US2485608A - Pulse modulator - Google Patents

Description

I Oct. 25, D Y. KEIM PULSE MODULATOR Filed May 24, 1943 FIG. 4.

FIG. 3.

INVENTOR DAVID Y. KEIM ATTORNEY Patented Oct. 25, 1949 PULSE MODULATOR David Y. Kcim, Garden City, N. Y., assignor to The Sperry Corporation, a. corporation of Delaware Application May 24, 1943, Serial No. 488,128

'9 Claims. 1

My invention relates broadly to modulators and, more specifically, to pulse modulators for ultra high frequency oscillators.

Modern ultra high frequency technique requires in some applications the operation of certain types of ultra high frequency osciliators such as the magnetron at comparatively high power levels. Since the maximum power levclat which continuous operation of these oscillatorsmay be safely maintained is limited by design, it is common practice to operate the apparatus with high power inputs for shcrt periods only, thereby keeping the average operating conditions within the limits of safety.

When thus operated, the oscillator is "pulsed by a high power level surge, the period of which determines the on time of the oscillator in which signals are transmitted. The interval between pulses, which is long compared with their period, then fixes the time the apparatus is turned off before signals are again transmitted. Modulators for supplying this high power pulse have taken two forms, namely, the vacuum tube type and the spark gap generator type.

The spark gap generator is not adaptable to systems in which the oscillator is triggered from a synchronizing source, but is used where the transmitter itself delivers the trigger pulse because of one of its inherent characteristics, termed in the art as jitter," which does not permit accurate timing of the spark. Because of high power requirements, the vacuum tube modulator has heretofore been rather bulky, which is equally objectionable.

It is therefore an object of my invention to provide a vacuum tube pulse modulator whichis simple in design, small and compact in structure, and adaptable to portable applications where minimum weight is essential.

Another object of my invention is to provide a method and apparatus for coupling a high power source and an ultra high frequency oscillator which does not require a direct current charging path for the filtering condenser in its power supply nor the damping devices made necessary by such a charging path.

A further object of my invention is the arrangement of a vacuum tube pulse modulator circuit in which a plurality of power pulsing tubes may be reliably triggered without distortion of the triggering pulse.

A still further object of my invention is to provide a pulse modulator in which a power pulsing network is inductively connected to an ultra high frequency oscillator.

2 Another purpose of my invention is to provide a high power vacuum tube pulse modulator in which medium power tubes may be used.

It is also an object of my invention to provide apparatus for accurately timing a high power pulse.

Yet another purpose of my invention is to provide a pulse modulator in which the potential of interruptively discharged cherry is increased and simultaneously transferred by induction.

These and other objects of my invention will become manifest as the description proceeds.

In carrying out my invention in a preferred embodiment thereof. I employ a line controlled blocking oscillator to drive a plurality of power pulsing tubes for controlling the time duration of pulse modulation. When the pulsing tubes receive a high amplitude pulse from the blocking oscillator, the period of which is determined by the length of a simulated transmission line connected inthe grid circuit of the blocking oscillator, a charging condenser in the power supply network of the circuit is caused to discharge through the primary Winding of an auto transformer used for coupling the power supply to the oscillator which is to be pulsed. The secondary winding of this transformer then delivers the modulating pulse.

In addition to acting as a coupling means for the power tubes and their load, the auto transformer also performs the function of an impedance-matching device which is necessitated by the number of power tubes employed in the switching network of the circuit. Here the number of tubes used is dependent upon the currentconducting requirements of this branch.

The fact that a number of power pulsing tubes are necessarily used also makes it impossible to drive these tubes directly and reliably by the output of the blocking oscillator. By connecting a cathode follower, which acts as a buffer stage, between these elements of the circuit, it is possible, however, to drive the tubes without distortion of the triggering pulse.

Also by using my improved coupling method, it is possible for me to use medium voltage tubes in the switching network having oxide-coated cathodes and to use these tubes in conjunction with a. high-powered system. This makes unnecessary the use of the gas-filled 'Ihyratrons andthe bulky, less efficienthigh voltage tubes used in previous systems.

This method of coupling further simplifies the circuit, since it makes it unnecessary to provide a direct current shunt path across the ultra high frequency oscillator. Consequently, I am able to omit the inductance coils and dampin devices used in prior systems.

By reducing the size and number of parts, I have produced a vacuum tube modulator which is adaptable to portable needs where the weight of the apparatus must be kept at a minimum.

