US2898505A - Electron discharge structure - Google Patents

Description

f Aug. 4, 1959 R. J. MEAGHER ELECTRON DISCHARGE STRUCTURE 2 Sheets-Sheet 1 Filed Sept. 28, 1956 INVENTOR. Robert J. Meogher ATTORNEY Aug. 4, 1959 R. J. MEAGHER 2,898,505

ELECTRON DISCHARGE STRUCTURE} Filed Sept. 2a, 1956 2 Sheets-Sheet 2 IN V EN TOR.

Robert J. Meogher ATTORNEY Patented Aug. 4, 1959 2,898,505 ELECTRON DISCHARGE STRUCTURE Robert Joseph Meagher, Peabody, Mass.,

lumbia Broadcasting System, Inc., corporation of New York assignor to Co- Danvers, Mass., a

The invention relates to electron discharge devices and particularly to the structure of multi-grid electron discharge tubes.

Multi-grid tubes may be used in circuits in which an incoming radio frequency signal is combined with a locally generated signal to produce a beat signal. On many occasions, it is more practicable to generate a beat signal than to try to amplify a radio signal to usable proportions. The tubes which perform the operation of generating a beat signal are commonly called mixers or converters in the art. Almost every known type of tube, except perhaps diodes, are adapted to perform the process of mixing signals, but it is usual practice to provide tubes that are especially designed to accomplish that particular function.

Although almost any type tube is applicable to circuitry for heterodyning a single received signal with a locally generated signal, when it is necessary to heterodyne two received signals with a locally generated signal, two separate tubes must be used. In the color television art, for example, two signals are conventionally employed to convey the chrominance information. For reasons that are not here pertinent, it is conventional to convert the hue and saturation of the colors of the scene being televised to two signals and impress both on a single subcarrier to modulate the subcarrier in both amplitude and phase. The resultant chrominance signal, must be demodulated at the receiver so that two separate signals, each of which corresponds to one of the two modulating signals at the transmitter, are obtained. The process by which such separation is accomplished is known in the art as synchronous demodulation and is effected in a synchronous detector or synchronous demodulator. Such demodulators as are known to the art require at least two tubes.

The use of two separate tubes to completely demodulate such a signal entails certain disadvantages. The physical size of two separate tubes makes reduction of the minimum size of the chrominance demodulating section of a receiver very difiicult. Furthermore, selection of tubes with similar characteristics is required in order that distortion be reduced to a minimum. In addition, the complexity of the circuitry of a color receiver is compounded by the necessity of supplying energizing potentials to tubes. Interaction between the signals has always been excessive when a single tube of any known type was used.

An object of the invention is to provide an electron tube by means of which two signals may be synchronously demodulated.

Another object of the invention is to provide an electron tube into which a number of signals may be injected and from which a plurality of independent signals may be taken.

A further object of the invention is to provide a simplified double balanced demodulator tube for a color television receiver.

According to the invention, there is provided a novel thermionic vacuum tube having a plurality of substan tially separate portions, each portion including an anode or output electrode, means for modulating the space charge in each separate portion simultaneously and means in each separate portion for modulating the space charge in that portion independently of all other por tions. The means for modulating the space charge si multaneously in each portion consists in a source of electrons disposed in a restricted opening between the separate portions, a control grid adjacent the source of electrons and a screen grid disposed around the control grid, while the means for modulating the space charge in the separate portions consists in a control electrode within each separate portion.

. Foruse in a conventional color television receiver, a tube according to the invention, may have but two separate portions. A signal containing chrominance information is impressed on .the common control grid, a locally generated signal of given phase is applied to the control electrode in one of the separate portions, and a locally generated signal of quadrature phase is applied to the control electrode in the other portion. With energizing potentials applied to the various elements, the average voltages on the respective anodes are proportional to the magnitude of the respective sidebands of the modulated chrominance subcarrier. This proportionality of the respective sidebands in practice means that the chrominance signal has been demodulated to provide signals corresponding to the hue and saturation of the colors in the televised scene.

