RF Measurement Unit Values

[Trebaol]

I am trying to process data collected with a PXIe-5663 and I cannot find any information explaining what the unit values are for the UINT16 values that are recorded.  I need to convert the measured data to dBm, uV, or mW.  Can anyone point me to documentation that specifies what the conversion factors are.

[Andy_Hinde]

Hello Trebaol,

I assume you are recording or saving the data from a 5663 in binary format to disk.

I recommend looking at the example 'RFSA Stream Binary IQ Data to Disk.vi' located at:

 C:\...\<LabVIEW>\examples\instr\niRFSA

This example shows how to convert between binary and scaled data formats. As for units, the units of the scaled I and Q data in this example are Volts.

Please reference 'RFSA Power vs Time (Zero-Span).vi' located in the same path for how to convert to units of dBm.

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

[Trebaol]

Ande,

We are streaming data from the PXIe-5663 to a RAID drive and then post-processing the data in Matlab. 

I took a look at the files you suggested and it looks like the units should be in dBm unless I am missing something.  Towards the end of the VI the data is corrected by adding the reference level which is also in dBm, so I assume that Labview at least operates with those units.

I was also curious about the how to create a scaling factor for the binary data.  Does all 1's equate to the reference level and all 0's equate to ~98 dB below that?  Does it scale linearly?

Thanks

[Andy_Hinde]

Hi Trebaol,

The units of the 'I data from File' and 'Q Data from File' indicators in the 'RFSA Stream Binary IQ Data to Disk.vi' example are in units of Volts, not dBm. 

The example  'RFSA Power vs Time (Zero-Span).vi' shows how to convert complex IQ data, which is what is returned from the NI-RFSA Fetch subVI, from units of voltage to units of power - in this specific case units of dBm.

I don't see what you're referring to when you say "Towards the end of the VI the data is corrected by adding the reference level which is also in dBm" in either of these two examples.

The scaling question you ask is answered in the example 'RFSA Stream Binary IQ Data to Disk.vi'. The scaling of the binary I16 samples to scaled voltage values depends on the 'gain' value present in the wfm info cluster output of the NI-RFSA FetchIQ subVI. This setting changes depending on different analyzer settings so to convert form binary to sclaed voltage values, you need to save the gain setting from the original acquisition - something that is demonstrated in the aforementioned example.

Notice in the example near the end of the code, to the right of the While loop, where the I16 array from the 'Read from Binary File.vi' subVI is multiplied by the 'Gain' setting stored in the file in the original acquisition inside the While loop. The offset value is then added to it, producing I and Q waveforms of scaled voltage (not dBm) values.

Regards,

Andy Hinde

RF Systems Engineer

National Instruments