US3134043A

US3134043A - Secondary-emission prevention circuit for use with vidicon tubes and the like

Info

Publication number: US3134043A
Application number: US138891A
Authority: US
Inventors: Horowitz Irving
Original assignee: BLONDER TONGUE ELECT
Current assignee: BLONDER TONGUE ELECT; BLONDER-TONGUE ELECTRONICS
Priority date: 1961-09-18
Filing date: 1961-09-18
Publication date: 1964-05-19
Anticipated expiration: 1981-05-19

Description

"May 19, 1964 HQROWITZ 3,134,043

SECONDARY-EMISSION PREVENTION CIRCUIT FOR USE WITH VIDICQN TUBES AND THE LIKE Filed Sept. 18, 1951 IV r . r1050 AMI? IN VEN TOR. l/zwms llaeowxrz ATTORNEYS United States Patent ()fiice 3,134,043 SECONDARY-EMISSION PREVENTION CIRCUIT FOR USE WITH VIDICON TUBES AND THE LIKE Irving Horowitz, Eatontown, N.J., assignor to Blonder- Tongue Electronics, Newark, N.J., a corporation of New Jersey Filed Sept. 18, 1961, Ser. No. 138,891 4 Claims. (Cl. 315-'10) The present invention relates to secondary-emission prevention circuits, and more particularly, to such circuits as may be employed with the targets of vidicon tubes and the like.

It has for some time been customary to connect the target of a vidicon tube or the like through a high-series resistance (of value comparable with that of the vidicon i.e. the target-to-ground resistance) to a source of potential for the purpose of providing automatic compensation for different intensities of light striking the target. The voltage drop across the series resistance will increase with increasing light intensity striking the target, and this increase will result in tending to reduce the actual target current toward its original value, thereby providing substantially the same response irrespective of the intensity of the light impinging upon the target.

This type of automatic light compensation circuit, however, is subject to certain practical difiiculties. When the vidicon is first turned on, its target will assume a high positive potential-as a result of the presence of the compensating circuit, which may be sufiicient to cause the target to emit secondary electrons in substantial quantity. No matter how long the vidicon remains on, therefore, it will then be completely incapable of operating in its intended fashion, since more electrons will be emitted by secondary emission than will be available to scan the effects of light impinging upon the target. In such instances, attempts are made at the factory to lower the voltage that is applied to the series resistance so that the initial vidicon target voltage, upon turn on, may be below the predetermined voltage at which substantial secondaryemission takes place from the target. Unfortunately, however, this adjustment does not always work for many vidicon targets in view of the differences in targets resulting from present-day manufacturing techniques, and many vidicons must accordingly be rejected when they are actually really operative tubes, but are just not adapted for operation in this type of circuit.

An objectof the present invention is to provide a new and improved circuit that enables such vidicons all to be employed in this type of automatic light compensation circuit, irrespective of variations in target construction as a result of manufacturing techniques and the like.

A further object is to provide a new and improved vidicon circuit.

Other and further objects will be explained hereinafter and will be more particularly explained in connection with the appended claims.

In summary, the objects of the present invention are attained by connecting the target to a threshold-voltageoperated device of a particular character that effectively connects the target to the vidicon cathode prior to the reaching of a condition where substantial secondary-emission current would be emitted from the target.

The invention will now be described with reference to the accompanying drawing, the single figure of which is a schematic circuit diagram illustrating the invention in preferred form.

A vidicon of conventional type is schematically shown at 1 provided with a light-sensitive target 3 upon which electrons from an electron gun, generally designated by the reference numeral 5, may impinge, in response to accelerating voltage indicated as applied at the terminal l-.

3,134,043 Patented May 19, 1964 While the invention is described in connection with this vidicon type of tube, it will be evident that it is applicable to other kinds of tubes of this character that become subject to the same type of difiiculty. The target 3 is connected by a conductor 7 through the before-mentioned series resistor R to a potentiometer P the slider S of which is adjusted to provide the compensating voltage before-discussed from a potential source labelled +ve, in order to enable automatic light-compensation effects to take place, as before described. The signal output from the target 3, resulting from scanning by the electron beam from the gun 5, is passed through capacitor C to the videoamplifier stages, so labelled, as is well known.

As before discussed, when the voltage is first turned on, the targets 3 of some tubes will assume the potential tapped off by the slider S, and this potential may exceed the predetermined threshold voltage at which the target 3 becomes an effective or substantial secondary-emitter. Under such circumstances, the vidicon cannot work. To salvage the tube, one attempts to lower the position of the slider S in the hope of finding a voltage below the predetermined secondary-emission Voltage at which operation can be obtained. As before explained there are many instances where the particular type of target 3 is not adapted for this particular circuit and no such lower voltage can be found that still provides the desired automatic light compensation effect.

