US2442769A - Electronic delay circuits - Google Patents

Description

June 8, 1948. D. E. KENYON ELE CTRONIC DELAY CIRCUITS 2 Sheets-Sheet 1 Fiied Dec. .30, 1942 FIG. 2.

a bum jzaq Trigger Pulse lnpuf INVENTOR DAVID E. KENYON.

June 8, 1948- D. E. KENYON I ELECTRONIC DELAY CIRCUITS 2 Sheets-Sheet 2 Filed Dec. 30, 1942 Cuf-off Voltage FIG. 3.

' INVENTOR DAVID E. KENYON BY g M A-wmmav Patented June 8, I948 UNITED STATES ATENT OFFICE nrlrzornomd it nifv omourrs V I 7 David E. Kenyon, Smithtown, N. Y., assignor to The Sperry Corporation, a corporation of Delaware My invention relates to electric discharge device I circuits and has for its principal object the provision of arrangements for producing pulses of precisely fixed duration or spacing.

It is an object of my invention to provide a circuit in which the time duration or the delay of a pulse is fixed by the constants of a tuned circuit. It is also an object of my invention to provide a circuit in which the interval between pulses is fixed by circuit constants. Still another object is to provide arrangements for readily adjusting or varying the pulse duration and the interval between pulses.

Other and further objects and advantages of my invention will become apparent as the description proceeds.

In carrying out my invention in one of its preferred forms, I provide a pair of grid-controlled vacuum tubes with cross couplings between grids and plates, analogous generally to the arrangement of two-tube resistance-capacity coupled relaxation oscillator circuits, known as multivibrators. However, I interpose a cathode-follower vacuum-tube stage in one of the couplings. The cathode lead of the cathode-follower stage includes a tuned circuit to produce a negative impulse applied to the grid of one multivibrator tube for holding the tube ofi for the duration of a half-cycle of the natural frequency of the tuned circuit. In consequence, a positive pulse is produced at the plate of the latter multivibrator tube, the time duration of which pulse is precisely fixed and depends upon the natural frequency of the tuned circuit rather than on the magnitude of the supply voltage or other current constants. Two positive pulses may also be produced which are separated by a time interval equalling onehalf cycle of the tuned circuit.

A better understanding of my invention will be afforded by the following detailed description considered in connection with the accompanying drawings and those features of the invention which are believed to be novel and patentable will be pointed out in the claims appended thereto.

In the drawings,

Fig. l is a circuit diagram of one embodiment of the invention in which recurring pulses are produced.

Fig. 2 is a circuit diagram of another embodiment of the invention in which pulses are produced when triggered.

Fig. 3 is a set of graphs explanatory of the principle of operation of my invention.

As shown-by the drawings, I provide a tuned circuit with means such as an GIEQtrqn charge device for normally through the tuned circuit and cutting oil the cur rent,.when the apparatus is triggered, in order, to induce an inductive voltage, which may in turn be used to control timing of circuit operation for producing a square wave or two SUGGPPUISBSQG'.

fining a, time duration determined by the natural period of oscillation of the tuned circuit. Furthermore, I arrange the circuit for fully damping out oscillation of the tuned circuit after the first half-cycle. In the specific embodiment of my invention, represented by Fig. 1, there are two current-controllingdevices in the form of electric discharge devices II and I2, connected in a manner analogous to the stages of a multivibrator circuit. In place of the usual feedback connection or coupling from the output of the device I2 back to the input of the .devicell, I interpose another stage, which may be called a feedback stage, and inwhich I include a tuned circuit and a current-controlling devicein the form of a third electric discharge device IS. The electric discharge devices I l, ['2 and [3 may be vacuum tubes of the triode type as illustrated. But my invention'is not limited to the use of triodes. The triodes ll, l2 and I3 comprise anodesor plates I4, Hand 16, cathodes l1, l8 audit, represented as being of the indirectly heated type with filaments not shown, and control electrodes or gridsZO, 2i and 22 respectively;

