Amplitude Modulation

Heising Modulation

Viking and Ranger Valiant
Modulation

modifications

One of the first
things we must
understand before
discussing
characteristics of
AM is power
measurement. First,
there is no such
thing as “RMS
Power”. We find
power by multiplying
RMS voltage times
RMS current, but
there is really no
such thing as “RMS
power”.

What does exist is equivalent
or heating power.
This is useful power over a defined
period of time, even a very short time. It
is power that
does, or can do, some amount of
actual work. Both PEP and average power are based on the heating or work power,
even if that heating or work power is taken over a single RF cycle.

The old common method of quantifying RF power was average power.
Average power is
same as equivalent
work power or heating power of each cycle averaged over a significant time
compared to the time when power level changes. With an unchanging
(during the measurement period) power level, such as a
carrier applied to a
constant resistance
load, average power and peak envelope power are the same. If we close
and hold-closed a manual telegraph key on a good
stable CW
transmitter, we will
see the
average power

displayed on a power
meter. It will not
be the “RMS power”.
It is also the peak
envelope power, because it is the
maximum stable heating power
level over some
period of time that we hold the key.

As for peak envelope
power, peak envelope
power is the very
maximum short term
peak reached of either
steady or varying heating power levels!

Consider a sine wave
with a peak voltage
of 100 volts. The
RMS voltage is
70.7107 volts, or
100 peak volts. If
we placed that
voltage across a 50
ohm resistance we
would have 70.7107 /
50 = 1.414214
amperes. That would
also be 100 watts
average power in one
complete cycle or
any number of equal
amplitude cycles
that follow. The
peak envelope power
is also 100 watts because the peaks are the same cycle after cycle.

If
we pulsed that power
off and on rapidly
with a 50% duty
cycle the average
power would be 50
watts. Half the time it would be 100 watts, and half the time zero watts. The peak
envelope power would
be 100 watts, because that would be the power at the crest of the envelope! The
envelope can be as short as one cycle, although no meters ever respond to that.

Power cannot be RMS power.
RMS is calculated by
squaring the function’s value,
taking the average
(mean value) of the
squared function,
and finally
converting that mean
value back by
finding the square
root of that mean. If we had a
peak power of 100
watts with a 50%
duty cycle the RMS
power, if there was
such a thing, would
be SQRT( (100^2 +
0)/2) = 70.71 watts.
We see that 70.71
watts is not the
average power, is
not the heating or
“work” power, and is
not the peak power.
It isn’t anything at
all useful! We can
have meters that
and we can also have RMS
current, but we don’t
with any of our
power measuring instruments. We can’t even calculate RMS power to be anything
useful or sensible at all.

Characteristics of AM (amplitude modulation)

Let’s consider the
case of perfect
undistorted sine
wave
modulation of an
amplifier stage. The
carrier, sidebands,
and power levels of
the various spectral
components making up
the signal have a
certain ideal
relationship.
Consider the case
below with
symmetrical sine
wave
modulation.

Unmodulated carrier
= 100 watts (PEP or)
average carrier
power.

Average is the same
as PEP because,
absent amplitude
modulation, the
carrier level is
unchanging over
time.

modulated 100 w
carrier = 400 watts
PEP or 150 watts
average or “heating” power. Of
this 150 watts
average or “heating” power, 100
watts is in the
carrier, and 25 watts
average power is in each of the two AM signal’s sidebands.

Carrier average power =
2/3 of the total 100% modulated average
power

Total of both sidebands, average power = 1/3 of
total average power
under 100% modulation

Average power one sideband =
1/6th average
power with 100%
modulation

Peak Envelope Power 100% symmetrical modulation = Four times
carrier power

Plate Modulator Requirements

100% sine wave
modulation of a 100-watt
carrier requires a modulator
sine wave power of 50 watts. This
to the PA’s RF power, making the total heating (or
average) power 100+50 =
150 watts.

On
100% positive
audio peaks
waveform, as seen on an oscilloscope, doubles in voltage.
Doubling load voltage on audio peaks also doubles
load current on audio peaks. This means any AM signal with symmetrical 100%
modulation, when measured using true PEP meters, has four
times carrier
power.