In accordance with the present invention, however, it is no longer necessary to reject vidicons i, that do not so Work. To the contrary, a supplementary circuit is connected by conductor 9 from the target 3 to a threshold voltage-operated device, preferably a device of the character of a tube N having a pair of cold electrodes 2 and 4 disposed within a neon or similar gas-filled envelope. The lower electrode 4 of the device N is connected to a slider S adjustable along a further potentiometer P that is grounded at G1, for applying a bias potential, below the conduction threshold voltage of the tube N, to the electrode 4, in view of the bi-lateral conduction properties thereof. The bias provided by the slider S on the potentiometer P will be adjusted so that, upon turn-on of the voltage supplied to the vidicon 1, the target 3 can never reach a potential sufficient to produce the substantial quantity of secondary electrons before referred to, since the neon device N will conduct before such potential is reached. The neon tube N, which initially presents an impedance very much greater than that of the seriesresistor R, will be caused to conduct if the target 3 starts to approach too high a potential, thereby effectively connecting the target 3 through the then low-impedance conducting threshold device N to the ground terminal G1, and thence through ground to the ground terminal G2 of the electron-gun cathode structure 5, Insurance is thus attained that the target 3, even upon turn on, will never reach that predetermined voltage at which the target would become an effective secondary emitter. The present invention therefore enables the use of all vidicons, irrespective of the peculiarities or variances in the characteristics of their target 3, with the type of light-compensating circuits before described.

It is important not only that the threshold-voltageoperated device N have an initial impedance, prior to conduction, that is high compared with the high-series resistance value R, required for automatic light compensation, but that the capacitance of the device N must be sufficiently small that it does not disturb the high-frequency performance of the video amplifier output circuits connected to the coupling capacitor C. It is fortunate that the threshold-voltage neon tube before described has such a lowcapacitance of the order of a A of a micro-micro farad. Other types of two-electrode devices and the like, however, such as the ordinary type of semi-conductor, on

the other hand, not only do not provide a sufficiently higher resistance compared to resistor R to effect the purposes of the present invention, but ordinarily they have far too high capacitance of the order of from two to ten micro-micro farads, Similarly, thermionic diodes or devices of this character also have too high a capacitance of the order of, say, four micro-micro farads, and, furthermore, have far too low a resistance in view of the heater-to-cathode leakage resistance of such diodes. The present invention, accordingly, requires threshold-voltageoperated devices that provide this very high initial resistance compared with the series-resistance R and this very low value of capacitance.

Modifications will occur to those skilled in the art, and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. Apparatus for preventing secondary-emission effects from a vidicon target and the like upon which electrons may be impinged from a cathode, having, in combination with the vidicon target, series resistance of value comparable with that of the vidicon and connected between the target and a source of positive potential, and a circuit for preventing the target from reaching a predetermined positive voltage value at which the target produces substantial secondary emission, the circuit comprising a normally non-conductive threshold-voltage-operated device of impedance large compared with the said series resistance and means for biasing the device to a degree sufficient to insure that the threshold conduction voltage thereof is produced for a value of target voltage below the said predetermined voltage value, the said circuit being connected between the target and the said vidicon cathode through the device when conductive.

2. Apparatus for preventing secondary-emission effects from a vidicon target and the like upon which electrons may be impinged from a cathode, having, in combination with the vidicon target, series resistance of value comparable with that of the vidicon and connected between the target and a source of positive potential, and a circuit for preventing the target from reaching a predetermined positive voltage value at which the target produces substantial secondary emission, the circuit comprising a normally non-conductive threshold-voltage-operated device of impedance large compared with the said series resistance and means for biasing the device to a degree sulficient to insure that the threshold conduction voltage thereof is produced for a value of target voltage below the said predetermined voltage value, the resistance of the device, when conductive, being negligible, and the capacitance of the device being sufiiciently small to present negligible impedance to the higher vidicon signal frequencies, and the said circuit being connected between the target and the said vidicon cathode through the device when conductive.

3. Apparatus as claimed in claim 2 and in which the said device is a gaseous-discharge device.

4. Apparatus as claimed in claim 2 and in which the said device is a two-electrode neon device and the like, one electrode of which is connected to the target, and the other electrode, to the biasing means.

References Cited in the file of this patent UNITED STATES PATENTS Shelton Mar. 24, 1960 Pike et a1 Dec. 20, 1960 OTHER REFERENCES

Claims

1. APPARATUS FOR PREVENTING SECONDARY-EMISSION EFFECTS FROM A VIDICON TARGET AND THE LIKE UPON WHICH ELECTRONS MAY BE IMPINGED FROM A CATHODE, HAVING, IN COMBINATION WITH THE VIDICON TARGET, SERIES RESISTANCE OF VALUE COMPARABLE WITH THAT OF THE VIDICON AND CONNECTED BETWEEN THE TARGET AND A SOURCE OF POSITIVE POTENTIAL, AND A CIRCUIT FOR PREVENTING THE TARGET FROM REACHING A PREDETERMINED POSITIVE VOLTAGE VALUE AT WHICH THE TARGET PRODUCES SUBSTANTIAL SECONDARY EMISSION, THE CIRCUIT COMPRISING A