A pair of supply terminals is provided comprising a positive terminal 23 and a negative or grounded terminal 24 to which a suitable source of energizing current,such as a. battery 25, may be connected. The devices ll andlZ, which may be referred to as multivibrator tubes, have their anodes l4 and I5 connected to the positive supply terminal 23 through load impedances or resistors 25 and '21, respectively, and have their,

cathodes I! and I8 grounded to the negative supply terminal 24. The control electrode 2!, ofthe second multivibrator tube 12, is resistancecapacity coupled to the output terminal or anode I of the tube II. The coupling elements, may take the form of a condenser 28 connected be tween the electrodes 14 and 2| and a grid resistor 29. If self-triggering or the recurrence of output pulses is desired, the coupling between the tubes H and I2 may be of the zero bias type or of the positive bias type, apositive .bias being accomplished in the specific circuit illustrated by connecting the grid resistor 29 between the control electrode 2| and the positive supply terminal23. I Y. The feedback coupling stage comprises the do;

passing direct current vice tube the control electrode E2 of which is coupled to the anode I of the second multivibrator tube I2. The coupling circuit comprises a condenser 3| connected between the anode I5 and the control electrode 22, and a grid resistor 32. For the purpose of rendering the tube I3 normally conducting, a connection is made from the grid resistor 32 to the cathode I3 and no source of negative biasing potential is employed. As illustrated, the resistance 32 is connected between the control electrode 22 and the ground or negative source terminal 24, the grid resistor connection being completed through the cathode lead.

The feedback stage including the tube I3, is arranged to maintain the first multivibrator tube I I, and likewise the tube I3, non-conducting for a precisely fixed time interval in order to prov duce a pulse output having a precisely fixed time duration. For this purpose, a tuned circuit is included in the connections of the tube I3. For example, as illustrated, the tube I3 may be arranged as a cathode-follower stage with a tuned circuit 33, connected between its cathode I9 and the negative or grounded terminal 24 of the supply source 25. The tuned circuit 33 includes an inductive reactance or inductance coil 24 and a capacitive reactance or condenser 35, connected in parallel between the cathode I9 and ground. For the purpose of providing adjustment of the time duration of the pulse, one of the reactances 34 or 35 may be made adjustable. For example, the condenser 35 may be of the variable type.

With the tube I3 connected as a cathode-follower stage, a load impedance in the anode connection is unnecessary to the functioning of the apparatus if no pulse is to be taken from the anode I6. However, for the purpose of limiting the current drawn andprotecting the tube, an anode resistor 36 is preferably included in the connection between the positive supply terminal 23 and the anode I6 in any event. With the specific arrangement shown, for obtaining a steep-front positive pulse, a pulse output terminal 31 is coupled to the anode I6 by a direct connection or through a coupling condenser. The anode resistor. 36' is then essential. When the feedback stage comprises a single tube, the cathode I!) of the feedback stage is directly connected by means of a. conductor 38 to the con trol electrode 2!! of the first multivibrator tube II for coupling the output of the second multivibrator tube l5 back to the input of the first multivibrator tube I I without change of phase.

The inductance coil 34 has a negligible resistance so that when direct" current only is flowing the control electrode 20 of the first multivibrator tube II is substantially at ground potential. The control electrode 22 of the tube I3 is also connected to ground through the grid resistor 32. Accordingly, the normal or quiescent condition of the circuit tends to be that in whichthe tubes II and I3 are conducting. Furthermore, since the control electrode 2| of the second multivibrator tube I2 is positively biased through the grid resistor 29, it would also tend to be conducting in a normal or quiescent condition of the circuit. However, as will be explained hereinafter, the operation of the circuit charges the condenser 28 and the charge collected thereon ne atively biases the control electrode 2| until the'condenser 28 has been discharged suflclciently by the resistor 29.

Assuming that initially the condenser 28 is charged and that the potential of the control electrode 2| is rising toward the cut-off potential of the tube, the initial condition will be that with the tubes II and I3 conducting and the tube I2 non-conducting. This is illustrated graphically in Fig. 3a by the three broken lines I, 2 and 3, lines I and 3 representing the tubes II and I3, being shown as solid initially to indicate that initially these two tubes are conducting and the tube I2 is non-conducting. At a later instant of time, represented in Fig. 3 by the time h, the potential of the control electrode 2|, represented by the curve G2, rises to the cut-off value of the tube represented by the dashed horizontal line 38 in Fig. 3d. At the instant t1, therefore, the second multivibrator tube I2 becomes conducting, as represented by the solid portion of the line 2 in Fig. 3a. This drives theanode potential P2 of the tube I2 negative at the instant n, as shown in Fig. 3d. Simultaneously, a. negative pulse is transmitted through the coupling condenser 2| from the anode I5 to the control electrode 22 which accordingly follows the curve G3 shown in Fig. 3e. The constants 0f the circuits are so chosen that the negative impulse G1 drives the tube I3 well beyond cut-off and assures that it will remain nonconducting for the pulse duration.

