Correct output power is handled by the signal generator ALC or by the signal generator power search function.
In many cases it is possible to leave the Signal Generator ALC enabled to get accurate output power. One or both of the and the properties must be selected to prevent the ALC from trying to level during the pulse off time. Enabling the prevents the ALC from trying to level individual pulses whose power scale values are not zero.
In general the ALC needs hundreds of milliseconds to level. This could translate into thousands of pulses depending on the pulse widths. So if you need accurate output power on the very first pulse or if the pulse width is less than the minimum leveled width supported by the signal generator, the ALC must be disabled.
The ALC is automatically disabled when downloading signals where all of the pulses are less than the minimum leveled pulse width supported by the signal generator, or when Antenna Scans have been enabled.
The ALC may be turned on by manually selecting and doing a play. However, enabling may not provide the desired results and could cause the output power to be higher than expected.
When the ALC is disabled, accurate output power is achieved using the signal generator's power search function. To provide accurate output power, the signal generator needs to know the RMS voltage of the input signal provided by the Arb.
Pulse Building includes two properties to provide accurate output power when the ALC is disabled:
- this system behavior property executes a power search on play when the ALC is disabled. It is available on the dialog box tab ( > ).
- this signal property automatically keeps track of the signal RMS voltage, which is pattern specific. It is available on the Advanced Pattern Library form.
The initial Pulse Building waveform has an average level of 1 volt (1 VRMS). This level can be affected by the following :
Type of intra-pulse modulation
Pattern item Power Scale value
Type of Antenna Scan
Waveform Corrections
Arb waveform predistortion
Arb output voltage
I/Q Gain Balance value (Implemented using Arb output voltage)
Maximize DAC value
The application automatically adjusts the waveform RMS voltages to account for all of the items listed. The adjusted value is what is loaded into the signal generator for the power search reference.
You can override the initial waveform RMS voltage. The value you enter manually is not put directly into the signal generator but is subject to all of the same adjustments discussed above.
The maximum RMS voltage allowed is 1.4142 V. The minimum RMS voltage allowed is 1 mV. However, the power search operation will typically fail when the RMS voltage is less than 100 mV. The application clips the RMS voltage between 100 mV and 1.4142 V. When the RMS voltage exceeds these limits the application uses the signal generator Amplitude Offset property to correct the actual output power.
When the RMS voltage is greater than 1.4142 V the power search reference level is set to 1.4142 V and the following adjustment is made through the Amplitude Offset.
Power Adjustment in dB = 20 * log10( 1.4142 / Waveform RMS Voltage )
When the RMS voltage is less than 100 mV the power search reference level is set to 100 mV and the following adjustment is made through the Amplitude Offset.
Power Adjustment in dB = 20 * log10( Waveform RMS Voltage / 0.1 )
The power adjustment is added to any user-specified Amplitude Offset.
If the RMS voltage is greater than 1.4142 V it is assumed that the initial maximum absolute waveform values were less than 1.0. This can occur when antenna scans are enabled or when pattern item values are set to less than 0. In these cases the I/Q waveform is normalized between ±1 to provide the best dynamic range of the DACs in the AWG. The normalization adds power to the signal that must be canceled with the Amplitude Offset.
If the RMS voltage is less than 100 mV it means that the I/Q waveform has a large crest factor (peak to average ratio). This can occur for some types of intra-pulse modulation or when using Custom Profile or Custom I/Q pulses. In these cases using 100 mV for the power search implies that the waveform has more power than it really does. The Amplitude Offset is adjusted to make up for the power that is not really in the waveform.
If power search fails when the manual power search reference level has been set to 100 mV there is not enough gain in the signal generator for the Amplitude requested. Lower the Amplitude level and execute play again.