July 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Author Howard Wright takes the opportunity here to distill the concept
of modulation down to its basic operation while dispensing with
the garbled mix of "graphs, formulas, charts, vectors, diagrams,
and Greek letters which often enter into various discussions of
modulation". Wright describes how to the uninitiated radio dial
spinner, the culmination of events occurring behind the scenes in
an AM reception is akin to knowing "that, to be reproduced, the
picture [in a magazine] was broken down into its primary colors,
if all we had to go by was the original print and the magazine?"
That is a very apt comparison.
Low-Pressure Modulation Facts
Down-to-Earth Talk About Radiotelephony
By Howard Wright,* W1PNB
Several years ago, having built and operated several successful
amateur radiotelephone transmitters, I was reasonably satisfied
with my knowledge of 'phone principles. After all, they didn't seem
too complicated, if one could manage to ignore the inconsistencies
that showed up now and then. To the best of my memory, I used to
consider modulation from about the following point of view:
"Sidebands a condition somewhat resembling
a case of measles."
"The r.f. section of a transmitter consists of a carrier-generating
exciter and a final amplifier that amplifies the carrier and passes
it along to the antenna. To use this typical c.w. transmitter for
'phone operation, we merely couple voice power to the final amplifier
through a modulation transformer. The voice power is then in series
with the power supply to the final. Therefore, the level of the
carrier is varied above and below its original value at an audio
rate. This is called 'modulating' the carrier and is done to allow
the voice signal to be recovered by a receiver."
As I said before, I was quite happy with the above understanding
of a 'phone transmitter. I suspect that there are many amateurs
who are getting along nicely today on similar ideas.
And then came single sideband! Formerly, I had considered sidebands
as a condition somewhat resembling a case of measles, occurring
only on unhappily adjusted transmitters. With the introduction of
amateur s.s.b. techniques, we were informed that sidebands are completely
normal and honorable properties of all stations. Concerning modulation,
as the saying goes, "I didn't know from nothing."
At this point you may suspect that I have become one of those
incurable single-sideband enthusiasts. To this thought I happily
plead guilty, but don't leave. I'll try to keep the propaganda to
This subject of modulation has been well covered in many excellent
articles in the past few years. If it hadn't, I would never be here
with pen in hand. I am not covering any new ground, but trying to
present the material in a form that may be helpful to those who
are not on the best of terms with the graphs, formulas, charts,
vectors, diagrams, and Greek letters which often enter into various
discussions of modulation.
In my estimation there are two main reasons for the lack of a
better general understanding of 'phone principles. The first is
that, in the hands of a person without much theoretical knowledge,
even the best of receivers tends to create a false impression of
the true nature of incoming signals. I will cover this more thoroughly
at a later time. The second reason seems to be our inability to
connect the modulation of a transmitter with the heterodyne process.
But there I go, taking for granted that everyone knows what the
heterodyne process is. Let's back up several steps.
At some point in our conversion from broadcast listeners to radio
amateurs, we discover a complicated electronic jargon. To most of
us, this amounts to a new language that must be learned if we are
not to be baffled by the simplest of statements. Among the new terms
that tend to add most to the confusion of a beginner are: beat,
heterodyne, convert, mix and modulate. The fact that most of us
never recognize is that all of these imposing terms mean exactly
the same thing. The different words are only used as a matter of
convenience, in indicating some general type of circuit.
Now it's not my purpose to try to explain why the heterodyne
process works. To put it briefly, however, here is how it works.
Combine any two a.c. signals (regardless of frequency) in a suitable
circuit and two new signals will appear that are the sum and difference
of the originals. Probably most of us are familiar enough with the
operation of modern receivers to let us stop at this definition
of the heterodyne process.
At this point, it may seem that I have strayed quite a way from
the subject of modulating a final amplifier. Not so! The term "modulate"
is included in the interchangeable group of words that includes"
mix" and" convert" - necessary processes in your receiver's operation.
I hope it is obvious that I am trying to point out that the action
of the little converter tube in your receiver is exactly the same
as the process taking place in your final during modulation. Of
course, there are vast differences of amplitude and frequency in
these circuits. Nevertheless, if you can see their basic similarity
and actually start thinking of modulation in terms of converting,
mixing, heterodyning, beating, or whatever you want to call it,
you have nearly won the battle of understanding 'phone principles.
"Their basic similarity..."
Think of that pair of tubes in the final of your 'phone transmitter
as a mixer. I'll give more details later, but now back to the subject
of deceiving receivers.
Before you rise to the defense of your particular high-priced
beauty, let me hurry to state that receivers only tell lies to people
who don't realize that a receiver, designed for broadcast-band type
reception, inherently disguises the true nature of incoming signals.
Here is a typical example:
A neophyte tunes his receiver across an unmodulated carrier.
The receiver tells him that the carrier is a certain number of kilocycles
wide. The neophyte immediately starts a frantic and futile investigation
to discover why one carrier is broader than another.
Now let's have a man who has studied receivers tune the same
receiver across the same carrier. He also sees that the carrier
occupies space on the dial but, knowing that a carrier has no width,
he realizes that the carrier is telling him the selectivity, or
"bandwidth," of the receiver.
This case of the unmodulated carrier is bad enough, but the receiver
is designed to perform a masterpiece of deception in the case of
a modulated 'phone signal. It does a perfect job of gathering in
the various parts of the signal, eliminating any evidence of the
presence of the sidebands theory tells us were transmitted, and
combining the sidebands with the carrier in such a way that it appears
that the voice is simply superimposed upon the space supposedly
occupied by the carrier. So complete is this deception, that it
might be compared to the reproduction of a color photograph in a
magazine. How would we ever know that, to be reproduced, the picture
was broken down into its primary colors, if all we had to go by
was the original print and the magazine?
"I can't quite give you a T9, OM"
Before we finish this business of generating sidebands, there
is one very important concept to grasp. No intelligence (modulation)
can be transmitted without taking up room in the spectrum. Couple
this statement to the previously mentioned fact that a carrier occupies
no space and there is only one conclusion to be drawn. The modulation
can in no way be" on the carrier." It must consist of appropriate
new signals at frequencies "alongside the carrier." If we recall
what has been said about heterodyning signals, it takes no genius
to see that the voice power from the audio system of a transmitter,
when applied to the final amplifier (or mixer) doesn't affect the
carrier in any way. It can't. Following the theory of mixing, it
combines with the carrier frequency to generate new r.f. signals,
both above and below the carrier, which can certainly be considered
as riding" alongside the carrier."
Now is the point where drawing a cute little diagram of carrier
and sidebands appears attractive, but let's proceed with just words.
Take the case of an ordinary amateur 'phone transmitter. For
the sake of discussion, let's say that the carrier frequency is
adjusted to exactly 3900 kc. Now this happens to be a fine transmitter,
except for one fact. There isn't enough filtering in the audio power
supply, Of course, the result is a signal plagued with 120-cycle
hum or, in the words of c.w., "I can't quite give you a T9, OM."
Constantly remembering all we know about mixing, let's take a
theoretical look at our hum-modulated signal. The hum voltage of
120 cycles should mix with the 3900-kc. carrier and produce new
signals of 3900.120 and 3899.880 kc. In other words, we should now
have three separate signals, the strongest being the original, flanked
on either side by a "hum side-frequency" 120 cycles away.
Until now, references to receivers may not have seemed too flattering.
This, however, had only to do with the listener's lack of ability
to interpret what he heard. Now, let's use our receiver to tie down
theoretical reasoning to what we actually hear. Simply turn on the
receiver's b.f.o. and tune carefully across the hum-modulated signal.
Presto! We hear three distinct points of "zero beat." We have three
signals. We have exact confirmation of the heterodyne theory of
If you're somewhat confused by my use of hum voltage in the above
example, don't be. It was simply used in place of "a single audio
tone," which is often used in explanations of sideband generation.
Of course, hum is a far cry from the actual voice signals we use
to modulate our transmitters, but the heterodyne principle remains
unchanged, regardless of the type or complexity of the modulating
signal. The voice contains a great number of individual frequencies
which beat with the carrier. Each resulting new r.f. signal generated
still maintains its original audio-frequency relationship with each
of its neighbors, even though the whole business has been shifted
up into the r.f. part of the spectrum.
Due to the heterodyne action, our complete band of audio frequencies
is reproduced, not only once, but in exact duplicate on either side
of the carrier. Thus we have the sidebands that have been discussed
so much in recent years. Considering the original audio frequencies,
we might think of the sidebands as being "back-to-back." The lowest-pitched
sounds are close together alongside the carrier and the higher ones,
progressively removed from each other, cause the complete signal
to be twice as wide as the highest tone transmitted.
That's about the story of conventional a.m. If propagation conditions
are good and no mishaps (such as interference) befall this complicated
group of signals, we are all set to have the receiver perform its
magic and restore human-sounding values to the finished product.
S. S. B. Techniques
Now, let's take a look at single-sideband techniques, which have
almost completely taken over transoceanic telephone service and
are enjoying ever-increasing popularity with radio amateurs. This
definitely is not a drop-off point into the mysteries of complicated
electronics. If you once manage to grasp a firm understanding of
the regular double-sideband signals we have been discussing, single
sideband is only a small step away. After all, if we understand
the whole of any subject, the study of one of its parts shouldn't
be too difficult!
Rather than start directly with s.s.b. transmitters, let's return
to the though of converting in a regular receiver. From one point
of view, every superhet is a s.s.b. receiver in two respects. The
first, one which seldom needs to be considered, is this: In converting
incoming signals down to the intermediate frequency, the new frequency
(or sideband) caused by the difference between the incoming signal
and oscillator is the one that is used. The theory of heterodyning
tells us that the sum of these frequencies is also present at the
output of the converter. This sum frequency is so far removed from
the i.f. that it is eliminated by the filtering action of following
The more important reason for considering a receiver as having
s.s.b. action concerns "image" reduction. Due to the heterodyne
process, if no selectivity precedes the converter, the receiver
is sensitive to two frequencies. One is above and the other below
the oscillator by an amount equal to the i.f. I believe that most
of us are familiar with the drawbacks of having bad r.f. "images"
or, in other words, having each signal appear at two points on the
Here is the connection between receiver images and s.s.b. transmitters.
The act of adding r.f. selectivity to the front end of a receiver
to reduce the image is exactly the same process, in reverse, as
adding a selective filter to a double-sideband transmitter to reduce
the" image sideband." The only difference is that the receiver is
purposely designed so that the image can be reduced by the use of
a few simple tuned circuits preceding the converter. In a transmitter,
the sidebands produced by modulation (conversion in a receiver)
are separated only by a relatively few cycles and are therefore
more difficult to divide by filtering methods.
Until recent years, equipment selective enough to separate and
suppress one sideband was either nonexistent or very complicated.
No doubt the basic advantages of transmitting only one sideband
were realized as early as those of having image rejection in a receiver.
However, in the case of the transmitter, the power in both sidebands
was recoverable by the receiver, plenty of space was available in
the spectrum, and no simple and effective way was available to eliminate
one of the sidebands. Thus, we have the predominance of a.m. as
we know it.
You may have noticed that I have not stressed the "suppressed
carrier" part of s.s.b. There is enough material contained in this
subject to fill a book, but it is distinctly a separate subject
from "single sideband." A very large part of both the superiority
of s.s.b. systems and the furor caused by the appearance of s.s.b.
signals on receivers tuned for regular operation can be attributed
directly to carrier suppression and not to the elimination of one
sideband. This, however, is an article on modulation, so let's stick
to the sidebands.
Now, if we had to give a definition of single sideband, we could
call it the suppression of an "image sideband" for the purpose of
reducing to a minimum the frequency band necessary to transmit a
given amount of intelligence. Because the filter method is used
for reducing the unwanted sideband or "image" in a receiver, we
will first consider this method as applying to transmitters.
A carrier is modulated in the ordinary way, producing identical
sidebands on either side of the carrier. These sidebands arid the
carrier are fed into a very sharp filter which passes one sideband
and suppresses the other even though they are very close together.
There are LC filters, crystal filters, and mechanical filters. They
can all be built to do a good job of separating sidebands, but all
have the common property of having better selectivity as their design
frequency is made lower. This is the reason why practically all
filter-type single-sideband transmitters use receiver-type heterodyne
methods to convert to the desired band from the lower frequency
at which the filter works well.
The "phasing" method of s.s.b. generation employs theories which
certainly seem to belong to people with engineering degrees. However,
the theory of filtering is also basically very complicated, but
we have been using different types of filters so long that we tend
to leave their mystic properties to the experts. Let's describe
the phasing system in terms similar to those used for filtering.
Each sideband is broken up into two parts by the use of a few
craftily chosen resistors and condensers, a couple of tuned circuits,
and a certain amount of adjustment. These parts of each sideband
differ from each other only in that the times when any given thing
happens are different ("phase shift" to an expert). The four signals
thus produced are combined in another tuned circuit so that the
parts of one sideband "beat each other's brains out." The parts
of the other sideband take an immediate liking to each other and
combine to form the signal intelligence to be transmitted.
The phasing method is not limited to low frequencies. It works
as well at 50 Mc. as at 50 kc. However, for reasons of operating
convenience, the signal is often generated at some point outside
the band and heterodyned in.
Before I finish, let me say that the previously mentioned s.s.b.
properties of receivers should, in no way, be confused with the
general meaning of the term, "selectable-sideband receiver." Such
a receiver is actually able to remove the "audio image" from any
incoming signal. In plain words, it listens to either sideband and
rejects the other. Either phasing or filter methods are used in
selectable-sideband reception. In fact, the very parts used in a
transmitter can almost always be used in a receiver.
In conclusion, if the above discussion of modulation differs
so widely from your ideas on the subject that you tend to become
discouraged, remember that not knowing all of the "lowdown" on such
matters can't stop us in getting a lot of pleasure from amateur
radio. However, there isn't any doubt that the mechanics of radio
communications are becoming more complicated every day.
If we are to have any chance of keeping up with technical developments,
the one thing we can't afford to overlook is this business of mixing
- sorry, I meant" modulation!"
Posted April 16, 2016