Stenode's Selectivity Revolutionary, August 1931 Radio-Craft

By the early 1930s when this Stenode vacuum tube article appeared in Radio-Craft magazine, 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.

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.

How station and other radio interference is overcome by the application of a novel, though simple, method of intermediate-frequency tuning and compensation

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 trade.

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.

Fig. 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.

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.

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 incredible.

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 proportions.

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.

One of the Stenode laboratory models, of the type familiar in commercial superheterodyne chasses. The crystal-tube is shielded, like the others.

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 end.

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.

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 quality.

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 virtues.

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 come up.

Stenode's Selectivity RevolutionaryAugust 1931 Radio-Craft

Stenode's Selectivity Revolutionary
August 1931 Radio-Craft