Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
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RMS Value - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
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Post subject: RMS Value Posted: Wed Apr 29, 2009 6:34 pm
Joined: Tue Sep 04, 2007 7:54 am
This is a simple one.
Just consider two
sine waves each with 1V peak value. Assume 1ohm resistor. The RMS value
of each sine is 1/sqrt(2) and the average power is 0.5W.
consider the signal which sum of the above sine waves. This will be
a sinewave with 2V paek and average power of 2W. But the individual
signls have only 0.5W so the sum should be 1W.... where am I missing?
Post subject: Re: RMS
ValuePosted: Wed Apr 29, 2009 11:29 pm
Sun Jan 11, 2004 4:44 pm
formula is as you know P=V^2/R
So you see the power is a function
of the square of the voltage. A 2x factor in voltage yields a 4x (2^2)
increase in power. That's why you go from 1/2 W to 2 W (1/2 x 4 = 2).
You are thinking of the two independent 1 Vpk sinewaves across two
independent 1 ohm resistors. It might not seem intuitive that impressing
2 Vpk across a single 1 ohm resistor would dissipate 2 W when individually
(1 Vpk and 1 ohm, twice) they would dissipate only 1 W total, but that's
Ohm's law (and real life). The two scenarios simply are not equivalent.
Post subject: Re: RMS ValuePosted: Thu
Apr 30, 2009 8:50 am
Joined: Tue Sep 04,
2007 7:54 am
Thanks for the
One morer question on this.
I have a two signal generator
set to 1Ghz and are connected through a combiner to the power meter.
Each of the signal generator levels are adjusted so that each signal
alone measures 0dBm on the power meter. Now, I turn on both signal generators.
As per your analysis, the power meter should 6dBm.... it reads only
Looking forward for your comments.
Post subject: Re: RMS ValuePosted: Thu Apr 30, 2009
Joined: Sun Jan 11, 2004 4:44 pm
Hi again ljoseph,
The 3 dB power increase is correct when
two equal amplitude, equal phase signals are added linearly. It is due
to the way decibels work for power and voltage.
For a factor
of 2 in power with units of power:
10 log (2) = 3.01 dB
For a factor of 2 in power with units of volts:
10 log (2^2)
= 20 log (2) = 6.02 dB
In your previous case from the first post,
you were doubling the voltage (two, 1 Vpk signals). In this one, you
are doubling the power.
subject: Re: RMS ValuePosted: Fri May 01, 2009 9:03 am
Joined: Tue Sep 07, 2004 3:09 pm
looking at adding voltage waveforms at a voltage summation point in
a circuit, it is right to assume that you will get 6dB increase in power
if the two waveforms are coherent.
But don't forget that your
combiner performs an impedance transformation, some voltage from each
waveform get's transfered to current by a squarerooth of 2 factor, and
therefore you fall again on 3dB increase.