A more comprehensive understanding of my invention may be obtained from the following detailed description, when taken together with the accompanying drawing, in which like reference numerals have been used throughout to designate like parts and in which,

Fig. 1 is a circuit diagram of an embodiment of my invention.

Fig. 2 is a representative showing of prior art methods of coupling high power sources with ultra high frequency oscillators.

Fig. 3 is a simplified showing of my improved coupling means for coupling power tubes with their load, and

Fig. 4 is a diagram showing the relationship between the power line frequency and pulse repetition rate.

In the embodiment illustrated in Fig. 1, a square trigger pulse is applied to the input terminals Ill of the circuit, from which point it is fed to a connection II on the simulated transmission line |2. This line is made up of sections comprising inductance coils l3 and capacitors M and is used to control the period of a pulse generated by a pulse amplifier network IS in whose control grid circuit it is connected.

The network |5 serves as a blocking oscillator transformer and comprises an electronic discharge device IB, which may take the form of a double tetrode, a multiple winding transformer l1, and the delay line l2. A grid winding |8 of the transformer l'l has a resistor I9 connected in parallel therewith and serves as a conductor for transmitting the trigger pulse from the delay line l2 to the grids 2| of the double tetrode it through grid resistors 22. A common winding 23 of the transformer I1 is inductively connected with the grid winding l8 and output winding 24, the latter being provided to obtain a positive voltage pulse as the output of the blocking oscillator transformer. Resistors l9 and 25, in parallel with windings l8 and 24, provide fixed loading for these windings and in performing this function improve the wave form of the pulse output of the network.

Positive voltage is supplied to the anodes 26 of tube l6, whose electrodes are parallelly connected, from positive energy source 21 through a common bus 28, transformer winding 23, and plate resistors 29. The screen grids 3| are tied together and positive voltage applied to them from source 32 through line 33. Cathodes 34 are connected directly to ground, and the screen grids by-passed to ground through condenser 8. Bias from negative source 2|! is applied to grids 2| through the resistor 9. l

The output of the pulse amplifier I5 is used to drive a switching device 36 comprising a plurality of parallelly connected power pulsing tubes All.

Interposed between the pulse amplifier or line controlled blocking oscillator l5 and the power pulsing tubes 41 is an electronic discharge device 35 which acts as a buffer stage. This device, which may be of the double tetrode type, is parallell connected and acts as a cathode fol-- lower. It receives the output of the blocking oscillator transformer H from the output winding 24 thereof upon its grids 3] through grid resistors 38. Anodes 39 are connected to positive energy source 21 through line 28 and plate resistors 40, and the screen grids 4| to positive energy source 32 by line 33. The cathodes 42 are grounded through the cathode resistor 43, and the screen grids coupled thereto through the bypass condenser 44. Negative grid bias for this stage is derived from source 20 through the output winding 24 of transformer H. The input impedance of this stage is relatively fixed and is matched with that of the blocking oscillator H: to provide maximum power transfer.

The pulse developed across the resistor 43 of stage 35 is applied through the grid resistors 46 to the grids 45 of the power pulsing tubes 41 which are connected in parallel and which are biased negatively from source an through the resistor 50. ground through the capacitor 60 and are supplied with positive energy from source 21 through conductor 28 and resistors 49. The plates 5| of these tubes are coupled in parallel to the high potential source 52 through the double throw switch 53 and the unilateral current-conductin element 54.

One terminal of the secondary winding 55 of transformer 52 is connected to ground and the other terminal to plate 56 of element 54. The cathode 51 of this device, illustrated as a vacuum tube of the diode type, is selectively connected to the plate circuits of tubes 41 through the twoway switch 53. The blade of this switch is normally closed to contact 58, but under certain conditions of circuit operation is closed to contact 59 to couple the inductance coil 6| into the power pulsing network.

An inductive device 62, which may take the form of an auto transformer, serves as a coupling element and impedance-matching device for the power pulsing tubes 41 and their load, the ultra high frequency oscillator 53 illustrated in the drawing as a split anode magnetron.

Coupled in the primary winding circuit of transformer 62 is a condenser 64 which serves the dual purpose of filter and coupling condenser. Common terminal 65 of transformer 52 is connected to ground and its secondary terminal 65 to the cathode and filaments B1 of oscillator 63. Condenser 64 discharges through the power pulsin tubes 41 into the primary winding of transformer 52 through ground and terminal 65. A modulating pulse is therefore induced in the secondary winding of transformer 62 which is impressed upon the ultra high frequency oscillator 63, the circuit being completed from terminal 65 through ground to anode 69, thence to cathode El and terminal 66.