The abrupt termination of current through the tube I3 and the tuned circuit 33 results in a sharp negative inductive voltage across the tuned circuit. A negative impulse is transmitted through the connection 38 to the control electrode 20 of the first multivibrator tube II. This is illustrated by the portion 39 of the curve G1 in Fig. 3b. Such an abrupt termination of current in a tuned circuit would normally give rise to a train of oscillations 0r sine wave voltage represented by the dashed line 4!] in Fig. 3b.

Normally, a slight damping of the wave 40 would take place. However, in the circuit illustrated, at the time 152, which is the end of the first half cycle 39 of the oscillatory wave train, the tubes Iland I3 again become conducting. The control electrode potential of the tube II rises to zero and above cut-off as shown by the curve G1 in Fig. 3b. The resultant anode drop of the tube II is inverted by the tube I2 and drives the'control electrode 22 of the tube I3 sharply positive. As tube I3 recommences conducting current, and as the grid 29 of tube II becomes positive relative to cathode 51 substantially at the time i2, tubes II and I3 are effective to damp the oscillations in the tuned circuit 33. The resistance presented by the grid-cathode circuit of tube II when the grid is positive is a very low resistance in shunt with tuned circuit 33; and because of its low output impedance, the cathode follower stage including tube I3, when returned to a condition of current conduction, supplements the shunting and damping action of tube II. Accordingly, the wave train is very nearly extinguished after the time t2 and only the first negative half cycle 39 appears across the control electrode 20 of the tube II. In Fig, 3b, a greatly attenuated positive half cycle 4| is shown following the time 152. For practical purposes, however, the curve G1 may be considered as flat along the zero line after the time t2. Accordingly, the potential at the anode I4 of the tube II is a wave P1 shown in Fig. 3b which rises and falls abruptly at the times t1 and times t2, respectively. The rising portion of the wave may be rounded at 43 owing to the charging of the condenser 28. However, sharp voltage variations are transmitted to the successive stages.

Owing to the fact that the magnitude of the inductive voltage 39 is far beyond the cut-oil value 44 of the tube I I, and the fact that the negative voltage half cycle 39, impressed on the control electrode 20, has very steep slope at the times 151 and 122, a pulse is caused to begin and end precisely at the times 151 and 152, respectively, falling abruptly at is. The time interval t1t2 is precisely fixed because, first, an inductancecapacity circuit is used, and second, the sinusoid 39 has maximum slope at the time instants t1 and t2. Steep voltage variations, separated by a time interval or delay equalling half the natural period of oscillation of the timed circuit 33 may be taken off at suitable points in the circuit. For example, if a sharp square wave pulse is desired rising at time t1 and falling at time is, it may be taken from the anode l6 of the tube E3 or the pulse output terminal 31 at which the wave P3 (Fig. 3f) appears.

The pulse duration. or delay l-t2 is determined by the constants of the tuned circuit '33 and is independent of the magnitude of the supply voltage 25' or other circuit constants.

If positive voltage variations are desired at both times 251 and t2, the first positive pulse may be taken from the pulse output terminal 31 and the second from another point in the circuit. For example, a differentiating circuit 42, may be connected to the anode l5 of the tube l2. The circuit 42, as shown, comprises a condenser 42a and a resistor 42b in series between the anode l5 and ground with a junction terminal 420 at which the differentiated wave appears, the resistor being at the grounded end of the circuit. It will be understood that the time constant of the circuit 42 must be short in relation to the time interval t1t2. A differentiated wave dPz will then appear at the terminal 420 as illustrated in Fig. 39.

Whether an output pulse is taken from the terminals 3'! or 42 it will have a precisely fixed time characteristic, expressed mathematically by the expression (t2t1), which is the time interval between an initial voltage variation and a second voltage variation. The second voltage variation is in one case the trailing edge of the pulse P3 of Fig. 3f, and in the other case the leading edge of the pulse dPz of Fig. 3g. Each such voltage variation has a fixed delay with respect to the leading edge occui'ing at the time 7 1.

Preferably, the design is such that the negative voltage G3 applied to the control electrode 22 when the tube l2 becomes conducting, exceeds the negative peak of the conductive half cycle 39. For example, the negative peak of the curve Ge may be made about 25% greater than that of the negative half cycle 39 of the curve G1.

