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[METER] (HP-IB:
A3) selects external power meter leveling of the HP 8340B/41B. A portion of the
RF output must be measured by a power meter, with the power meter also connected to the EXT
INPUT BNC connector to form an output-input feedback loop (typical bandwidth 0,7 Hz). Press
[METER], which will cause (after a brief delay) "ATN: —xx dB, REF: —xx.xx dBV" (where x is the
last-entered value) to appear in the ENTRY DISPLAY. Use the rotary [KNOB] or the numerical keys
with the [dB(m)] terminator key to set the REFerence level, and the [STEP] keys to set the ATN while
watching either the POWER dBm display (allowing for coupler or splitter losses) or the power meter.
The "Externally Leveling the HP 8340B/41B Synthesized Sweeper'' Operating Guide located at the
end of this section fully explains power meter leveling.
[SHIFT] [METER] (HP-IB:
SHA3) bypasses the ALC (automatic leveling control) to allow direct con¬
trol of the linear modulator circuit. This is useful when very narrow pulses are being generated in pulse
modulation mode. In this mode there is no limit on the minimum pulse repitition frequency. Press
[SHIFT] [METER], and "ATN —xx dB, mod; x.x dB'' (where x is the last-entered value) will appear in
the ENTRY DISPLAY. To set the power, place the HP 8340B/41B in CW mode, or in pulse modulation
mode with pulses wider than 2|isec. Then use the [STEP] keys to set the ATN (attenuator), and the
rotary [KNOB] or numerical keys with [dB(m)] terminator key to set the MOD (linear modulator), as
follows: Set MOD entry at 0 dB, increment ATN until the POWER dBm display shows a level 5 dB to 15
dB higher than the desired output power, then reduce the power to the desired level by changing the
MOD value. The POWER dBm display shows actual power when the HP 8340B/41B is in CW or wide-
pulse pulse modulation modes; this actual power changes very little as the pulse width is narrowed,
even though the POWER dBm reading drops. Therefore, at this point reduce the pulse width to the
desired value and ignore the POWER dBm display. The ATN and MOD values in the ENTRY DISPLAY
also have a limitation: Although the ATN displayed value is always accurate, the MOD becomes
saturated in the top 10 dB (approximately) of its range at which point no change occurs in the true
power, furthermore, the modulation entry is only approximately calibrated. Consequently, rely on the
POWER dBm display for the true power level instead of the MOD value. See Figures 3-14 and 3-15 for
additional pulse modulation information [SHIFT] [METER] can also be used as a diagnostic function
for the ALC circuits, as described in Section VIII (Service).
Figure 3-16.
Leveling Keys/Input (2 of 2)
Operating Information
HP 8340B/41B
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