Instrument Function
R&S
®
SMBV100A
179Operating Manual 1407.6062.32 ─ 08
Automatic Level Control (ALC) is an adaptive control system to stabilize the RF output
level. It continuously monitors the current level and adjusts it to keep a steady state over
temperature and time.
ALC is active in almost all applications by default. However, some operating modes
exclude ALC, as the control loop would detect incorrect values and result in level devia-
tions. These are:
●
Pulse modulation
●
I/Q modulation
Also note that ALC may detect incorrect values in multi-transmitter test setups. If mul-
tiple generators are coupled, reverse power may affect the ALC readings. Based on
incorrect values, ALC would have an impact on the signal to intermodulation ratio.
ALC States
The following description basically explaines the ALC states and their principle of oper-
ation. In particular ALC OFF (Sample & Hold) gives an overview on the function in terms
of the equipment of the generator.
The R&S SMBV offers the ALC states:
●
AUTO
automatically adjusts the output level to the operating conditions.
●
On
●
enables ALC permanently, regardless of the currently selected mode.
●
Off
deactivates ALC.
To maintain a defined output level in Off mode, you can choose either Sample &
Hold (S&H) status or Table.
– "S&H"
activates ALC briefly when frequency or level settings change.
– "Table"
uses predefined values from a table to adjust the level at changes.
The following section explains the functionality of "Sample & Hold", to provide an overview
and to indicate what is to be considered. "On" and "Auto" require no additionial explan-
ation. Furthermore, find the ALC state settings described in detail in State - ALC.
ALC OFF (Sample & Hold)
In "S&H" mode, the signal generator switches for a short period of time into CW mode
and activates ALC. ALC adjusts the level to the set value and the generator holds the
value (freeze). Then, the generator switches ALC off again and back to the operating
mode.
RF output behaviour during Sample & Hold depends on the configuration of your instru-
ment. Instruments equipped with...:
●
an electronic step attenuator
The level is decreased by 30 dB.
●
a mechanical step attenuator
RF Signal and Analog Modulations - RF Block
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