The Designer's Guide Community Forum - Average Power Vs. RMS power

crhu

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Average Power Vs. RMS power
Dec 22^nd, 2004, 11:11pm

Hi, forum,
I am wondering what is the criteria to use average power
or RMS power in power simulation.

Acordingt to Poynting's theorem, Sav=Re(EXH), average
power should be enough for power measurement all the EM field.
However, I find that there are many places, esp. in RF ckt, RMS power
is used instead.

Any inputs are highly appreciated.

Thanks.

--crhu

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Ken Kundert Global Moderator Offline The Spectre Posts: 1442 San Jose	Re: Average Power Vs. RMS power Reply #1 - Dec 27^th, 2004, 1:13pm Charlie, Average and RMS power are the same, are they not? The equation you gave for Poynting's theorem assumed the use of phasors, which assumes sinusoidal signals. When using RMS you are allowing signals other than sinusoids to be used. -Ken
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crhu

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Re: Average Power Vs. RMS power
Reply #2 - Dec 27^th, 2004, 5:58pm

Hi, forum,

I googled "RMS average power" and here is something
giving me some hints:
http://www.dartmouth.edu/~sullivan/techniques.pdf

"More generally only components of v(t) and i(t) at
the same frequency contribute to average power"

If it is true ( I do not know how to derive it yet),
average power can not characterize the power for
non-linear circuit. Am I right?

Thanks.

--Charlie

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crhu

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Re: Average Power Vs. RMS power
Reply #3 - Dec 28^th, 2004, 2:02pm

Hi, Ken,
I went through the Poyting's Therom one more time, and
you are right that the Saverage = Re(SXconjugate(H)) is
in the format of phasor. As any periodic inputs can be decomposed into sinusoids of different frequency, the
Therom should be able to apply to all the periodic/quasi-
periodic inputs.

I do think right now that there is a hidden assumption in
Poynting's Therom: the changing E field only generates
the M field with the same frequency; so does the changing
M field, as the linear, homogeous medium is assumed.

So, seems to me that the average power is not a good
enough measurement for a non-linear system.

Thanks.

--Charlie

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