line type modulator

8/3/2019 Line Type Modulator

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LINE-TYPE RADAR MODULATORS

Garrison Brown

LibraryU. S. Naval Postgraduate SchoolAnnapolis. Md.


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LINE-TYPE RADAR MODULATORS>

by. - ~ .,"Garrison. BrownLieutenant Commander, United Sta te s Navy

>

Submitted in p a r t i a l , f u l f i 1 l m e n to f the requirementsfo r th e degree o fMASTER OF SCIENCEinENGINEERING ELECTRONICS

Uni ted S t a t e s Nav alP o stg rad u ate SchoolAnnapo1is,,-'Mary1and1949


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This work is accepted as fulf i l l ingthe thesis requirements for the degree of

MASTER OF SCIENCEin

ENGINEERING ELECTRONICS

from the'United States Naval' Postgraduate School

" ,', '" ,.... " ;" /. . '

//Department of Elect ronics and Physics

Approved:~ ' - - L ."Y!\t'I=-r: ::

Academic Dean

i


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PREFACE

A' portion of the mater ial herein presented wasobtained while on duty a t the GlennL"., Martin Company,Baltimore, Maryland, during t he period 3 January - ,18March, 1949. Work. was conducted on several l ine-typeradar modulators under, .the guidance, and with the ass i s t -ance of, the engineers in , ~ h ~ r o d u c t i o n Design Groupof the Electronics Section, S p e c i ~ l , Weapons Division.The author wishes part icular ly .to;, acknowledge the help-

, 1 '_, ', , . ' ' ,' "

fu l advice and assistance rendered him by Mr. L. J .Hruska, the Group Engineer, and Messrs. D. A. Bourne,

'> ",.... -"",,' ,

J . MarkWalter, and,B. f I ~ y e ~ , A s ~ i ~ t E 3 . n t . E n g i n e e r s o


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TABLE OF CONTENTS

I lliITRODUCTIONI . The Purpose of Radar Modulators2 . Fundamental Considerations3 . The Basic Circuit of the Line-typeModulator

I I CHA..l1.GING METrlODS1 . General2 . Charging from a D-c Source30 Charging from an A-c Source4. Inductive Impulse Charging5 . A Comparison of Charging Methods

I I I S ~ ~ T C H I N G 1 ~ O D S1. Requirements of the Switching Device2. The Nature of the Spark Discharge3 . Rotar;r Spark Gaps4 . Enclosed Fixed Spark Gaps5 . The Hydrogen Thyratron6. A Comparison of Switching Methods

IV PULSE FORMING NETvVORKS1 . Elementary Theory2. Line-simulating Networks3. Guillemin Lines4. Practical Considerations

V THE LOAD1 . The Pulse Transformer2., The Magnetron

( i i i )

pg. 1pg. 1pg. 3

pg . 8pg. 7pg. 10pg. 14pg. 19

pg. 21pg. 21pg. 23pg. 28pg. 30pg. 32

pg . 34pg . 37pg. 40pgo 46

pg. 50pg . 53


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LIST OF ILLUSTRATIONSFIGURE 1 . Idealized Pulse Waveform pg. 2FIGURE 20 Basic Block Diagram of the Line-type 9g 2

PulserFIGURE 3. Equivalent Cirduit and Naveforms for pg. 8D-c ChargingFIGURE 4. Equivalent Circuit and Vlaveforms for pg. 12A-c Diode ChargingFIGURE 5. Eauivalent Circuit and Vvaveforms for pg. 13A'::c Resonance ChargingFIGURE 6. Full-cycle and T".No-cycle Charging pg. 15FIGURE 7 . Half-cycle Charging pg. 16FIGURE 8 . Simplified Circuit for Inductive pg. 16Impulse ChargingFIGURE 9. Waveforms for Inductive Impulse pg. 17ChargingFIGURE 10. Circuits for Series Gaps, and Cross- pg. 27section Views of Switch Tubes.FIGURE 11. Circuit and Waveforms for Ideal Trans- pg. 35mission Line Pulse Forming NetworkFIGURE 12. Three Line-simulating Networks pg. 39FIGURE 13. A-a Waveforms for Determining theGuillemin Line s pg. 42FIGURE 14. Four Canonic Forms of the Guillemin 9g 45LineFIGURE 15. Equal-capacitance and Type "E" Ne"tworks pg. 47FIGURE 16. Equivalent Circuits for Ideal and pg. 51Pulse Transformers