In operation, the blocking oscillator network |5 performs the function of timing the plus modulation of oscillator 63. A trigger pulse of suitable characteristics is impressed upon the circuit through input terminals III for delivery to device it which is normally cut of: by the fixed negative grid potential supplied from source 28. When the trigger pulse is received by grids 2|, they are driven less negative than the cut-oil value, and plate 26 start to draw current. The flow of plate current through plate winding 23 of transformer ll induces a voltage in grid winding IB which in turn drives grids 2| more positive with the result that the plates draw more current. When this regeneration starts, a steep negative wave front, that is, negative from the grid side of line to ground, is sent down the blocking lin l2. This wave front hits the open Screen grids 48 are by-passed to of Wievmm and =18! b80152: Withhthifli SWMHBSHMVB: phase: to lbw-811G191 ofi time I8; Wi'lem this negative woltage appears ow the .grids, the griii voltage becomes less positive with: a; rosuitant decrease iniplatewcucrent. plate currentdecreases,- regeneration isrestabllshed'iin thevoppositerdirectiom and the grids: are: driven negatiuoly-beynndfiplate current cutwflt: During: tha tinwthat the grids arm-drawing current, anegative -voliiage ia bufliflupwcross the ccndensers: rod lim 121' whloln actsas-adiisd= negative grid: biasio keepl-th e tubesh'uii oitfduving the: re-- mainder of the: triggerpulsei timaconstant oi th e grid circuit: together-withing add adltime diieto -trigger input impedance is mads--- short compared itathe time between trigger signals soz that the grid voltage will have attained its origsimi vzalucbetore the next trigger signaliappears.

wmdihg fliis emplbyed in: the transformer-cla cuiir-to -dch'lvem a positive pulse-output: to butter stagesa which OPGIEtiEW-fifi =a =cathbde follower. Since the L cumeniirequirementsl the switching branch-38 necessitate the paralrellm oi a pin-- fixedand lsmatthed to-that ofblooltlng oscilla town" Its grids are-biased to cut-om'from negative source-2 and} when the'pulsefrom the blocking oscillator is rooeived on'them they aredriven posiiirve anda-high peale current is drawn through the rmo -inductive resistor-43$ is=-therof0re developed at the cathodes 4! of" elemerittimed delivercdasatrigger for the tubes 41 of switchlhg deviceifitthe function of' which is bestdescribed in conneetion with the power supply-branch.

The circuit is so'synchronized' that the trigger puma Marcitubes fl is reeetvecl aftei the line voltage-Es reaches a positive-peak vaiue; as shownin .Fig. 4. It follows therefore that the tubes 4!: :are. non -cond'llclii've durihg' the positive rise in' linevoltagoin: the-secondary winding 55 of transformer" 52; since: thesetubes are normallyneg atively. biased: to cutsofi by source as, and accordingl y they' act'at this moment asan open swituhn.

When the line voltage Er. goes positive; plate 56 of tube 54 goes-positive; and the tube becomesconductlng and current is stored in the filtering condenser ttgithwfiow: being: from ground through secondarmwinding '5, the tube, switch'53,'con= denser Bill and the primary winding of transformerl 62', determined-by the tap -68 and-"tom minainfiibaclczto ground;

Thercaditer, pulse 1 from linezconttolledblockingzoscillator lmis reoelved on the gridsof tubes 41'; driving-them positivebeyond cut-off. As these'tubesbecome conducting, condenser Mdis charges-. through themarrd: the primary winding otitransfonmer 62 This develops a-pulse, whose period is determined by and-equai to that of the trigg-ering pulse received-by tubes 41," whichv is induced in'the-secondarywindlng of transformer 62 'andtransmittedtherefrom to the ultra. high frequency oscillator as a modulating voltage.

Asstat'ed inthe description of the circuit,

A pulse smitchzitlis normally closed to contact 58, andthe operation. of the circuit under these conditions is predicated on the fact that the power supply frequency is substantially equal to the pulse repetition. rate.

the power circuit, since element 54- is;conducting. only during the positive half cycle of the voltage EL. Since tubes." act as a switch, current flows from power source 52 through them, thence over ground to the secondary winding 5570f transformer 52. This is objectionable, otcourse, since charging current is diverted from the condenser 64. and since it also results in an unnecessary dissipation of power. The currentconducting capacities of the elements 41 are limited by their inherent characteristics, and the additional current drawn from the power circuit by them: is therefore necessarily dissipated in the'form of heat energy.

