Example - Agilent Technologies X Series User Manual

Optimizing Performance
Using the Power Meter Servo
Power Meter Continuous performs the adjustment as in Once mode, and continues to adjust the
power periodically if the value differs by more than the specified Settling Error.
Once these parameters are set, the servo loop engages and levels the DUT's output power.

Example

The following example emphasizes the importance of setting the amplitude offset, as it protects the
DUT from being exposed to too much power.
For this example, the source amplitude offset is 20 dB and the source amplitude is programmed to
25 dBm. The offset is subtracted from the programmed level, making the actual source output power
5 dBm. If the power meter measures 24.5 dBm, for example, the output power will be adjusted by
0.5 dBm since the power meter measurement is 0.5 dBm lower than the desired 25 dBm. The new
source output power is 5.5 dBm. If the power meter then measures 24.97 dBm, no further
adjustments will be made since the measured value is within half of the settling error of 0.2 dB.
If using the continuous leveling mode, the power meter readings are monitored and the output power
of the source will be adjusted if the measurement drifts outside the specified settling error.
Continuing with this example, if the amplitude offset remained at the default of 0 dB, the output
power would have been 25 dBm, resulting in a power meter measurement of 40 dBm (assuming the
amplifier already went into compression). The algorithm would have reduced the source power by
15 dB, thus outputting 10 dBm and resulting in a measurement of maybe 29.5 dBm. The additional
reduction of another 4.5 dB would have lead to the same outcome as with the 20 dB amplitude offset
(source output power of 5.5 dBm). However, in- between, the DUT was stressed possibly past its
specified operating range.
96 Agilent X-Series Signal Generators User's Guide