If we monitor
transmitter output
current on a
typical slow-response RMS
current
meter, like a
typical thermal RF ammeter, we should observe current rising to 1.22475
state current with
modulation. We should
also observe an
average RF voltage
of 1.22475 times
the unmodulated
carrier voltage, when
steady sine wave modulation of 100%
is applied. This is
1.5 times the
unmodulated carrier
average power, and fully
accounts for the
carrier power plus
the 50% audio power
required to modulate
that carrier at 100%
modulation.

One word
of caution, measured
values
are affected by the
type of meter we
use, and the
modulating waveform!
Some metering
schemes don’t fully
respond to peaks,
the average either.
This is a metering
problem.

We will
not see
the 100% sine wave
modulated average
power levels with
perfect 100%
modulated speech,
although PEP will
indeed reach four times the
carrier on a good
sample and hold
meter. This is
because speech has a
larger peak power to average power ratio, when compared to peak-to-average power
sinewave
modulation. A true peak reading meter with adequate peak-hold time is, by far, the
most reliable way to measure positive modulation peaks. A true peak-reading RF
power meter, with
is actually a much more
accurate indicator
of 100% positive
peaks than a
conventional oscilloscope.

The best overall
modulation
percentage indicator
would be a
specialized device
that sampled and held negative
peaks, and also sampled
and held positive
peaks. This would
not indicate
bandwidth of course, only
percentage of
modulation! Actual bandwidth
would only be
indicated by using a
peak holding
spectrum analyzer
or, in a pinch, a very
narrow bandwidth
with a peak
responding and
slow decay
calibrated signal
level meter.

True Plate Modulation

Plate modulation
might well be
the most common
method of obtaining
amplitude
modulation. Most amateurs consider plate modulation in a very favorable manner,
and many of us are capable of
building a simple rig we call “plate modulated”. Most
older tube gear
uses a system called “plate modulation”. What isn’t often understood is the
system often called plate modulation is often not true plate modulation, but rather
a combination of plate and screen modulation!

In order
to be purely plate
modulated
without
distortion, a
plate modulated RF power amplifier
stage must maintain a
square law

power output
function with
varying plate
or anode voltage. As a matter of fact this can actually be called parabolic, or square
law, modulation.  If peak PA anode voltage increases
50% from modulator
power (50%
modulation), peak envelope RF output
power should increase
225% over the
carrier value. If
the modulator
doubles plate
voltage (100%
modulation), positive peaks of envelope power
output should

The reason for this squaring of power is logical and easily
understood. With 100% symmetrical modulation, the
plate modulator system doubles anode
voltage on positive peaks. When tube
anode voltage doubles, assuming the tube behaves like a constant average
resistance, this also doubles
anode current. Doubling current and voltage at the same time obviously means plate input power
is
four-times initial resting power.
Efficiency remains
constant at a pretty
high value in a class C stage, causing peak RF output power to ideally be four-times the
unmodulated carrier value with a 100% modulation and symmetrical modulating
waveform signal!

The above assumes real 100% plate modulation. Most amateur
rigs and not pure plate modulation, but are a combination of screen and plate
modulation. We just call them “plate modulated” rigs.

True
plate modulation has the same PA load, or PA operating impedance, all
through the modulation cycle. This occurs because any change in anode voltage
results in a proportional change in anode current. The product of anode supply
E/I is constant across all voltage swings. Doubling anode voltage proportionally
doubles anode current.

Methods of “Plate”
Modulation

There are several
methods of obtaining
“plate” modulation.
The two most common
in amateur use are
Heising modulation
and standard plate
modulation. Heising
modulation is also
called constant
current modulation.
There are details of
Heising modulation
on my

Heising Modulation

page.

It is very popular among amateurs to call the modulation reactor a Heising
choke, and to call a conventional transformer coupled modulator Heising
modulation when it has a modulation reactor, but it is NOT Heising modulation.
Contrary to what
amateurs popularly
claim, plate
modulation with a
transformer and
modulation reactor
is not Heising
modulation
and the
modulation reactor is not a
“Heising choke”. The
reactor serves only
PA current out of
the modulation
transformer, giving
the transformer more
magnetic saturation
is reached.