For best operation of the circuit it is preferable to make the time constant of the charging circuit for the condenser 28 determined by the resistor 26 and the capacity of the element 28, comparable with thetime constant of the tuned circuit 33, or substantially no greater in order that the condenser 28 will become substantially fully charged during the time interval t1-tz. As illustrated in Figs. 3b and 30, at the time h, when the tube H iscut-Ofi, the voltage of-its anode M rises to the potential of the positive terminal 23 and the condenser 23 is charged through a circuit including the resistor 25 and the grid-cathode space of the tube l2, following the curve V0, shown inFig. 3c.

-At the time t2, when the tubes II and I3 are rendered conducting, the voltage applied to the condenser 28 reverses, discharging of the condenser 28 commences and the potential of the condenser 28 falls exponentially along the curve 65 shown in Fig. 3c.

The condenser 28 loses but a fraction of its charge, however, before the potential of the control electrode 2! has risen to cut-on. The utilized portion 45 of the discharge curve is therefore substantially a straight line. The corresponding portion d6 of the curve G2, shown in Fig. Ed, is also substantially a straight line. Consequently, it has a relatively large slope where it intersects the cut-on line 38, as compared with a curve becoming asymptotic to a horizontal line, such as produced by a condenser which is allowed to discharge completely.

At the time is the action which was initiated at the time ii is repeated and a second pulse of precisely fixed duration ts-t4 is produced. The fact that the curve 46 has relatively large slope when it intersects the cut-off voltage line 38, results in the time duration tz -ts between pulses also being rather exactly fixed.

The time interval'tz-tz between pulses may also be made adjustable if desired by providing an adjustable time constant for the discharge circuit of the condenser 28, for example, by making the grid resistor 29 a variable resistance-as shown.

My invention finds its greatest usefulness in connection with the production of pulses having a short duration (or pairs of pulses close together) in relation to the interval between pulses (or pairs of pulses) and ordinarily would be used in practice with such constants as to render the time interval t1tz much smaller in proportion to the time interval t1-ta than as shown in the drawing, where the small interval has been exaggerated for the sake of clarity. My invention is not limited to production of pulses of any predetermined duration. However, I have found it useful in producing pulses of the order of 10 microseconds duration. For pulses of such length, the constants of the tuned circuit 43 may, for example, be micromicrofarads and 100 milli-henries giving a tuned wave period of 20 microseconds, the pulse duration being one-half the wave period of the tuned circuit.

In Fig. 1, I have illustrated an embodiment of my invention in which the circuit is self-triggering and recurrent pulses are produced automatically. However, my invention is not limited thereto and may also be arranged to permit the roduction of pulses only when the circuit is triggered. For example, the coupling circuit 23 and 29 between the tubes H and !2 of Fig. 1 may be modified to eliminate the positive biasing feature. A triggering pulse may then be applied at any suitable point in the circuit of one of the discharge devices. As illustrated in Fig. 2, the tube i2 is provided with a negative bias for its control electrode 21 by connecting a grid resistor 48 between the control electrode 2! and the negative terminal 49 of a source of negative bias or a C battery 50 having its positive terminal 5| grounded to the negative supply terminal 24, and having suificient voltage to bias the tube l2 beyond cut-off. A trigger pulse input terminal 52 is provided which is coupled through a condenser 53 to a control electrode of one of the tubes, in this case, the tube I3.

As previously stated, my invention is not limited to the use of triodes and multi-grid tubes may alsobe employed. For example, as illustrated in Fig. 2, one of the tubes, namely the tube (3, has been replaced by a pentode l3 having a grounded suppressor grid 54 and a second control electrode or screen grid 55, in addition to the number one control electrode or grid 22. The screen grid 55 may be connected in the conventional manner through a dropping resistor 58 to the positive source terminal 24.

In the arrangement illustrated in Fig. 2, trigger pulse input terminal 52 i coupled to the second control electrode or screen grid 55 of the tube 13 and negative pulses are employed for triggering. The quiescent condition of the circuit of Fig. 2 is that in which tubes l I and I3 are conducting and the tube i2 is non-conducting. The negative pulse applied to the terminal 52 drives the screen grid or second control electrode 55 of the tube l3, sharply negative, extinguishing current therein and producing the negative half cycle 39 (see Fig. 3b) which is applied to the control electrode 28 of the tube H to drive it sharply negative and produce a single pulse at the pulse output terminal 31. This pulse has the shape and duration represented by P3 in Fig. 3e, but is not repeated because, after the termination of pulse, the control electrode 2| of the second multivibrator tube l2 remains negative beyond cutoif by reason of the bias source 50.

