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DETECTOR
Figure A-2. Typical External Leveling Hookup
ALC Disabled
-
Leveling Mode ALCoff , Leveling Mode Search
ALC Off. In this configuration, the ALC is disabled, power is
not sensed at any point, and therefore the absolute power level is
uncalibrated (see Figure A-l). Direct and separate control of the
RF modulator (p/o RF Components) and the attenuator is possible.
The synthesizer’s front panel indicates the attenuator setting and
a reference level. The reference level is an approximate indication
of the attenuation provided by the RF modulator . Typically the
RF amplifier that follows the modulator is saturated for modulation
levels near 0 dB. Therefore the actual change in the RF output power
will not track the indicated reference level until the amplifier is out of
saturation.
The ALC off mode is useful for applications that involve pulse
modulation with extremely narrow pulses. If the pulse is narrow
enough, the ALC may be unable to provide accurate leveling due to
bandwidth limitations.
Search.
Search mode is similar to the ALC off mode in that,
the ALC is disabled in order to remove bandwidth limitations.
The essential difference is that, when search mode is enabled, the
synthesizer searches out the appropriate modulator level such that
the RF output power after the ALC is disabled closely matches the
power prior to search mode being enabled. Specifically, when search
mode is selected the synthesizer follows this sequence of steps:
1. All modulation is disabled and the ALC system is closed to
provide a calibrated reference power.
2. The output power is measured using the internal coupler/detector.
3. The ALC system is disabled (opened).
4. While monitoring the internal detector, the RF modulator level
is varied until the detected power is equivalent to the reference
power measured in step 2.
A-8
Operating and Programming Reference
HP 8360
User’s Handbook