in this narration of mine about a beginner's approach to an unknown digital oscilloscope (which I hope will be useful for future beginners), I came to the measurements; I was curious about the Stdev (standard deviation) measurement.
I saw this video from Dave:
But I'm still not a first level Nerd, not to mention that I followed the translated subtitles ...
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/xcool.gif.pagespeed.ic.cOxV6sPNRd.png)
Let's see if I understand: Stdev in practice would be a value that we can use when the signal we are measuring has a dc Offset: the RMS measurement is wrong (due to the offset) while the stdev value does not take into account the dc offset, and there is a more truthful RMS value.
I have attached an image:
-in the upper test the signal has no dc offset (stopped by the ac coupling) and we can see that the oscilloscope guesses the RMS value (in this obvious case it also coincides with the stdev value).
-in the lower test the signal has a dc offset of about 1.5v, we can see that the oscilloscope misses the RMS value (2.15v, therefore higher ..), while the detected stdev value is 1.51v, therefore very close to the right RMS value.
In conclusion we can say that it is better to use the stdev value as the RMS value?
![Azn ^-^](https://www.eevblog.com/forum/Smileys/default/azn.gif.pagespeed.ce.7t5xC2ZH_H.gif)
I add: it seems that with a signal with dc offset, the stdev value indicated is the RMS one, while where the RMS value indicates it corresponds to the measurement AC + DC = [sqrt (ac * ac) + (dc * dc)]