US2083926A - Screen grid oscillator - Google Patents

Description

H. SHORE SCREEN GRID OSCILLATOR Filed Jan. 30, 1932 OUTPUT 00" !O|OUOOOOO"'1'OO June 15, 1937.

VOLTAGE E AND E56 INVENTOR HENRY SH E BY /7 g M ATTORNEY Patented June 15, 1937 SCREEN GRID OSCILLATOR Henry Shore, Elizabeth, N. J., assignor to Radio Corporation of America,

aware a corporation of Del- ApplicationYanuary so, 1932, Serial No. 589,858

24 Claims. ((1250-36) The present invention relates to a thermionic oscillator and is directed primarily to an oscillator of great inherent. stability.

In the prior art, oscillators of great stability and constancy of frequency have been provided through the use of piezo electric control devices. These oscillators, while very efllcient, nevertheless require great care in the selection of a suitable crystal and design of the crystal mounting,

as well as great care to provide ways and means of neutralizing temperature change effect on the crystal to maintain the constancy of generated frequency desired.

It is also known that oscillators which are crystal controlled are limited to relatively small output energy for the most part because a high current output from the oscillator would p du e or generate sufiicient heat to endanger cracking the crystal.

Therefore, it has been an object of this invention to provide an oscillator which is of substantially the same stability as the crystal oscillator; to provide an oscillator which is capable of a high current output so as to be usable as a master oscillator; to provide an oscillator wherein'the effects of temperature changes are substantially neutralized and made ineffective to produce changes in the resulting output frequency; to provide an oscillator which will main- 39 tain the generated frequency constant irrespective of variations in the voltages of the sources supplying operating potentials thereon; and to provide an oscillator which will be simple in the circuit design and operation.

Many other advantages and further objects of the invention will become apparent from a reading of this specification and the appended claims in connection with the accompanying drawing, wherein Figure 1 shows a preferred form of cira cult arrangement for an oscillator using a screen grid or four element thermionic tube, and Figure 2 represents a series of curves plotting frequency against voltage to describe the operation of the device shown by Figure 1. Reference may now be made to the drawing for an understanding of the present invention, and first to Figure 2 thereof.

It is known in the prior art that in any type of oscillator, such for example, as has been disclosed by Hartley, Colpitts and others, a change in the voltage supplied to the plate electrode of the oscillator tube will produce a change in the output frequency. Thus, if the voltage supplied to the plate should increase in value the output frequency will decrease. This is true because "oscillator tube be held constant and the voltage the greater output current resulting fromincreased plate voltage produces greater loads in the systemand these loads must be absorbed in the oscillator load circuit which includes the usual capacity and inductance elements. Simi- 5 larly, if the plate or operating voltage should decrease then the reverse, or an increase in the output frequency, takes place.

In accordance with this understanding of the prior art, as has already been determined by experiment, it can be seen that by referring to the curves of Figure 2, wherein the ordinates represent frequency and the abcissae represent voltage, if the screen voltage, represented by Esc, is maintained constant and the voltage Er applied to the plate is increased it then can be seen that the output frequency will decrease. By assuming, for example, four different values for the screen voltage 'on-the electron tube it can be seen, where ESG equals C1, Ese equals 2. etc., that in each case the tube output frequency 'will decrease with an increase of plate voltage EP in the tube. Now, let it be assumed that the potential Er applied to the plate electrode of the 5 applied to the screen electrode be made to change from a minimum to a maximum, it then can be seen from experiment that the output frequency of the oscillator tube will increase. This is in accordance with the curves showing an increase in, frequency with respect to increasing screen grid voltages which are designated by the characteristics Er equals K1, Er equals K2, etc.

