FFT in oscilloscope measuring Frequency spectrum

Thread Starter


Joined Oct 26, 2017
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)

My questions :

  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?



Joined Oct 8, 2019
The Threshold and Excursion settings are only relevant for Search of frequency peaks.
You need to slow the timebase and then adjust the Hz/div in FFT mode for more meaningful results.
Amplitude levels are dependent on signal source impedance and connection type.

If you have a sig gen or AWG use that to help get your head around FFT.