Heising modulation is constant current
modulation, in that the modulator tube and PA tube share the same reasonably-constant power supply
current level. The power supply current shifts back and forth between the PA tube and
the modulator tube, but the supply load is for all practical purposes constant.
Heising modulation never uses a transformer, but rather parallels the PA stage
and a modulation with both being fed through a modulation reactor or choke. One
of the few rigs to employ Heising Modulation is the
Globe Scout.

The modulation choke, when used with transformer coupling from
the modulator system, simply serves to keep dc plate current out of the
modulation transformer and keep flux levels low in the modulation transformer.
The modulation reactor improves performance of the modulation transformer by
lowering flux levels cause by dc current flowing through the secondary winding.
It serves no Heising function at all.

Plate Modulated
Triodes

As
a general rule only
plate modulated
low-mu or medium-mu
triodes provide
needed square-law
power response with
modulated anode voltage
variation. The ideal
response generally occurs
only when a triode
is operated well
into class C with
short conduction
angles. A
triode operating in
this manner behaves like a
rapidly off-and-on
switched resistance (switched
at the RF excitation
rate). This means
the triode presents
resistance to the
much lower frequency
plate modulator system.
Some of the
cleanest, least
critical to tune,
high-level modulated
AM transmitters use
low-mu triodes.

 A B C 1 .4 .05 0 2 .6 .1 0 3 .7 .2 0 4 .83 .3 .05 5 1.0 .42 .1 6 1.2 .58 .2 7 1.4 .7 .3 8 1.5 .8 .4 9 1.7 1.0 .55 10 1.9 1.2 .67

We can see a
low-mu triode, as
plate voltage
increases, has a
substantial increase
in plate current.
If we pick the
the input power will
approximately
doubling of anode
voltage.

For example at A5
we have 1 amperes at
2500 volts, or 2500
watts input power.
At A10 we have 1.9
amperes at 5000
volts, or just under 10,000
watts
instantaneous

plate power.

Remember this is
instantaneous

power, since the
anode current is in very
short pulses in
class C. The average
power is MUCH less.
Tube anode heat is
an integration of
these short pulses
of very high
dissipation. The
thermal mass of the
anode averages the
heat.

Linearity is not
perfect with the
100TL, but if we
pick the correct
the tube provides
very close to square-law response. If the
modulator doubles
anode voltage, peak
power would nearly

Let’s  look
at a tetrode.

Plate Modulated
Tetrodes (and pentodes)

An ideal tetrode or pentode tube with fixed grid voltages has
constant current characteristics. The anode current does not track with anode
voltage. A “plate
modulated” tetrode or pentode,
without the aid of
supplemental grid modulation,
power performance. This is
because screen
grid voltage dominates
cathode-to-anode current in a
tetrode (or pentode).
The tube does not operate at a constant, stable, E/I ratio (impedance) over the modulation cycle.
Production of linear undistorted modulation requires a combination of grid and anode modulation.
The tank circuit tuning is also critical, since the tube functions partly as as
a grid modulated stage.

Deriving screen voltage through a high
resistance, so the screen has a high audio frequency impedance to ground, is
common method of adding grid modulation to a “plate modulated” tetrode or
pentode. A better way is to add an intentional amount of audio to the screen.

Whatever method is used to get around the constant current effects in the
tube the stage also becomes sensitive to tank loading. Unlike with a low mu
class C triode, tetrode and pentode operating impedance varies with supply
voltage. The tank has to be “tuned” (as does a linear) for the lowest impedance
(highest peak power).

Let’s look at a
commonly used amateur
beam power tube, the 6146.

Curves A through G
represent anode
current as anode
voltage is varied
with constant bias
and screen voltage
applied. Notice how
flat the plate
current curves are
as anode voltage is
varied.

If
the modulator doubles
anode voltage in an
ideal tetrode or
pentode amplifier, plate
current would not
change at all! Most
tetrodes are not
perfect and will do
a little better than
this, but still
have considerable
distortion when
exclusively plate
modulated.