This result, it will be appreciated, takes place by virtue of the fact that in a. screen grid type vacuum tube, if the plate voltage is increased there is an increase in the current flow through the tube and, if the screen voltage is held constant, for each increase in the plate voltage the strength of the electron stream to support the current flow through the tube increases in accordance with the voltage on the plate and consequently with increases in plate voltage more and more electrons reach the plate electrode. However, if the plate voltage on the tube is held at a constant value and the voltage on the screen electrode is increased from a minimum to a maximum, it is seen that due to the increase in screen voltage/less and less electrons reach the plate electrode and therefore the current flow through the tube output circuit decreases. Such -findingssupport the above outlined conditions that for increased current flow through a vacuum tube oscillator the frequency will decrease but for decreased current flow through the tube the output frequency will increase.

Bearing these features in mind, the invention herein disclosed has been applied for the purpose of illustrating a concrete example of one form which the oscillator constructed in accordance with the present invention may assume, as comprising a circuit arrangement based substantially upon the teachings disclosed by Hartley, although it is understood that a form of oscillator using the principle of the Colpitts oscillator could be substituted without involving any changes. In accordance with the preferred arrangement of this invention, the vacuum tube I which is of the screen grid type and which comprises a filament electrode 3, and three cold electrodes, such as a grid electrode 5, a screen electrode I and a plate electrode 9, has its plate electrode connected through a source of potential II to the filament electrode through a choke coil II. The screen electrode I is also connected with the positive side of the plate supply battery through a resistor It for a purpose which will hereinafter appear. The oscillator or tank circuit coupled with the tube oscillator comprises an inductance I! which is grounded at its midpoint I! and which inductance is shunted by a capacity element I! so that the oscillating or tank circuit comprises the inductance l5 and the capacity l9, each of which may be of variable nature so as to provide for different selected output frequencies.

The output circuit is coupled through thewinding It with the inductance ii of the oscillator circuit. To provide for coupling connections between the grid and plate circuits of the tube i with the oscillator circuit well known capacity couplings are provided by way of capacities 23 and 25. A grid leak resistor 21 connects the grid electrode 5 with the filament. From this arrangement it is seen that the direct current supplied as the tube plate voltage is blocked from the oscillator circuit by means of the capacity 25 and the high frequency output of the tube oscillator is blocked from the direct current plate supply by way of the choke i 3. v

Having determined in accordance with the curves illustrated by Figure 2 the operating features it can be seen that of the various curves designated Esc equals C1, etc., and Er equals K1, etc., there are two curves which intersect each other at substantially right angles, or in other words, these curves intersect at a point such that the slope of one curve at the point of intersection'is substantially the negative of the slope of the other curve. This can be expressed mathematically by the equation of) 3 sa Er-K 5E? Era-C wherein f is the frequency and having determined the voltage at the point of intersection of these two sets of curves meeting the conditions of the above equation it can be seen that for normal operation the battery ii should be of a value equal to that of the voltage at the point of intersection of these curves.

Now, having determined this value of the voltage for the source ii, it is possible to calculate by Ohm's law the value of the resistor element It necessary to produce a voltage drop sufiicient to place the screen electrode I at the proper potential with respect to the plate potential, assuming the voltage drop in the choke it to be negligible, of course, to provide for an output frequency corresponding to that shown at the point of intersection of the two curves. This preferred operating voltage having now been determined, assume, for example, that the voltage of the source should increase. This increase, of course, will increase the potential on the plate electrode with respect to the cathode but, as this potential increases, the screen potential also increases and consequently the screen electrode tends to rob someof the electron stream that tends to flow normally toward the plate electrode. Thus the current output of the tube remains constant.

Now assume that the potential of the source ii should decrease so as to cause a decrease in plate voltage and also a decrease in screen voltage. Under this operating condition it can be seen that with the screen potential decreasing the screen will tend to rob less and less of the electron stream and that the number of electrons reaching the plate electrode will thus remain substantially constant, and, as a result, the output current will remain constant so as to maintain constant generated frequency.