For a ready application of the invention to the problem of the artisan, an express embodiment, given by way of example only, is described with reference to the accompanying drawing in which:

Fig. 1 is a side view, partially cut away, of an electron tube constructed in accordance with the principles of the invention;

Fig. 2 is a base diagram of the tube shown in Fig. 1;

Fig. 3 is a cross-sectional view of the tube shown in Fig. 1; and

Fig. 4 is a schematic diagram of a synchronous de modulator utilizing a tube according to the invention.

Referring now to the first three figures, an embodiment of the invention is shown comprising an envelope 11, which may be made of rality of lead-in-wires 12 to 20 sealed to one end thereof in a known manner. Separate ones of each of the lead-in wires 1220 are formed and secured to separate elements of the tube as shown. Among the elements are several that are conventional in the construction of electron tubes, such as a heater 22, a cathode 23, a control grid 24 and a screen grid 25. While these elements are shown having particular shapes, it is understood that the illustrated shapes are not the only ones the elements may assume; for example, the cathode 23; the control grid 24, and screen grid 25 may be circular in cross-section. An electron confining electrostatic shield 33 is also connected to a lead-in wire or to the cathode 23 by means of a strap 21 as shown. The shield 33 may be fabricated of any desired electrically conductive material or of a non-conductive material and then coated with an electrically conductive coating. As shown, the electrostatic shield 33 is cylindrical in configuration, being formed from two identical semi-cylindrical portions and is positioned about all the elements of the tube including those to be described hereinafter. Afiixed to the inner wall of shield 33 and projecting radially inwardly thereof is a pair of opposing planar rectangular fins 31, 32. These fins preferably are made integrally with the shield 33 as shown. The fins 31, 32 are identical in dimensions so that the inside of the glass, and which has a plu- .3 shield 33 is divided substantially into two semi-cylindrical portions 34, 35 having a longitudinal opening 36 thercbetween. The heater 22, cathode 23, control grid 24 and screen grid 25 are concentrically disposed :in opening 36 with-the supports for each of these elements in the plane of the fins 31, 32. Such positioning, in a practical tube, essentially :fills the opening 36, even though Figs'l and 2 show, for clarity, a space between thescreemgrid 25 and the 'inner'edges of the fins 31, 32. In fact, it is desirable on occasion actually to terminate the inner :edges of the :fins 31, 32 at the supports 25a for the screen :grid 25. To 'one side :of the tube, a control electrodeldand'an anode 29 are positioned as shown. A similar control :electrode'28 and an ode 30 are also positioned on the opposite side, control electrode 28 being connected to ilead-in wire 20; control electrode 26 to lead-in wire 19;.anode 30 to :lead-in wire 17; and anode 29 to lead-in wire 14. The various elements are held in position with respect :to each other and to the envelope '11 by appropriately-shaped projections and support rods which cooperate with insulating spacers 37. The=eflfect of the electrostatic shield 33 may also be augmented by means of a top shield 40 and a bottom shield .59, each-of which'may be disposed as shown.

While a .tube having two separate portions of equal size is shown by'example, it should be understood that any number of such separate portions may be provided and that the :-portions'may be of 1difierent size depending on the application.

'In operation, potentials 'are applied across the heater lead-in wires 12, :13to energize the heater 22. The heater 22, in turn, :causes the cathode 23 'to inject a cloud "of electrons substantially isotropically in both portions 34, 35. The zcontrohgrid 24, being properly biased, has impressed'on it a first signal which modulates the electron cloud :equally in both portions 34, 35. The screentgrid 25, in turn, has a potential impressed-on it-to accelerate the individual electrons that pass through the :control grid 24. Up until this point, theelectronsemitted from the cathode 23 have been subjected to substantially 'identical electric forces so that identical time-varying currents :exist in both portions 34, 35 .of the tube directly outwardly of the screen grid 25. It may be considered for all practical purposes that two virtual cathodes -or sourcesof electrons exist, theifirst in portion 34 and the second in portion-35. The-control electrode 26 in portion .34 is .also properly biased and has impressed on it a second independent time-varying signal, which usually is different from thesignal-impressed on the control grid 24. This latter time-varying signal, in turn, modulates the electron cloud in one portion 34. Anode '29'servesto collect the electrons after passage thereof through control electrode 26. .It .may be seen that normal heterodyning between-the first and second time-varying signals occurs so that at the anode 29 there appear components corresponding to the impressed signals and components whichcontainfrequencies equal to thersum and difference of the two impressed signals. Any of many well-known load circuits may beconnected to the anode to effectuate coupling of any o'fthe frequencies to other circuits. The original time-varying electroncloud also is injected into the other portion 35 where the control electrode 27 and the anode 30 operate in a manner similar tocontrol electrode 26 and anode 29. However, a second independent signal maybe impressed upon the control electrode '27 so that the signal on the anode 30 difters from the signal on .the anode.29.

