US2301820A - Modulation of photo-current amplifiers - Google Patents

Description

7 Nov. 10, 1942. K, SCHLESINGER MODULATION 0F PHOTO-CURRENT AMPLIFIERS Filed July 8, 1938 Patented Nov. 10, 1942 MODULATION OF PHOTO-CURRENT Kurt Schlesinger,

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. min by Berlin, Germany assignments, to Radio Corporation of America, New York. N. Y., a corporation" of Application In July a, ma, Serial No. 21am Germany October 5, 1937 2 Claims. (Cl. Pia-7.2)

The present invention relates to television transmitting arrangements and more particularly to such arrangements, wherein a photoelectric cell combined with an electron current multiplier is used for generating and amplifyin the picture signals.

In arrangements of this type it is desirable for numerous reasons to modulate a high-frequency carrier wave by the signal currents derived from the photo-sensitive cathode. This principle results automatically in a conjoint amplification of the D. C. component, is devoid of interfering slow building-up transients, is very insensitive to microphonic noises, mains hum and other external interferences. The carrier frequency principle is particularly suitable. for transmission of the signals on line-channels.

In known arrangements of the above mentioned type serious difilculties are caused by the problem of modulating the carrier frequency in accordance with the varying potentials supplied by the photocell or the multiplier, the primary current of which represents the signals to be modulated upon the carrier wave and still contains the D. 0. component. 7

Care must be taken that the carrier frequency is brought completely to zero when a black image point is to be transmitted. Usually this is accomplished by special bridge-connections, i. e. by compensating methods. In the case of multiple rid tubes a complete control is not readily obtained, so long as the tubes act as class A amplifiers, as there always fiows a feed current which would result in a strong signal.

It is one object of the present invention to provide in an arrangement for television transmitting purposes in combination an electron multiplier comprising a photosensitive cathode, a plurality of impact anodes adapted to emit secondary electrons and a collecting anode, and means for varying the current to be picked-up by the collecting anode in accordance with a carrier frequency to be modulated.

It is another object of the present invention to provide in an arrangement for television transmitting purposes, in addition to the Just mentioned means, additional meansior causing said current to have an extreme value during the synchronizing signals.

The invention will be better understood, and further features of the invention will be seen from the following more detailed description and the accompanying drawings, of which in a purely diagrammatic fashion and by way of example,

comprising a photo-electric cell combined with an electron current multiplying system, a control grid within the multiplier, means for generating a carrier frequency and means for varying the potential of the control electrode in rhythm with the carrier frequency.

Fig. 2 shows another embodiment of the invention comprising a photo-cell similar to that shown in Fig. 1 and means for varying the potential of one of the impact anodes of the multiplier in rhythm with the carrier frequency,

Fig. 3 shows a further embodiment of the invention comprising a photo-cell similar to that shown in Fig. 2 and means for varying the potential of a plurality of impact anodes in rhythm with the carrier frequency,

Fig. 4 represents a cross-sectional view through a tube and illustrates a modification of the modulating system; and,

Fig. 5 shows a still further embodiment of th invention similar to that of Figs. 1 and 3wherein means are provided for influencing the output current of the multiplier by the synchronizing impulses. 1 i

Referring first to Fig. l, the multiplier I is of the type wherein a plurality of net-shaped electrodes 2 activated for secondary electron emission are used as impact anodes. The photo cathode 3 of the multiplier receives the light proceeding from the scanning device. A control grid 4 is'arranged between two impact anodes and influenced by carrier frequency potentials from the generator 8. The control grid 4 is so well screened f om the output circuit I by means of special screens provided inside and outsideof the tube that between the control grid and the impact anode situated next to the same. a carrier frequency potential can be applied indirect fashion without this potential being capacitively transmitted directly to the output I. -'I'he introduction of the carrier frequency-potential can take place, for example, by an inductive coupling 25, the carrier frequency generator 8 being Y disposed, for example, at a remote place. Preferably, however, the generator'and, in any case. the lead, are furnished with a separately earthed screening means I. The control grid 4 itself is Y already screened from the output I byusing activated nets as impact anodes, the grid being situated in the space between two capacitatively Fig. 1 shows an embodiment of the invention plicity, thoseof the impact anodes I which are 7 or other methods known in the high-frequency art. the operation of the arrangement is as follows: Electronic currents of different strength reach the impact anode 2a in proportion to the exposure of the photo-cell 3 upon the scanning operation. So long as the comparatively large surface grid 4, which is situated in front of this anode and is inactive in itself, is biassed sufilciently negatively in relation to the impact anode 2a the suctional field, which normally conveys the electrons to the next impact anode 2b, is

