US2072315A - Push-pull detector - Google Patents

Description

I March 2, 1937. scHLESlNGER 2,072,315

PUSH-PULL DETECTOR Filed NOV. 7, 1934 On are/7200:

Patented Mar. 2, 1937 UNETED STATES PATENT orrrce Application November '7, 1934, Serial No. 751,927 In Germany November 10, 1933 4 Claims. (Cl. 250--27) .In the rectification of carrier waves modulated with image currents there are employed with advantage push-pull rectifiers, as these remove the carrier wave away from their output circuit by compensation, a disturbing grain in the image thus being avoided. In this connection it is necessary, generally speaking, assuming it is desired toavoid an expensive reversing tube, to employ transformers, as already shown by the applicant in the patent application Ser. No. 747,011 and patent application Rectifier for wireless television reception of 17th October 1934, Ser. No. 748,644.. With transformers of this nature, however, there is encountered the the secondary potential is in no way symmetrically positive and negative against earth, but always reveals a one-sidedly directed additional potential. These conditions are. explained with reference to theaccompanying drawing, in which-- Fig. 1 shows a connection system with reference to which the principle of the invention will be described and which constitutes an exemplary embodiment of the invention.

Fig. 2 illustrates a special form of a transformer preferably employed in connection with the device according to the invention.

In Fig. 3 a connection device is shown as an exemplaryembodiment of the invention using a special twin grid tube.

In particular with reference to Fig. 1 there is shown a preliminary amplifier tube I, which is connected with the primary winding 2 of the push-pull transformer, this winding practically being earthed over the block condenser 3. The secondary winding 4, as shown in the patent application Ser. No. 749,511 of Oct. 23rd, 1934 is tuned to a higher frequency than the highest side band frequency, and the winding 2 on the other hand to a lower frequency than the lowest side band frequency. Owing to the unavoidable stray capacities 5 and 5' there is superposed to the induced potential an electrostatic potential.

In consequence the distribution of the potential is not the ideal one, which may be considered as being represented by the straight line 6, the latter passing through the centre point of the 50 coil i and the terminal potentials of l being equal and anti-symmetrical to earth, and in place thereof the potential will be according to the line i, because over the capacity 5 an additional potential is superposed on the coil 4 from the end,

55 assumed to be positive, of the coil 2. The terminal potentials at the coil 4 are now no longer equal against earth; as a matter of fact they do not even necessarily possess opposite signs with respect to earth, and it may even occur that the virtual point of intersection of the potential 5 straight line 1 falls outside of the coil entirely. In consequence a push-pull rectifier, for example the tube 8 in Fig. 1, is no longer in a position to compensate the carrier wave in its anode circuit, and there result the known disturbances in the 10 image and also the danger of self-oscillation, and furthermore a decreased degree of efiiciency of the rectification.

According to the invention, the coil 2 is constructed as a distributed winding with strongly marked stray effect, for example as a straight cylindrical winding open at the top and bottom. Over this coil (Fig. 2) there may be placed the secondary coil 4. The secondary coil 4, in accordance with the invention, is divided into two 20 separate winding halves 9 and ID, the spacing between which is no longer small in relation to the height of the primary cylinder 2. There is then found in practical agreement with the followingtheory that under the given tuning con- 25 ditions of the primary winding with axial displacement of the secondary coil in the direction of the arrow as shown a point may be found at which the potentials at the terminals H and I2 of the coil halves 9 and Ill, which are connected 30 in series, are towards earth exactly equal and opposed. By a displacement of this nature the induced potential of the lower secondary coil 9 becomes always greater, as the same embraces an increasing number of primary lines of force, 35 until when reaching the middle of 2 it takes over a maximum potential. At the same time, however, the induced potential of the partwinding l0 continuously becomes less, the more the secondary is moved towards the plate end of 40 the primary coil because of the increasing stray effects weakening the effective induction flux. Since, however, the primary potential distribution in accordance with the stated tuning is divided over the entire coil with the same sign corre- 45 spending with the potential curve I3, the electrostatic potential remains practically unaltered upon this displacement. In consequence the potential line I in Fig. 1 is turned back in the direction of the arrow It in Fig. 1 until the desired balance has been attained. The method has the advantage:

1. Of not increasing the loading earth capacity of the winding 9, I 0, as no screening is introduced,

2. Of supplying a comparatively high potential,

as the loss of induction potential is compensated by the capacitative electrostatic coupling.

