Unexpected Spurs appearing on PXIe 5646

Updated May 21, 2018

Reported In


  • PXIe-5646


  • LabVIEW



Issue Details

 I have a PXIe 5646 VST and I am getting unexpected spurs in my signal when taking measurements from certain frequencies. The power level of the spurs changes if the SPAN or RBW changes, or if the FFT settings change from "default" to "flat top". I only see this behaviour when using Soft Front Panel, but not in IQ mode using the NI-RFSA driver.


If you are experiencing unexpected spurs in your signal, consider doing a multiple acquisition spectral sweep manually instead of allowing NI-RFSA to perform the sweep. This will allow the user more control over the sweep and minimise the digitally created spurs.
Please refer to the below example VI to manually perform the spectral sweep for spur mitigation. This example demonstrates how to perform one spectral sweep with the LO placed below the centre frequencies of acquisition and then one spectral sweep with the LO placed above those centre frequencies. Since the spurs are a result of interactions between the input signal and the 5646 LO, the spurs will move based on this change in LO placement. Comparing the two sweeps and only keeping the signals that exist in both is one method for deciphering between true RF Input signals or acquisition related spurs.  

Additional Information

When NI-RFSA performs a multi-span acquisition on devices such as the VST, it operates in a Low IF Offset mode. This helps to remove any LO leakage from being present within the desired acquisition bandwidth. However, images can still be a complication when using low IF offset. Any signals present within the instantaneous bandwidth of the device will be converted down to baseband (FLO); in addition to signals within the instantaneous bandwidth, any signals present at the RF input to the VST which do not get filtered by the band select filters have the potential to digital fold (Nyquist Aliasing) into the acquired passband.

The Band Select Filters can be seen below for reference or by looking at the RF Input Subsystem of the 5646.



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