(iv)


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TJ\BLE OF SYMBOLS .AND ABBREVIATIONS

~ - - - - exponential damping factorCN--- storage capacity of PFNS---- pulse width in microsecondsALm- - incremental change of magnetron currentAVm- - incremental change of applied magnetron voltageEbb-- d-c power supply voltageEm- - - peak value of a-c power supply voltage~ t - - - pulse transformer efficiencyiLs- - current in saturable-core reactorI L- -- d-c current in loadI m- - - magnetron currentI p- ...- transformer primary currentI s - - - transformer secondary currentK!l--- kilovoltsID'f--- kilowattsRjd-- ratio of length to diameter (of a coil)L---- inductanceLc- -- charging reactorLr - -- value of charging reactor for resonance chargingLs- - - saturable-core reactorma--- milliamperesn---- transformer turns rat iopp.s. - pulses per secondPFN-- pulse forming networkPRF-- pulse recurrence frequency

(v)


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(

R---- resistanceRc- - - charging circui t resistanceRC--- characterist ic resistanceRL--- load resistances-- - - Laplace-transform operatort - - - - timeTo- - - time at which charging current is maximumTr -- - pulse recurrence periodvN--- voltage on the PFNV L - - ~ load voltageV ~ - - - applied magnetron voltageVp - - - transformer primary voltageVs- - - transformer secondary voltageyes)- Laplace-transform admittance functionz---- impedanceZo--- characteristic impedanceZp--- transformer impedance viewed from primaryZs--- transformer impedance viewed from secondaryZ{s)- Laplace-transform i m p e d ~ n c e function

(vi)


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CHAPTER IINTRODUCTION

1 . The Purpose of Radar ModulatorsPresent day radars are very nearly a l l of the p U l ~ e d

type, and with few exceptions employ cavity magnetronsto generate electromagnetic oscil lat ions. In order toexcite the magnetron at the proper frequency and a tthe proper power level, provision must be made forsupplying high-voltage, high-power pulses to i t . Inaddition to proper pulse voltage and power, the shape ofthe pulse is of primary importance for proper magnetronoperation. The unit which furnishes these pulses ofvoltage to the oscil lator i s known as the modulator; i tis also called the "pulser" or "keyer" 020 Fundamental Considerations

In order to discuss modulators, certain parametersaffecting the design must be considered: pulse ~ i d t h ,peak power, pulse recurrence frequency, duty ratio, andaverage power. Since the output waveform is not rectangular, i t is necessary to define various terms connected with pulse shape. An idealized waveform is i l lus t ra t -ed in Figure 1. For purposes of discussion, the followingterms are defined:

Pulse width - elapsed time in microseconds betweenleading and trai l ing edges, measured at .707 ofmaximum pulse amplitude (unless otherwise stated)

..-"Rise time - elapsed time in microseconds between

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15% and 85% of maximum pulse amplitude, measuredon leading edge.Fall time - elapsed time in microseconds between85% and 15% of maximum pulse amplitude, measuredon t rai l ing edge.Maximum pulse amnIi tude - maximum value of thepulse i.n volts (or amperes), measured to theaverage of any osoillations that exis t on the topof the pulse.Peak power - the product of maximum pulse amplitudeof voltage and m a x i m Q ~ pulse amplitude of ourrent.Pulse recurrence frequency !PRFL - the number ofpulses occurring in on e second.Duty rat io - the product of pulse width and PRF.Also called "duty cycle".Average power - the product of peak power and dutyra t io .