This advantageous feature is material because it can readily be appreciated that with a system so constructed there is practically no limit to the output current which can fiow and thus the tube can be used as a master oscillator. Furthermore, by a mere variation of the values of L and/or C (that is, the inductance i and the capacity I!) the tube may be caused to generate oscillations throughout a wide range of frequency, although always inherently stable at the particular frequency selected.

Furthermore, the oscillator may be made substantially independent of temperature changes by a suitable design of the inductance and capacity elements. This stability is not accomplished as easily with the crystal oscillator, because when using a crystal to control the oscillation frequency if it is desired to change the frequency limits within a small range the frequency change is normally produced by causing a small change in the crystal temperature. Since the crystal has high thermal inertia the time period to produce the desired frequency change is often of the order of hours, whereas for the arrangement shown herein the frequency change may be made instantaneously by a mere variation of the value of the inductance or capacity or both.

If this invention-were applied to the form of oscillator of the type disclosed by Colpitts, it can be seen that in place of the single capacity It, two series connected capacity elements would be provided, and in place of grounding the inductance ii at its midpoint it would be desirable to ground at a point intermediate the two series capacity elements, of course maintaining the same filament ground 29 as is necessary in arrangements using the Hartley type circuit.

It, of course, will be understood that while the invention has been disclosed using the well known type of screen grid tube as the oscillator, the pentode type tube may be used. When so connected the suppressor electrode or grid positioned between the screen and the plate electrode is usually connected directly with the cathode within the tube. The pentode tube is ideally adapted for use as an oscillator since the function of the grid or electrode intermediate the screen and the plate is to suppress secondary emission from the plate and thus it gives greater stability of operation.

Other modifications and changes in the invention will, of course, become apparent and suggest themselves to those skilled in the art from a reading of this specification, and therefore I believe it to be within the scope of my invention to make and use any or all of these modifications provided, of course, they fall fairly within the spirit and scope of this invention as defined by the hereinafter appended claims.

Having now described my invention, what I claim and desire to secure by Letters Patent is the following:

1. An oscillator circuit comprising a four element thermionic tube, atank circuit connected between grid and plate electrodes of said ther-- mionic tube, a common source of potential connected between the plate and an intermediate electrode of said tube and the filament electrode thereof, and means for producing on said intermediate electrode changing voltages of a value to maintain the current output from the tube for all changes in voltages of the source to maintain constant frequency of oscillations in the said tank circuit.

2. An oscillator circuit comprising a four electrode tube, a tank circuit connected between the plate electrode of said tube and an intermediate electrode thereof, a common source of voltage connected at one terminal with the tube cathode and at the other terminal to serve as a common energy source for the plate electrode and a second intermediate electrode of said tube, and a resistor connected in series with the voltage source and ode and at the other terminal with the plate electrode and the screen electrode of said tube, and a resistor connected in series with the voltage supply source and said screen electrode for controlling the potential of the screen relative to the plate so that at time periods of changes in plate potential to provide for varying the screen potential by increments sufficient to provide a constant output frequency from said vacuum tube oscillator.

7 A vacuum tube oscillator comprising a screen grid type thermionic tube, a tank circuit connected between modulating grid and anode of said tube, a common so'urce of potential connected between the anode "and the screen electrode of said tube and the cathode thereof, and a resistor connected in series with the screen electrode and the ource for producing on said screen electrode at ti e periods of change in supplied anode voltage fluctuating voltages of a value suitable to maintain the frequency of the current appearing said second named intermediate electrode so as to provide constant output current from said vacuum tube.

3. An oscillator circuit. comprising a multielectrode thermionic tube, a tank circuit connected between an intermediate electrode and the anode electrode of said thermionic tube, a common source of potential connected between the anode and a second intermediate electrode of said tube and the tube cathode, and means for varying the voltage'supplied to said second named intermediate electrode upon changes in anode voltages by increments suflicient to maintain the frequency of the current in the tank circuit constant irrespective of changes in applied voltages.

