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Describing the operation and advantages of the electron multiplier, with speculations as to the possibility of using this device for television, not with the radio carrier of convention, but with a beam of light.
A short while ago Dr V K Zworykin demonstrated his Electron Multiplier during a lecture before the Institution of Electrical Engineers in London (February, 1936). This instrument is of the greatest possible interest to television engineers, for it will enable them to provide better television pictures; it is the object of this article to explain the reasons for this improvement and to outline a method of television transmission, using visible or invisible light instead of wireless as a carrier.
The electron multiplier consists essentially of a photo-cell and a current amplifier in one envelope and is used instead of a photo-cell and at number of separate valves (voltage amplifiers).
Fig. 1. - Arrangement of cathodes and deflector plates in the electron multiplier. The electron path is shown in dotted lines.
The electron multiplier as developed by Dr Zworykin consists of an evacuated glass bulb containing a number of cathodes about one centimetre square in area arranged in a row, each cathode being coated with a photo-electric material such as caesium oxide. Opposite the cathodes and about one centimetre away is a row of anodes, or deflector plates, of about the same size. The anodes are situated as shown in the diagram, Fig. 1, in which the photo-cell cathode is marked A. When light falls upon this cathode, electrons are emitted and are attracted towards the first anode but are bent back by the action of a magnet, so that they strike the next cathode.
When electrons are made to strike a photo-electric surface in a certain manner each electron can give rise to the emission of a number of secondary electrons from the surface. Because of this secondary emission effect a greater number of electrons leaves the second cathode than originally reached it.
The electron stream leaving the second cathode is then attracted towards the second anode, but is again bent towards the next cathode by the magnet, and the number of electrons is once more multiplied. After passing from stage to stage and being amplified in each one, the electrons finally strike the anode of the final stage which may be arranged in triode or pentode form. The final anode is provided with an external anode load and is therefore a voltage amplifier.
Fig. 1 shows also the method of application of the necessary accelerating potentials. The potential is adjusted to about 40 Volts per stage. The actual value depends on the field strength of the magnet and must be accurately adjusted to bend the electron stream so that it strikes each cathode in turn. It is also of importance that the magnet be correctly oriented, as, if this is not done some or all of the electrons in the beam will fail to strike the next cathode and the overall amplification will be very much reduced. It is necessary to focus the light on to the first cathode to avoid electron dispersion in the later stages, since the electron focusing is not perfect.
Johnson Noise
The reasons for the increased band width and reduced noise/signal ratio are as follows. In the absence of light on the first cathode no electrons are emitted and no anode current flows in the final anode circuit, so that the noise voltage is then solely that due to Johnson noise in the final anode load. Johnson noise is that due to potentials set up in the circuit by thermal agitation of the particles composing the material of which the circuit is constructed. The amplitude of this potential at any given frequency is dependent only upon the impedance and temperature of the circuit. For this reason, the signal/noise ratio obtained with an electron multiplier is very much higher than is the case when an ordinary amplifier is used.
Since there is only one circuit across which a potential has to be developed (the final anode circuit), the high-frequency loss is greatly reduced by comparison with that attainable in an ordinary vacuum tube amplifier and, as a result, a flat frequency characteristic can be produced over a much wider band of frequencies. This wide frequency characteristic enables pictures of much better quality to be transmitted, especially as the noise is not increased as a result of the increased band width. It is an unfortunate fact, however, that the present band width of 2 MHz is closely approaching the limit of expediency with the method of transmission at present in use, partly due to the difficulty of constructing high-gain amplifiers of extremely wide band-width and partly to the ether congestion which will undoubtedly result from the multiplication of services in the future. It will also be a difficult matter to damp the radio transmitters and receivers sufficiently to enable much wider band-widths to be employed without materially reducing the amplification obtainable, unless shorter carrier waves are used.
Since there is still room for improvement in the definition obtainable in television transmission, it will be of interest to examine the problem in order to determine in what way the Zworykin electron multiplier may further the end in view.
Improvement in Definition
A ten-stage Zworykin Electron Multiplier. This particular specimen is on view at the Science Museum, South Kensington.
Consider first the transmitting end with relation to the possibility of taking advantage of the increased band-width obtainable with this new tool. Since greater amplification is obtainable it follows that the source of light may be reduced in power, and it is, in this case, a fortunate fact that with a cathode-ray tube film scanner, or with an Iconoscope, the spot becomes considerably smaller and better focused when the light output in the former case and the beam current in each case are reduced. Whether the fineness of the mosaic in the Iconoscope can be increased sufficiently to enable full advantage to be taken of the smaller spot remains to be seen. In any case, even with present methods, the use of the electron multiplier at the transmitting end is able to provide improved results. This is so because, especially with the higher numbers of lines, the size of the scanning spot and its brilliancy have at the moment to be pushed above the optimum value.
It is clear, however, that at least with cathode-ray tube scanners a material improvement in definition could be obtained if we could arrange to transmit the increased band-width necessary. One method of doing this would be to employ a light beam as a carrier wave instead of wireless. Such a system would become inoperative in fog or in smoke-laden atmospheres, but an infra-red beam might be used to overcome these difficulties in the future. Infra-red beams such as would be required for the system cannot at present be produced, and therefore this article will be limited to a discussion of the possibilities of visible light beam television.
The Zworykin electron multiplier opens up possibilities of such a system, since it has undoubted advantages at the receiving end as well as at the transmitter.
Let us suppose that one or more cathode-ray tubes be employed as light sources. Bearing in mind the fact that the brilliancy of cathode-ray tube screen materials has been enormously increased during the past two years, it is not unreasonable to suppose that sufficient brilliancy might be obtainable for night-time operation of such a system in the near future. This is especially reasonable in that the need for a focused spot disappears, and therefore the energy in the cathode-ray beam could probably be increased a thousand-fold or more without damaging the screen.
Presuming, therefore, that such a tube were to become available, it would be necessary to modulate the ray with a wave-form similar to that at present in use. For the white parts of a picture the beam would be modulated to provide maximum brilliancy over the whole surface of the screen, and it would be adjusted to a dim value for the black portions. The beam would require to be completely modulated out in order to transmit the synchronising signals. The potentials for modulating the cathode-ray tubes would be obtained from the output of an electron multiplier, the light input of which would be controlled by the scene to be transmitted. The synchronising signals would be injected into the anode circuit of the electron multiplier.
At the receiving end another electron multiplier mounted in a parabolic or other suitable reflector would be arranged on the roof, and would be oriented towards the transmitting cathode-ray tube. The output circuit of the multiplier would yield a television signal similar to that obtained from the radio set of the present type of television receiver, and this signal, after being passed through a suitable feeder, would be translated into a picture by means of a cathode-ray tube and two time bases, exactly as is done to-day.
There are many advantages in such a system which may be outlined as follows:-
- Considerably better quality of reproduction could be obtained on account of the wider band width obtainable, While at the same time ether congestion would not be aggravated.
- Interference by one television system upon another, especially during freak conditions, would be entirely avoided.
- Interference due to motor cars and other electrical sources of disturbance would be avoided. This forms the greatest obstacle to the enjoyment of television pictures at the present time.
- Inherent noise in the system would be very greatly reduced owing to the use of the electron multiplier at each end.
- The initial cost of the receiver would be considerably reduced, as also would the running costs.
The accompanying sound transmission could take place over a radio channel or via another light-beam system.
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