As an alternative remedy a screening means may be employed between the primary and secondary coils using a wire net cut opening axially, With this, however, there is usually associated an increase in the capacity against earth, which is not permissible in numerous connection systems. An alternative remedy consists in the use of variable loads consisting of a differential condenser with the capacities l5 and I6, by which the E. M. F., of the two ends of the secondary winding, may be equalized against the pole H, which may be connected over an audion resistance I8 with cathode and earth. I5 and I6 are of the order of approximately 3 cm. maximum.

The stated methods are of significance only in connection with exposed coils, the centres of which cannot be earthed in direct fashion. This connection arises positively upon use of the grid current detector arrangements without grid condensers. In the connection system set forth in Fig. 3 there is employed as audion, according to the invention, a tube l9 having an insulated grid, or preferably two insulated grids 20, 2|, which are arranged with the same reciprocal of the amplification factor over the common cathode 23, and are furnished with a good insulating coating, on the surface of which, however, there is provided a conductive layer 24, which is common to both grids and may be connected with a terminal 22. Tubes of this character offer for television purposes, in accordance with the invention, the following advantages:

A potential produced in the counter-cadence transformer coil 4 may be applied in unweakened state to the grids 20, 2| without a special audion condenser, which on account of its requisite small size would mean a considerable loss in sensitiveness. Since these grids, however, according to the invention, are insulated against the interior of the tube, the middle of the transformer 4 in tubes of this nature may be connected firmly with earth. Under the action of the particular positive half-waves the outer coating 24 common to both grids is charged negatively. This charge is leaked off by a resistance 25, which may either be provided outside of the tube, or may also be fitted in the tube as conductor for the insulating layer. As will be apparent, it is possible, particularly in the latter case, to reduce considerably the capacity of the arrangement, and in consequence to employ a comparatively larger audion resistance 25 than in the connection system for the ultra-audion in Fig. 1, so that the sensitiveness of the arrangement may be further increased as compared with the previous arrangement with the same lack of inertia. Owing to the possibility of earthing the transformer coil 4 in direct fashion, the danger of lack of symmetry is practically obviated, so that proper compensation of the carrier wave is possible. On the other hand, employing a transformer construction as in Fig. 2, a balancing of the connection is always possible also in the connection system in Fig. 3.

I claim:

1. A push-pull detector arrangement for television purposes comprising a transformer and a detector valve in push-pull connection, said transformer consisting of a primary and a secondary coil, a carrier frequency amplifier valve, the one end of said primary connected to the anode of said amplifier valve, the other end of said primary winding grounded, said rectifier valve having at least a cathode, a plate, and two grids, the one end of said secondary winding connected to the one of said grids, and the other end to said second grid, said secondary being spacially displaceable with respect to said primary coil, and consisting of two separate winding halves connected in series and wound such as to form a spacing between said halves which is great in relation to the height of said halves, said primary winding being constructed as a coil with large straying.

2. An arrangement according to claim 1, characterized in that said primary winding and said secondary winding are decoupled as much as possible against capacitative influencing from said primary to said secondary winding.

3. An arrangement according to claim 1, characterized in that said primary winding and said secondary winding are decoupled as much as possible against capacitative influencing from said primary to said secondary winding, said decoupling means consisting in a wire-net of cylindrical form arranged between said primary and said secondary, said wire-net being cut open axially.

4. An arrangement according to claim 1, characterized by variable loads at the ends of said secondary coil, to enforce equality of the output

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