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POWER SEARCH
Unleveled mode may also be accessed by pressing [SHIFT] [INT]. The ENTRY DISPLAY now reads
"POWER SEARCH_dBm" and the "INTERNAL" annunciator comes on. The power reading that
appears is the same as the internal leveling power, and it can be changed via keypad, [KNOB], or
[STEP] keys. In this mode, the instrument is unleveled as in the [SHIFT] [METER] mode, but the
microprocessor automatically searches out the correct power level. This search occurs with each
power entry change or frequency change, and' requires about 200 ms. Once the search is completed,
the instrument behaves exactly as with [SHIFT] [METER], In this mode the attenuator is set automati¬
cally. For decoupled operation while unleveled, the [SHIFT] [METER] mode must be used.
EXTERNAL LEVELING
In externally leveled operations, the output power from the HP 8340B/41B is detected by an external
sensor. The output of this detector is returned to the HP 8340B/41B's leveling circuits, and the output
power is automatically adjusted to keep the power constant at the point of detection. Figure 3-32
shows a basic external leveling arrangement. The output of the detected arm of the splitter or coupler
is held constant, if the splitter response is flat, then the output of the other arm will be constant also.
This arrangement offers superior flatness over internal leveling, especially if long cables are involved.
For best flatness a good resistive splitter with power meter detection should be used.
HP 8340B/41B
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EXT
INPUT
RF
OUTPUT
LEVELED OUTPUT
POWER
SPLITTER
(OR DIRECTIONAL
COUPLER)
CRYSTAL DETECTOR
(OR POWER METER)
Figure 3-32.
Typical External Leveling Hookup
Reference to Figure 3-29 inciates that when externally leveled, the power level feedback is taken from
the external detector input rather than the internal detector. This feedback voltage is what the loop is
trying to control. For a given ALC input, the loop will adjust its output until the feedback is, for instance,
10 mV. The type of coupler or detector has no influence on this — if the loop is able, it will drive the
feedback to the requested level. Since there is no inherent relationship between ALC input and the
amount of externally leveled RF power, the power level ENTRY DISPLAY shows the feedback voltage
which the loop is seeking. This voltage is called the reference voltage and is displayed alongside the
attenuator setting as ATTN:_dB, REF:_._dBV. The entry units are dBV (dB relative to
1 volt), where 1.00 V = OdBV, 0.1 V = —20 dBV, etc. As with the decoupled mode of internal leveling,
the REF is set with the keypad or [KNOB] while the attenuator is set with the [STEP] keys. The
attenuator will normally be left at 0 dB. Its use is described below.
3-104
Operating Information
HP 8340B/41B
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