Vector Signal Transceivers (VST) Fluctuation Between Different of Reference Level

Issue Details

We performing loopback test of NI VST device such as PXIe-5841 by generate CW RF signal and measure it under different reference level. 
However the power reading between different reference level is fluctuated. We expecting It should be have a trend across different reference level instead of fluctuated.

Solution

The PXIe 5841 output (NI-RFSG) has an accuracy error value, so there will be a range of possible actual output power level. After going through cable loss, the CW signal is received by the PXIe 5841 input (NI-RFSA). PXIe-5841 input (NI-RFSA) also has an accuracy error value, so there will be also a range of power level that it might read.

When changing the reference level, the internal of PXIe 5841 will undergo adjustment, resulting in differences in the power readings, which may appear to be unstable but it will still fall within the range of the PXIe 5841 error value (NI-RFSG-> cable loss-> NI-RFSA), as shown in the figure below.

As for the fact that there is no mention in specification that the power reading of the PXIe 5841 will have a trend across different reference level. As long it is still within the error value range, this phenomenon is expected.

If the difference of the power reading is greater than twice of the error of the PXIe-5841 input under conditions of good environment, configuration and effective calibration. Then, the input signal power reading is abnormal, as shown in the figure below.