If we plate
modulated a typical
pentode or tetrode
like the 6146,
the system will only achieve
50-60% positive peaks
(200 watts PEP for a
100 W carrier) by
the time negative
peaks reach 100%.
For example at -30
volts bias with 200
volts on the screen
(curve G), anode
mA whether anode
voltage 200 volts or
700 volts. If only
the anode was
modulated, audio
would be highly
distorted. With true plate
modulation of a
tetrode, it impossible to
obtain 100% positive
peaks. Even
negative peaks would
be grossly
distorted.

The easiest way to
properly plate
modulate a tetrode is to screen
modulate
at the same time the
PA stage is anode modulated. By applying
the correct
proportion of
modulating voltages
to the screen grid and
anode, with neither
element actually modulating
100%, the system can
come very close to
producing the
desired square law
power response. The
exact ratio of
modulation applied
to the screen and
anode varies with
grid drive, tube
type, and operating
voltages.
A
properly designed plate
modulated tetrode is
actually not plate
modulated, but
rather is partially
plate and partially
screen modulated. We
could also modulate
the control
grid along
with the anode,
leaving the screen
fixed in voltage.
We could modulate an
earlier exciter
stage in combination
with the plate of
the final amplifier. Still, the most common
plate modulation method
of tetrodes, and the
method
that seems to work
combination of
screen grid and anode
modulation.

There are two ways
to combine
screen and anode
modulation. One
method “forces” the screen
by supplying
the screen from
voltage taken on the
power amplifier side of the
modulation
transformer. This
method is shown
below:

Components of note are:

C4 (screen
bypass) must have
high reactance at
the highest audio
frequency when
compared to the
parallel combination
of R1 and R2.

C6 (screen supply
blocking capacitor)
must have low
reactance at the
lowest audio
frequency when
compared to R2.

R1 determines
screen operating
current and voltage.

value to provide the
best audio linearity
at the design value
of plate and grid
operating currents
and voltages. This
resistor determines
the amount of audio
supplied to the
screen grid.

A
second method is to
let the screen
self-modulate:

Components of note are:

C4 (screen
bypass) must have
high reactance at
the highest audio
frequency when
compared to R1.

C6 (screen supply
blocking capacitor)
must have low
reactance at the
lowest audio
frequency when
compared to R1.

R1 determines
screen operating
current and voltage.
This resistor must
be very high in
value, probably over
5,000 ohms in most
cases. If the
resistor cannot be
a series choke that
has very high
reactance across the
audio spectrum must
with R1. R1 is often
compromised in value
to provide the best
audio linearity at
the design value of
plate and grid
operating currents
and voltages. This
resistor determines
the amount of audio
supplied to the
screen grid.

Self-modulation of
the screen works
because screen
current varies with
anode voltage. The
screen system, in
effect, becomes a
constant current
supply for the
screen grid. As
the plate modulator pushes
anode voltage
higher, screen
current decreases.
Decreased screen
current causes
screen voltage to
increase. This
system, when a
series resistor is
used, normally
requires the screen
supply voltage to be
at least 50% higher
than required. With
a screen choke, the
screen supply
voltage can be equal
to the desired
screen operating
voltage. For a
6146 with the
resistor method of
supplying constant
current, the screen
source voltage
should be at least
300 volts. Screen
voltage is often
sourced from the
anode supply line.

Since the tetrode
(or pentode) stage is
both screen and
anode modulated, high voltage
supplied to the
anode system does not
reach double carrier
voltage on 100% positive
peaks. More
important, and a
point many people
seem to miss, is the
modulated anode
source voltage does not
reach zero for 100%
negative peaks! If a
negative peak
limiting circuit is
installed in the
modulator system of
a “plate modulated”
tetrode (or pentode), the stage
will normally go
well beyond 100%
negative peaks without
crossing over to
negative voltage.
The scope display
below is a good
typical
representation of
modulator voltage
supplied to the 6146
power amplifier in a
Johnson Ranger II.

Point C shows the RF envelope cutting completely off (over-modulation) even though modulated high voltage never goes below 40 volts positive.

For more details
see

Johnson Ranger and
Valiant Audio mods.