- missing. If on the other hand the control grid I has the same potential or even apositive potential in relation to 2a, the electrons are able to .pass to the next impact anode 2b, are additionally multiplied, and finally reach the collector. The electronic current reaching the latter will accordingly in the first place always be proportional to the. light exposure and secondly it will have the value 0 if the light exposure ceases, and also the excitation of the carrier frequency amplifier will be at zero if no light falls on the photo-cathode. In this way the desired carrier frequency wave form of the television transmission is-produced in simple fashion without bridge connections or compensation.

Various modifications of. this method are possible. Thus, for example, in place of one single controlled discharge path 20, 2b there can also be employed a plurality of series-connected discharge paths operated in phase with one another, whereby the full-range control to zero can be additionally improved. Further, the method can also be employed in the case of magnetic multipliers, there simply being connected in front of an amplifying surface a control grid, which is connected to carrier frequency potentials in re lation to the surface.

Within certain limits, the location of the controlling meanswithin the multiplier can be selected as desired. It is, however, not desirable to include the control space in the first stages of the multiplier, as small electronic rays which still manage to pass through the blocking field are so amplified by the adjoining part of the multiplier that they are able to interfere with the black and thus produce a considerable background noise. It is also not desirable to perform the modulation in the final stage preceding the collector, :as the screening of the oscillatory grid is more and more diiilcult. A spreading of electrons also cannot be so very easily avoided in the case of comparatively high intensity of current. It is accordingly advisable to perform the control approximately at the centre of the cascade.

Further, in place of a special control grid; one

7 of the impact anodes itself can be furnished with alternating potentials as compared with the preceding anode. Generally' speaking, this method will require higher controlling potentials. An arrangement of this kind is illustrated by Fig. 2.

- 'In this embodiment of the invention, no special inactivated control grid is provided. One of the impact anodes themselves (anode 2c in Fig. 2)- is supplied with varying potentials by means of the carrier frequency generator 6. Of course; it is a possible to impress varying potentials to more than one of the impact anodes.

Fig. 2 shows a particular collecting system which is preferably used in net multipliers. The

unactivated collecting anode 25 is of grid like structure and is mounted between the last of the net shaped impact anodes 2 and a solid further impact anode 23 which, for example, consists of a metal plate activated for secondary emission.

In place of the inductive coupling it is also possible to use another form of coupling in the case of control both by means of grids'as 'well as impact anodes, for example the simultaneous capacitative connection in parallel of a plurality of impact anodes with the interposition of an impact anode earthed by way of a condenser between each two of the parallel impact anodes connected with the same high-frequency generator, as shown in Fig. 3. The number of impact anodes earthed with respect to alternating current is preferably as large aspossible in all cases.

Further, there can also be performed a defiection control, this being possible in both types of multipliers employing electromagnetic or electrostatic fields for guiding the' electrons, 'and it one impact anode to the next only in a certain phase of the high-frequency oscillation, andin the remaining phases moves past the next impact anode following a curved path dicated by b.

There can also be provided a special control coil, not shown in the drawing, the axis of which, for example, forms a right angle with that of the coil 20, and which deflects the electronic current laterally out of the plane in which it normally passes from impact anode to impact anode.

The known undesirable, increasing dispersion of the electronic current can be prevented ac cording tothe invention by a sleeve electrode l9 surrounding the entire multiplier electrode system. This electrode. in the case of the deflection control described in the above can be employed electrons passing by the impact anodes.

