August 1931 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
By the early 1930s, commercial broadcast stations were still working
out what would be the best combination of channel bandwidth and
spacing to enable a maximum number of adjacent transmissions while
achieving sufficient selectivity to enable acceptable reception.
5 kHz was deemed reasonable to reproduce the human voice as
well as musical instruments. An accompanying 10 kHz channel
separation (still in effect today) was adopted to accommodate upper
and lower sidebands that amplitude modulation creates. Interestingly,
if you read carefully, the Stenode's high level of selectivity,
made possible by an integrated crystal, was intended to remove modulation
sidebands and thereby significantly narrow the required bandwidth.
A debate within the engineering community existed then regarding
the usefulness of and need for upper and lower sidebands.
The Stenode's success was dependent upon being able to design
a receiver circuit with the ability to 'recreate' sidebands from
the carrier. That presents a dilemma mathematically since the information
in an ideal AM scheme is contained entirely in its sidebands. If
the sidebands are removed, that information is lost forever unless
it is present somewhere else, like on the carrier itself, which
means pure amplitude modulation is not occurring. It is commonplace
to recover a suppressed carrier from the sideband(s), but recovering
the sidebands from a pure sinewave carrier is impossible.
Two separate advertisements by the
Corporation of America appeared in this issue.
Stenode's Selectivity Revolutionary
How station and other radio interference is overcome by the
application of a novel, though simple, method of intermediate-frequency
tuning and compensation
Fig. A - The 175-kc. crystal is C, supported
by the cord F between electrodes B-D. E is an insulating holder,
and A the upper lead.
The significance of the Stenode principle, in the development
of both broadcasting and television, promises to be very considerable.
Its general extension means great enlargement of transmitting facilities,
less interference, bigger and better television images.
While these things, so far as transmitting technique is concerned,
are some distance in the future, the Stenode receiver is now an
accomplished fact, having reached the stage when it is ready for
the application of production methods.
Stenode receivers, built up of standard parts, with the exception
of the crystal-control tube, have been demonstrated to representatives
of Radio-Craft. Their selectivity, in the presence of a high-power
local (WABC) is great; the local could be tuned in and out in a
twinkling, seeming to occupy but a portion of its channel on the
dial. The marked feature, however, in comparison with standard receivers,
is the apparent great reduction of noise, permitting a distant station
to come through on the Stenode while, on the other sets, it is lost
in its background.
While constructional data and circuit constants are not available
for this article, it will be but a short time before manufacturers
are ready to release the necessary material for constructors and,
later, superheterodynes of different Stenode models for the set
No radio invention of the past few years has occasioned such
general interest and discussion, in the engineering world, as the
Stenode system of transmission and reception; of which a general
explanation was given some months ago in the pages of Radio-Craft*.
However, it is now assuming, not only technical but commercial interest
in the United States, as the result of systematic work in which
the engineers of the Stenode Corporation have been quietly engaged
during the past year.
As the original article explained, the Stenode principle, invented
by a British scientist, Dr. James Robinson, was conceived in an
endeavor to make more efficient use of the wavebands allotted for
transmission purposes. For that purpose, Dr. Robinson designed a
transmitter which is to make use of an extremely "narrow channel"
(in Greek, stenos odos, from which the name); and thereby make it
possible to operate without interference twenty, or perhaps a hundred,
stations where one was hard-pressed for room before. However, such
a transmitter requires a special receiver; for it cannot be received
intelligently on one of ordinary type.
The first interpretation of his idea, therefore, by the radio
profession was that a complete revolution in radio, and simultaneous
scrapping of all existing material, would be demanded. However,
the very interesting development has been brought out, that not
only is the Stenode receiver suitable for the reception of any existing
broadcast stations, but it has an actual superiority for that purpose,
by reason of its extraordinary selectivity. For that reason, the
Stenode receiver has assumed an immediate importance, without waiting
for the development of the special transmitter.
At the recent radio trade show in Chicago, in fact, the Stenode
demonstration receivers were subjects of principal interest; as
a considerable number of manufacturers, both of receivers and of
components, have been investigating the commercial possibilities,
and several of them have arranged for licenses under the basic patents.
For the set constructor, first of all, and very soon the radio dealer
and Service Man, some further explanation of the Stenode is therefore
of practical value.