There is an
important lesson to
be learned from
this. Some articles
claim a negative
peak limiter, or
blocking diode, can
be used to prevent
over-modulation on
negative peaks. That
claim is clearly
false because a
properly configured
6146 (or any other
tetrode) reaches
100% negative peaks
while the anode is
still significantly
positive. In the
case of the Johnson
Viking Ranger a
properly designed
negative peak
limiter would hold
the anode at least
at 40 volts
positive, and have
filtering to round
the transition off
and not create sharp
waveform edges near
the negative limits.
This is almost
impossible to do at
high levels. It
would be much better
accomplished in low
level stages.

Efficiency Modulation (Grid Modulation and Low Level modulation with Linear Amplifiers)

There are two basic
systems that use
efficiency
modulation, grid
modulation and
linear amplifiers.
A linear amplifier or grid modulated stage has constant anode voltage. Just as
changes in anode voltage from modulation do not change current linearly, changes
in drive level can only change anode current. When we vary current
in a device without a corresponding change in voltage, we
normally do
not have a square
law power response. With
fixed coupling to
match at full peak
power, lower drive
levels cause a
reduction in
efficiency. This is
because, unlike with
plate modulation,
anode supply voltage
does not vary. With
a conventional deep
class-C triode with
plate modulation,
the output device
acts presents a
constant
time-averaged
resistance to
the modulator. If
the modulator
doubles anode
voltage, anode
current also
doubles. Since anode
voltage and anode
current changes in
the same proportion,
they maintain the
same ratio. Anode
impedance is E/I,
and if the ratio
remain constant the
anode RF impedance
also remains
constant. The output
device sees the same
match (or mismatch)
at zero modulation
as the output device
sees at modulation
positive peaks or
modulation negative
peaks, and with the
same conduction
angle efficiency
remains the same.

Screen modulation,
control grid
modulation, or
linear amplifiers
all have a constant
anode or collector
voltage. This means
output device
impedance, or E/I of
the output device,
varies over the
audio cycle. The
device has highest
current on
modulation positive
peaks, and lowest
current on
modulation negative
peaks. This means
the mismatch between
the output device
over the modulation
cycle. The normal
tuning procedure is
to match the output
device at maximum
positive modulation
peak. As the
modulation positive
peak is reduced the
output device has a
higher impedance,
and this mismatches
the device to the
tank. The result is
a reduction in
efficiency as the
system moves below
the peak positive
modulation level,
reaching minimum
efficiency at
maximum negative
peak.

The approximate rule
with 100% modulation
is device
carrier efficiency
is half of the
device positive peak
efficiency. Let’s
70% efficiency at
the anode of a tube
with 4% tank and
other losses for 66%
total efficiency. At
carrier, plate
efficiency will be
losses of 4% we have
an overall
efficiency of 31%.
This means on
carrier 2/3 of the
plate input power
will be heat, or
twice as much heat
as carrier power
into the tank. With
500 watts PEP output
on modulation peaks,
the tube anode
watts of heat.

This is true for
screen modulation or
linear amplifiers!

Many linear
amplifiers with high
conduction angles
efficiency on peaks,
plus the normal
procedure is to
slightly
over-couple to
ensure linearity on
occasional
exceptionally high
peaks. This means
with an unmodulated
carrier, that same
amplifier might be
down around 25%
plate efficiency. A safe
general rule
for linear
amplifiers is output
device power
dissipation is three
times the carrier
power when
amplifying the
unmodulated carrier.

This means a
legal-limit AM
linear could have
dissipation during
carrier conditions
of 375 watts, and on
positive modulation
peaks output power
watts with 1500
watts of short-term
dissipation. This is
a reasonable safe
estimate.

If
a conventional AM
linear or screen
modulated stage is
making more than
half the peak
efficiency on
carrier, odds are
very good it has
excessive distortion
and splatter.

Linear Amplifiers on AM, or the Difference between Low Level and High Level Modulation

It may not seem like it at first glance, but grid modulation and linear
amplifiers use similar principles.  As such, they can be discussed
together.

Low level modulation
often has much less
distortion and more
fidelity than high
level modulation of
tetrodes, and more
faithfully
reproduces the audio
input. It is much
easier to have low-distortion high-fidelity audio using
low-level
modulation. To be
sure, some of the
cleanest AM BC
transmitters ever
built were low level
modulated systems.
Unfortunately the
low efficiency
resulted in high
energy consumption,
causing most
stations to use more
energy efficient
high level
modulation.

