NI Vector Signal Transceiver Returns -Inf in Terminated Noise Floor Measurement

Updated Mar 22, 2018

Reported In

Hardware

  • PXIe-5644
  • PXIe-5645
  • PXIe-5840

Issue Details

I’m attempting to characterize the noise floor of the instrument (terminated input), but for certain combinations of Frequency Span and Reference Level, the measurement fails. Specifically, this occurs at higher Ref Level (e.g. 0 dBm) and low Frequency Span (e.g. 10kHz). However, the driver does not return an error or warning. Why does this happen and how can I avoid it?

Solution

What is happening here is that that ADC onboard 564x VST devices are not being driven by sufficient input power to output a non-zero number. When the ADC outputs a zero (or the ADC's output is coerced to a fixed-point value within the signal path), it is eventually put through a logarithm and Log(0) = -Inf. As we increase the span, the amount of power that the ADC sees for each sample increases proportionally per the definition of thermal noise (Noise = kTB) where the increasing span is increasing measurement resolution bandwidth (B).

This can be avoided by avoiding the combinations of RBW and Reference Level that cause the driver to return -Inf. It is recommended that a custom error / warning be implemented in LabVIEW in the event that a -Inf return will cause issue.

Additional Information

It should be noted that, under the same conditions as PXIe-565x, the PXIe-5840 has been shown to avoid this behavior in several low-span, high-reference level situations. While this is not exhaustive, it may provide guidance in future product selection.

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