3 0 The Basic Circuit of the Line-type ModulatorFundamentally, there are two types of pulsers:(a) Those in which a small fraction of the storedenergy is discharged into the load during the pulse,(b) Those in which a ll of the $ored energy is discharged during each pulse.

The former are c aIled "hard-tube" modulators, since ahigh-vacuum tube i s used for switching purposes. Thela t te r type are known a s "line-type", since the energystoring d ~ v i c e is basically an ar t i f ic ia l -transmission

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l ine . I t i s with the second type that this paper isconcerned, and further discussion wil l be limited tothat sUbject.

The basic block diagram of the line-type pulser isgiven in Figure 2. The operation of this circui t is asfollows: An ar t i f ic ia l transmission l ine (generally knownas a pulse forming network, and hereinafter termed "FFN")i s charged through an isolating element during the interval between pulses when the switching device i s nonconductive. At the instant the pulse i s desired, thel!Iwitching device is made to conduct, thus effectivelyestablishing a short circuit across the load. This causesthe PFN to discharge and thereby form the pulse, whichi s coupled to the magnetron through an impedance matching t ransformer . Directly after the discharge occurs,the switch again be comes non-conductive, the PFN geginscharging again, and the cycle is repeated.

As shown in Figure 2, the line-type modulatorconsists essentially of five sections exclusive of thetransmitter, viz . , power supply, isolating element,switching device, PFN, and pulse transformer. Each ofthese oomponents, except the power supply, wil l betreated in a separate chapter of th is paper') Althoughthe power supply is a v i ta l part of the modulator system,i t i s usually quite conventional, and merits l i t t lefurther attention. Unless otherwise specified, thepower supply will be considered to be a voltage source

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of zero internal impedance 0 I t is not the purpose ofthis treatise to discuss magnetrons per se , but as thesedevices present a non-linear load to the pulser andtherefore influence pulser design to a great extent, i tis necessary to consider them from that point of view.Consequently, i t should be borne in mind that this isthe type load into which the modulator wil l work, andthat certain requirements, not otherwise found withl inear resis t ive loads, are therefore imposed on themodulator system and i t s components.

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!CHAPTER I ICHARGING METHODS

1 . GeneralBecause charging and discharging in a line-type

pulser are independent of one another, i t is convenientto t reat these operations separately.Three iwportant considerations in the design of

the charging circuit are:(a) Regularity-(b) Isolation(c) High efficiency-

By regulation it is meant that the PFN must be chargedto the same energy level on each cycle. The chargingelement must isolate the power supply from the discharging circui t during the pulse in order to preventshort circuiting the source during the interval .Isolation is also necessary for a short period i m m e d i a t e ~ly following the pulse to permit the gaseous-dischargeswitch to deionize. A high efficiency is particularlydesirable since the line-type modulator operates a thigh power levels .A resistance may be used as the charging element,

since i t fulf i l ls the f i r s t two requirements l is ted above;however, the maximum efficiency is but 50%, and hence th ismethod i s precluded in practice. More eff icient methods,generally employing an inductanc'e a the is ola tingelement,' are discussed in the fo llowing secti on s.

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2. Charging from a D-C SourceIn order to analyze the d-c charing circuit using

an isolating inductance, the following assumptions are made;(a) The PFN is represented by the capacitance, eN'appearing across i t s terminals; the inductances ofthe PFN are short circuits a t the low frequency ofcharging.(b) The inductance of the charging reactor, Lc ' isconstant.(c) The pulser switch is ideal .(d) The inductance of the pUlse transformer primaryi s negligible compared to that of the chargingreactor.