The electrostatic shield 33 and cooperating fins 31, 32 serve 'to isolate one portion 34 'from the other portion 35. Such isolation .is desirable in order to eliminate cross-modulation (or cross-talk) between the two vportions. As mentioned heretofore, the fins 31, 32 may, on occasion, be extended inwardly to terminate in the supports'for the screen grid '25. This "modification of the illustrated structure increases the degree -of isolation -of the separate portions. However, if the fins 31, 32 are so modified, it will be foundnecessary to elevate the fins 31, 32, the electrostatic shield 33, including the top shield 38 and the bottom shield 39, to the same potential as the screen grid 25. It may be :necessary in this event to increase the area of the anodes 29, 30 to maintain the efficiency of the tube.

Referring now to -Fig. 4, a circuit incorporating a tribe according to the invention is shown in schematic form. For simplicity, those 'portions of a color -te1evision receiver which are unaffected by the invention have been omitted. The signal containing the 'chrominance information to be demodulated is applied at terminal 51 to the control grid 24 of a tube 50 through an input circuit 52, or, if desired, the signal may be injected in the cathodecircuit in known manner. The output signal of a local oscillator (not shown) synchronized 'with the color burst signal in any known way isapplied at terminal 53 and phase displaced waves derived, as by the separate coupling and phase shifting .circuits '55, 56 shown. One of the waves is injected into tube 50 on the control :electrode 26 while the-other=is injected into *the tube 50 on the controlelectrode -27. The'demodulation-componerlts at the anodes 29, 30 are presented to output terminals ;'57 and 58, respectively, when proper potentials 'are'applied to the-anodes 29, 30-and thescreen grid 25 as shown.

The invention claimed is:

A space charge controlled'electron tubecomprising the combination of an evacuated enve'lope, a-plurality of'electrodes being disposed within said envelope, an electro-- static shield and means projecting through said envelope separately energizing each said -electrode and said elec' trostatic shield, said plurality of electrodes consisting of -a source "of electrons centrally disposed within said envelope, a first control electrode disposed around said sourcefof electrons andcoa'xial therewith, ascreen-grid disposed around said ='first control electrode and -coaxial therewith, a second and a third control electrode disposed outwardly of said screen grid and equa'lly spaced therefrom, said second control grid corresponding to a first i predetermined portion of the area ofsaid screen grid, said third c'ontrol-electrode-corresponding to a second'predetermined portion of the area of said -screen grid oppositely of said first predetermined portion and similar thereto, a first =and a second anode, said first anode disposed outwardly of said second control electrode and equally spaced therefrom and corresponding'to apredetermined portionof the area thereof, said secondanode disposed outwa'rdly of said third control electrode and equally spaced therefrom oppositely of said first anode and being similar in -areathereto, said'electrostatic shield having-a 'firstand-second fin a'nda curved portion,said

curved portion being disposed 'around said plurality of: electrodes and coaxial tlierewith, said first fin being integrally attached to the inner'wall of :said curved portion and "extending radially inwardly thereof bet-ween said second .and said third control electrode to coincidence with'said screen grid, said second fin being integrally attached to the inner' wall of said curved portion oppositely of 'said first fin and extending radially inwardly thereof between said second and-said third control electrode to coincidence with said screen grid oppositely of said 'first fin.

References Cited in the file of this patent UNITED STATES PATENTS

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