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A step towards better television. A new type of electron multiplier tube specially suitable for television needs was described and demonstrated at a recent meeting of the Institute of Radio Engineers in America, A general description of the valve and its possibilities is given by our correspondent in this article.
Significant advance in television technique was disclosed and demonstrated. before the TIRE in New York, October 23, 1935, by Dr V K Zworykin, Dr G A Morton and Mr L Malter, of the Electronic Research Laboratory of RCA - Victor in Camden, New jersey. This advance, heralded as placing high intensity high detail television images one step nearer realisation, is a new valve that may ultimately rival the thermionic valve used to-day in such vast numbers. This new valve uses secondary emission for purposes of electron multiplication, which is not new. In this case, however, a new technique has been worked out in such a way that the electrons are under complete control to such an extent that valves in quantity may be constructed with the knowledge that they will be interchangeable.
These valves may have a voltage amplification of several million in a single envelope with a signal-to-noise output that is from 60 to 100 times better than any existing amplifier. Furthermore, such an amplifier has a very wide frequency response, making it valuable for television.
The Demonstration
Dr Zworykin, already well known for his kinescope and iconoscope, cathode-ray devices used in transmitting and reproducing moving images, demonstrated a combined photocell and amplifier capable of replacing present-day complicated high-gain amplifier systems of many stages. The valve was not much larger than the ordinary receiving valve, and its output was much quieter than an amplifier made up with an equivalent gain with existing apparatus.
When the light beam was cut off there was no sound from the loud speaker, indicating that no noise was generated within the valve itself. Anyone who has built and operated an amplifier with a voltage gain of one million (120 dB) knows the output noise produced by the 'shot' effect in the input plus the interstage coupling impedance noises all along the line of amplification. There is so little noise in this new valve because there are no coupling impedances; the total noise output is that produced in the input due to shot effect.
Although the immediate application of the valve is to television, or sound movies, where a light source is the actuating impulse, Dr Zworykin stated that thermionic cathodes can be used as well as photo cathodes. Thus it can be seen that the new valve may become a rival of the thermionic amplifier.
In television this amplifier would enable engineers to pick up cleaner signals from the transmitter cathode ray tube and on the output to reproduce the images with less background visible noise. The amplifier would be simple in that it would have but a single valve, which would be about the size of modern receiving valves; it would be noise free, and it would have a wide frequency response. It seems, therefore, fair to suggest that Zworykin and his associates have produced a new device of most significant possibilities. A new technique of low intensity electronics may result, certainly television is brought one step nearer for the man in the street.
How the Tube Works
Fig. 1. - Diagram to illustrate the general arrangement of electrodes and focusing cylinders in an electrostatic type of tube.
Consider any sort of cathode, or electron emitter, say, a photo-cathode for simplicity. Supply an anode on which electrons released from the photo surface may strike. Each electron, if it has the proper accelerating potential between cathode and anode, will liberate several other electrons when it strikes the anode. As many as eight or ten may thus be liberated. In the usual tube, a triode, for example, these new electrons (known as secondaries) immediately fall back upon the anode because there is nothing else for them to strike - this anode is the most positive surface within the field.
In a four-element valve in which the plate may at some instant have a potential less than that of another element, the screen grid for example, these secondary electrons may not go to the anode but to the screen grid, with the familiar result of a dynatron characteristic. A decrease in plate current is produced by an increase in grid voltage.
Theory of Operation
Fig. 2. - Arrangement of electrodes where the target electrodes are self-focusing.
Now, in this new type of valve, suppose we place a second anode with a potential somewhat higher than that of the first anode. Suppose, too, that the first anode is specially coated so that it is a good emitter of electrons. Now, when the first electron strikes the first anode, secondaries are produced, say, eight of them. These eight electrons may be attracted toward the second anode, and upon striking it produce eight new ones for each of the eight original secondaries. Thus the current from the second anode is of the order of 82 or sixty-four. If there are ten such anodes, each emitting secondary, electrons, the gain of the tube, that is the electrons in the final anode circuit produced by the single electron leaving the photo-cathode at the input, would be 810, a very great number.
This is the fundamental process involved in the new valve. In practice, of course, the process is not so simple as this. For one requirement is that the surfaces upon which the electrons strike and which become new cathodes for the following anodes, must be good producers of secondary electrons. And all efforts up to the present time have been towards the development of surfaces which emit few secondary electrons, for in modern valves these secondaries are a nuisance and are to be avoided.
The surfaces worked out to the best advantage by Zworykin and his associates do not seem to differ much from those in high vacuum photocells used industrially or in the sound moving talking picture theatre. Cesium on an oxidised plate of silver seems to be as good an emitter as any. Such a surface will emit as many as 8 to 10 secondary electrons for each primary striking it at voltages of the order of 400 to 600.
Now having secured a good source of secondary electrons it is necessary to make them go where they are to perform their function of producing still more secondary electrons. Here the knowledge gained in making cathode-ray tubes work comes into play. The new art of electron optics is most important in this phase of the multiplier tube of Zworykin.
The electrons must be directed into a beam by electron lenses and then aimed at the proper emitting surface which acts as anode for the first stage and as cathode for the second. This focusing of the diverse emission into a beam may be carried out by electrostatic means, by electromagnetic fields or by combinations of these two effects. An electrostatic type of tube is shown in Fig. 1. Here primary electrons are drawn to the first target. The secondaries are focused into a beam by cylinders to which the proper potentials are applied, and then strike the second anode or target.
Progress in Design
Fig. 3. - Characteristic stage gain where n = number of stages of multiplication. G = Rn.
In another type of valve, more recent than that of Fig. 1, the targets themselves are given such shape and orientation and voltage that they do their own focusing. Such a valve is structurally simpler, naturally, and is the type that will come into active service.
In still another type both electrostatic and electromagnetic fields are used to focus the electrons to force them to their proper anodes. Acting on the suggestion of Slepian, who in 1919 developed a structure shown in Fig. 2 for the purpose of getting a high current cathode, Zworykin and his co-workers have made valves of great interest and scientific value. In this valve there are two rows of parallel surfaces. The lower surfaces are coated and act as emitters or sources of secondary electrons. The top plates are merely deflecting surfaces which, with the aid of an external electromagnetic field, force the electrons to traverse a curved path, first toward the upper deflecting plate and then downward to the appropriate anode (which then becomes a cathode).
Each time an electron strikes one of these surfaces it liberates 8, say, secondaries, and thus if there are 10 such stages the amplification of the tube will be 810. Actually valves have been made, in which gains of several million have been attained.
The lack of noise in such an amplifier is due to the fact that no coupling impedances exist in it, and, contrary to conditions existing in the conventional multi-stage amplifier, no noises additional to the shot effect originating in the input are added as the amplification is built up, stage by stage. Practically, the signal-to-noise ratio in these valves seems to be better by 100 times than that in present-day amplifying circuits. The virtue of this feature in a television scheme cannot be over-emphasised, in fact it seems to be the most important feature of the new valves.
The frequency response of this amplifier seems to be essentially flat over an extremely wide range, sufficient for high-fidelity television systems. The upper limit seems to be that controlled by the transit time of the electrons.
Valves shown by Zworykin about the size of ordinary receiving valves had an output sensitivity of 10 Amperes per lumen of light input - compared with the output of a good vacuum photocell of about 10 microamperes per lumen. The characteristics of the valves are such that they can be made to oscillate, detect, amplify, or modulate.
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