The RF output power of the signal generator is constantly monitored and controlled by the ALC circuit. Its purpose is to hold output power at a desired level in spite of drift due to temperature and time.
Pulse modulation is accomplished using a modulator that precedes the ALC detector. The ALC measures the power of the pulsed signal, and can do this if the pulse width exceeds 1 µs. For narrower pulses, set the ALC Off and use the signal generator's Power Search function.
The ALC is part of the signal generator calibration process. Even when the ALC is turned off, the value for the output gain correction is held (but not updated) in a digital-to-analog converter and applied to the output. If the ALC is turned off for an extended period of time, the output calibration may drift. It is good practice to turn off the IQ modulation and press the manual calibration softkey under the Power hardkey to update the calibration about once a day.