US2914734A - Pedestal modulator - Google Patents

Description

Nov. 24, 1959 P T. J. PARKER 2,914,734

PEDESTAL MODULATOR Filed Jan. 19, 1956 2 Sheets-Sheet 1 TRIGGER 7 DELAY UN |T.

' INVENTOR AMPLITUDE I THOMAS J PARKER, DECEASED I B) SUE E'. PARKEEEXECUTR/X A T TORNEYS 1959 T. J. PARKER 2,914,734

PEDESTAL MODULATOR Filed Jan. 19. 1956 2 Sheets-Sheet 2 TRIGGER Fig. 3

IN VE N TOR moms J.P4RKR 05054550 BY suz. E. PAR/(ER,

E XE CU TRIX n zm'm ATTORNEYS 1 PEDES A M D T fT eeE-T a h e ute San Diego, Ca Application January 19, '1 95 6,-"Serial No. 560,267;

f fic im'- (dia -$1)? s (Granted-=uniler Title 35, 'U.S.'- C oiie (1952), see. 266) j nventiorijmay be manufactured and" used by or for the Government of the United ,States any royalties thereon or therefor.

pa arlyftojalpedestal rno'dulator' capable of driving a n iagii'etronto'producevery short pulses of RF energy..

The problem of producing in high powered magnetron applications short pulses of duration of less than .1 microsecond has in the past generally been resolved by theuse of hard tube modulator techniquesMThese hard tube.

modulators liay manylf'advanta'ges such as being capable of operating into magnetronsj with a wide range of v impedance mismatch yet still forming good rectangularpulses. The pulse ,durationcan be, changed readily by changingtlow voltagecomponents. The hard tube modulatoralso .can produceseries of short pulses; with very cinge The hardftube modulation, while in many agetobeapplied ,to the magnetronoscillatorh Also, large transmitting type vacuumtubesmust be used to' pass the laie of San Diego, Calif., by

, rica forgovernmentalfpurposes without the paynvention'relate s fto pedestal modulators and more i H f quiteifle xible requires a high foverhead; i.e.,; tlieplate powersupply voltage must exceed the peak volt- 2,914,734 Patented Nov. 24,1959

former. 'Also the problem of saturation in the outer laim' nations with magnetic flux barely penetrating to the'inner-j most laminations restrict the upper power level as well as the upper pass band frequency. For these reason s the upper practical limit for pulse transformer operation at medium and high power has been for pulse durations greater than approximately .1 microsecond.

Consider next the case where the pulse forming network is allowed to discharge directly into the magnetron, this procedure eliminating the lossy. pulse trans forme r. For I conditions of impedance match, which case will cause the transfer of energy stored in the pulse forming network to the magnetron load in asingle rectangular pulse; the voltage developed across the load will be This reouires that the network be charged to a voltage twice as high asrequired for full power level in the magnetron. Thus, a magnetron requiring 22 kv pulse the pulse forming network must be charged to 44 kv.:-

pea yaluerof current drawn by the magnetron during'its pul du e 'to plate powerfdis'sipated in the keyer tubes in passingflarge pulses ofcurrent; r l v "'Witlithe conventional line type pulse'r the pulseflduran being the-round trip time ofrt ravel of a voltage wavel rnpressed upon thenetwork; In this function'the pulseformingnetwork can b'eeither a real or an artificial tfa'nsmissiomlineQ The pulse developed by the two way tim of travel in 'the'transmission line serving as pulse H rn'gnejtwo'rkis coupledby'means o'f avolta'ge step, up transformer to the magnetro'nfload. 'For the very short pulse case, however, thepulse applied to thejmagfor ga agains wide bandwidthj. The larger cor Sizes aii wire s'iz es an'd the heavier interlayer insu latio'n cause an increased leakage reactance as well as increased dielectric losses both ofwhich factors reduce the elfe'cti've frequency pass band-charactristics of the "trans-- These two factors alone require, that the hard tube'fmodulator be rathertbulky in spaceand ineificient is established in thegpulse forming network, this pulse";

