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High Power HF Generators Demonstrated
Recent advances in 'multipactor' tube design, announced by P T Farnsworth before the Institute of Radio Engineers in New York on March 4, make it possible to generate kilowatts of power in a cold-cathode tube, at frequencies as high as 300 MHz. Applications of the new tubes in oscillators and radio-frequency amplifiers give efficiencies not possible with conventional hot-cathode thermionic tubes.
1 kW multipractor amplifier.
New tubes, capable of producing several kilowatts of high-frequency power without the aid of a hot filament, at frequencies even higher than the 'ultra-high' region, with efficiency as high as 90%, were demonstrated in New York on March 4 by Philo T Farnsworth, Vice-president in Charge of Research of Farnsworth Television Incorporated. Following closely on the heels of a paper delivered by Dr V K Zworykin on Electron Multipliers (see report in The Wireless World, November 22, 1935), Mr Farnsworth's demonstration held unusual interest because, while using the same basic phenomenon, his tubes perform in a manner quite different from those of Dr Zworykin and lead to results quite as spectacular and perhaps of even greater immediate practical value.
Both the Zworykin and Farnsworth tubes depend on 'secondary emission' of electrons from a cold cathode. In the conventional thermionic valve, the electrons are emitted by heating the cathode to a sufficiently high temperature; in the electron multiplier tube electrons are caused to bombard the cathode. As each electron hits the specially prepared surface of the cathode it liberates from three to ten 'secondary' electrons - that is, the original electron is multiplied, or amplified, three to ten times. By this method the electron current in the tube is amplified, and if the newly liberated 'secondaries' are made to strike the cathode again, each one liberates from three to ten additional secondaries. By successive impacts of this kind the electron multiplication is made to build up cumulatively until the original electron impact has been amplified many millions of times. This manner of operation is the basis of all electron multiplier tubes; it permits exceedingly great amplification to be obtained in a single tube, and because it amplifies the current directly (without the necessity for converting the current into a voltage drop and then amplifying the voltage, as is done in conventional circuits), the signal-to-noise ratio of the electron multiplier tubes is many times higher than that of the usual valve amplifier. A tube of this kind has provided several Watts of audio power when its input was a few microamperes of photoelectric current; this feat, accomplished by Dr Zworykin, was hailed as a great advance. But the power output of the Zworykin tubes was not more than a few Watts, although they could probably be made to deliver higher power.
Up to Four Kilowatts
4 kW multipractor with air cooling fins.
The Farnsworth tubes, on the other hand, are expressly designed for high-power output. Three distinct forms of 'multipactors', as Farnsworth calls his tubes, were shown; their tentative power output ratings are 200, 1,000, and 4,000 Watts. The largest size was about a foot long and tour inches in diameter. This great increase in power-handling ability is based on two factors - the method of obtaining the electron-impacts, which is described below, and the material used in the cathode surface which emits the secondary electrons. The surface used by Dr Zworykin and other workers in the field of electron multiplication is a caesium-oxide-silver deposited on a silver base, and is very similar to that used in high-vacuum photocells of the caesium type. The disadvantage of this type of surface is the fact that it cannot stand high temperatures, and that it can be destroyed by bombardment of positive ions formed within the tube, because of these limitations, high-power tubes, which must necessarily operate at high temperatures, cannot be built using the caesium-silver surface. A new material, the details of which are not yet available, has been developed by Farnsworth, however, which will operate satisfactorily up to 1,000 °C, and which will deliver as many secondaries per electron impact as the caesium-silver surface. The material, which is an alloy having a special surface treatment, can be used even at red heat, making possible compact and efficient high-power multiplier tubes.
Fig. 1. - Principle of the electron multiplier.
The electron multiplication action depends upon-many successive impacts of secondary electrons. Various arrangements have been used to obtain the required action. In the Zworykin tubes many separate cathodes are used, each maintained at successively higher positive voltage. The original electron, attracted to the first cathode and liberating several secondaries, produces an amplified electron flow, which is then directed to the second cathode, and so on, each cathode liberating a current multiplication of from three to ten times. This action is shown in Fig. 1. The final amplified current is collected at the plate and conducted to the external circuit. The original electron is produced by illuminating the surface of the first cathode, which is photo-electric.
Fig. 2. - The new Farnsworth tube in its elementary form.
In the Farnsworth tube, in its elementary form (see Fig. 2), only two cathodes are used, and they are maintained at the same potential. The electrons are caused to fly back and forth from one cathode surface to the other (multiplying as they go) by applying a very high frequency voltage between the two cathodes. First one cathode and then the other is made positive by this HF voltage, and the electrons fly back and forth in response to the pull of whichever cathode is positive at the time. The central anode is maintained positive at all times by the battery, and serves to attract the electrons in their flight. To prevent the electrons being collected by this anode, in the simple form of tube, a magnetic field has been used to direct the electrons, away from the anode and toward the cathodes. In this manner the number of free electrons inside the tube is enormously increased, until a cloud of electrons (a space charge) is formed. This cloud prevents further increase of current (space-charge equilibrium) unless it is removed through the anode to the external circuit.
Fig. 3. - Arrangement of electrodes in a practical multipactor.
