US2817821A - Grid magnetron frequency pushing controls - Google Patents

Description

Dec. 24, 1957 D. B. HAAGENSEN GRID MAGNETRON FREQUENCY PUSHING CONTROLS Filed Sept. 17. 1954 l4 /0 Mnamsmau MCDIA'ATOR' /3 i k k 3 D g E 3 Q Q Q 1% g 5 FREQUENCY FREQUENCY FREQUENCY /N VENTOI? DUANE B. HAAGENSEN United States PatentO GRID MAGNETRON FREQUENCY PUSHING CONTROLS Duane B. Haagensen, Wayland, Mass., assignor to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application September 17, 1954, Serial No. 456,851 9 Claims. (Cl. 332-) This invention relates to means for substantially reducing frequency pushing in a grid controlled amplitude modulated magnetron by a simple external adjustment, and to means for producing a frequency variation with power changes which is in opposite directions with respect to a given power level.

Even in the amplitude modulated grid magnetron, the problem of frequency pushing, that is, the change of frequency with magnetron current, becomes troublesome.

As the modulation input voltage changes in a grid magnetron, the magnetron current also changes. The change in current drain through the direct current anode voltage source alters the anode voltage of the magnetron. Since the magnetron frequency is partially dependent upon the anode voltage, changes in degree of amplitude modulation is accompanied by an undesirable shift in operating frequency. Moreover, pushing in the grid magnetron as ordinarily connected is dependent upon the particular magnetron under consideration and is not controllable.

in accordance with the invention, an auxiliary electron discharge device, such as a vacuum tube tetrode or pentode, having approximately the same anode voltageanode current characteristic as that of the magnetron, is connected in circuit therewith, so that the anode voltage source, in series with a variable resistor, is in a portion of the circuit common to both the magnetron and the auxiliary control device.

An adjustable portion of the voltage derived across the aforesaid variable resistor is supplied to one of the grids of the auxiliary control device and is of such polarity as to change the current in the auxiliary control device circuit in the opposite direction to that of the anode current in the magnetron circuit. In this way, the algebraic sum of the currents, that is, the current in the common path including the direct current anode voltage source, remains substantially constant in spite of amplitude modulation. Since the total current in the circuit, that is, the current through the direct current anode voltage source, remains substantially constant, the direct current anode voltage also is maintained substantially constant and the frequency pushing, which is a function of anode voltage, is substantially reduced.

In the darwing:

Fig. l is a circuit diagram of an embodiment of the subject invention;

Fig. 2 is a curve showing the pushing effect in grid magnetrons of the prior art;

Fig. 3 is a curve illustrating the pushing effect in grid magnetrons when incorporated into the compensating circuit of Fig. l; and

Fig. 4 is a curve illustrating the effect of amplitude modulation upon the magnetron operating frequency when ovcrcompensation is achieved.

Referring to Fig. l, magnetron includes, in addition to the usual anode l2 and cathode 13, a grid electrode 14. This grid magnetron may be of the type shown and described in detail in an article entitled. Grid magnetron delivers modulated UHF output," appearing on vpages "ice I 148 to 531 of May i953 issue of Electronics. A modulating voltage from modulator 15 is applied to the input or grid circuit of magnetron ill between grid 14 and cathode 13. The anode 12 of magnetron 10 is connected to ground.

The auxiliary control tube 2b is an evacuated electron discharge device, shown by way of example as a tetrode .249, comprising cathode 21, control grid 22, screen grid 23 and anode 2d. The cathodes of magnetron 1t and auxiliary control tube 20 are interconnected while the screen grid 23 of control tube 20 is maintained preferably at a potential intermediate that of the cathode and grounded anode. The screen grid potential will determine the operation of a device in a manner to be described later.

A potentiometer 25 is connected in series with a direct current anode voltage source, such as a battery 2'7 between the common cathodes and ground. Although the anodes and positive terminal of source 27 are shown connected to ground, other reference levels than ground may be resorted to, it being necessary only that the anodes of electron discharge devices 10 and 20 be maintained at a potential sufficiently positive with respect to the cathodes of said corresponding electron discharge devices.

T he arm of potentiometer 25' is connected to a suitable biasing resistor 28 and battery 29 to the control grid 22 of auxiliary control tube 20. The current 1 flowing through potentiometer 25 in the path common to both electron discharge devices is in the direction indicated by the arrow and is such as to produce a voltage drop across said potentiometer of the polarity shown in Fig. 1, that is, the end connected to the cathodes of electron discharge devices it and 26 is positive relative to the opposite end.

