US2791741A

US2791741A - Voltage amplitude control

Info

Publication number: US2791741A
Application number: US414913A
Authority: US
Inventors: Nathaniel L Cohen
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-03-08
Filing date: 1954-03-08
Publication date: 1957-05-07
Anticipated expiration: 1974-05-07

Description

May 7, 1957 N. L. COHEN 2,791,741

VOLTAGE AMPLITUDE CONTROL Filed March 8, 1954 W TEH-E I N V EN TOR. NATHAN/EL L. COHEN f/TTORNEYS United States Patent "ice VOLTAGE AMPLITUDE CONTROL Nathaniel L. Cohen, New Milford, N. J., assignor to-the United States of America as represented by the Secretary of the Navy A plication March 8, 1954,'Sen'al No. 414,913 1 Claim. (Cl. 323*30) This invention relates to a circuit for voltage amplitude control and more particularly to a circuit for voltage amplitude control or limiting, relying for its operation on a gated beam tube, adapted to eliminate voltage amplitude variations from a frequency modulation signal or the like, and in addition adapted to generate rectangular pulses.

it is well known that the advantages of frequency modula-t-ion are best utilized when the received FM signal waveform is limited to preclude amplitude variations. The gated beam tube has several characteristics which makes it admirably suited for eliminating amplitude variations froma frequency modulation signal. It is also useful for generating rectangular pulses. For a discussion of a gated beam tube of aparticular type known by the commercial marking G l 3N6, reference is made to Proeeedin-gs of .the :National Electronics Conference, volume September 2 6, 27, 28, 1949, pages 4 08 to 426, and Electronics, volume 2 3, Number 2, February 1950, page 82.

Generally, the gated beam tube includes an electron gun having a focusing means. By designing the lens of the electron gun so that it suitably shields the cathode from other associated electrodes it is possible to maintain a constantbeam. current regardless of changes in the poten'tials of the other electrodes. In the path of the beam current, comprising a stream of electrons issuing from the electron gun, there is disposed an accelerator electrode formed with a suitable restricted slot or aperture. Beyond the slot in the accelerator-electrode and in the path of the beam current there is disposed a signal grid otherwise called the control grid. If the potential of the signal grid is made suiiiciently negative, no beam current passes through the signal .grid. correspondingly, if the potential of the signal grid is raised sufficiently positive, a maximum percentage of the bearn current passes through the signal grid and on toward the subsequent electrodes. This generally applies to all electron tubes, hereinafter called tubes, having control grids. However, what is remarkable in the case of the gated beam tube is that the necessary potential swing on the signal grid forproduc- 'ing this change is'verystnall. A slight negative potential on the signal grid causes the latter to act as a perfect barrier to the beam current; a slight positive potential on the signal grid causes the latter to be effectively transparent to the beam current and the aforementioned maximum percentage of the beam current passes through the signal grid on toward the subsequent electrodes in the path of the beam current. It is this sharp transitional change, namely, that the beam current passing through the signal grid varies between zero and a maximum under the control of a small change in potential of the signal grid, which is one of the important characteristics relied upon for the proper operation of the circuit of this invention.

Due to the fact that the electrons comprising the beam current in the gated beam tube do not return along the same path that they follow in moving from the cathode 2,791,741 Patented May 7, 1957 to the signal grid substantially all the beam current that is turned back by the signal grid when its potential swings slightly negative is captured by the accelerator electrode. Therefore the current in the circuit of the accelerator electrode also varies sharply between minimum and maximum.

The gated beam tube is further provided with a second grid, generally known as the quadrature grid, followed by an anode (or plate) both of which are in the path of the beam current issuing from the electron gun.

The quadrature grid affects the tube operation in a manner similar to the signal grid. Within narrow limits, the amount of anode current is controlled by the quadraturegn'd. However, if the potential of the quadrature grid is made .sufiiciently positive, it no longer exercises any control over the anode current since the current can never rise above a certain amount which is a fixed percentage of the beam current; the amount of beam current issuing from the electron gun is fixed.

Both the signal grid and the quadrature grid are adapted to cut off the anode current.

(The characteristics of the gated beam tube utilized are (l) the constant beam current issuing from the cathode, (2) thevsteep rise of plate current in the signal grid-plate transfer characteristic from zero to a maximum due to a very slight rise in signal grid potential (3) the steep rise of plate current in the quadrature grid-plate transfer characteristic due to a slight rise in quadrature grid voltage.

The section of the gated beam tube comprising the cathode, signal grid, and accelerator acts in a manner similar to that of a dynatron. The section including the quadrature grid and anode acts like a tiiode.

The circuit of this invention adapts the gated beam tube for very effective use as a limiter. With no bias on the signal grid and with a positive bias on the quadrature grid, a signal grid potential swing in the general vicinity of one volt causes a considerable variation in accelerator current which is accompanied by a reverse considerable variation in accelerator voltage. The variation in accele'r'ator voltage is coupled through a condenser to the quadrature grid to cause the anode current to be either reduced to zero or raised to a maximum. The resulting voltage swing at the anode is coupled to another stage through a coupling condenser.

An object of this invention is to provide a voltage amplitude control.

Another object is to provide a voltage amplitude control adapted to function as a limiter.

Another object is to provide a voltage amplitude control adapted to function as a limiter for generating rectangular pulses.

A further object of this invention is to provide a volt age amplitude control for use in eliminating amplitude variations from frequency modulation signals or the like.

A further object is to adapt the characteristics of a 6BN6 gated beam tube to voltage amplitude control.

