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FAQ 118-2002-en

I am using the spectrum analyser to measure a digital signal.

I have set it to:

  • RBW=300KHz
  • VBW=300Hz
  • SWP=2sec

It seems that the VBW has an effect on the digital level value.

Are you sure that the VBW must be set at 300Hz?

I have done this before, and I have not included this value.




-->The resolution bandwidth is not important when measuring digital signal levels.

The only thing you must do is apply a correction factor according to the resolution bandwidth selected.

This factor is calculated by using the following formula:

Real level = analyser reading + 10log (7800000/RBW)

Therefore, the correction factor is:

  • 14 dB for 300 KHz
  • 19 dB for 100 KHz

However, in order to be able to see the intermodulation products of the digital signal on the analyser screen, the maximum resolution bandwidth must not exceed 300 KHz.

The video filter is a function of the spectrum analyser being used.

In practice, the measurement can differ depending on the video filter that is being used:

If a wide video filter is used. the reading can be a couple of dBs higher than the real value.

The best settings for the video filter are:

  • 1 KHz or
  • 300 KHz.

The sweep time SWP is a parameter that is selected automatically by the spectrum analyser depending on the previous settings.

It never affects the measurement results.

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