The sole shortfall
with linear amplifier or
grid modulation schemes is
efficiency. In order
to reproduce the
input faithfully,
the amplifier has to
the PEAK power. This
is normally four
times the carrier
power (or more in
some cases). This is
because the linear
has to be
“efficiency
modulated”.
A safe estimate is
25% carrier
efficiency. This
be making three
times the heat as
carrier power. An
SB220 can safely
dissipation
to fully use the
tubes) so it is safe
at 125 watts carrier
when properly tuned.

Very few amplifiers
can safely
handle legal limit
AM. Legal limit AM
requires
375 watts of carrier
power,
and three times
carrier power would
be a safe power
amplifier
dissipation estimate
for
carrier-level heat dissipation.
Typically, with a
375-watt carrier,
over 1100 watts
of heat is produced.
This
takes a lot of air
and a 1200-watt or
higher plate
dissipation tube. An
8877 at full rated
airflow, or a
3CX1200 series tube, would work.

A rig certainly does
NOT need to be plate
modulated to sound
perfect, and as a
matter of fact most
amateur plate
modulated
transmitters have
terrible distortion
as a percentage of
modulation. It’s
just that most
people can’t
actually hear the
distortion, they
listen to and enjoy
the frequency
response and might
actually “like” a
little distortion,
and they confuse
distortion with good
sound.
Contrary to popular myth,
there is no
difference in the
sound of any AM
transmitter when
amplified in a
properly tuned and
operated linear
amplifier. This is
because a properly
tuned and operated
linear, be it a
Heath SB220 or
anything else, has
much less modulation distortion
than the typical
boatanchor rig. The
real problem with a
linear is NOT the
sound. The
real problem is

heat

caused by poor
carrier efficiency.

It’s certainly
low level
modulation, but
plate modulating a
tetrode also guarantees
we have to do
special tuning and
to avoid significant
distortion. While
the plate modulated
tetrode system
reduces problems
power, and heat, it
does not eliminate
these problems.
high-level
modulation requires
a high power
modulation source
with low distortion
fidelity.

To be linear all stages must be tuned or loaded at full peak
envelope power, plus a little safety factor. In other words if we are going to
1500 watts PEP output, we must load the amplifier stages to 1500 watts carrier
or more! After loading at full peak power, carrier is set at less than 25% of
the peak power. Failure to do this will result in
modulation distortion called “flat-topping”. The result will be very wide
bandwidth splatter and “downward modulation”.

If we are going to run 100 watts AM carrier, all stages must
be tuned for at least 400 watts of peak power.

HF SSB rigs on AM

transceivers are
excellent on AM,
with much less
distortion and
better fidelity than
most older amateur
AM rigs. The Ten-Tec
Orion and the Yaesu
FT-1000D are two
examples of very
good AM
transceivers.

Peak envelope power,
with 100%
modulation, is four
times carrier power.
For 100% modulation
must run 25 watts or
less carrier. With a
power on voice peaks
should be held to
100 watts or less.

I used an
IC-751A or IC-706
ICOM on AM. The
problem with that
ICOM, like with many
uses ALC to
limit output power. Turning
the output power
down or mic control
up will not increase
the percentage of
positive peaks. This
is because the
ALC system in most
SSB rigs almost
always detects
peak power. If we
watts and try to
modulate 100% (100
watts), the peaks
reduce gain until
peaks are back at
the 25 watt carrier power
level. The positive
peaks stay at 25
watts or so…and
the carrier drops to
7.5 watts when
modulated!

The cure is to run
the power level all
the way wide open
and apply an
external stable
negative voltage to
the external ALC
external negative
ALC until the
carrier is 20-25
watts, and then the
mic control until
we have 100 watts
on peaks using a
meter.

An external ALC
carrier control can
be a 9V battery
across a 500k pot in
a voltage divider.
The positive battery
and the ALC output
comes from the pot
wiper. P1 goes to
the EXT ALC jack on

Remember to
disconnect the
battery when using
other modes or when

To
use this circuit,
normal power control