# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: JimKnopf on April 29, 2019, 07:00:56 pm

Title: Rigol 1054Z Vrms Math function sqrt
Post by: JimKnopf on April 29, 2019, 07:00:56 pm
Hello,

i got a new Rigol 1054Z Oscilloscope. Setting up an Oscilloscope for the first time, i tried to measure AC Power of the Doorbell. It show me 19,2Vmax,  38,4V Peak to Peak. Vrms value is 13,4x V. Sine wave looks great.  Now i want an additional wave which displays Vrms from math function.
I thought it's the sqrt function that i can use isn't it? It just doesn't work. Shows me "NAN" but no wave.

Any Idea how i can display  Vrms wave from any Channel?
Title: Re: Rigol 1054Z Vrms Math function sqrt
Post by: barry14 on April 29, 2019, 10:27:20 pm
I believe you are trying to display a time waveform for which the amplitude is the RMS value of another waveform.  This cannot really be done on an instantaneous basis because the RMS value is a measurement over a period of time.  The M in RMS stands for mean. To find the RMS value of a waveform you need to square the waveform (hence the "S"), take the mean value of that square (which means the average over some time period which is usually at least one cycle of the waveform), and then take the square root (hence the "R") of that mean value.  For a sine wave the RMS value over one cycle is a constant (the same for every cycle) so that there is really nothing to display. If the waveform were something like random noise, then the RMS value (measured over some time period of the noise) would vary a bit with time (depending on the particular characteristics of that noise). But it would be difficult to display this variation on the same display as the noise waveform since it would consist of a series of numbers spaced at the time period selected to do the averaging.
Title: Re: Rigol 1054Z Vrms Math function sqrt
Post by: rstofer on April 30, 2019, 04:12:23 pm
But it would be difficult to display this variation on the same display as the noise waveform since it would consist of a series of numbers spaced at the time period selected to do the averaging.

You could make a moving calculation.  I wonder what happens with the integral function over a 1 cycle waveform?  Alas, we don't have the squared values.

Quote
RMS can also be defined for a continuously varying function in terms of an integral of the squares of the instantaneous values during a cycle.
https://en.wikipedia.org/wiki/Root_mean_square