119, 121 through 123, 134, and 135. (Sometimes all these special functions will
not be turned off if the instrument state is dependent on a special function. A message will be displayed
to notify you if a special function remained on after you selected Special Function 100.) Special
functions can also be turned off individually. (Refer to page 3-169.)
111.
Disable the Automatic Special Function Verification Display. This special function disables the
automatic display of the special function description that normally occurs after each special function is
selected.
3-172
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