What is often confused is the fact that many people think that RMS power is the
amount of power needed to be dissipated in a resistor to produce the same amount
of heat that an equivalent amount of DC power produces. This is not the case.
The correct term is "the amount of RMS voltage needed to apply to the resistor
to
produce the same amount of heat as an equivalent amount of DC voltage applied."
You will find that the peak AC power applied to produce the same heat as DC is
two times the value of the DC.
Therefore the average power of the AC is 1/2 of the peak power to be equivalent
to the average DC power.
If it was RMS power it would have to be 70.7% of the peak power. After all, to
find RMS from peak you multiply peak by .707!
more below.....
MorgusMagnificen@aol.com wrote:
> (This is a correction to my previous note).
>
> To those who say there is no such thing as RMS power, its time for you to go
> back and re-learn your basic AC theory. RMS power is defined simply as the
> time-average value of the energy in a circuit. There is much subtlety to what
> is the appropriate time interval for this average, and causes most of the
> confusion I see going on here.
Could the FCC be wrong in their definition of PEP ?
"peak envelope power (of a radio transmitter) [PEP, pX, PX]: The
AVERAGE power supplied to the antenna transmission line by a transmitter
during one radio frequency cycle at the crest of the modulation envelope taken
under normal operating conditions. "
I don't see RMS mentioned here. If you look in any radio text book you will not
find RMS power mentioned. Look for RMS power in the index of the ARRL handbook.
It's not there!
>
>
> In the practical world, RMS power is readily measured by a device (meter,
> numerical integartor, etc.) that can measure the average value of V times I.
> If the waveform is periodic, the average over many cycles approaches a
> definite limiting value and this is referred to as the RMS power. If the
> periodic waveform is also sinusoidal, this value will turn out to be (voila!)
> .707(I) x .707(V)=.5VI where V<I are peak values. So for convenience, .707V
> or I are referred to as the RMS values because they will properly calculate P
> when used in P=VI.
They will properly calculate to AVERAGE power.
If you have peak power and try to take .707 of it you will come out with a
different number. If you take .5 times peak power (.707 times .707 = .5) you
will
come out with AVERAGE power.
>
>
> Another not so well know fact is the actual measurments made by ordinary
> voltmeters. When you measure an AC sinusoidal voltage with your Fluke or
> Simpson260, what that instrument measures is actually the AVERAGE of V over a
> half cycle, which is actually .626 Vpk. The scale of the meter is then
> printed with a hidden scale factor of (.707/.626) so that a 1 volt sine-wave
> will read 1 VAC. This calibration is ONLY valid for a sine-wave and other AC
> waveforms will be read in error. For this reason, serious engineers also
> possess and use "true RMS" meters which actually measure the avg. value of
> Vsquared.
Meters like the simpson 260 do indeed work on the average voltage value as no
capacitor is involved. Just the mechanical inertia of the meter movement is used
to average the rectified voltage. Then the scale is calibrated as RMS volts. But
most meters that have an amplifier circuit in them first detect the peak AC
voltage by letting the diode charge a capacitor to the peak voltage applied. The
peak voltage is then converted to RMS on the scale. As you say, the calibration
is only valid for true sine waves. True RMS voltmeters work differently.
Most watt meters work on the principle of charging a capacitor to the peak
detected voltage and current (Bird watt meters). The scale is then calibrated to
read average power.
>
>
> The problem with all of this discussion of SSB power is that the RF waveform
> is madly varying on two different time scales: the RF carrer frequency and
> the audio modulating frequency. In fact, your SSB transmitter with VOX input
> does not have a well-defined average power of any kind. PEP is the true RMS
> RF power over any time interval of several RF cycles (several microseconds )
> But that average value will vary with the voice waveform and can be easily
> averaged with an AF integrator driven by an RF RMS detector. That will give a
> meaningful average power over millisecond time inervals, which is the best
> you can hope for.
>
> Eric von Valtier K8LV
The fact that two different time intervals are involved in PEP is the reason for
the FCC definition. "AVERAGE power over one RF cycle at the peak of the
modulation envelope."
A modulation peak of full power or maximum height can let many RF cycles through
or it can be as little as one RF cycle as far as PEP calculation is concerned.
The fact that the modulation peak is at maximum means that the RF cycle(s)
during
that time will also contain the maximum power that will occur in them. Finding
the average power of that RF cycle(s) at that time determines peak envelope
power.
The average power of the transmitter must be measured on a much longer time
period. A time period long enough to average the modulation envelope time.
73
Gary K4FMX
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