3-14
LEVELING
OPERATIONS
The 681XXA generates leveled output power over a range of up to
28 dB (up to 135 dB with option 2 step attenuators installed). It uses
an automatic level control (ALC) system to control the amplitude and
power level of the RF output. The 681XXA lets the operator select the
ALC mode of operation—internal, external (detector or power meter),
or fixed gain (ALC off). In addition, it provides an ALC power slope
function that provides compensation for high frequency system or ca-
ble losses and a decouple function that allows decoupling of the step at-
tenuator (if equipped) from the ALC system. The following paragraphs
provide descriptions and operating instructions for the power leveling
modes and the ALC power slope and decouple functions. Use the Level-
ing Modes menu map (Chapter 4, Figure 4-12) to follow the menu se-
quences.
Selecting a
Leveling
Mode
Output power leveling is controlled by the ALC sys-
tem. It is a feedback control system, in which the
output power is measured at a detector and com-
pared with the expected power level. If the output
and desired power levels do not equal, the ALC ad-
justs the power output until they equal. The feed-
back signal can be provided by either the internal
detector or external detector. Alternately, 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
CONTROL
At the Level/ALC Control Menu display, press
ALC Mode
. The ALC Mode Menu (shown below) is
displayed.
LOCAL (FRONT
LEVELING
PANEL) OPERATION
OPERATIONS
3-56
681XXA OM
