Instrument Function
R&S
®
SMB100A
152
Operating Manual 1407.0806.32 ─ 21
ALC is active in almost all applications by default. However, the
Pulse Modulation
mode excludes ALC, as the control loop would detect incorrect values and result in
level deviations.
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 explains the ALC states and their principle of opera-
tion. In particular
ALC OFF (Sample & Hold)
gives an overview on the function in
terms of the equipment of the generator.
The R&S
SMB 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.
The instrument switches to
Sample & Hold (S&H)
state, which still allows to main-
tain a constant output level.
The following section explains the functionality of "Sample & Hold", to provide an over-
view 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
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 behavior 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
By default, the mechanical step attenuator is not switched during S&H cycles to
optimize the settling time. The instrument provides the output power for 3 ... 5
ms.
However, you can affect the attenuation at the output by the setting "RF during
Power Search" to "Minimum", see
. Then the gen-
erator decreases the level by 30
dB with the mechanical attenuator. Note that this
may take a certain period of time.
High frequency instruments, such as the R&S
SMB with one of the high frequency
options R&S
SMB-B120 or R&S
SMB-B140, are equipped with a
mechanical step
attenuator.
●
no step attenuator
RF Block
