The VXG signal generator has a distributed level control system with level detection possible in three modules or locations, depending on the requested power, frequency, or mode of operation. Level detection and control in the source output is used for frequency bands between 1 MHz and <19.5 GHz. Level detection and control in the upconverter is used for frequency bands from >19.5 GHz to 44 GHz. Level detection in the amplifier is used for detection from 1 MHz to 44 GHz and provides supplemental power level information for the total system.
When in CW/pulsed operation, automatic level control (ALC on) is a hybrid of both open-loop and closed-loop operation, depending on the requested power level:
For CW frequencies ≤ 3200 MHz the amplifier is switched in when the power is ≥ +10dBm.
For CW frequencies > 3200 MHz the amplifier is switched in when the power is at -3dBm.
In this high power mode, the system will not compensate for ambient temperature changes unless the frequency or amplitude is changed.
When in IQ mode of operation, the ALC should be disabled and the power search routine used to set power (This avoids possible signal distortion, especially for wideband noise like IQ signals). Power search is a routine that uses the set or calculated rms (sqrt(I^2+Q^2)) value of the IQ data to create an equivalent DC drive voltage through the IQ system. You can manually set this value via the front panel GUI or SCPI. If you do not set this value, it will automatically be calculated at load time by the source firmware. The DC equivalent voltage is created by the baseband IQ system via the IQ DACs and results in a CW tone at the set frequency and power requested for a small duration of time. Depending on the requested power level and frequency the appropriate level detection scheme will be used:
For IQ mode, the switch point is based on the system lookup table, and varies depending on set frequency, power level, and waveform bandwidth.
When operating the VXG signal generator in dual-channel coherent mode of operation with synchronized baseband IQ signals, the power-search routine can still be executed without disrupting the time synchronization of channel one vs. channel two. However, when ALC and power search are in Auto mode, there will be a momentary switch from IQ data to equivalent rms DC CW operation, if the requested frequency or power level is changed. The transition from IQ data to equivalent rms DC CW operation is seamless in time. To avoid this CW to IQ transition, when changing power, you can use the Baseband Power control feature. This feature uses digital-baseband IQ scaling to tune power in dB, relative to absolute set power. It is good for approximately a 20-dB power adjustment range. After this point, signal-to-noise ratio degradation will occur, ultimately degrading signal fidelity.
This section describes the ALC system's modes of control.
Sets the detector feedback loop bandwidth, depending on the modulation type running on the system, to reach the requested absolute output power. Very narrow loop bandwidths (Very Slow) cause the leveling loop to pass rapidly changing amplitude levels, but still corrects for slow amplitude drift due to temperature variations or other effects over longer durations of time. Slow or Medium increases the loop bandwidth to partially correct/level for rapidly changing amplitude fluctuations. Fast increases the loop bandwidth, providing rapid correction of rapidly changing amplitude fluctuations. For IQ modulation, the ALC and Power Search should always be set to Auto.
Very slow – use for CW or modulation with low rate amplitude variation
Slow or Medium - use for CW or modulation without amplitude variation
Fast – use for CW, FM/PM and pulse modulation
Configures the ALC Hold operation for the indicated channel. The hold operation allows the instrument to use the ALC with a signal that has significant power variation in time; a marker signal is used to indicate when the signal power is stable. This allows the ALC to track the signal when requested. When modulation power is not stable, the ALC is placed into hold mode so the signal does not become distorted. The ALC must release from hold mode within 100 ms; if ALC is configured to hold or track on a trigger, it is important that the trigger arrives within 100 ms. Refer to ALC Hold for trigger source configuration.
ALC Hold can be used in at least two different ways:
When CW Pulse Modulation (PULM) is enabled, ALC Hold will be automatically enabled during pulse off time. In essence, the ALC Hold will track the pulse modulation envelope duty cycle during the off periods. This ensures that every CW pulse is properly leveled. The off period must be <100 ms, otherwise you must turn ALC off.
When playing Pulsed CW IQ waveforms and a marker file is played in concert with the IQ data, where the marker file represents the on/off duty cycle of the IQ pulse data, if the marker output is routed internally to the ALC hold function, then the IQ data can be properly leveled during the on time of the IQ pulse. The off period must be <100 ms, otherwise you must turn ALC off.
Enables or disables ALC On and ALC Bandwidth couplings. Setting ALC On, ALC Bandwidth, or ALC Hold Mode turns this parameter off. For IQ modulation, the ALC and Power Search should always be set to Auto.
Enables or disables the automatic leveling control (ALC) circuit on the indicated channel. The purpose of the ALC circuit is to hold output power at a desired level by adjusting the instrument’s power circuits for power drift. Power will drift over time and with changes in temperature. For IQ modulation, ALC and Power Search should always be set to Auto.
Executes a single power search calibration at the current RF output frequency and amplitude setting.
This button is equivalent to [:SOURce][:RF<channel>]:POWer:ALC:SEARch ONCE
Power search is a calibration routine that improves level accuracy with ALC off. The instrument microprocessor momentarily closes the ALC loop to find the modulator drive setting necessary to make the quiescent RF level equal to an entered value, then opens the ALC loop while maintaining that modulator drive setting. When executing power search, RF power will be present for typically 10 to 50 ms. Power search can be configured to operate either automatically or manually at the carrier frequency. Power search may not operate above the maximum specified output power.
Toggles Power Search Mode on or off. When enabled, the power search calibration routine is executed whenever an instrument setting is modified that affects RF output power. This includes changes to frequency, amplitude, and modulation. For IQ modulation, ALC and Power Search should always be set to Auto.
This button is equivalent to [:SOURce][:RF<channel>]:POWer:ALC:SEARch ON|OFF