In article ,
Gary Schafer writes:
Well Chris, you got a lot of good advice and some completely wrong
advice. Funny thing is nobody really answered your question!

[lots of useful information snipped...]

This is how you convert average power to PEP. I say average power
because RMS power is really a misnomer. There is no such thing as RMS
power as so many commonly refer to. It is really AVERAGE power. It is
derived from RMS current and RMS voltage but what you get when you
multiply RMS voltage by RMS current or resistance is an AVERAGE value,
not an RMS value.

I completely agree about RMS 'power'. RMS applies to a voltage or
current waveform.

Caveat: you cannot simply multiply RMS voltage by RMS current unless
you have a purely resistive load. Oddly enough, if the load is a
fixed resistance (measured as a conductance G) in parallel with a
reactance then the E_rms^2 * G (E_rms^2/R when purely resistive)
value continues to apply. When it is a fixed resistance in series
with a reactance, I_rms^2 * R continues to apply. And these extend
to the nonsinusoidal case as well, again under the restrictions
given above (fixed G in the first case, fixed R in the second).

This may seem trivial but it becomes important when trying to convert
from one form to another. Using the wrong notation can give you wrong
answers.

As to the above, the average voltage or current of a sine wave is 50%
of the peak.

Actually, it is .6366 times the peak. The averaging is done continuously
over the positive half cycle of the sine wave, and is essentially the area
under the sine curve divided by the length of the half-cycle (2/3.14159,
or, if you calculated in degrees, 114.592/180).

The true average value of a sine curve is to average over a full cycle,
but is not very interesting: it's zero. The .6366 number is useful
because it is what a VOM typically measures - average DC voltage
after passing through a full-wave rectifier and before applying a scale
factor of .7071/.6366 to produce E_rms - all based on the assumption of
a sinewave input.

Average value of voltage or current waveform is of limited use; but
RMS value is directly related to the ability to deliver power to a
resistive load.

The rms value of a sine wave is .707 of the peak voltage or current.
If you multiply .707 (rms voltage) by .707 (rms current) you get .5 or
50%. This is AVERAGE. It is no longer an rms value.

By the way, the definition of peak envelope power (PEP) is: "The
average power contained in one RF cycle at the crest of the
modulation envelope". (note that the definition says "AVERAGE power"
not RMS power)

The average power is in fact all that is of interest in AC circuits,
and is the average over time of "instantaneous power" E*I.

If you do not use average power you have to deal with the fact that
reactances accept power during half of the cycle and then feed it
back during the other half; and for nonsinusoidal waveforms do the
same but in not so regular a pattern.

PEP is just averaging over the shortest reasonable interval (one
cycle of the carrier frequency), then keeping only the largest
such value seen. Useful to regulators because it is a measure
of your maximum capacity to interfere with other signals.

[...]

73
Gary K4FMX