I'm observing the noise of a voltage regulator output in AC coupling on a scope. Shown is a photo from the measurement:

enter image description here

The FFT for the Vrms shows explicitly on the screen that there is around a 6.25kHz noise on pink plot.

But on the bottom right of the screen there is another frequency shown in yellow color as f = 96.2818kHz.

What is the meaning of that frequency?

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    \$\begingroup\$ What does the oscilloscope’s manual say? \$\endgroup\$ – winny Sep 18 '18 at 13:38
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    \$\begingroup\$ You've got your 'scope set too "zoomed out" on the time scale and anything you're seeing is probably aliased to death. Reduce your time/div. It also looks to me as though the purple "6.25kHz" is telling you something about the resolution of the FFT and is not the result of a measurement. \$\endgroup\$ – brhans Sep 18 '18 at 13:42
  • \$\begingroup\$ It is not in a log scale so hard to tell. But looks like SMPS noise with some dominant switched f load . Where it comes from is needed, but 1mVrms is a good result for SMPS. My guess is 6.25kHz/cm and a spurious peak at 6kHZ \$\endgroup\$ – Sunnyskyguy EE75 Sep 18 '18 at 13:43
  • \$\begingroup\$ @brhans f doesn’t change with time scale but voltage scale \$\endgroup\$ – HelpMee Sep 18 '18 at 14:04
  • \$\begingroup\$ Scope is a Siglent SDS 1102CML \$\endgroup\$ – HelpMee Sep 18 '18 at 14:08

Final update

If you refer to manual, it says on one of its first screen shots that this frequency is

Readout shows trigger signal frequency

Previous (first totally wrong and second somewhat wrong) guesses:

Educated guess - it is range of FFT display from 0 (left) to ~15 segments * 6.25 kHz = 93.75 kHz (right). Rotating the respective knob you must be able to change the scale.


Ok, I was proven to be wrong, and looking to manual I see no direct clarification how to interpret the data.

Here's my second guess: yellow f= is not related to FFT at all, it is average, median, or whatever frequency measured by the scope using its yellow waveform. That's why f= is yellow color, not pink. How noisy signal frequency is being measured may be a big secret up to scope developers :)

Do not ask me how scope measures it; may be it takes some "prevalent" frequency of noisy signal, or it uses some AI to guess what you want displayed frequency to be... As a test - use scope's calibration output (usually of 1 kHz) and this yellow f= will show exactly 1 KHz.

  • \$\begingroup\$ No I just checked it has nothing to do with that. \$\endgroup\$ – HelpMee Sep 18 '18 at 13:54
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    \$\begingroup\$ I think @winny made a very good suggestion to look into manual and instead of guessing find out exactly meaning of this frequency. Would also be great if you share what you did to prove that I am wrong. \$\endgroup\$ – Anonymous Sep 18 '18 at 14:03
  • \$\begingroup\$ I changed the resolution of the FFT by the knob the yellow f stay same. It is not 15 x f_res(pink). \$\endgroup\$ – HelpMee Sep 18 '18 at 14:06
  • \$\begingroup\$ Updated the answer. \$\endgroup\$ – Anonymous Sep 18 '18 at 14:16
  • \$\begingroup\$ Yes correct, Im able to change vis trigger menu. I left it ser to %50 trigger. I dunno what is it used fir though besides bring Auto \$\endgroup\$ – HelpMee Sep 18 '18 at 14:31

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