3-11
LEVELING
OPERATIONS
The 681XXB generates leveled output power over a maximum range
of up to 28 dB (up to 131 dB with Option 2). Instruments with Option
15A provide leveled output power over a maximum range of up to
22 dB (up to 125 dB with Option 2). An automatic level control (ALC)
system controls the amplitude and power level of the RF output. The
operator can select the ALC mode of operation—internal, external (de-
tector or power meter), or fixed gain (ALC off). In addition, the
681XXB provides (1) an ALC power slope function that provides com-
pensation for high frequency system or cable losses, (2) a decouple
function that allows decoupling of the step attenuator (if equipped)
from the ALC system, and (3) a user level (flatness correction) calibra-
tion function that provides for calibrating out path variations with fre-
quency in a test setup.
The following paragraphs provide descriptions and operating instruc-
tions for the power leveling modes and functions. Use the Leveling
Modes menu map (Chapter 4, Figure 4-9) to follow the menu se-
quences.
Selecting a
Leveling
Mode
The ALC system is a feedback control system, in
which the output power is measured at a detector
and compared with the expected power level. If the
output and desired power levels do not equal, the
ALC adjusts the power output until they do. The
feedback signal can be provided by either the inter-
nal detector or an external detector or power meter.
Alternatively, the output power can be set to a fixed
level without using the normal feedback (ALC off).
The ALC mode menu lets you make the selection of
a leveling mode.
To go to the ALC Mode menu, first press the main
menu key
LEVEL/ALC
SELECT
At the Level/ALC Select Menu display, press
ALC Mode
. The ALC Mode Menu (below) is dis-
played.
LOCAL (FRONT
LEVELING
PANEL) OPERATION
OPERATIONS
3-48
681XXB OM
