Configurable RBW Values and Practical Considerations for NI Vector Signal Analyzers

Issue Details

What are all the RBW (resolution bandwidth) values available for a Vector Signal Analyzer (VSA)?

Solution

NI VSAs allow you to set the RBW to any value less than the sampling rate. For example, with the PXIe-5842 VST (Up to 2 GHz BW), you can theoretically set any value from 1 Hz to the sampling rate of the same model, 2.5 GHz. However, you should be aware of the following points:

[Notes]

• Actual RBW value may not match the set value <br>The RBW value adopted by the hardware driver does not necessarily match the value set by the user. This is because the RBW value is determined discretely in relation to the number of elements used in the FFT (Fast Fourier Transform) calculation (FFT size).
• The lower the RBW value, the longer the measurement time .<br>The lower the set RBW value, the larger the FFT size and the longer the measurement time.
• Increased memory usage and potential for errors
  Increasing the FFT size also increases memory usage, which can lead to memory overflow errors.
• Differences in RBW step width depending on frequency band <br>The minimum step width of the RBW value that can be set varies depending on the frequency band used.

Verification example: RBW setting using PXIe-5842 VST

We verified the relationship between the RBW setting value and the RBW value actually adopted by the driver under the following conditions.

• Hardware: PXIe-5842 VST (Up to 2 GHz BW)
• Center frequency: 10 GHz
• Bandwidth: 1.5 GHz
• Program used: RFSA Getting Started Spectrum.vi (example program included with the RFSA driver)

The verification results under these conditions are as follows. It can be confirmed that the "Set RBW value" is not the "Actual RBW value." Also, when the RBW was set to 10 Hz on the program, the FFT size became very large and a memory error occurred.

[Verification results]

Also, under the same conditions, when the RBW value was swept in 1 kHz increments from 1 MHz, the relationship between the "Set RBW value" and the "Actual RBW" was plotted as shown in the figure below.