Other objects and many of the attendant advantages of this invention'will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a graph showing a signal grid-plate transfer characteristic of a 6BN6 gated beam tube, for three levels of quadrature grid potential,

Fig. 2 is a graph showing a signal grid-accelerator transfer characteristic of 6BN6 gated beam tube for one particular potential on the quadrature grid, and

Fig. 3 is a schematic diagram of a preferred circuit according to this invention.

There is shown in the schematic diagram a gated beam tube 12. Included within the envelope of the gated beam tube 12 is a cathode 14. Connected to the cathode and at the same potential as the cathode are a focus electrode 15. The tube 12 further includes a signal grid 16, a lens 17 surrounding the signal grid, an accelerator electrode 18 having a screened opening, a quadrature electrode 22, an anode 24, and a shield 25. There is no bias provided between the cathode 14 and the signal grid i6. The input signal voltage is applied to the circuit at the

terminals

26 and 28. A potential is provided between the cathode 14 and the accelerator electrode 18 by means of the battery 32. In the circuit with the battery 32 between the accelerator electrode 13 and the cathode 14 is a resistor 34. A voltage dividing means such as a potentiometer'36 having a tap 37 is connected in shunt across the battery 32 to provide for a selectively variable bias on the quadrature grid 22. In series circuit between the quadrature grid 22 and the tap 37 of potentiometer 36 is a resistor 38. A coupliug condenser 42 is directly connected to the accelerator electrode 1% and the quadrature grid 22 at the ends of the

resistors

34 and 38, respectively. Resistor 38 and condenser 42 provide a difierentiator circuit for the quadrature grid 22. 'One end of load resistor 44 is connected to the anode (or plate) 24 and the other end 'of the load resistor 44 is connected to the positive terminal of the plate supply 46. The other terminal of the plate supply 46 is connected to ground. A coupling condenser 48 is provided for coupling the output of the circuit to another stage. In operation, assuming a negative goingportion of an FM signal is being applied to the signal grid, as the grid 16 goes negative the current in the accelerator increases rapidly causing a rapid drop in accelerator voltage. The magnitude of the drop in accelerator voltage is a function of the accelerator current and the size of resistor 34. V The drop is considerably greater than that on the signal grid. The drop is instantaneously coupled to the quadrature grid 22 through the resistor 38-condenser 42 difierentiator. The magnitude of the drop is snfiicient to cut oi? the plate current causing the potential at the plate to rise to that of battery 46. Assuming that a positive going portion of an FM signal is applied to the signal grid 16, the immediate result is that the current through the accelerator 18 and the resistor 34 decreases resulting 'in an increase in voltage at the accelerator 18. This change in voltage on the accelerator 18 is communicated to the quadrature grid 22 through the resistor 33-condenser 42 coupling causing the plate current to rise to a maximum and the potential to -drop accordingly.

Change in potential at the quadrature grid 22 acts to reinforce the effect of the signal grid voltage on the plate current. Therefore, though theamplitude of the original input signal at 26-23 may vary more or less, the output amplitude isfixed. The transition from zero to maximum plate current takes place over a very limited range of signal grid voltage swing whereby the output plate voltage remains constant for all values of grid voltage above one limit and is zero for all values of grid voltage below the other limit. The eifect of the connection from accelerator to quadrature grid is to increase the efiectiveness of the signal grid voltage. Essentially it may be considered a form of positive feedback. Because of the inherent shieldmg of the quadrature grid on the anode, it is possible to obtain apparent positive feedback efifects far greater than could normally be obtained in a conventional amplifier without reaching instability. In addition, because of the independence of the plate circuit from the feedback circuit, there is little or no effect on the output impedance of the device. Hence the frequency response is not noticeably affected. Through proper design of the elements of this invention, it is possible to convert an input signal into a series of rectangular pulses corresponding to the former in repetition rate.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

I claim:

A frequency modulation limiter circuit comprising a gated beam tube, said gated beam tube including a cathode, a signal grid, an accelerator electrode, a quadrature grid, and a plate; a coupling condenser connected between the quadrature grid and the accelerator electrode whereby variations of the potential of the accelerator electrode are immediately transferred to the quadrature gr a first direct current power supply having a positive sue a negative terminal, the negative terminal of said first battery connected to said cathode; a first resistor, one end of said first resistor connected to the positive terminal of said battery and the other end of said resistor connected to the accelerator electrode; a voltagedivider connected in shunt across said first direct current power supply and having an adjustable tap; a second resistor, one end of said second resistor connected to the quadrature grid and the other end of said second resistor connected to the tap of said voltage divider; a second direct current power supply having a positive terminal and a negative terminal, the negative terminal of said second. supply connected to said cathode; a third resistor constituting a plate load resistor, one end of said third resistor connected to said plate and the other end of said third resistor connected to the positive terminal of said second battery whereby when a signal whose voltage range includes the eifective signal grid voltage range is applied to the signal grid, current in the accelerator circuit changes rapidly causing an opposite change in the accelerator voltage, the latter being coupled by the RC network consisting of said condenser and said second resistor to the quadrauture grid, the action of the quadrature grid being such as to reinforce the effectrof the signal grid voltage on the plate current to cause the output plate voltage to be constant at one limit for all values of signal grid voltage above a certain value and to be constant at another limit for all values of signal grid voltage below a certain value.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,919

OTHER REFERENCES PublicationAdler: A Gated Beam Tube; Electronics, vol. 23, No. 2, February 1950, pp. 82435.