4. An oscillator circuit comprising a four electrode tube, a tank circuit connected between the plate electrode of said tube and an intermediate electrode thereof, a common source of voltage connected at one terminal with the tube cathode and at the other terminal to serve as a common energy source for the plate electrode and a second intermediate electrode of said tube, and a resistor connected in series with the voltage source and'said second named intermediate electrode so as to provide constant plate current output from said vacuum tube.

5. A vacuum tube oscillator comprising a screen grid type thermionic tube, a tank circuit connected between modulating grid and anode of said tube, a common source of potential connected between the anode and the screen electrode of said tube and the cathode thereof, and means for producing on said screen electrode at time periods of change in supplied anode voltage varying voltages of a value suitable to maintain the frequency of the current appearing in the tube output circuit constant for substantially all applied operating voltages. v

6. A vacuum tube oscillator comprising a screen grid type vacuum tube, a tank circuit connected between the plate electrode of said tube and the grid electrode thereof, a common source of voltage connected at one terminal with the tube oathin the tube output circuit constant for substantially all applied operating voltages.

8. Means for generating oscillations of constant frequency comprising a vacuum tube having a cathode, an anode, and a plurality of intermediate'electrodes, an oscillation circuit containing capacity and inductance elements connected with said anode and one of said intermediate electrodes, a lead from said cathode to said oscillation circuit, a common source of potential for supplying operatingvoltages to said anode and one of said intermediate electrodes, said source being connected at one terminal with the tube anode and one intermediate electrode and at the other terminal with the tube cathode, and means connected in series with said last named intermediate electrode and said voltage supply source to change the voltage on said intermediate electrode at time periods'of change in voltage upon said anode by an amount suflicient to compensate for resulting anode current changes so as to produce constant frequency output from said oscillator.

9. In an oscillating electron tube circuit'including an electron tube having a cathode element, a control grid element, an anode element and an auxiliary element associated with the electron stream of said tube, means including said auxiliary element and the main source of electric energy for said oscillating electron tube circuit for compensating for changes in the frequency of oscillation arising from variations in the voltage of the said main source of electric energy, through the application of such increments of potential to said auxiliary element as substantially neutralize the tendency to frequency change caused by simultaneous increments in potential applied to said anode element.

10. In an oscillating electron tube circuit including an electron tube with at least two anode elements associated with a common source of electric energy, the method of minimizing changes in the frequency of oscillation arising from variations in the voltage of said common source of electric energy which consists in the selection of such operating potentials for said anodes as to cause the change in frequency due to a change in the voltage applied to one of said anodes to be substantially compensated for by the opposite effect upon frequency of a change in voltage applied to the second of said anodes.

11. In an oscillating electron tube circuit including an electron tube having a cathode element, a control element and anode element associated in circuit for the generation of oscillations, an auxiliary element associated with the electron stream of said tube and having a connection to an impedance to one extremity of which a variable voltage is applied, and means including a device for applying to said anode element certainv selected' voltages, the values of which are a function of said variable voltage whereby the frequency of oscillation of said electron tube circuit is maintained substantially constant.

12. In an oscillating electron tube circuit including an electron tube containing a cathode, a control grid, and two additional elements, a source of potential and connections between said source of potential and said two additional elements, the potentials for the said two additional elements being selected for causing the frequency of oscillation to remain substantially constant for changes in the potential of said potential source due to the compensating effect of the said two additional elements.

13. In an oscillating electron tube circuit including an electron tube containing a cathode, a control grid, an inner anode and an outer anode, a circuit between said inner and outer anodes, a common potential source connected in said circuit, said source delivering such operating potentials to said anodes for causing the frequency of oscillation to remain substantially constant for changes in the potential of said common potential source due to the compensating ciifect of the said anodes.

14. In an oscillation generator, an electron tube including a cathode, a control grid and at least two additional elements, a resonant circuit included between said cathode and one of said additional elements, a source of electric energy included in circuit between said cathode and one of said additional elements, and means including one other of said additional elements and a connection to said source of electric energy for selectively applying such potentials to said last mentioned element as cause the frequency of oscillation to be substantially constant for variations in the voltage of said source of electric energy.