As is well known in the art, the plate current in a tetrode vacuum tube decreases as the control grid becomes more negative, assuming, of course, that the plate voltage and screen grid voltage are held constant. If the magnetron current I increases, the current flow in the common path increases and the negative voltage existing across potentiometer 25 also increases. A portion of this increased negative voltage is applied to the control grid 22 of auxiliary control tube 20, thereby reducing its plate current I whose direction or flow is indicated by an arrow in Fig. l. The increase in magnetron anode current I is thus offset by the decrease in auxiliary control tube plate current I so that the resultant current l (the algebraic sum of currents I and I flowing through the common path including direct current power source 27 is held substantially constant. Since the current drain in direct current source 27 is relatively constant, the magnetron anode voltage, and hence the frequency, likewise remains substantially fixed.

Although a tetrode vacuum tube has been shown and described heretofore, it should be understood that any electron discharge device whose plate voltage-plate current characteristic is substantially identical to that of the magnetron may be used.

It has been found in practice that the minimum pushing does not necessarily occur when the magnetron anode voltage is held absolutely constant. Diiterent magnetrons require varying degrees of destabilization of the anode voltage to obtain the least pushing and an optimum performance. For this reason, it is necessary to adjust the setting of the potentiometer arm for any given magnetron until the frequency variation with changes in amplitudemodulating voltage or power output is a minimum.

Because of the similarity of the plate characteristic.

curves of the auxiliary control tube and magnetron, the control tube plate current will follow closely in an inverse direction to the magnetron anode current. If the slope of the characteristic curve of the control tube decreases, the

greater is the change in its plate voltage for a given change in control grid voltage. As the screen voltage is increased, the ratio of the change in magnetron plate current to the change in auxiliary control tube plate current decreases. The setting of potentiometer 25 will depend not only upon the screen voltage of control tube 20, but also upon the various parameters of both electron discharge devices it; and 20.

Inspection of Fig. 1 will indicate that, if the arm of potentiometer 25 is in the extreme left position, the voltage across the potentiometer is inelfective in controlling the current in auxiliary control tube 20. The magnetron 10 will then operate in the normal fashion, as shown in Fig. 2, wherein pushing with changing power output is considerable. As the potentiometer arm is moved toward the right in Fig. 1, the voltage applied to control grid of control tube 20 becomes increasingly negative, thereby reducing the plate current I in the manner already de scribed. As the potentiometer arm is moved toward the right from the aforesaid extreme left position, an improve ment in pushing will be achieved until, at some point. determined in part by the parameters of both electron discharge devices, the pushing will be reduced to a mini mum, as shown in Fig. 3.

By means of an easily accessible adjustment external to the grid magnetron, pushing may be readily reduced to a minimum for any magnetron.

If the potentiometer arm is moved still further toward the right in Fig. 1, the effect of changes in magnetron rent with modulated input voltage will be overcompensate-d and a power output versus frequency characteristic, such as shown in Fig. 4, will be obtained. it will be noted that the effect of overcompensation is to cause gnetron frequency to increase with power output up to a certain value, shown as Point A in Fig. 4, and corresponding to a given modulating input voltage, and to decrease with power output above this level. This is in contrast with the uncompensated magnetron of the prior art in which the magnetron frequency always changes in the same direction as the power output (a function of the amplitude of the modulation input voltage) increases.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In combination, a magnetron including an anode, cathode, and a control electrode, a source of anode voltago for said magnetron connected between said anode and said cathode, means for varying the current and power output of said magnetron in accordance with a modulation input voltage of variable amplitude, an auxiliary control electron discharge device having at least a cathode, an anode, a control grid, and an additional grid, the cathode of said auxiliary control device being connected directly to the cathode of said magnetron, a variable impedance device, said impedance device and said source being connected in a path common to said magnetron and said auxiliary control device, means for deriving across said impedance device a control voltage in response to change in current in said magnetron, the control grid of said auxiliary control device being supplied with said control voltage for changing the current in said common path in a sense opposite to that of said magnetron current.

2. In combination, a magnetron including an anode, a cathode, and a control electrode, a source of anode voltage for said magnetron connected between said anode and. said cathode, means for varying the current and power outputof said magnetron in accordance with a modular tion input voltage of variable amplitude, an auxiliary control electron discharge device having at least a cathode. an anode, a control grid, said auxiliary device having an. anode voltage versus anode current characteristic subs-tam (ill tially identical to that of the magnetron, and an additional grid, a variable impedance device, said impedance device and said source being connected in a path common to said magnetron and said auxiliary control device, means for deriving across said impedance device a control voltage in response to a change in current in said magnetron, the control grid of said auxiliary control device being supplied with said control voltage for changing the current in said common path in a sense opposite to that of said magnetron current.

3. In combination, a first closed series network including a magnetron, a source of voltage and a potentiometer having a movable arm, said magnetron having at least an anode, a cathode and a control electrode, means for altering the current and power output of said magnetron in accordance with a modulation input voltage of variable amplitude, a second closed series network including an auxiliary control electron discharge device, said source of voltage and said potentiometer, said auxiliary control device having an anode, a cathode, a control grid an additional grid, means for interconnecting said mov. arm to said control grid of said auxiliary Control device, and means including said potentiometer and said auxiliary control device and responsive to a variation in the current flowing through said magnetron and said source for compensating at least partially for said variation.