To do away with the necessity for the magnetic field, and for other reasons, the form of tube shown in Fig. 3 was developed. It consists of an outer cylindrical cathode, with the special electron-emitting surface turned inward. Inside is the anode, a helical coil of wire, very similar in appearance to the grid of conventional valves. Because of the open spaces in this anode, electrons liberated from the surface of the cathode can pass across the tube and hit the cathode on the other side, there liberating still more electrons. The frequency of the HF voltage applied between the cathode and earth is so chosen with respect to the time it takes the electrons to fly across the tube that the electrons make many trips across the tube, each time adding greatly to the current, before they are finally collected by the anode. The same space charge formed in the simple tube of Fig. 2 is thus formed within the cylindrical cathode and is eventually collected, at least in part, by the anode.
A Thermionic Multiplier
Fig. 4. - Typical thermionic multipactor combining thermionic action with electron multiplication.
In another form of tube a second cathode is placed in the centre of the tube (Fig. 4). This second cathode does not emit secondaries, but it does emit by thermionic emission, being an ordinary heated filament. The use of this secondary cathode, in addition to supplying initial electrons, is to prevent the electrons from flying clear across the tube, resulting in improved performance, especially for radio-frequency amplification.
If the HF voltage used on the multipactors is obtained from the action of the tube itself, the result is a self-sustaining oscillator circuit which takes energy from the battery in the anode circuit and converts it into radio-frequency energy in a tuned circuit. Such an arrangement is shown in Fig. 3. The initial electron flow needed to 'start the ball rolling' is freed by the shock to the tube when the HT battery is suddenly connected across it. Thereafter, the flow of electrons across the tube is so timed that it gives energy to the tuned circuit, and, as a result, oscillations are sustained. Frequencies from as low as 100 kHz to as high as 300 MHZhave been produced by this method. In the demonstration before the Institute of Radio Engineers a one-tube transmitter, consisting of a multipactor, a single tuned circuit and a HT power supply, was used to transmit music on a carrier frequency of about 10 MHz. The modulation was obtained from a gramophone record, the modulation voltage being applied in the anode circuit. The power output of the tube thus connected was about 25 Watts. Such multipactor oscillators can be crystal controlled, giving all of the desirable frequency stability available in this system, and at the same time can deliver very great amounts of power. The frequency is controlled either in the tuned circuit or by means of a previous driving stage.
Fig. 5. - Farnsworth tube used as an HF amplifier.
The second practical application of the multipactor tube demonstrated by Mr Farnsworth was its use as a radio-frequency amplifier. When so used, the output HF current from the tube must be proportional to the input electrons, which means that some means of controlling the input electrons must be provided. Two schemes are available for doing this. The multipactor may be made in two stages (both inside a single glass envelope), the first stage supplying the input electrons to the second. A diagram of such a double tube is shown in Fig. 5. The other system making use of a more conventional method uses a heated filament surrounded by a grid (Fig. 4). The input radio-frequency energy is applied between the grid and filament, resulting in a flow of electrons having the same frequency as the input, in a manner exactly analogous to that of the thermionic triode valve. The electrons flowing through the grid strike the surrounding cathode of the multipactor, which then proceeds to amplify the current by electron multiplication, finally delivering the amplified radio-frequency energy to the tuned circuit connected to it. From this tuned circuit the energy may be fed to the aerial or wherever else needed. Such HF amplifiers display efficiencies of from 60 to 90%.
Future Possibilities
The high efficiency and high-power output available with the new multipactor tubes indicate that they may become serious competitors to the now universally used thermionic triodes. This statement applies especially to the production of radio power at ultra-high frequencies, where it seems certain that the electron multiplier has many advantages over the thermionic valve. The multipactor is essentially an electronic oscillator, in the same sense that the Barkhausen and magnetron tubes are. It depends for its frequency of oscillation on the transit time of the electrons flying across it, and since this time can be made very small by high voltages and small spacings the upper frequency may be higher than 1,000 MHz.
The importance of the new tubes in television transmission cannot be overlooked. Not only do they supply power at the required high frequencies, but they are capable of passing the very wide band of frequencies made necessary by high definition visual transmission without attenuation.
It seems likely, also, that the electron multiplier principle will be applied to everyday amplification problems, with gratifying results. A single multiplier tube can be made to have a mutual conductance measured in amperes per volt rather than in the microamperes per volt of typical thermionic valves. Likewise frequency conversion, as used in superheterodynes, can be accomplished by the same multiplication principle, providing conversion gains of 10,000 in a single stage and thus eliminating the necessity for IF amplification. Also, because the output of multipactor tubes can be made very rich in harmonics, a single tube may use as low-frequency crystal control and still supply large amounts of high-frequency power in the output.
The prospect of such revolutionary developments is not immediate, because there are many difficulties in the manufacture and practical application of the tubes to be cleared up first. The preparation of the electron-multiplying surface, on which the characteristics of each tube depend, must be carried out with great care to prevent variations and such factors as length, of life and other service requirements must be established. But there seems to be no doubt that the electron multiplication principles, as exemplified by both the Zworykin and Farnsworth developments, can be expected to take the place of the valves now used in many important applications, particularly for photo-electric purposes and ultra-high-frequency transmission.
A master oscillator unit for a frequency of 5 MHz.
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