With: inductance coillfizi in the power circuit, noappreciablepcurrent is drawn by the tubes 41 from the power source, since itofiers a high impedance to pulses which are in the order of the triggering pulse delivered to the power pulsing tubes. Thus, current flow from plate 55 to cathode Slrthrough inductance coil 6! to tubes 41 over ground and back through winding 55 is impeded until tubes 41 are non-conducting, and no appreciable amount of current is drawn. by

them; This allows the output of the power circuit to; be stored in the condenser 64.

Where pulserepetition rates exceed approximately; two and one-half times the power supply frequency, it is advisable to resort to an alternate power-supply method such as a voltage doubling circuit. This arrangement lends itself more readily to proper synchronization for these conditinns.

Thusfar in: describing the operation of the circult, reference has not been made to the fundtinns of;transformer 62. other than that of stepping; up: the modulating pulse delivered to the ultra high frequency'oscillator 63. In addition tosthis function, it .also serves as an impedance- 'matching device and; coupling means for the tubes -41. andzrtheir: load 63.

Rreviously;. the: power tubes of vacuum tube modulators: fornultra high frequency oscillators were connected: directly to'their load in. a. manner similamtolthatssnown in Fig. 2, which fact necessitates theiuse oii large tubes. In actual practice, a-nnmbenzofizthese tubes are necessary, although only'one; I1, is shown .in the figure. This tube is greatly enlarged for the purpose of comparing it with tube 41 in Fig. 3 to symbolize, though not to: scale, the reduction in size of these elements made possible by my improved coupling, diagrammecl in Fig. 3.

In the old coupling method illustrated in Fig. 2, attention is direct'ed'to two elements, coil 13 and unilateral current-conducting device 14, whose functions may be described by first considering the operation of the circuit without them.

As the. line voltage EL starts positive, plate 56 of tube 54 goes positive and so does plate 15 of condenser 64. Since plate It is therefore necessarily negative, anode 69 of element 63 is also negative, and the polarity of this device is reversed. Accordingly, no current flows and condenser v54 is not charged but is left floating in the line. In order that charging may be effected, it is therefore necessary to provide a shunt path for the direct current and accordingly an element such as the coil 13 is placed in the circuit. When this is done, the resistance of the shunt path together with the capacitance of element 64 form a tuned circuit, and as soon as condenser 64 discharges, oscillations are set up in the circuit which cause an over-shooting in the pulse. To damp out these oscillations and to prevent over-shooting, the unilateral current-conducting element 14 is placed across the coil 13.

In applying my improved coupling means, which is shown in Fig. 3, I am able to simplify the circuit by omitting both of the elements 13 and 74. Here the primary winding of the inductive coupling means 62, defined by the tap E8 and terminal 65, in addition to other functions provides a' direct current path and no coil, such as 13 (Fig. 2), is necessary. Likewise, element M (Fig. 2) is unnecessary, since the direct current resistance and the inductance of this primary winding is low, and the objectionable oscillations found in the prior art circuit are not present. Of itself, this is a marked improvement, since element 14 must be specially constructed to withstand the high inverse voltage impressed on it and also to meet the high current-conducting requirements of the circuit.

Further, since the power pulsing elements H are inductively connected to their load, it is possible to use medium voltages tubes and thereby further reduce the size of the apparatus.

To summarize the salient features of the circuit's operation, it may be pointed out that the coupling device employed embodies in one single element, illustrated as an auto transformer 62, a, means for directly performing the multiple functions of coupling tubes M with their load 63, of providing a direct current shunt path for the filtering condenser (i l, of providing an impedance-matching element for tubes 41 and oscillator 63, and that of stepping up the modulating pulse supplied to this latter element. Collaterally, it permits the use of medium voltage tubes in the place of high voltage tubes heretofore necessary under direct connected operation.

Likewise, the interpositioning of the buffer stage 35 between the line controlled blocking oscillator l and the switching network 35 serves several purposes by providing for the accurate pulsing of the oscillator, by acting as an impedance-matching means for the networks I5 and 36 and by providing for the delivery of an undistorted trigger pulse to the tubes 41, irrespective of the fact that the high current-conducting requirements of this branch of the circuit necessitate the paralleling of a plurality of these devices.

Modifications of my invention are, of course, possible and may suggest themselves from the foregoing disclosure. Accordingly, the embodiment herein described and represented in the accompanying drawing is to be regarded as illustrative. and the spirit and scope of my invention to be limited only by the appendant claims.

