The explanations make sense to me.

The average *voltage* of an amplitude modulated signal equals the
carrier voltage, provided that the modulation has no DC offset (that is,
its average value is zero). This would be true for any waveform,
provided that it's AC coupled and it's not DC level shifted, clipped, or
otherwise distorted after the AC coupling. (It wouldn't be true of a
classically overmodulated carrier, for example.) The average of an AC
coupled waveform is always zero, if averaged over a time period that's
long compared to the time constant of the coupling network.

So a power meter that's really reading the voltage should stay at the
carrier level, provided that its time constant is comparable to or
longer than the time constant of the AC coupling of the modulation
signal. I'd expect this to be the case for a typical meter and typical
audio modulation.

Even if you're monitoring the true power level, the average typically
wouldn't get much greater than the carrier if you were modulating it
with a normal voice. Compression would tend to raise the average some,
though.

As I understand the Bird wattmeter, it basically takes current and
voltage samples and adds them (rather than vectorially multiplying them,
as a true power detector would have to do). What this would do to its
response to average power I'm not sure, but I wouldn't expect it to give
an accurate indication of either the carrier power or total power of a
modulated signal.

We're confronted all the time with measurements that seem to contradict
established theory. Some people regretfully are quick to embrace these
as evidence that established theory is wrong. It does take some effort
and knowledge to dig a bit to find out why there's a disagreement. But
with a miniscule frequency of exceptions, the digging always reveals
that we're not measuring what we think we are, we're using the wrong
theory, or we're applying it wrong.

Roy Lewallen, W7EL

Bill Turner wrote:

Isn't the S-meter in a typical receiver voltage driven? They do kick
upwards with modulation.

--
Bill W6WRT


S-meters are driven by the AGC voltage, which is usually derived from
the peak voltage of the signal. The peak value of an AM signal does
increase with modulation, so it wouldn't surprise me to see one kick up
a little once in a while on voice peaks when listening to AM.

Of course, they always kick upwards when listening to SSB -- just about
none of what I've written applies to that mode.

Roy Lewallen, W7EL

In message , Bill Turner
writes
On Fri, 22 Oct 2004 19:34:26 -0700, Roy Lewallen wrote:

So a power meter that's really reading the voltage should stay at the
carrier level, provided that its time constant is comparable to or
longer than the time constant of the AC coupling of the modulation
signal. I'd expect this to be the case for a typical meter and typical
audio modulation.

_________________________________________________ ________

Isn't the S-meter in a typical receiver voltage driven? They do kick
upwards with modulation.

--
Bill W6WRT


Try making a simple RF voltmeter consisting of a diode detector driving
a sensitive moving coil meter (say 100microamp fsd) or a multimeter
reading volts. If the RF source doesn't already have a DC path to ground
(ie it is AC coupled), you will also need to add a shunt resistor or RF
choke. Make the resistor as low as possible without killing the RF too
much.

Apply sufficient RF to give (say) a half-scale reading. Now modulate the
signal with audio (to 100% if possible). The meter reading will stay the
same. Now add a fairly large capacitor across the meter (say 1uF). The
reading will increase. If the RF source impedance is low compared with
the resistance of the meter, the capacitor will discharge very little
when the diode is 'off', and the cap will charge up to the peak voltage.
With a good modulation waveform, 100% mod will double the reading.
(Note: Any capacitor value between 'far too small' and 'more than
enough' will give a reading somewhere between.

I did this about 30 years ago to add a simple mod depth indicator to and
old signal generator which already had an RF level meter.


Anyway, all this talk about power and PA efficiency tends to complicate
the explanation. Just think of the spectrum of the signal.
No mod = 0dB reference = the carrier.
Add mod. Sidebands appear each side of the carrier. The carrier level
stays at 0dB.
Set mod at 100%. Each sideband should be 6dB below the carrier level.
Total power = 0dB + -6dB + -6dB = 1 + 1/4 + 1/4 = 1.5
(Note: This is regardless how the modulation was applied).
Peak power can be got from the voltage waveform. The peak RF voltage
doubles with 100% mod. The peak power is proportional to the square of
the voltage, so it quadruples.