I have herein shown and particularly described certain embodiments of my invention and certain methods of operation embraced therein, for the purpose of explaining its principles and showing its application but it will be obvious to those skilled in the art that many modifications and variations are possible that I aim therefore to cover all such modifications and variations as fall within the scope of my invention which is defined in the appended claims.

What is claimed is:

1. Apparatus for producing a pulse of precisely fixed time characteristic comprising first, second and third. electric discharge devices, each of said devices comprising an anode or plate, a cathode, and a control electrode or grid, positive and negative supply terminals adapted to be connected to a source of direct current, a pair of anode resistors each connected between said positive terminal and the anode of one of said first and second dis charge devices, the anode of the first of said discharge devices serving as a pulse output terminal, a coupling condenser connected between said last-mentioned anode and the control electrode of the second discharge device, said condenser having a leakage circuit, a second coupling condenser connected between the anode of the second discharge device and the control electrode of the third, a grid resistor completing a circuit between said last control electrode and the oathode of the last discharge device, a connection between said positive supply terminal and the anode of said third discharge device, a tuned circuit comprising an inductance and a condenser in parallel, connected as a unit in series between said negative supply terminal and the cathode of the third discharge device, tuned for a wave period equalling twice the desired pulse characteristic, and a connection between said cathode and the control electrode of the first discharge device, whereby the second and third discharge devices are normally conducting, the second tends to approach a quiescent condition in which it becomes conducting and drives the third non-conducting, thus inducing a large negative voltage in said cathode inductance continuing for a half cycle of the said tuned circuit and driving the the anode of the first discharge device positive to produce a positive pulse for the duration of said halfcycle, subsequent half cycles of such nduced voltage being damped out by the conducting condition of the third discharge device which obtains at the termination of said first half-cycle.

2, Apparatus for producing a pulse of precisely fixed time characteristic comprising first and second vacuum tubes referred to as multivibrator tubes and resistance capacity coupling between the multivibrator tubes, the coupling being of the positive biasing type, and an additional stage of feedback coupling between the second multivibrator tube and the first, said stage including a tuned circuit for rendering said feedback coupling effective during a time duration fixed by a halfcycle of the natural period of oscillation of the tuned circuit.

3. Apparatus for producing a pulse of precisely fixed time characteristic comprising first and second vacuum tubes referred to as multivibrator tubes, resistance capacity coupling between the first and second multivibrator tubes, a third vacuum tube referred to as a cathode-follower with coupling between the second multivibrator tube and the cathode-follower, a tuned circuit connected in series with the cathode of the cathodefollower, a, coupling between the cathode of the cathode-follower and the first multivibrator tube, the cathode-follower being zero biased, whereby it is normally conductive, and the production of a condition of conductivity in the second multivibrator tube drives the cathode-follower to the non-conducting condition, extinguishing the current in the tuned circuit and producing a halfcycle of negative induced voltage transmitted through the coupling from said cathode-follower to the first multivibrator, thereby producing a pulse with a characteristic fixed by the natural period of the tuned circuit.

4. Apparatus for producing a pulse of recisely fixed time characteristic comprising a pair of electric discharge devices referred to as multivibrator discharge devices, a third electric discharge device referred to as a cathode-follower,

coupling between the first and second multivibrator devices and between the second multivibrator device and the cathode-follower with feedback coupling from the cathode-follower to the first multivibrator device, the cathode-follower including a tuned circuit connected in series with its cathode, and the cathode-follower being zero biased whereby it is normally conducting, and'a negative voltage of half cycle duration is fed back to the first multivibrator whenever the cathode-follower is rendered non-conducting, thus producing a pulse in the multivibrator having a time characteristic fixed by the natural period of oscillation of said tuned circuit.

5. Apparatus for producing a pulse of precisely fixed time characteristic comprising a multivibrator circuit with two stages having resistance capacity coupling between the first two stages, and a cathode-follower stage with a cathode connection for feedback coupling between the second and the first stages, said cathode-follower stage including a tuned circuit in series with its cathode connection.

6. Apparatus such as set forth in claim 5, in which the coupling between the first and second multivibrator stages is of the positive bias type for intermittently restoring the second stage to a conducting condition for producing recurrent pulses.

