US2118866A - Detector - Google Patents

Description

May 31, 1938- K. SCHLESINGER 2,118,866

DETECTOR Filed Oct. 20, 1934 Patented May 31, 1938 UNITED STATES PATENT OFFICE Application October 20, 1934, Serial No. 749,236 In Germany October 23, 1933 1 Claim.

As is well known, there arises in the reception of wireless television transmissions the problem of demodulating a carrier oscillation modulated by the image current, and at the same time of eliminating the carrier wave in the anode circuit by the known push-pull circuit.

The subject matter of the present invention is an improved method and a new arrangement which involves the possibility of removing disadvantages arising in the case of circuit arrangements according to U. S. patent application Serial No. 717,310, Patent No. 2,068,768, dated January 26, 1937.

When using these arrangements for the reception of high-quality television transmissions, for example a 240-line image, i. e., with a maximum image-current frequency to be transmitted amounting to 10 periods, it has been found that the sharp change-over in contrast from black to white occurs with sufficient sharpness with suitable dimensioning of the entire earth capacity of the grid circuit and suflicient capability of electron emission on the part of the cathode, but that the change-over in an opposite sense appears all the more blurred the greater the contrast produced between black and white.

In case, for instance, a. white portion of the image has been reproduced, an immediately following black portion cannot be reproduced with the extreme sharp contour and suddenly occurring contrast, as would correspond to the original transmitted from the transmitting end. This effect occurs if the usual, well known grid leak detector connection system is employed.

This invention is intended to overcome this defect and will be best understood by reference to the accompanying drawing, in which in Fig. 1 there is shown a fundamental audion connection suitable for television purposes as set forth by the applicant in the patent application Ser. No. 717,310.

Fig. 2 shows the characteristic of charging and discharging of the working circuits.

Figs. 3 and 3a show circuit arrangements em- 5 bodying the idea of the present invention.

Fig. 4 is a further suitable embodiment.

In particulars in Fig. 1, l is a push-pull de tector audion tube, the grids 2 and 3 of which should possess the smallest possible operating capacity to earth or the cathode 4 respectively. The tube is operated by an amplifier tube 6 through the medium of a push-pull transformer 5.

The applicant has ascertained the above mentioned disadvantages to be founded in the properties of the charging and discharging working circuits. The following is short explanation of the reasons which have led to the present invention and which will be described with reference to Fig. 2.

The absolute grey value of the particular image point is entered as ordinate in Fig. 2. This value is proportional to the negative charge of the transformer coil 5 or the grid of the detector tube I in Fig. 1. As abscissa there is entered the time or the declination of the image point from the point designated 6, in which the original contour is required to change suddenly from white to black. In all detector connections, in which the discharge of the grid circuit is performed by means of an ohmic grid resistance 1, the two discharge time curves 8 and 9, which proceed from a very white image point of the intensity value I 0 and from a merely grey intensity value ll, must obviously reach the same intensity values at different times, i. e., the absolute value black must be reached by the curve 8, i. e., behind merely one grey image point, more quickly than by the curve 9 behind a very white image point. This effect which may be referred to as echo effect occurs, as careful investigations by the applicant have shown in every detector circuit, in which discharging of the grid circuit is produced by means of ohmic resistances the value of which being independent of the size of amplitude of the operating potential.

Now the idea according to the invention is to conduct the discharge according to an exponential curve, but rather quickly for example according to the steeper time curve l2. A curve of this nature would be obtained if the discharge resistance 1 had the property of becoming all the smaller the more powerful that the current passing the grid circuit 5 has been. The useful effect of an amplitude-dependent resistance of this nature is already apparent in the circuit according to Fig. l 1, consisting in the fact that the charging over the electronic path from 4 to 2 and 3 does not reveal an echo effect of this kind in the image.

According, therefore, to the invention, a connection system set forth in Fig. 3, which is in- L tended merely as an example of a large number of similar connections embodying the idea. according to the invention.

For discharge purposes there is employed, for example, a special electronic tube I3, the cathode M of which is connected with the centre of the transformer coil, whilst its anode I5 is connected with the cathode 4 of the audion tube l and earth. The discharge output of this tube is so selected that the resulting echo effect in the image corresponds at the most with the width of an image point. Since the discharge takes place under constant potential, whilst charging occurs through the medium of the tube i upon interruption on the part of the carrier wave, the emission of M may be made smaller than that of Alternatively, there may also be employed in place of a high-vacuum tube 13 a glow lamp with or without a hot cathode, whereby a suitable working point may be adjusted shortly prior to the ignition by means of an auxiliary potential 16. In the case of indirect heating the cathode M, according to a feature of the invention, is so constructed that its capacity is as small as possible against earth. In Fig. 3a there is shown a further exemplary embodiment of the circuit arrangement of Fig. 3, wherein as tube !3 there is used a cold cathode glow discharge tube.

According to an additional feature of the invention, the electrodes M and 55 are provided together with the audion in the same vacuum container l, and the heating of i4 is effected by the heating means for 4. An example of this is the arrangement according to Fig. 4, in which the cathodes and M are heated by a common insulating heating element I! having a filament I8 as thin as possible, and in which preferably also the cathode M together with the control grids 2 and 3 are conducted in direct fashion through the glass bulb through separate leading-in points.

I claim:

In a television receiving system an arrangement for rectifying and amplifying the imagecontents-currents of very high frequency and of greatest difference in their amplitudes comprising a twin-grid grid-leak detector valve containing at least a hot equi-potential cathode, two control grids and an anode, an electronic discharge device comprising an anode and a hot cathode in the grid circuit of said valve, said discharge device being arranged in one and the same container together with the electrodes of said

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