High powered magnetrons present a special problem in the formation of-short pulses. Consider, for example, the 4150 magnetron which requires" that therrate of rise of voltage at the 80% point'offthe voltage pulse not exceed llO kv. per' rnicro'second. Should the voltage pulse to this magnetron exceed this rate of rise, there is a marked tendency for the electron cloud not toforrn in the 1r-rnode, the preferred mode of operation, this causingthe dynamic impedance of the magnetron, tobe improperly high. The end res ult occasioned by a too rapidly applied voltage pulse to ahigh powered magnetron Will be that of themagne-j tron failing to establish bscill'ationin its r-mode. Witlif the line' typ'e pulser this causes the voltage to ris'tofa. Value considerably greater "than normal, since thevmagnetro n' willeither be operating "as a' diodeforwill be car-f: riedto a cutoff condition. If the magnetron fails topper-f ate in its proper 'modqand'thus take powe'r from ,the' pulser, the pulse voltage applied tothe magnetron will? approachtwice the normal operating value. 1 For the; 4150 V magnetron application using aline type pulser the slope of the pulser voltage curve atabout 80% of full fvoltagej should not exceed the rateof rise of 110 kv. per micro second. This value of voltage rate of rise here limitsjthe shortness of the pulse to be produced,since a pulserwithf a single section network will produce the minimum' dura-f ""tion pulseof about .3 microseconds, This pulse width" tion requires that a low powered pulse, well rounded, applied to the magnetron as a pre-operating'pulse; It the' pu'r'pose of this'pulse to bring the magnetron to thej can be reduced slightly givinga pulse of .25 microseconds using a'mb'dified line type pulser in which an inductor. connected across the primary of the transformer serv 'e s r, as a tail, biteri, to cause the magnetron pulse to be ter minated somewhat more sharply than otherwise would be) the case. However, this'modification indicates. that;

appro irirriate'limit has been reached for shortpulse power modulators of the line type. I p I The pedestal voltage technique employed in this inve threshold of operation with a wave'shapewhich aucws the electron cloud to become sorted into the 1r-rnode and to be producing RF p'oweiflatfabout 5% to 10% of the full power level. After the magnetron is passing current due to the pre-op erating pulse, a short high powered pulse is applied to carry the magnetron voltage to the full power v leyelQflSince the magnetron is already in the oscillating, modeat the time of application of the high powered pulset thefrate of rise of magnetron current and also v ofRF curl-j;

7 rent will ,be'limited almost entirelyby/therate of build T resonant c'a upf'of the rotating electron cloud and by'the. Q of the vities and of the output circuit of the magn g a 'periodof .15 microseconds.

pedance, Z

An object of this invention is the provision of an improved pedestal modulator capable of producing an oscillator to produce very short pulses of RF energy.

A further object is the provision of a voltage pulse for application to a magnetron within its prescribed rate of ri'se'yet having a durationof .l microsecond or less.

A further object is the provision of a pedestal voltage technique whereby a well rounded low powered preoperating pulse is applied to the magnetron, upon which a short high powered pulse is applied to carry the magnetron voltage to full power level. 1

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

Fig. 1 is a simplified schematic of the pedestal modulator;

Fig. 2 is a schematic of the pedestal plus Darlington line pulser;

Fig. 3 shows the pedestal modulator with exponential pulse transformer in a passive delay circuit;

Fig. 4 is a schematic of the series-generator pedestal modulator; and

Fig. 5 shows the RF pulse envelope produced by the magnetron.

Referring now to the drawings wherein like numbers designate like parts there is shown in Fig. 1 a simple circuit for the production of the short pulse on a pedestal. The pedestal is formed by discharging the single section network upon firing hydrogen thyratron 11. This produces a magnetron current pulse approximately 1 microsecond in duration and approximately a half sinusoid in shape. The magnetron current is carried to approximately the 1.6 ampere level at which valve there is 'a jump in magnetron current, indicating a buildup of oscillations inthe 1r-mode. The trigger pulse is also sent through a delay .unit 12. At a time approximately .6

' microseconds after the triggering of thyratron 11, thyratron 13 is triggered, allowing network 14' to discharge into magnetron 16, causing its current to increase to the.

operating power level of approximately 30 amperes for Fig. 5 shows the RF pulse envelope produced by the .15 microsecond 30 ampere magnetron current pulse as viewed with a spectrum analyzer.

v The pedestal modulator of Fig. 1 presents three serious drawbacks; viz, (1) two thyratrons 11 and 13 are required, (2) accurate trigger sequencing is required and (3) the lower time limit of short pulse operation is established by the pass band characteristics of the pulse transformer 15. The Darling ton line (Glasoe and LebacqzPulse Generators, Radiation Laboratory Series, vol. V, McGraw-Hill Book Co., 1948, pp. 464-465) offers a technique which can be employed to overcome these disadvantag es. The Darlington line' employs the characteristic that where a voltage wave traveling along i i mission line. Each section has the same two way time of travel of a voltage wave.