4. In combination, a first closed series network includ ing a magnetron, a source of voltage and a potentiometer having a movable arm, said magnetron having at least an anode, a cathode, and a control electrode, means for alternating the current and power output of said magnetron in accordance with a modulation input voltage of variable amplitude, said means for altering causing an attendant variation in the operating frequency of said magnetron. a second closed series network including an auxiliary con trol electron discharge device, said source of voltag... and said potentiometer, said auxiliary control device having an anode, a cathode, a control grid and an additional. grid, means for interconnecting said movable arm to said control grid of said auxiliary control device, and means including said potentiometer and said auxiliary control revice and responsive to a variation in the current flowing through said magnetron and said source for reducing said variation in magnetron operating frequency.

5. In combination, a first closed series network including a magnetron, a source of voltage and a potentiometer having a movable arm, said magnetron having at least an anode, a cathode, and a control electrode, means for altering the current and power output of said magnetron in accordance with a modulation output voltage of variable amplitude, said means for altering causing an attendant variation in the operating frequency of said magnetron, a second closed series network including an auxiliary control electron discharge device, said source of voltage and said potentiometer, said auxiliary control device having an anode, a cathode, a control grid and an additional grid, said auxiliary control device and said magnetron having substantially similar anode voltageanode current characteristics, means for interconnecting said movable arm to said control grid of said auxiliary control device, and means including said potentiometer and said auxiliary control device and responsive to a variation in the current flowing through said magnetron and said source for reducing said variation in magnetron operating frequency.

6. In combination, a first closed series network including a magnetron, a source of voltage and a potentiometer having a movable arm, said magnetron having at least an anode, a cathode, and a control electrode, means for altering the current and power output of said magnetron in accordance with a modulation input voltage of variable amplitude, said means for altering causing an attendant variation in the operating frequency of said magnetron, a second closed series network including an auxiliary control electron discharge device, said source of voltage and said potentiometer, said auxiliary control device having an anode, a cathode, a control grid and an additional grid, said auxiliary control device and said magnetron having substantially similar anode voltageanode current characteristics, means for interconnecting said movable arm to said control grid of said auxiliary control device, and means including said potentiometer and said auxiliary control device and responsive to a variation in the current flowing through said magnetron and said source for altering the manner of variation of said magnetron operating frequency.

7. In combination, a magnetron including at least an anode, a cathode and a control electrode, a source of anode voltage for said magnetron connected between said anode and said cathode, means for varying the current and power output of said magnetron in accordance with a modulation input voltage, a potentiometer inserted between said cathode and said source, means for deriving a control voltage across said potentiometer which is a function of the magnetron current, an auxiliary control electron discharge device having at least an anode, a cathode, a control grid and having an anode currentanode voltage characteristic substantially identical to that of said magnetron, said cathodes of said magnetron and said auxiliary control device being at substantially the same reference potential, said control grid of said auxiliary control device being electrically connected to said potentiometer and responsive to a portion of said control voltage for altering the current flowing in said auxiliary control device in the opposite sense from the variation in magnetron current, and means responsive to the variation of current in said control device for maintaining substantially constant the current flowing through the anode source and consequently the anode voltage of said magnetron.

8. In combination, a magnetron including an anode and a cathode, a source of voltage connected between said anode and said cathode of said magnetron, a variable impedance means, an auxiliary electron discharge device including an anode, a cathode and at least one control electrode and having an anode voltage versus anode current characteristic substantially identical to that of said magnetron, said source of voltage also being connected between said anode and said cathode of said auxiliary electron discharge device, said variable impedance means being connected in the anode current path of both magnetron and auxiliary device for deriving a control voltage for application to said control electrode of said auxiliary device which varies with the anode current of said magnetron, said control voltage determining the resultant current flowing through said source of voltage and hence the magnitude of the voltage supplied to the magnetron by said source of voltage.

9. In combination, a magnetron including an anode and a cathode, means for supplying an anode voltage between said anode and cathode of said magnetron, an auxiliary control electron discharge device having at least a cathode, an anode and a control grid, said cathodes being directly interconnected, said anodes being connected to one terminal of said means for supplying, a variable potentiometer, said cathodes being connected through said potentiometer to the other terminal of said means for supplying, said potentiometer having produced thereacross a control voltage in response to the anode currents flowing in said magnetron and in said auxiliary control device, and means for supplying a portion of said control voltage to said control grid of the auxiliary control device for varying the manner of deviation of magnetron operating frequency with magnetron anode current.

References Cited in the file of this patent UNITED STATES PATENTS 2,149,080 Wolff Feb. 28, 1939 2,461,642 Hayes Feb. 15, 1949 2,620,467 Donal Dec. 2, 1952 2,688,705 Fundingsland Sept. 7, 1954

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