Your signal generator is equipped with an automatic level control unit to obtain best RF level accuracy.
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 deviations. These are:
Also note that ALC may detect incorrect values in multi-transmitter test setups. If multiple 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. |
The following description basically explains the ALC states and their principle of operation. In particular ALC OFF (Sample & Hold) gives an overview on the function in terms of the equipment of the generator.
- AUTO
automatically adjusts the output level to the operating conditions. - On
enables ALC permanently, regardless of the currently selected mode. - Off
deactivates ALC.
The instrument switches to Sample & Hold (S&H) state, which still allows to maintain a constant output level.
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 additional explanation. Furthermore, find the ALC state settings described in detail in "State - ALC".
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.
- an electronic step attenuator
The level is decreased by 30 dB. - a mechanical step attenuator
- no step attenuator
The signal generator outputs the set level for 3 to 5 ms after level or frequency setting during a Sample & hold measurement.
Instruments equipped with one of the options R&S SMA-B103L or R&S SMA-B106L come without step attenuator.
The level control status is permanently displayed as a status message in the info line.
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