15. In an oscillation generator, an electron tube including a cathode element, a control grid element and at least two additional elements, a resonant circuit included between two of said elements for determining the frequency of oscillation, a source of electric energy included in circuit between said cathode and one of said additional elements and means including one other of said additional elements and a connection to said source of electric energy for applying such variations of potential to said last mentioned additional element as will substantially counteract the frequency change tendency due to variations in the voltage of said source of electric energy;

16. In an oscillation generator comprising an electron tube having a cathode, a control grid, an anode and an additional element, the method of compensating for changes in the frequency of oscillation resulting from variations in potential of the source of anode potential which consists in so choosing the potentials of the anode and additional element that the net effect upon frequency caused by a change in the potential of said anode potential source is substantially zero.

17. In an oscillating electron tube system comprising an electron tube having a cathode, a control grid, an inner anode and a main outer anode, the method of compensating for changes in the frequency of oscillation resulting from variations in potential of the source of anode potential,

which comprises applvins to the inner anode a potential proportional at all times to the potential applied to the main anode and of such value that the change in frequency due to a change in the outer anode potential will be substantially compensated by the eifect of a proportional change in the inner anode potential.

18. In an oscillation generator comprising an electron tube having at least four separate elements, the method of compensating for changes in the frequency of oscillation which comprises applying to one of said elements a positive potential, and applying to another one of said elements a different positive potential proportional at all times to the first-mentioned applied potential and of such value that the change in frequency due to a change in the first mentioned positive potential is substantially compensated by an opposite effect upon frequency caused by the proportional 013K188 in the second mentioned positive potenti 19. In an oscillating electron tube system comprising an electron tube having a cathode, a control grid, an anode and an additional element, a

resonant circuit connected between said cathode and said control grid, an output circuit and a source of anode potential, the method of compensating for changes in the frequency of oscillation resulting from variations in potential of the source of anode potential which comprises applying to said additional element a potential proportional to the potential applied to the anode and of such a value that the frequency change tendency caused by a variation of the anode potential is substantially offset by an opposite frequency change tendency caused by a proportional variation of the potential of the additional elemen 20. In an oscillating electron tube circuit having an electron tube containing a cathode and two electrodes electro-positive with respect to said cathode, the method of stabilizing the frequency of oscillation which consists in applying to the said two electrodes such electro-positive potentials as to cause an increase in frequency due to an increase in potential applied to one of said electrodes to be substantially compensated by a decrease in frequency resulting from a change in potential applied to the other of said electrodes.

21. In an oscillation generator including an to one of said electrodes to be substantially oifset by the opposite effect upon frequency caused by a change in potential applied to the other of said electrodes.

22. In an oscillation generator having an electron tube including a cathode element, a control element and two anode elements, the method of avoiding frequency modulation resulting from changes in a common anode potential source which consists in so choosing the ratio of the potentials applied to the two anodes that the frequency remains substantially constant due to the compensating eflect of the two anodes.

23. In an oscillation generator having an electron tube containing a cathode and at least two elements electro-positive with respect to said cathode, the method of compensating for a change in frequency due to a change in the electro-positive potential of one of said elements which consists in applying such change in the electrc-positive potential of the other of said elements as to cause an opposite frequency change tendency from that caused by the change in electro-positive potential of the first mentioned element.

24. In an oscillating electron tube circuit including an electron tube having a cathode and at least two elements electro-positive with respect to said cathode by virtue of their association with a common power source, the method of minimizing changes in the frequency of oscillation arising from variations in the voltage of said common power source which consists in the selection of such electro-positive potentials for said elements as to cause the change in frequency due to a change in the electro-positive potential of one of said elements to be opposite to the change in frequency caused by the change in the electropositive potential of the other of said elements.

HENRY SHORE.

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