A particular embodiment of arrangement ac-.

cording to the invention takes shape :in connection with television transmitters if at the same time there is considered the transmission of synchronisation signals. According to a method which has been adapted in Germany and in England, the so-called positive transmission system," it is'required, for example, that the carrier wave should be modulated to zero only for the synchronisation signals. whilst in the case of black in the image it should assume a prescribed partial value of the maximum, for example 20%. With an arrangement according to the invention a complicated diaphragm of thiskind can also be produced in a simple manner. For this purpose, for example, it is merely necessary by means of a second light source to allow a constant light to fall on the photocathode. In Fig. 5, for example, an arrangement of this kind is illustrated diagrammatically. The modulation of the carrier wave by the photo-currents takes place in a manner described in connection with Figs. 1 to 4 and preferably according to the arrangements showninjlg. 2. The light source ll lights the somewhat as in-- object to be transmitted which, for example, is a film l I. The image of the film is reproduced on a the Nipkow disc l2 and the modulated light beam when the scanning light Illa has disappeared in the case of black in the image there'is still a residual illumination of the cathode 3 by the auxiliary light l5. This residual illumination corresponds to a residual current, which continues even during the black in the image and only disappears when the second light source I5 is also disconnected. This disconnection can take place, for example, in optical fashion by means of a slotted disc l6, which rotates in front of the opening M. It can, however, also take place electrically by supplying to a special impact anode biases from a synchronisation impulse generator, these biases ceasing when a synchronisation signal is to be transmitted. There can also be employed the same impact anode 20 which, as shown, is already connected with the carrier frequency generator, if the same is so biassed that in the case of ceasing excitation the cell current to this impact anode is blocked. For this purpose the impact anode is strongly biassed negatively, and during the transmission of the synchronisation signals the carrier frequency generator 6 is blocked. All of these methods are known to those skilled in the high-frequency art. Further, it is readily possible to set forth reversals of these arrangements for cases in which synchronisation signals are to have an overwhelming amplitude and the image is transmitted as a negative (American method of transmission). In the American transmission method in particular, in which the synchronisation signals are transmitted with full transmission current intensity, care will require to be taken'that the auxiliary light I5 is more powerful than the strongest main light Ill, and also only lights up during the synchronisation periods. This is capable of being accomplished by exchanging the requisite spokes of the siren It for openings when employing a constant light source, or also by using a glow lamp as light source l5 and by means of numerous other known, electricaliy controllable light sources (Braun tubes, Kerr cells and the like). I Iclaim: 1. In a television transmitter system, an electron multiplying tube comprising a photosensiof mesh-like impact anodes adapted to release secondary electrons when struck by impinging primary electrons supported in substantially parallel relationship and substantially perpendicular to the tube axis, said impact anodes being located intermediate the cathode and the output electrode, a mesh-like modulating electrode positioned within the tube and located intermediate the impact anodes, means to generate acarrier frequency, a'scanning means, means to illuminate the photosensitive cathodeof the multiplier by the scanning means with progressively changing light intensities measuring the intensity of the image points of a subject as analyzed by a scanning operation so that the electrons released from electrode to vary thereby the amplitude of the electron current in the tube in th region between the impact anodes, and an output circuit connnected to the output electrode,

2. In a television transmitting system, an electron multiplying tube comprising a photosensitive cathode and an output electrode, a plurality of-mesh-like impact anodes supported in substantially parallel relationship and substantially perpendicular to the tube axis and adapted to release secondary electrons when struck by impinging primary electrons, saidimpact anodes being located intermediate the cathode andthe output electrode, a mesh-like modulating electrode positioned within the tube and located intermediate the impact anodes, means to generate a carrier frequency, means to project light on the photosensitive cathode or the multiplier so that photoelectrons will be released from the photosensitive cathode proportional to the intensity. of the light projected thereon, means for applying operating voltages to the impact anodes and the output electrode so that the photoelectrons released from said photosensitive cathode are caused to move toward the output electrode and to release secondary electrons upon impact with the impact anodes, and means to supply the gen-. erated carrier frequency to the modulating electrode to thereby vary the amplitude of the elective cathode and an output electrode. aplur'ality tron current in the tube in the region between the impact anode; and an output circuit connected

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