I am using a Keysight InfiniiVision MSOX3054T Scope having Bandwidth of 500MHz and Sampling rate of 5Gsa/s.

I am learning to use the FFT Option in the scope.

Below is the waveform I captured of my QSPI Clock signal (48MHz)

enter image description here

My start frequency is 30MHz and Stop Frequency is 60MHz.

  1. What is the threshold and the excursion levels (marked in red) mean? How to play around this value?

  2. And how is the -4.97dBV calculated (again marked in red)? Can someone show the calculation for the -4.97dBV value?

  • \$\begingroup\$ Are you using a 10X probe? \$\endgroup\$ Dec 17, 2019 at 13:36
  • \$\begingroup\$ Yes. I understood my 2nd question. Regarding the dBV. The reference is taken as 1V and the other value is taken as the average of the QSPI Frequency value \$\endgroup\$
    – user220456
    Dec 17, 2019 at 14:00
  • \$\begingroup\$ Could you help with my first question \$\endgroup\$
    – user220456
    Dec 17, 2019 at 14:00
  • \$\begingroup\$ I don't have that scope, but I'm willing to guess that Threshold is the minimum value a peak can have--if it's lower than that, it's not counted. Not sure what Excursion is, but if forced to guess I'd say that's how far the FFT has to go down in between peaks for them to be counted separately. \$\endgroup\$ Dec 17, 2019 at 14:14

1 Answer 1


Searching for FFT Peaks
5) Press Threshold and turn the Entry knob to specify the threshold level necessary to be considered a peak.
6) Press Excursion to specify the amplitude above the FFT waveform's noise floor necessary to be recognized as a peak.

Source: Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide p. 95

  • \$\begingroup\$ Or in simpler terms, threshold is the minimum absolute magnitude to count as a peak, and excursion is the minimum magnitude relative to the noise floor to count. \$\endgroup\$
    – Hearth
    Dec 17, 2019 at 15:43

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