RFmx Warning 377101: Detector Operation is Bypassed

Issue Details

I am using the Soft Front Panel for RFmx SpecAn in InstrumentStudio. I configure my analyzer to use the Peak detector type and request 1001 detector points. However, when I start the spectrum measurement, I get the warning number 377101 with the following description: Detector operation is bypassed as the requested detector number of points is greater than or equal to the number of input sample points. 

Why is this happening and how can I resolve it?

Solution

Vector Signal Analyzers or Vector Signal Transceivers rely of the Fast Fourier Transform (FFT) to sample, process, and display spectrum acquisitions. During this process, multiple settings of the instrument like the span and the RBW will dictate how many samples are feed into the FFT algorithm. In some occasions, given the settings chosen, the total samples taken by the instrument to be processed into the FFT is less than the request detector number of points. In those situations, the RFmx driver reports this warning.

To avoid it, the instrument must be configured in such a way where the settings chosen ensure the same or a greater number of points than the ones requested by the detector operation. This can be achieved by increasing the span to add more data to be sampled, decreasing the RBW to ensure a finer sampling, or increasing the sampling time to add more information to the FFT.

If the settings of the instrument cannot be changed because the measurement must adhere to a standard, then FFT padding must be used to ensure the FFT has enough points to match the points requested by the operator.  There are several key points to discuss around the FFT padding setting:

1. In order to use the FFT Padding setting, the Measurement Method setting must be configured to Normal. This is because the other option, Sequential FFT, does not support FFT Padding. By default RFmx SpecAn uses the Measurement Method setting as Normal. To verify it, go to the Instrument Settings >> Spectrum >> Advanced >> Measurement Method. 
2. Once the Measurement Method is set to Normal, we also need to disable the Cleaner Spectrum setting to be able to use FFT Padding. This is because in the Cleaner Spectrum mode, RFmx requests spectrum data from the NI-RFSA driver. That means the FFT is performed by NI-RFSA instead of RFmx, and NI-RFSA does not support FFT Padding. By default, RFmx sets Cleaner Spectrum to true. Therefore, it must be manually disabled. To do so, go to the Instrument Settings >> Instrument >> Advanced >> Cleaner Spectrum.
3. Once the above changes are done, the FFT Padding setting can be modified. To do so, navigate to Instrument Settings >> Spectrum >> FFT >> FFT Padding and change the value to anything greater than 1. By default, the driver uses -1. That tells the driver to choose the optimum padding factor for the requested FFT size. 

Finally, once important aspect to take into account is that manually adjusting the FFT Padding setting may have an impact on the measurement speed. This is because the setting is using the requested value instead of letting the driver choose the optimal value.