# Sawtooth waveform RMS - Multisim

Discussion in 'General Electronics Chat' started by xxxyyyba, Sep 18, 2014.

1. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
It's well known that RMS voltage of Sawtooth wave is Urms=Up/sqrt(3), where Vp is peak voltage.
Why Multisim gives me wrong RMS of Sawtooth wave? Here is my circuit and settings for sawtooth wave:

Voltmeter should measure 5V/sqrt(3)=2.886V

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Depends upon whether the waveform is DC offset or not. The waveform you have created has zero average offset which is the reason for the discrepancy.

xxxyyyba likes this.
3. ### crutschow Expert

Mar 14, 2008
12,977
3,221
Yes, the zero voltage point affects the RMS voltage. Put the 0V point at the negative peak and you should get the answer you calculated.

xxxyyyba likes this.
4. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
Sorry but I'm not sure I understood what to do for correct measurement

5. ### crutschow Expert

Mar 14, 2008
12,977
3,221
The Voltage Offset parameter must be set to -2.5V for a 5V sawtooth.

6. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
I tried it already but I still get incorrect result

Aug 7, 2012
249
2
8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
I'm not a Multisim user.
Probably because the AC measurement ignores the DC component.
Plus the scope measurement also looks to be AC coupled so we can't really tell what the generator is producing.

9. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
Ok, thanks anyway

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,377
494
Try voltage offset of 5.

Here is my sim:

xxxyyyba likes this.
11. ### xxxyyyba Thread Starter Member

Aug 7, 2012
249
2
I get correct result using function generator (I tried different values for voltage offset and every time I get same result):

12. ### Aerb New Member

Jul 9, 2014
12
4
hey dude, channel a of the oscillator is set to measure ac component. Click on dc and your problem will be solved

xxxyyyba likes this.

Jul 9, 2014
12
4

14. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Not sure why offsetting by -2.5V constrains the lowest signal point on the signal to zero V which is what the sawtooth definition requires for your original statement regarding the "well known" RMS value to be true.

15. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Depends if you are talking about a sawtooth which has or doesn't have a zero DC (or mean) value.

The RMS value of the classic sawtooth is indeed

$V_{rms}=\frac{U_p}{\sqrt{3}}$

The Mean value of the same waveform is Up/2.

If the classic sawtooth is level adjusted such that its mean value is zero then the RMS value is altered to

$V_{rms}=U_p\sqrt{$$\frac{1}{3}-\frac{1}{4}$$}=0.2887U_p$

16. ### Aerb New Member

Jul 9, 2014
12
4
Oh i love the way people approaching to situations here.

Did you really get what is wrong in xxxyyyba's problem? Did you get what the solution is? I know about voltage offsetting, sawtooth waves, rms and mean values and the calculations above are not connected to problem here. The problem is just about the usage of the oscillator. When the problem is simple, please give simple solutions.

17. ### crutschow Expert

Mar 14, 2008
12,977
3,221
And I'm really happy for you that you know all those things. But it's not good form to criticize the way other's answer questions. We all answer them the best way we know how.

18. ### vk6zgo Active Member

Jul 21, 2012
677
85
Sorry,but that isn't a sawtooth,it's a triangular wave.
OK,you can call it a special case of a sawtooth,but it is sufficiently dissimilar as to be regarded as a different waveform.
A normal sawtooth has a faster return to zero than its rise.

19. ### Aerb New Member

Jul 9, 2014
12
4
Ok, now you really blew this out of portion. But instead clarifying myself again, well, i'm outta here.

20. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello everyone

Have written and asked a lot with respect to: What value does ammeter or voltmeter Measures (RMS, Average or Peak)?.

Just as an example visit this link and see what I think is a tremendous discussion: http://electronics.stackexchange.com/questions/18368/what-value-does-ammeter-or-voltmeter-measures-rms-average-or-peak

We can say that all AC voltmeter show a reading of the RMS voltage of a pure sine wave. This excludes the called True RMS Voltmeter.
If we try to measure another type of signal which is not purely sinusoidal reading will be erroneous. according to what we expect.
And calculate the reading that should give us our AC voltmeter is not as easy as it sounds.

Now, the original question asked by xxxyyyab was: Why Multisim gives me wrong RMS of Sawtooth wave?
The question is somewhat difficult to answer.
Since the simulator for electronic circuits leave much to be desired, some more than others, until the complex to measure with a AC voltmeter a signal that is not purely sinusoidal.
most likely We can find the answer in this video: