October 1930 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Frequency crowding has evidently been an issue since the early
days of radio according to this 1930 article in Radio-Craft
magazine. The situation was really bad in the earliest times
when unfiltered spark type transmitters were the norm. Those
pioneers could be credited, I suppose, with being the first
users of wideband communications, but it was not because they
chose to do so. Here author Clyde Fitch discusses the debate
over whether there really were such things as sidebands from
modulation (yes, it was questionable
in the day) and makes an argument for their existence
based on analysis of various types of modulation. In particular,
he predicts the coming popularity of single sideband receivers
with crystal-filtered channels (also
a relatively new concept), and the need for matching
SSB transmitters with... wait for it... carrier and sideband
The "Stenode Radiostat" System
The invention which promises to revolutionize radio practice
is explained here
By Clyde J. Fitch
Since radio broadcasting struck its stride, the ether has
seemed too narrow for all who wish to put it to use. The system
described here promises to make it possible to put a hundred
stations on the air, where there was one before. Yet its adoption
requires not only special transmitters, but costly crystal-controlled
receivers; and will therefore be a matter of time.
Fig. 1 - The superheterodyne action above
creates a beat-frequency c, as a transmitter creates a wave
d, which is similarly modulated in amplitude. The "Stenode,"
on the other hand, uses a frequency-modulated wave e.
Still, it must come; and the alert radio man will therefore
desire to familiarize himself with it at the earliest moment.
The recent announcement that Dr. James Robinson, an English
inventor, has perfected a receiver which gives high-quality
reproduction of radio telephony on what is practically a carrier
wave alone (the frequency "channel" required is stated to be
but from 20 to 50 cycles wide, instead of the 10,000-cycle channel
which is standard in the United States, or the 9,000-cycle channel
used in Europe) has revived throughout radio circles the discussion
of the much-mooted question of "sidebands."
Do sidebands in radio waves really exist? And, if so, what
is their nature?
The answer is of the highest practical importance. Throughout
the world, nations and individuals are endeavoring to increase
the number of radio stations to the maximum which is practicable
without causing undue interference and, in the case of broadcast
stations, impairment of the tone quality of the reproduced voice
To explain the points in controversy among radio engineers
and physicists, as well as to describe the principle of the
new invention, is the purpose of this article.
What are Sidebands - If Any?
It was once customary, in the early days of radio, to think
of a wave as having only a single frequency, with current and
voltage variations proportioned in amount or amplitude, to the
audio-frequency impulses with which it was modulated. But, after
hundreds of stations came on the air, it was obvious that a
station of one frequency interferes with a station on another
frequency and that, when a receiver is tuned so sharply that
the interfering station is cut out, something is lost also from
the higher notes of the station to which the set was exactly
tuned. We express this fact by saying that the loss of tone
quality is due to "cutting sidebands."
The simple explanation, which satisfied most radio fans who
could not go into the mathematics of the thing, is that: to
the carrier-wave frequency there is added a whole band of frequencies
on either side, which are equal numerically to twice the highest
audio frequency used in modulation. That is to say, if a 1,000-kilocycle
carrier wave is modulated by audio frequencies up to 5,000 cycles,
there results a compound wave varying in frequency from 995
to 1,005 kilocycles and, consequently, 10 kilocycles wide. The
five-kilocycle bands of frequencies, above and below the carrier
frequency, were known as "upper and lower sidebands."
However, many radio engineers have never been satisfied with
such an explanation; and others, admitting the existence of
sidebands, saw no reason why they should be necessary in broadcasting.
Sir Ambrose Fleming, inventor of the two-element vacuum tube,
and a noted mathematical authority, doubts as to whether the
sideband is anything but a convenient mathematical assumption,
or has any real existence. Sir Oliver Lodge, the dean of radio
research, says that a deep philosophical question seems to be
However, let us consider the matter in the simplest possible
The Heterodyne Theory
Suppose we forget our old ideas of "modulated radio waves"
and look at the situation from a different viewpoint. Take the
superheterodyne; we know that the superheterodyne works,
and we claim to understand its theory. Well then, apply the
same theory to the radio broadcast station and our problem is
partly solved. This idea was described by the writer, at some
length, in the November, 1929, issue of Radio-Craft.
In accordance with the heterodyne theory, which has been
verified by the action of the many superheterodyne receivers
now in use, when two frequencies a and b are combined, two others
or "beat" frequencies are produced, namely, a+b, and a-b. Harmonics
may also appear; but they are of no importance in this discussion.
It will be observed, when analyzing the superheterodyne circuit,
that the beat frequencies a+ band a-b appear only after the
frequencies have been passed through a detector - the first
detector. This is shown in Fig. 1. Here the frequencies a and
b are combined or mixed to produce the composite effect shown
at c. After passing through the first detector, the beat or
difference-frequency a-b appears. For simplicity, the sum-frequency
a+b is omitted; as, usually, only one beat note of the superheterodyne
We can apply this theory to the broadcast transmitter, and thereby
explain its action clearly. In the transmitter, we have the
carrier frequency, a' combined with the audio frequency b',
producing the composite effect shown at c' (the frequencies
are added algebraically on a graph paper) and this, in turn
is converted into that shown at d by means of the limiting action
of the transmitter's oscillating tubes, In other words, these
tubes also perform a function similar to that of the first detector
in a superheterodyne. The wave that leaves the transmitting
aerial is therefore of the shape indicated at d. This is in
accordance with observed facts.
Now if you will compare them, you will note that the wave
shape at d is the same as that at c. Since c is composed of
two radio-frequency waves, so also is d. Therefore d must have,
in addition to the carrier frequency, another radio-frequency
or "sideband" wave.
This reasoning, at least, ought to prove that sidebands exist.
The next question is, are sidebands necessary?
Of the four frequencies generated at the transmitter (namely
the audio frequency, the carrier frequency, and the two sideband
frequencies) all except the audio frequency are radiated into
space. This is because audio frequencies do not radiate very
far and the antenna system is not tuned to the audio frequency.
Therefore, three frequencies reach the receiver in a form something
like the composite group indicated at c.
lf we should reverse the process, and subtract the various
sideband waves from the carrier wave (d or c) the result would
be a single "sine wave," representing the unmodulated carrier
wave of the station.
Therefore, if we took the sidebands away from the carrier
by means of suitable filter circuits, the result would be an
unmodulated carrier wave. In other words, removing the sidebands
would demodulate the wave. Partial removal (as in a highly-selective
receiver) would partially demodulate the wave, and result in
poor tone quality. This is actually the case; there is no question
It is safe to say that a pure sine wave has no sidebands,
or harmonics. Altering the shape of the wave, no matter how,
is the same as adding sidebands, or harmonics. Removing these
sidebands or harmonics by means of filter circuits, brings the
wave back to its original pure sine-wave shape.
Carrying our heterodyne example farther, at the receiving
set both the upper and lower sidebands heterodyne with the carrier
frequency, and produce beat-notes, which are intercepted by
the detector and represent the original audio frequencies.
Special Transmission Systems
We can even suppress one of the sidebands at the transmitter;
then let the other heterodyne with the carrier at the receiver,
and produce the audio frequencies without impairing the tone
quality. A single-sideband system of transmission was described
in the June, 1925, issue of the Proceedings of the Institute
of Radio Engineers.
We can even suppress one sideband and the carrier frequency
also, at the transmitter, and radiate the other sideband only.
In this case a new "carrier" frequency must be generated at
the receiver, by means of a local oscillator, in order to produce
the heterodyne effect with the received sideband. This oscillator-frequency
must be exact; if it is too near the nearest sideband frequency,
the audio-frequency beat notes will be too low. They will sound
something like running a phonograph record slowly; except that
the length of time required to complete the message is not reduced.
If the oscillator frequency is too far from the nearest sideband
frequency, the audio-frequency beat notes will be too high and
a "tinny" sound will be the effect. If the frequency is on the
wrong side of the sideband, the high audio notes will be low,
and the low audio notes will be high. This is what is called
"inverted speech"; the system has been demonstrated and described
by Bell telephone engineers, and has been used in transatlantic
telephony to obtain secrecy.
Now comes the demonstration of Dr. Robinson's "Stenode" (narrow
path) system; and it has been hailed as disproving the existence
of sidebands by its operation, apparently without them. However,
the receiver, which is demonstrated together with a transmitter
made by the same inventor, depends upon the principle of "frequency-modulation"
rather than that of "amplitude-modulation," which we have described
as producing sidebands.
The idea of frequency-modulation is an old one - so old that
patents issued on such a system have already expired. But there
was then no commercial opportunity for its use. Several experimenters
have worked on the possibility of a crystal-controlled receiver
which would make possible reception of a greater number of stations,
each on a frequency corresponding exactly to one to which the
receiver would be held; this was discussed at the time of the
institution of the Federal Radio Commission. But Dr. Robinson
has incorporated the crystal control of the receiver into an
intermediate-frequency amplifier, and thereby is able to receive
many frequency-modulated transmissions with a single crystal.
Referring again to Fig. 1, at e' we have the frequency-modulated
wave; compare it with the amplitude-modulated wave d. In one,
the current is constant in frequency and varies in amplitude
or height. In the other, the current remains constant, while
the frequency is varied; as shown by the varying distance between
How this is accomplished is shown in schematic form in Fig.
2, representing a portion of the Stenode transmitter. The speech
currents, impressed through the microphone input on the amplifier,
produce corresponding mechanical vibrations of a plate, which
acts like a loud-speaker diaphragm operated by the electromagnet
Fig. 2 - This diagram indicates the essential
parts of the "Stenode" transmitter, the action of which depends
on the variation of the capacity of the crystal Q and modulation
thereby of the transmitted frequency. (Adapted from Die Woche.)
The width of the air gap separating the plate from the quartz
crystal Q is therefore varied at audio frequencies; and this
affects the period of vibration of the crystal, which is the
master control of the oscillator tube. With this system, a very
slight frequency-modulation is needed - only about 1/300th of
one per cent., as will appear. The variations of the crystal's
oscillating frequency are in this order.
At the receiving end, we find the superheterodyne circuit
of Fig. 3. As stated above, the purpose of employing a double-detection
circuit is to use a single quartz crystal to govern reception
at all broadcast frequencies. Otherwise, it would be necessary
to have a separate crystal for every station to be received-a
physical and economic absurdity. In addition to this, it is
almost impossible to bring two crystals so exactly into unison
as would be required at transmitter and receiver.
Fig. 3 - The "Stenode" receiver shown differs
from ordinary superheterodynes in the circled filter, which
accepts frequency variations not exceeding a hundred cycles.
By the process indicated in Fig. 4, these are turned into a
full band of audio frequencies.
The quartz crystal circuit acts as a highly-selective filter
or "gate" (hence the name Stenode ) through which only a very
narrow band of frequencies may pass - less than a hundred cycles
wide. If the received wave is of constant frequency, no sound
will be heard from the speaker; but if it is varied in frequency,
at intervals comparable to the periods of audio frequencies,
a response will be heard.
Consult Fig. 4. Here we have the selectivity curve of the
quartz crystal of the response is plotted against frequency.
At the bottom, T indicates the signal wave-train, which has
been frequency-modulated at an audio frequency indicated by
the curve A. The response of the crystal, to this variation
in radio, frequency, is to pass current in the manner indicated
by the curve W. This modified signal is then fed into the grid
circuit of the second detector tube, as indicated at the right.
The result is that the presence of the crystal has changed
the frequency-modulated waveform of the input into an amplitude-modulated
wave W; which is detected in the usual manner and converted
into a suitable audio frequency for reproduction.
It will also be seen that, if the frequency of the transmission
varies too much, or shifts beyond the points f1 or
f3, the station will fade. This would appear to be
the case in actual practice. One of the greatest problems of
the Stenode Radiostat is to maintain a constant average frequency
at the transmitter. Slight shifts in frequency will impair the
audio quality. The heterodyne oscillator at the receiving end
must be carefully adjusted and maintained constant, in order
to keep the intermediate frequency at the proper average value
- f2 in Fig. 4. These problems are being, or have been,
solved by Dr. Robinson.
Delicacy of Tuning
The extreme selectivity of this set has made it necessary
to incorporate unusual tuning controls. The ordinary type of
vernier-dial control is entirely too crude; the slightest touch
would tune the station in and out. Therefore, a single-plate
condenser has been placed in parallel with the main tuning condenser;
and this single-plate condenser is equipped with a slow-motion
control, so that one complete turn of the knob varies the capacity
only 1 mmf. Even with this delicacy of tuning, a slight turn
will pass completely by a station, from silence to silence;
for the width of the band passed by the quartz gate is not over
50 cycles - or one two-hundredth of that used in the ordinary
broadcast receiver. This extreme selectivity, of course, is
due entirely to the quartz crystal, and not to the fact that
a superheterodyne circuit is employed. For, if the quartz is
removed and a piece of inert material, such as glass, substituted,
energy passes through (by virtue of the electrostatic capacity
of the electrodes when this capacity is not neutralized); and
the set functions as an ordinary superheterodyne, with the degree
of selectivity common to superheterodynes.
Fig. 4 - The frequency-modulated wave T corresponds
to audio modulation A. Passing through a crystal, with
response characteristic C, it is weakened in proportion to its
variation from the frequency f3, giving the amplitude variations
shown in W.
When the quartz is in place, however, it acts as a gate,
allowing only the narrow 20- to 50-cycle band to pass. To prevent
capacity coupling through the electrodes, a balanced-bridge
circuit is employed. In this bridge circuit the electrode capacity
is neutralized by the condenser NC2; so that the only energy
that can possibly reach the second detector must pass through
the quartz, by piezoelectric action. In Dr. Robinson's laboratory
models, an intermediate frequency of 107 kilocycles is employed.
It is evident that the ordinary, broadly-tuned receiving
set will not give reproduction when tuned in to a frequency-modulated
station. But the question is, will the Stenode receive an amplitude-modulated
station? Theoretically, no. But tests prove otherwise, and we
are interested in facts. These show that, in the modern amplitude-modulated
station, a slight frequency-modulation also exists. And this
slight frequency-modulation is sufficient to actuate the Stenode.
The test, however, would not prove that sidebands are not necessary.
One of the main points of this article is to show that the
results obtained with the Stenode Radiostat fail to prove that
sidebands do not exist or are unnecessary. It is true that the
Stenode receives ordinary stations with a great degree of success
- and it receives those stations whose frequency is controlled
by quartz crystals much better than the others, which fade,
or are hard to tune in. What the tests do prove is that the
ordinary type of station, even though quartz-crystal controlled,
is frequency-modulated as well as amplitude-modulated. The slight
frequency-modulation which exists is due, probably, to some
unexplained circuit conditions.
The amplitude-modulated component of the wave could be entirely
eliminated; its existence is detrimental, since it causes the
low notes to be disproportionally amplified; since the quartz
circuit is one of low damping and the persistence of the lower
notes is considerable. Hence, a special filter is used in the
Stenode's audio amplifier to compensate for this low-note frequency
How about sidebands when using a frequency-modulated transmitter?
Primarily, we radiate waves ranging in frequencies from
f1 to f3, (Fig. 4) a difference of 20 to 50 cycles.
This band of waves we know to exist. But, since changing the
wave-shape adds sidebands, or harmonics, as stated previously,
other frequencies may be introduced. To what extent, if any,
these widen the band is a problem for the mathematicians. Dr.
Robinson's tests indicate that the effect must be negligible.
At least, the improvement over the old method is enough to allow
some hundred times as many stations to transmit at the same
time without interference.
How soon the system will be placed in commercial use, is
difficult to predict. It would necessitate revolutionary changes
in all receiving sets, as well as transmitters, and the expense
would be enormous. Television, which requires the greatest space
in the ether, may be the first to benefit by this system.
Posted September 24, 2015