The Limits of Selectivity
1 (right) - The dark area C, indicating the selectivity of a crystal-controlled
Stenode, is compared with a very sharp-tuning superheterodyne of
normal circuit (B) and a band-selector tuned R.F. set (A). The area
between curves indicates interference eliminated by the Stenode.
First of all, in every technical discussion, comes the question:
sidebands or no sidebands? This has been debated by the best mathematicians
in the profession, and the formulas which have flown back and forth
are entirely too complicated for general exposition. Suffice it
to say, however, that calculations may be made on the basis of almost
any conception of the radio wave - as a carrier of constant frequency
hut varying amplitude - as a carrier of constant amplitude but varying
frequency - as a carrier accompanied by sidebands - or an undivided
complex wave - and all will bring us finally to the same point.
However, when a carrier, or continuous radio-frequency wave,
is modulated by audio frequencies of varying pitches, timbres and
intensities, the effect we call "sidebands" is created; if this
carrier, so modulated, is received by a properly-tuned circuit connected
to a faithful amplifier and reproducer system (including of course
a detector) the modulations are reproduced in their original form
- the carrier wave being cancelled out. Furthermore, with a system
of this kind, the result of tuning which is "too sharp" is to suppress
the higher tones in the output; this fact was early discovered in
the effort to obtain selectivity through cascaded R. F. amplifiers.
As a practical compromise, it has been assumed that audio tones
above 5,000 cycles may be dispensed with in radio reproduction;
that spacing the carrier-wave frequencies of broadcast stations
ten kilocycles (10,000 cycles) apart will then prevent their interference;
but that the number of broadcast stations is thereby naturally limited
and, to increase their number, the quality of broadcasting must
needs be reduced.
So, to make full use of the ten-kc. broadcast channels, for musical
transmission, it was necessary that receivers should be correspondingly
designed. We therefore find in modern receivers the "flat-top" band-pass
filter; designed to permit practically equal amplification of all
the sidebands, and proper reproduction of the higher audio frequencies.
It therefore seemed like flying in the face, not merely of tradition,
but of the laws of nature, when Dr. Robinson produced his Stenode
and proposed to receive the program of a broadcast station, with
all the high tones of articulated speech and overtones of its musical
instruments, yet without the aid of its sidebands. The public, and
even the radio experts, had been so long told that they must choose
between sensitivity and selectivity, and that they must give up
one, so long as they demanded the other, that the proposition seemed
Fig. 2 - The Stenode intermediate amplifier;
varying C regulates the sharpness of the tuning. The compensator
is a very low-impedance coupling, reducing the low notes to proper
Restoring the Quality
Yet the fundamental principle is a simple one; that, even though
the sidebands are cut off, by a circuit tuned far more sharply than
the standard three- or four-stage R. F. amplifier with its low-loss
coils and condensers, and only the naked carrier is received, that
carrier is still modulated, if only by the ghosts of its sidebands.
And, after that carrier has been separated from all interference,
those sidebands can be restored to their full original values by
the employment of proper compensation in the audio amplifier.
There is no gain without some loss; when the higher frequencies
have been brought down to the points of suppression, additional
amplification is needed to bring them back up to full value. But,
in the standard modern receiver, there is already a high reserve
of amplification which is thrown away (as in the volume control,
the variable-mu tubes, etc.) simply for the purpose of getting rid
of interference. This reserve may well be transferred to the audio
Fig. 3 - The principle (though not the actual
curve) of the Stenode's compensator is shown here. It brings up
to proper volume the high, notes suppressed in the crystal circuit.
What advantage is thus gained? Has it been simply a matter of
filling Paul's pockets at the expense of Peter? No.
The selectivity thus gained in the R. F. channel has enabled
us to escape, not only the interference of adjacent stations, over-powering
in the urban areas containing the largest number of receivers, but
also a large portion of the thousand and one other forms of natural
and artificial interference. Since the last named are distributed
over all parts of the broadcast spectrum, the narrower our tuning,
the greater will be the ratio of signal to static. In this connection,
Fig. 1 is of interest.