The delay produced in the Darlington line can be used to establish the time interval between the establishing of the pedestal and the short pulse. Consider the circuit of Fig. 2 using a 4 network Darlington line to produce a modulator pulse of .1 microsecond duration. Each individual network is a 4 section networkand during pulsing combines to produce-a rectangular pulse. This pedestal modulator of ,Figalis designed to Pulsejthe '4J50 magnetron for which the rate 'of rise of pulser voltage must not exceed 11 0' kv. per microsecond. The single section network in Fig. 2 is chosen to produce a half sine wave of voltage 'for the magnetron while in the diode current region. This 'voltage wave will be e =E sin wT where E represents the magnetron voltage at the threshand must not exceed 110 kv. per microsecond of the threshold voltage, 16 kv. Choosing as a safe value the slope at the 70% voltage level we get which, neglecting the diode operation loading gives as a realistic value of frequency for the L-C' network of 1.5 megacycles. of a period of .33 microseconds for the pedestal voltage'pulse. q 7 I p The triggering of the hydrogen thyratron 11 initiates the action of the single section network in producing a 16 kv. pedestal voltage which reaches its maximum value at a time .17 microseconds after the triggering of the thyratron. During the formation of thepedestal, the voltage wave produced by the firing of the thyratron travelsdown the Darlington line and is transformed up'fin voltage.

By. the time the voltage wave reaches the end' of the third network in the Darlington line it is delayed in timev by ,7 three times the one way time of travel (or by three halves the pulse duration) which for a .1 microsecond pulse is at a time..15 microsecond after the trig'geringfof the thyratron 11. The short pulse produced in the Darling-f ton line thus is delayedin time-to arrive at the peak of the pedestal pulse and is coupled by the diode 18 to the a transmission line meets a discontinuity, the boundary conditions at the discontinuity cause the magnitude of the wave traveling down the line to be changed as well "as to cause a voltage wave to be reflected back toward the source. Transmission line theory gives the magnitude of the reflected wave as Rp-z where R 'is the load impedance and V is the voltage ture is employed in the 'Darlington line 17 used in con junction with the voltage pedestal technique shown in Fig. 2 as being composedof 4 sections of artificial transsuggested as a pulse transformer for shortpulse's.

theoretical discussion reference is made to ,Shatz and q magnetron to produce a 250 kw. pulse of .1 microsecond duration;

It should be noted that v the firing "of a single thyratron produces both pedestaland powei'phlse and that the time sequencing isestablished totally by passive'elements, i.e., in 'Fig. 2, the Darlingtonline. Note, also, that the short pulse is developed'without requiring a pulse transformer. The pedestal pulse, however, is coupled to the magnetron 16 through a pulse transformer, 19 to permit th'elsin'gl'e section pedestal network to becha'rgedin parallel with the Darlington line but this transformer is required to pass only 'a .33 microsecond half-sinusoid pulse at a ratherlowp'owerleveh. I 7

The exponential-tapered rransmission line has been For a Williams, Exponential Pulse Transformer, Proceedings IRE, October 1950.. The exponential.pulseQtransforrner 21 in Fig. 3 produces short exponentialpulses.by the .action of triggering hydrogen thyratron ll tol discharge energy stored as electric charge on the distributed capacitam e in the 'transmissionline. With voltage step up of the order of 6 to 7, quite short pulses of the order of fractions of tenths of microseconds can be produced. The trailing edge of the pulse however will be of exponential order and thus will not decay rapidly. The voltage pulse to the magnetron load is shaped with a passive R-L circuit 22 as in Fig. 3. The exponential pulse transformer 21, when applied in the passive delay circuit such as in Fig. 3, provides the voltage step up while the open transmission line section 20 establishes the pulse duration by applying a step decrease in voltage to the magnetron load and thus terminating the magnetron current pulse. The time of travel of the voltage wave through the exponential pulse transformer 21 is used to establish the delay necessary to place the short pulse on the pedestal. I

