Vector Signal Transceivers (VST) Single Tone Generation: Fluctuations on Power Levels Generated by the Signal Generator

Issue Details

We're using VST to generate CW RF signals at a specific power level, and frequency using the NI RFSG example named RFSG Getting Started Singe Tone Generation.vi. Upon capturing the output of the VST, there are fluctuations (swings) on the VST's output power. What could we do to eliminate or minimize these fluctuations?

Solution

These fluctuations are caused by a leakage from the local oscillator (LO). To fix this, you could use the niRFSG property, Upconverter Frequency Offset (Hz), to keep the local oscillator (LO) leakage at a determined offset from the generated RF signal.
To ensure output power level consistency with the Output Power Level Accuracy values specified on each device's specifications, it is recommended to set an offset of 3.75 MHz from the configured RF frequency.

Note: Should you require greater offsets, consider targeting your offset to be within your desired RF ± [Instantaneous Bandwidth(IBW) / 2] for more accurate CW signal generations. You may refer to the specifications sheet of your VST for the instantaneous bandwidth for each frequency band. Below is an example IBW table from PXIe-5646's specifications sheet:
 

Additional Information

The transmitter architecture of NI Vector Signal Transceivers uses a direct (homodyne) or zero-IF upconverter, which upconverts the baseband signal from DC to RF at a configured LO frequency. Although there are several major advantages of using this architecture, there are challenges as well that may be needed to overcome. One of which is the local oscillator (LO) leakage.

The LO leakage refers to unwanted signals from the LO leaking to the mixer's input and output ports. The energy leaked on the baseband signal input of the mixer can be ignored as it could be filtered out by the RF components located after the mixer output. However, the LO's leaked energy on the mixer's output is troublesome as it could not simply be filtered out, doing so may end up filtering out your desired signal as well.
Offsetting the LO frequency from your desired RF frequency prevents LO leakages on output port of the mixer from interfering with the desired RF signal, just like the example below (3.75MHz LO offset). For more information on LO Leakage, you may refer to the external link on the Related Links section of this KB.
Note: As the VSTs' receiver also have the same architecture, it is advisable to set a value as well for the Downconverter Frequency Offset property when using the VST's signal analyzer.