Let's review some definitions to start:

AVERAGE POWER

Average power is found by squaring RMS voltage and dividing by
resistance. Or RMS voltage times RMS current.


PEP

It helps to fully understand exactly what PEP is in an SSB
transmitter. Then it is easier to see in an AM transmitter.

Peak envelope power is important because that is how the FCC defines
how much power we can run.

Let's look at the FCC definition of Peak Envelope Power:

"Peak envelope power output of a transmitter is the AVERAGE power at
the crest of the modulation envelope over at least one rf cycle."

NOT TO BE CONFUSED WITH AVERAGE POWER READ ON A METER as it swings
around!

If you think about what that is saying it will make sense. If you
transmit just a carrier with an SSB transmitter of say 100 watts. That
is 100 watts average power output. It is also 100 watts PEP output.
(in this case the envelope is infinitly long) If you were to key it on
and off in the CW mode the power relationship would be the same. 100
watts PEP on each CW dash or dot sent.

If you now switch to the SSB mode and modulate the transmitter so that
the peaks on the scope looking at that signal reach the same height,
the transmitter will be putting out 100 watts PEP.

If you were to modulate that same SSB transmitter with 2 equal
amplitude tones you would get a scope pattern that looks similar to an
AM signal modulated with a single tone. The crest (or peak) of the
waveform represents 100 watts PEP same as with voice but with the
tones it is easier to see as the waveform will be stable.

If you were to increase the speed of the time base on the scope and
spread the waveform out you would see that each crest of the audio
wave form has within it many cycles of the RF frequency. These many
cycles of RF are the AVERAGE power contained in the signal.

You will note that the maximum AVERAGE power is only reached for
several RF cycles at the crest of each audio cycle.

This is what is known as PEAK ENVELOPE POWER. (see definition above
again)


PEP WATTMETERS

A true PEP reading watt meter will show the peak envelope power of the
above signals as described.
There are a lot of so called PEP watt meters on the market. Not all
are able to properly read. Even the Bird meters have problems with
some types of wave forms.


S METER READINGS

S meter readings will vary according the particular receiver being
used but most all S meters are peak responding circuits. Most will
read pretty close to the peak values, depending on the decay time of
the circuit some may not hang up there like others do. If you think
about it if you have ever been plagued with pulse noise like ignition
noise it only takes a very narrow pulse occurring at a rather slow
rate to hold the S meter up high. Increasing the rate will not
increase the meter reading.

Likewise with an SSB signal, once the station is transmitting his peak
power on a regular basis, increasing mike gain or increasing
compression will not raise the S meter reading maximum.

The AVC circuit in the receiver must respond to the peaks or the
receiver would overload the detector if the gain was not cut back when
a peak was received. The S meter reads AVC voltage.


AM TRANSMITTER

It is best to try and understand the output signal of the AM
transmitter before trying to coralate it with what goes on at the
input side. Swapping back and forth can be confusing.

Take our 100 watt carrier output transmitter again. Measuring the
output voltage of the RF we find that it is 70.7 volts RMS across our
50 ohm load. P = I squared / R so 70.7 x 70.7 = 5000. 5000/50 = 100
watts.

Let's modulate 100% with a single audio tone. We get out of it a 100
watt carrier and two 25 watt side bands. 3 distinct signals. As you
stated before the carrier always remains constant.

If we look at the output signal on our scope we will see that it looks
similar to the SSB signal that was modulated with 2 tones. We see the
modulation envelope. We can again expand the scope's time base and
look at the RF cycles within each modulation peak. Same as with the
SSB signal, at the crest of the modulation envelope is the peak
envelope power of the composite signal.

Now let's get back to measuring that PEP. We know that the carrier
alone had a power of 100 watts which produced 70.7 volts across a 50
ohm load. If we look at the scope with and without modulation we see
that the voltage output doubles with modulation so it will be 141.4
volts RMS at the crest of the modulation wave form.
Again P = E squared /R. 141.4 x 141.4 = 20000. 20000 / 50 = 400 watts
PEP.

If we were to measure this with a good PEP wattmeter we would see the
meter also indicate 400 watts PEP.
Again some so called PEP meters do not do well on this type of wave
form. The carrier tends to confuse the meter as it causes an offset
in the voltage being read by the meter and the meter tries to average
it so the net result is some gets subtracted from the reading. It is
due to the way in which the particular meter circuit operates.


CONVERTING RMS TO PEAK

It would seem at first glance that you could find the peak power of
the 25 watt side bands and add things together but that doesn't work.
You can not convert power. THERE IS NO RMS IN POWER!

There is a wide misconception that there is something called RMS
power. There is no such thing! There is only AVERAGE power and PEAK
power. (note the FCC definition of PEP)

You find average power by using RMS voltage. But once you multiply or
divide, RMS term, the RMS goes away.
So once you have power you can not multiply it by 1.414 to find peak
power.

To find peak power you must first add the voltages together or find
the peak voltage of an rms voltage by multiplying by 1.414 then
finding power.


AM LINEAR

Operating an SSB transmitter and amplifier in the AM mode, if properly
set up, will produce exactly the same looking output signal as a plate
modulated AM transmitter.

If we have an SSB amplifier that will put out 1000 watts PEP on SSB it
will also put out 1000 watts PEP on AM.
But in order to do so the carrier output must be limited to 250 watts
output. It must be tuned up in the CW mode for 1000 watts output. Then
when switching to AM the carrier is reduced to 250 watts output
without touching any tuning controls. The amplifier must still be
tuned to be able to produce the 1000 watt peak envelope output.

When we modulate the 250 watt carrier with AM the peak envelope power
at 100% modulation will reach 1000 watts pep (or 4 times the carrier)
just as it did with the plate modulated AM transmitter. Looking at the
output with a scope we will see the voltage double with modulation
verses just the carrier.


POWER IN SIDE BANDS

As a note is seems that having 2 side bands with the same information
in an AM signal is useless but it is not.
In the detector of the receiver the energy in both side bands combine
and add together. So rather than using only 1 side band of 25 watts
you are really using 50 watts of side band energy. So a 3 db addition.
There is also another 3 db gained in the detector because of the
voltage doubling with the side bands being coherent in the detector.
So the carrier is really the only thing wasted.


PLATE CURRENT AND VOLTAGE DOUBLING

It is easiest to see with a triode tube that is plate modulated.
Doubling the plate voltage will cause the plate current to also
double. That is if the tube is capable of providing enough emission.
This must be a linear function in order to avoid distortion when
modulating.

Tubes that are weak may not be able to provide this. That is one
reason that PEP may not fully reach 4 times the carrier power with
100% modulation.

Screen grid tubes are not linear in this respect. Plate current is
somewhat independent of plate voltage. That is why you must also
partly modulate the screen along with the plate when using a screen
grid tube in the final. You want to have a linear plate voltage to
plate current relationship.

This is also why a lot of broadcast transmitters use triodes in the
final. Easier to maintain linear modulation.


HANDBOOK

All this can be found in the AM section in some of the older
handbooks. The newer ones do not cover AM very well.

73
Gary K4FMX