The pedestal modulators just discussed were of the either-or type since the power to the magnetron load wastbeing received from either the pedestal or power pulse circuit. This was particularly the case with the Pedestal-Darlington Pulser of Fig. 2 where the diode 18 functioned to isolate the Darlington circuit from the pedestal circuit to prevent premature discharging of that circuit. Fig. 4 shows a simplified schematic diagram for the series generator pedestal modulator. The modulator pulse is formed by means of two generator circuits in series, one (26) of which forms the pedestal, the other (27) the power pulse. The impedance of the pedestal generator is such that the pedestal voltage to ground will remain essentially constant during the power pulse. This is accomplished by charging a capacitor 23 to produce the pedestal voltage. The power pulse is applied to the pedestal modulator by means of a coaxial cable 24, the length of which is chosen to provide proper time relations between the pedestal and power pulses. During the pedestal operation, however, the cable provides a voltage drop in the pedestal voltage due to the flow of magnetron diode current.

After the pedestal pulse is formed, the power pulse is applied to the magnetron 16. The pulse is formed by firing thyratron 11 to form the pulse at the input to the delay cable. The length of the delay cable is chosen to cause the power pulse to arrive at the peak of the pedestal voltage. Here the action of a single thyratron produces a pedestal as well as a power pulse while the length of delay cable is chosen to establish proper time sequencing to cause the power pulse to be applied at the peak of the pedestal pulse. For impedance match during the power pulse the characteristic impedance at the coaxial delay cable must equal the resistance of the magnetron load. The pulse forming network must also have this same impedance.

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

What is claimed is:

l. Energization means for obtaining a short pulse from an oscillator comprising an oscillator, pedestal pulse means for energizing saidoscillator at a low power level, power pulse means for supplying a sharp pulse for operating said oscillator at full power level, said pedestal pulse means and said power pulse means being activated by the same source, and delay and amplifying means for amplifying and superimposing said power pulse upon said pedestal pulse.

2. Energization means as claim 1, said power pulse means comprising a Darlington circuit for amplifying the voltage level of said sharp pulse and delaying said sharp pulse until said pedestal voltage has been efiected.

3. Energization means as in claim 1, said power pulse means comprising delay means connected to said source in parallel with said pedestal pulse means for delaying said sharp pulse until said pedestal voltage has been effected.

4. Energization means for energizing a magnetron within its prescribed rate of rise yet having a very short. duration, comprising pedestal circuit means responsive to a triggering pulse for applying a well-rounded low powered pre-operating pulse to the cathode of said magnetron to bring it into operation at 5% to 10% of its peakpower and pulse means for applying to said preoperating pulse a short high powered pulse to carry the magnetron voltage to full power level, said pulse means including a delay circuit connected with said pedestal circuit means and responsive to said triggering pulse, said delay circuit applying said short high powered pulse to said pre-operating pulse at the peak thereof, said delay circuit including a Darlington delay line for amplifying and delaying said high powered pulse a predetermined time from the start of said pedestal pulse.

5. Energization means for energizing a magnetron within its prescribed rate of rise yet having a very short duration, comprising pedestal circuit means responsive to a triggering pulse for applying a well-rounded low 1 operating pulse a short high powered pulse to carry the magnetronvoltage to full power level, said pulse means including a delay circuit connected with said pedestal circuit means and responsive to said triggering pulse, said delay circuit applying said short high powered pulse to said pre-operating pulse at the peak thereof, said delay circuit including a delay cable for doubling the applied voltage and delaying said high powered pulse a predetermined time from the start of said pedestal pulse.

6. Energization means for obtaining a short pulse from an oscillator comprising an oscillator, curved pedestal pulse means for energizing said oscillator at a low power level, power pulse means for supplying a sharp pulse for operating said oscillator at full power level, said curved pedestal pulse means and said power pulse means being activated by the same source, and delay and amplifying means for amplifying and superimposing said power pulse upon said pedestal pulse, said power pulse means comprising an exponential pulse transformer and open transmission line for amplifying the voltage level of said sharp pulse and delaying said sharp pulse until said pedestal voltage has been effected.

7. Energization means for obtaining a short pulse from an oscillator comprising an oscillator, curved pedestal pulse means for energizing said oscillator at a low power level, power pulse means for supplying a sharp pulse for operating said oscillator at full power level, said curved pedestal pulse means and said power pulse means being activated by the same source, and delay and amplifying means for amplifying and superimposing said power pulse upon said pedestal pulse, wherein said pedestal pulse means and said power pulse means are serially connected between said source and said oscillator.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS 1,080,269 France "1;. May 26, 1954

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