So, to sum up... for AM:
100% mod, power increases x1.5 wrt 0% mod.
100% mod, peak power is x4 wrt carrier.

Ian.

--

My friend, N7TCY wrote this bit regarding RMSpower a few years ago.
We work with inverters so there was some call for it.

http://www.eskimo.com/~bgudgel/cgi-bin/rms/rms.html


boB
K7IQ








On Thu, 21 Oct 2004 10:49:39 -0500, "Steve Nosko"
wrote:


"Roy Lewallen" wrote in message
...
Steve Nosko wrote:
. . .

I am not not trying to prolong the pain (or this thread), it is just
that I
was born with a bone in my head that makes it hard for me to give up
explaining some basic concept like this. (yep, it can be a curse) . . .

Egad, another person with the same genetic defect! Welcome!
Roy Lewallen, W7EL

So it's genetic!...

Yea. And I do this in front of a class or 5-15 green students. It SURE is
rewarding when someone says :"Oooh! NOW I get it!"

73,

boB_K7IQ wrote:
My friend, N7TCY wrote this bit regarding RMSpower a few years ago.
We work with inverters so there was some call for it.

http://www.eskimo.com/~bgudgel/cgi-bin/rms/rms.html

That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to me.

Roy Lewallen, W7EL

Hm, my browser doesn't like the name I gave the file. So I've uploaded a
second copy with a different name. You can also get it as

http://eznec.com/Amateur/RMS_Power.pdf

Roy Lewallen, W7EL

Roy Lewallen wrote:


That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to me.

Roy Lewallen, W7EL

Both the Rosenbaum and Lewallen papers look very good and well written.

I would, however, Roy, make a more explicit statement that while RMS power
can be calculated, is has no practical value for the normal considerations
of power...and add a note indicating something like that:

"The phrase "RMS Power" has been used in some circles not as an exact use of
the term RMS, but rather as an informal "standard" that actually means
"average power under specific text conditions". (I refer to an earlier post
telling of the single channel, steady state audio power amp measurement)
This use of the term "RMS" was originally initiated to call attention to the
specific test. Unfortunately, this use has caused some confusion in the use
of this terminology and is some of the motivation for the paper."

73, Steve, K9DCI

--
Steve N, K,9;d, c. i My email has no u's.




"Roy Lewallen" wrote in message
...
Hm, my browser doesn't like the name I gave the file. So I've uploaded a
second copy with a different name. You can also get it as

http://eznec.com/Amateur/RMS_Power.pdf

Roy Lewallen, W7EL

Roy Lewallen wrote:


That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to
me.

Roy Lewallen, W7EL

Thanks for the suggestions, Steve. I've updated the file to incorporate
them. The new file is at http://eznec.com/Amateur/RMS_Power.pdf.

During recent trips to Powell's Technical Bookstore and the library, I
looked through a number of books about electrical circuits which are
oriented toward hobbyists and technicians (that is, ones lacking the
math of a college level circuits text). I'm glad to say I didn't find
any which were plainly wrong about average and RMS power (like the Joe
Carr book quoted earlier here). But what nearly all of them do is to
introduce RMS voltage and current pretty early on in the text, and
explain that the RMS values of voltage and current are important because
they represent equivalent heating values (which is correct). From then
on, they simply use E and I with the assumption that they represent RMS
values of voltage and current. At some point, they introduce the
equation P = E * I or, in the more advanced ones, E * I * cos(phase
angle), and maybe at that point mention that P is the equivalent heating
power (which is also correct). What I didn't see in any of them was the
fact that the product of the RMS values of E and I is the *average*, and
*not* the RMS value of P. It's easy to understand, then, why a lot of
people, like a number of the folks who posted comments and questions
here, naturally (and incorrectly) assume that the product of Erms and
Irms is RMS power. The books simply don't contain the information you'd
need in order to discover that Erms * Irms = Pavg. Hopefully the paper
posted by Rosenbaum and the one I did will help fill the void.