The two outer curves are those, respectively. of a high-grade
"band-selector" R. F. receiver and of a high-grade superheterodyne,
as plotted by their manufacturers. According to the previously-accepted
principles of design, they present the most favorable compromise
of selectivity with quality. The dark, shaded center area is the
selective curve of a Stenode, as determined in the laboratory of
a large manufacturing company which was testing it. It is a visible
demonstration of literal "razor-edge" selectivity.
This curve is obtained, as explained in previous articles, by
the introduction of a quartz crystal or "gate" (Q) into the intermediate-frequency
amplifier of the Stenode (Fig. 2). The crystal has an extremely
well-defined natural frequency of its own, serving here to pass
signals very close to that value and to suppress others almost completely.
The crystal is the heart of the Stenode circuit. To grind it
to an exact assigned frequency, as done for a transmitter, would
be a work of very exacting nature ; but it suffices to bring the
crystal approximately to a frequency of 175 kilocycles (which is
in accordance with American commercial superheterodyne practice),
and then to adjust the amplifier to the small extent necessary to
give maximum efficiency with the particular crystal.
The frequency of the intermediate amplifier being thus determined,
it is necessary merely to tune the frequency-changer (of any standard
type) to the carrier of the station which it is desired to receive,
and to bring the oscillator frequency along with it until the intermediate
frequency reaches that of the crystal gate. This is a point of great
exactness; with an ordinary dial, the station would jump in and
out almost simultaneously.
One of the Stenode laboratory models, of the
type familiar in commercial superheterodyne chasses. The crystal-tube
is shielded, like the others.
Precision Tuning Controls
Amateurs and short-wave fans are familiar with fine tuning; but
the broadcast listeners of today have not been brought up to this
necessity. However, to meet the situation, the Stenode engineers
have worked out a tuning dial with a super-vernier; and this will
be put on the market by one or more manufacturers. It has a ratio
of 10 to 1, for "rough" adjustment; and of 200 to 1 for the fine
setting required to hit a station "right on the nose." An extremely
low capacity trimmer is adjusted for the oscillator. 'When this
is done, the astonishing freedom from background noise and the clearness
of reproduction are quite surprising, well worth the added control.
The next question arises, what is to be done with this "razor-edged"
selectivity? (From an ordinary receiver, it is obvious, there would
be no reproduction whatever of speech.) The answer is found in a
compensating circuit which brings up the detector output on the
high tones and suppresses it on the low tones; thus exactly reversing
the effect of the crystal on the modulated intermediate-frequency
signal. The idea is illustrated, without reference to an actual
characteristic curve, in Fig. 3. The output, if amplified in the
ordinary way, would be all low-note - lower perhaps than the lowest
musical tone; while an ordinary receiver, feeding into the Stenode
coupling device, would give nothing audibly but the highest whistles.
The composite effect of the two is to produce a straight line, theoretically;
as a matter of fact, straight lines are not found in practical radio
engineering, and the output overall characteristic of the receiver
shows the rises and dips usually associated with an output of high
The crystal, it was observed, is the heart of the Stenode. Any
type of frequency changer, built with the necessary fine-tuning
controls, may be used; and any type of intermediate amplifier, the
tuning of which also need not be critical. The audio amplifier after
the compensating coupler may be of any type; it is interesting to
note that certain audio devices of poor quality (because of their
discrimination against low notes) have been found very useful in
making up experimental models, in which their former failings became
The first crystals employed in Stenode models were ground to
a low intermediate frequency; but those employed in later work conform
to American practice. A number were manufactured in England, under
the direction of the British Radiostat Corporation, which first
brought out the Stenode; and it is one of these which is illustrated
in Fig. A. Arrangements, however, were completed recently for their
manufacture in America. The vacuum-tube mounting shown is the most
efficient method of protecting the crystal and ensuring it practically
unchanged operating conditions.
The Stenode Corporation, which is introducing the new system,
is a holding and development corporation only, and not a manufacturer.
It is stated that, within a very short time, several types of kits
and commercial receivers will be available from their licensees.
Other developments, such as Stenode television receivers, for which
special high-frequency compensators will be necessary, and Stenode
telegraph and printing equipment, especially for land-line work,
are still in the laboratory stage; but promise to be highly interesting.
Radio-Craft will as usual, keep its readers informed as new developments
* See "The
Stenode Radiostat System." by Clyde J. Fitch, in Radio-Craft
for October, 1930.
Posted November 18, 2015