What is claimed is:

1. In combination with an ultra high frequency oscillator, a blocking oscillator having connected in the grid circuit thereof a simulated transmission line, a butler stage comprising an electronic discharge device connected as a cathode follower for receiving the output of said blocking oscillator, a power pulse switching device comprising a plurality of electronic discharge elements, each with a separate control grid driven by the output of said blocking oscillator, an energy storage condenser, a slnusoidally varying power-supply source, and interconnections between said switching device, said condenser and said source whereby said condenser is alternately charged from said source and discharged through said switching device to release energy to said oscillator. said discharge being synchronized to occur after the peak value of said source has been attained.

2. In combination, with an ultra-high-frequency oscillator, a blocking oscillator, a bufler stage having an electronic discharge device connected to receive the output of said blocking oscillator, a power pulse switching stage having at least one electron discharge device driven by the output of said buffer stage, a sinusoidal power-supply source, a storage condenser connected to receive energy from said source, said storage condenser being further connected to release energy thereof through said switching device to said oscillator after the attainment of the peak value of said source.

3. In combination, an oscillator to be energized at regular intervals, 2. pulse generator, a power pulse switching stage having at least one electron discharge device driven by the output of said generator, a sinusoidal power-supply source, a storage condenser connected to receive energy from said source, an inductive device having primary and secondary windings, said secondary winding being connected across said oscillator said storage condenser being further connected to release energy thereof through said switching stage to said primary winding after the peak value of said source has been attained.

4. In combination, an oscillator to be pulsed, a sinusoidal power-supply source, a storage condenser connected to receive energy from said source, an electron tube switching device having grid and plate circuits, a pulse generator connected to control the grid of said switching device and an inductive device having primary and secondary windings, said oscillator being connected to said secondary winding, the plate circuit of said tube switching device being connected between said condenser and said primary winding for periodically releasing the energy of said storage condenser to said oscillator during each positive second quarter cycle of said source.

5. Apparatus for supplying unidirectional electric pulses to a unidirectional voltage responsive radio-frequency energy generator, comprising first transformer means for receiving alternating current primary power and delivering high tension alternating voltage at the terminals of its secondary winding, a rectifier tube and a grid controlled tube connected in series between the secondary terminals of said first transformer means, the cathode of one of said tubes being connected to the anode of the other and the remaining anode and cathode being connected to the secondary terminals, a second transformer having two winding terminals for connection to said radio-frequency energy generator, a capacitor, said second transformer further having a winding portion connected in series with said capacitor, and means connecting said series-connected capacitor and winding portion between 2,4s'aaoa the cathode and the anode of said grid controlled tube.

6. Apparatus as defined in claim 5, further including means for supplying to the control grid of said tube pulses recurring at the frequency of the alternating current in said first transformer means, said pulses being timed with the quarter of each cycle during which the voltage applied to the anode of said rectifier tube is positive but decreasing in magnitude.

7. Apparatus as defined in claim 5, wherein said second transformer is an auto transformer having a terminal at each extremity of said winding and a connection to an intermediate position of said winding, both end terminals being connected to said radio frequency energy generator, a capacitor, and means connecting said capacitor in series with one of said terminals and said connection, thereby series connecting said condenser and a portion of said winding, said means being further connected between the cathode and anode of said grid controlled tube.

8. In combination with a load to be impulsively energized at regular intervals, an inductive device having primary and secondary windings, a sinusoidal power supply source, energy storing means connected to receive energy from said source, and a unidirectional electronic discharge device having a control grid, said device being connected in series with said energy storing means and primary winding, said load being connected to said secondary winding, and said control grid of said electronic device being synchronized with said sinusoidal source for unidirectionally releasing the energy from said storing means to said load during each cycle of the sinusoidal variations of said source after the peak value thereof has been attained.

pulse generator for periodically releasing energy to said oscillator after the attainment of the peak voltage of said source.

DAVID Y. HIM.

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

UNITED STATES PATENTS Number Name Date 1,978,461 Hoover Oct. 30, 1934 2,153,756 Hunt Apr. 11, 1939 2,212,420 Harnett Aug. 20, 1940 2,276,994 Milinowski Mar. 17, 1942 2,295,585 Lindquist Sept. 15, 1942 2,351,439 Livingston June 13, 1944 2,405,069 Tonks et al July 30, 1946 2,405,070 Tonks etal July 30, 1946 FOREIGN PATENTS Number Country Date 460,562 Great Britain Jan. 25, 1937

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