Rohde & Schwarz R&S SMB100B - Page 154

Improving level performance
R&S
®
SMB100B
154User Manual 1178.3711.02 ─ 08
About ALC
ALC is an adaptive control system to stabilize the RF output level. It continuously moni-
tors 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, when pulse modulation
with short pulses is used, the ALC uses table-based level settings with table values
depending on the pulse width.
Also note that ALC can detect incorrect values in multi-transmitter test setups. If mul-
tiple generators are coupled, reverse power can affect the ALC readings, which leads
to an incorrect RF output power.
ALC states and their effects
The following description basically explains the ALC states and their principle of opera-
tion:
●
"Auto"
Adjusts the output level to the operating conditions automatically. ALC is active in
almost all operating modes.
●
"On"
Activates the internal level control permanently, regardless of the operating condi-
tions.
Note: This mode is not useful for pulse modulation with narrow pulses and low rep-
etition rates, since the automatic level control cannot settle in time. Set ALC
"Table & On" instead.
●
"Off (Sample & Hold)"
Deactivates ALC, but still allows you to maintain a constant output level.
The R&S SMB100B switches briefly in the CW mode and enables ALC to adjust
the level to the set target value. The instrument freezes this setting, turns off ALC
again, and switches back to the operating mode.
●
"Off (Table)"
Deactivates internal level control loop. The instrument calculates every level setting
using the attenuation values from the internal (ALC) table. Level attenuation
between two table values is done with linear interpolation. This mode enables you
to perform a fast and strictly monotonic, but not quite as precise level adjustment.
●
"Table & On"
Starts with the associated value of the internal level table and then activates the
automatic level control. This mode achieves maximum level accuracy and fast set-
ting times.
If pulse modulation is applied, this mode ensures fast level settings even with nar-
row pulses at low repetition rate.
The R&S SMB100B displays the level control setting as a status message in the info
line.
Automatic level control (ALC)