'7. Apparatus for producing a pulse of precisely 9 fixed time characteristic comprising a multivibrator with a first stage zero biased for rendering it normally conducting, a second stage negatively biased for normally rendering it non-conducting,

coupling between the first and second stages, and an additional stage interposed between the second and first multivibrator stages for feedback coupling, said additional stage including a tuned circuit and being normally zero biased, whereby it is normally in conducting condition, and a triggering impulse terminal coupled to one of said stages for changing the conducting or nonconducting stage to the opposite condition in re sponse to a triggered impulse whereby such triggerin renders the feedback stage non-conducting, interrupts a current in the tuned circuit and produces a sharp inductive voltage applied to the first stage having a time duration fixed by the natural frequency of the tuned circuit to produce an output pulse having a time characteristic also fixed by the natural period of oscillation of said tuned circuit.

8. Apparatus for producing steep voltage variations separated by a precisely fixed time interval comprising a pair of supply terminals for connection to a current source, a tuned circuit having a natural period of oscillation twice the desired time interval, a current-controlling device connecting said tuned circuit to said terminals, said current-controlling device including a voltage responsive controlling element for rendering it non-conducting and cutting ofi current through said tuned circuit at the beginning of a desired interval, thus inducing voltage oscillation in said tuned circuit, and a coupling between said tuned circuit and said control element for application of control voltage to said control element and restoration of current in said currentcontrolling device upon voltage reversal in said tuned circuit at the end of a half-cycle of voltage oscillation, such restoration of said current-controlling element to conducting condition serving to damp out any further oscillation in said tuned circuit.

9. Apparatus for producing two successive pulses with precisely fixed time delay therebetween comprising a plurality of stages of electric discharge devices, each discharge device comprising an anode, a cathode and a control electrode, one of said discharge devices being impedance coupled to an adjacent stage, a pulse output terminal coupled to the anode of one of the latter two discharge devices, a differentiating circuit connected to the anode of the other of the latter two discharge devices and having an output terminal serving as a second pulse output terminal, one of said stages including a tuned circuit and means for cutting off current therein for producing a pulse at one of said terminals and inducing an oscillatory voltage in the tuned circuit, and a coupling from the stage containing the tuned circuit to another stage for reversing the condition of conductivity or non-conductivity of such last-mentioned stage at the termination of a half-cycle of such oscillatory voltage for producing a second pulse with a time delay fixed by the tuned circuit constants.

10. Electrical timing apparatus comprising a tuned circuit having a predetermined period of oscillation, and electron discharge device having a cathode connected to said tuned circuit, means connected to said tuned circuit and said electron discharge device for maintaining a unidirectional electric current flowing throughsaid device and part of said tuned circuit, means operatively coupled to said device for periodically varying the flow of said current at a frequency lower than the resonance frequency of said tuned circuit to initiate oscillations in said tuned circuit, and amplifier means coupled to said tuned circuit and responsive to said oscillations for suppressing said oscillations after a predetermined interval following each successive initiation of oscillations.

11. A multivibrator comprising a first electron discharge device and a second electron discharge device each having a cathode, a control electrode and an anode, resistance-capacitance means connected between the anode of said first device and the control electrode of said second device, a tuned circuit coupled to the anode of said second device to be excited thereby, said tuned circuit being connected directly between the cathode and the control electrode of said first electron discharge device.

12. A multivibrator comprising a first electron discharge device and a second electron discharge device each having a control electrode and an anode, resistance-capacitance means for coupling the anode of said first device to the control electrode of said second device, an amplifier including a tuned. circuit coupled to the anode of said second device whereby oscillations are produced in said tuned circuit in accordance with voltage variations in said second device, and means for applying a version of said oscillations to the control electrode of said first device, whereby voltage variations in said multivibrator are timed in accordance with the oscillatory period of said tuned circuit.

13. Electrical timing apparatus comprising a tuned circuit having a predetermined period of oscillation, a first electron discharge device having a current-carrying electrode connected to said tuned circuit for providing current therethrough, and a second electron discharge device coupled to said tuned circuit and to said first electron discharge device, said second electron discharge device being so coupled to said first discharge device as to provide a feedback path thereto for periodic current changes in said first discharge device at a frequency appreciably lower than the frequency to which said circuit is tuned, thereby initiating oscillations in said tuned circuit, and said second electron discharge device being so coupled to said resonant circuit as to absorb the energy therefrom immediately following the first reversal of oscillation polarity therein.

DAVID E. KENYON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,063,025 Blumlein Dec. 8, 1936 2,153,202 Nichols -1 Apr. 4, 1939 2,155,210 Young Apr. 18, 1939 2,181,309 Andrieu Nov. 28, 1939 2,182,555 Geiger Dec. 5, 1939 2,220,712 Geiger Nov. 5, 1940 2,259,891 Hunt Oct. 21, 1941

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