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Cursor jumps off Spectrum in Ascent/MAS VB when I click a Peak


QUESTION

Why is that the first time I click on a peak the cursor jumps to a point like this:



But when I click in the same place again, the cursor jumps off the chart (sometimes right off the top of the screen)?



ANSWER

The spectrum is just a 'connect the dots' approximation to a smooth curve that lies underneath. In the case of the example above, if the spectrum is 'smoothed out' you can see that the true peak is actually where the cursor is.



Here is another chart showing two peaks on top of each other, one smoothed and the other normal.



This loss of true peak amplitude is sometimes called the picket fence effect.

Regarding Ascent/MAS VB and the vb instrument

The vb instrument uses the same approximation formula when balancing, so you do not lose out in any way there. The 'smoothing out' operation requires a lot more memory so it is not performed in the instrument when recording spectra. No other vibration data loggers that we know of do this either.

If you record a time waveform in the instrument then transfer it to Ascent/MAS VB and observe it as an FFT it will be smoothed depending on the Options>Automatic FFT Options>FFT Oversampling option on the main menu.

The largest amount of error you will ever get with the Hanning Window is 17% (and you're quite unlucky to get that). With the Rectangular window it is 36% (max).