Roy Lewallen, W7EL

Steve Nosko wrote:
Both the Rosenbaum and Lewallen papers look very good and well written.

I would, however, Roy, make a more explicit statement that while RMS power
can be calculated, is has no practical value for the normal considerations
of power...and add a note indicating something like that:

"The phrase "RMS Power" has been used in some circles not as an exact use of
the term RMS, but rather as an informal "standard" that actually means
"average power under specific text conditions". (I refer to an earlier post
telling of the single channel, steady state audio power amp measurement)
This use of the term "RMS" was originally initiated to call attention to the
specific test. Unfortunately, this use has caused some confusion in the use
of this terminology and is some of the motivation for the paper."

73, Steve, K9DCI

On Thu, 18 Nov 2004 15:52:41 -0800, Roy Lewallen
wrote:

Thanks for the suggestions, Steve. I've updated the file to incorporate
them. The new file is at http://eznec.com/Amateur/RMS_Power.pdf.

During recent trips to Powell's Technical Bookstore and the library, I
looked through a number of books about electrical circuits which are
oriented toward hobbyists and technicians (that is, ones lacking the
math of a college level circuits text). I'm glad to say I didn't find
any which were plainly wrong about average and RMS power (like the Joe
Carr book quoted earlier here). But what nearly all of them do is to
introduce RMS voltage and current pretty early on in the text, and
explain that the RMS values of voltage and current are important because
they represent equivalent heating values (which is correct). From then
on, they simply use E and I with the assumption that they represent RMS
values of voltage and current. At some point, they introduce the
equation P = E * I or, in the more advanced ones, E * I * cos(phase
angle), and maybe at that point mention that P is the equivalent heating
power (which is also correct). What I didn't see in any of them was the
fact that the product of the RMS values of E and I is the *average*, and
*not* the RMS value of P. It's easy to understand, then, why a lot of
people, like a number of the folks who posted comments and questions
here, naturally (and incorrectly) assume that the product of Erms and
Irms is RMS power. The books simply don't contain the information you'd
need in order to discover that Erms * Irms = Pavg. Hopefully the paper
posted by Rosenbaum and the one I did will help fill the void.

Hi Roy,

there does seem to be an unsettling amount of misinformation, errors
and poor explanations in the majority of text books I've encountered,
I'm sorry to say. No wonder there's such a huge amount of confusion
surrounding these aspects of our hobby. :-(
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Ok Chris...here's the deal with "swing"...it's pretty much BS. By turning the power down to 2 watts and letting the xmtr go to 10 watts is just going to cause splatter, TVI and a whole host of other problems just so you can show off to your CB buddies how much you can make the s-meter swing on thier end. Anything in excess of 100 percent modulation is a waste of power and will make you sound like ca-ca. Unless you want to be a smartass and a prick, follow the guide below.

100 percent AM modulation as far as us hams are concerned is unmodulated carrier times 4. So if you have a 375 watt "dead carrier" with 100 percent modulation you will show a peak reading of 1500 watts with modulation (ham legal limit, BTW)...so a 4 watt dead carrier...well 100 percent modulation will be 16 watts. That's with a linear response out of the modulator and final circuitry...FCC rules say 4 modulated 5 as far as wattage for CB. The AMC (automatic modulation control) does this (4 watts dead key, 5 max "swing") by providing a non-linear response out of the modulator and final circuitry combination, accomplished usually with a diode or diode and potentiometer. Removal or adjustment of these devices, along with realignment, (peaking out to coin CB terms) can acheive the desired modulation result described here. Type in CB mods into google and you'll find a bunch of stuff if you're so inclined.

To also pick another statement out from your post, "That's just one of the reasons I'm looking to amateur radio."

That's your best bet...get your ham license. Blows 11 meter away. Take it from an ex freebander who now is a licensed ham.

john

Hope that answers your question