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Playback Unscaled RF Data at the Same Power Level

Primary Software: Driver Software>>NI-RFSG
Primary Software Version: 1.3
Primary Software Fixed Version: N/A
Secondary Software: N/A

After acquiring and saving unscaled data with my Vector Signal Analyzer (VSA), I do not regenerate the signal at the same power level with the my Vector Signal Generator (VSG). Why is this?

The VSA always tries to use the entire dynamic range of the ADC and adjusts the gain of the input RF signal chain to try and make this happen.  This issue is already taken care of or your when working with floating point, scaled IQ data using NI-RFSA.

When using binary data, you must do the following to ensure the signal being regenerated out of the VSG is at the same power level at which it was acquired.
  • Save the gain attribute from the RFSA Fetch IQ (1D I16).vi to file.
  • Calculate the required power level attribute setting for the RFSG Configure using the saved gain value.

When you record the data with the VSA, you will need to save the gain attribute to file to use in the generation example to properly scale the data.  If you do not save the gain value, you will lose all reference of the actual power level of the acquired data.  You can save the gain value from the wfm info output from the niRFSA Fetch IQ (1DI16).vi, as shown below.

When regenerating the data, the power level of the VSG needs to be calculated to regenerate the data at the same level at which it was recorded. You may use the VI Snippet below to calculate the required power level input for the niRFSG Configure The formula to set the power level is: 10 + 20 * log (gain * 32767).

A +30 dBm/1 W signal would be attenuated in the RF chain to bring the signal power down to the input range of the ADC. A -30 dBm/1 µW signal would be amplified in the RF chain to bring it up to the (full scale) input range of the ADC.

In both cases, you could theoretically end up with binary data from the ADC that is the same, even though the actual power levels differ by 106.  Without the additional information of the RF input signal chain gain, you will no longer know what the true signal power of the acquired data.

Related Links:
Product Manuals: NI RF Signal Generators Help
Product Manuals: NI RF Signal Analyzers Help
Products and Services: NI Vector Signal Generator
Products and Services: NI Vector Signal Analyzer


Report Date: 08/22/2007
Last Updated: 10/06/2017
Document ID: 4CLG6TY5

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