Under the foregoing conditions, the equivalent cmrgingcircui t is as shown in Figure 3(a)o Ebb is the d-cpower 8 U p ~ l y voltage and Rc is the to ta l resistance thatcauses damping in the circuit , in this case the ohmicresistance of the charging inductance. This circuitbehaves as a damped ringing circui t which is triggeredby t ~ opening of the switcho The voltage on the PFNreaches a maximum value a t time To equal to one-halfthe natural period of oscil lat ion. I f the pulse recurrence period Tr (the reciprocal of the PRF) is takenequal to To, the voltage on the network a t the time ofdischarge i s given by (Glasoe (2)):

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where,

and vN(O): the in i t ia l voltage on the network.At this instant the charging current is zero, and noin i t ia l current through the inductance will ex ist forthe next cyole. This condition is known as "resonancecmrging tt , and the following equation is then satisf ied:

L = L :::. T Zoc r _.::.r_ _1r'l.CNI f Tr is different from To, an in i t ia l current wil lexist , and may be either positive or negative. For Trless than To, the voltage on the network is s t i l l risingwhen the switch is fired, and so-called "linear charging"occurs. This is obtained in practice by using a valueof inductance larger than t1B.t which muld produceresonance charging. By using lar'ge inductance anduti l izing l inear charging, some degree of f lexibi l i tyof PRF and/or pulse width (a function of ~ may besecured. For inductances smaller than Lr however, thein i t ia l current i s negative, with the result that circuitlosses are increased, and the PFN voltage is past thepeak a t time of discharge. For these reasons , thiscondition i s avoided in actual circui ts . Figure 3(b)(c)& (d) i l lustrate the current and voltage waveforms for

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the three cases discussed abovej for simplicity inplott ing, the circuit is taken as lossless, am. thein i t ia l voltage on the PFN is considered to be zero.

In cases where greater f lexibil i ty of PRF or pulsewidth, or both, is desired, or where a small value ofcharging inductance is indicated, a hold-off diode isemployed in series with the charging reactoro Thisdevice prevents the flow.of negative current and maintains the network voltage very close to the peak valueunt i l fir ing occurs. Equation (1) remains valid forthis case, but Rc is increased by the effective resistance of the diode. This results in a slightly lowerefficiency than in the case of ordinary resonance charging; typica l figure s being 95% for the la t ter and 90%for the former. Current and voltage waveforms for thecase of d-c resonance charging with a diode are given inFigure 3(e}.3. Charging from an A-C Source

I t i s possible to use an a-c voltage source, such asan ordinary high-voltage transformer, to charge the PFN,provided the charging frequency i s integrally related tothe PRF. An isolating element is necessary as in the caseof d-c charging; i f the network were directly across thesedondary terminals, and discharged a t peak transformervoltage, it would tend to recharge immediately, and shortcircui t the transformer during the ensuing quarter cycleo

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One method of isolation is to place a diode inseries as shown in Figure 4(a). Negleoting the smallamount of damping, the PFN will charge to the positivepeak of supply voltage, Em' and wil l be held at thatpoint by the diode unt i l the switch closes. Dischargemust be made to occur during that part of the supplycycle in which the switch is non-conducting, otherwise theswitch and diode would complete a short-circui t acrossthe power supply. The maximum PRF is limited to thefrequency of the supply voltage; repetit ion r a ~ e s a t anySUbmultiples of the supply frequency are also possible.Figure 4(b) i l lustra tes the voltage waveforms involved inthis method of charging.

Another method of charging from an a-c source is touse a series inductance as the in te rmedia te element. Bymaintaining the proper relation between PRF and supplyfrequency, and between and Lc ' i t is possible to disoharge the PFN at a voltage peak and yet preserve stablecircui t conditions. The most common method is to chooseLc to resonate with eN a t a frequency equal to tha t ofthe source; this 1s known as "a-c resonance charging t t .In this oase the charging circui t 1s a tthigh-Q." seriesresonant circuit , as shown in Figure 5(a). The voltageacross eN will build up as shown in Figure 5(b), beingmaximum a t the zeros of the impressed voltage. Dampingin the circuit limits the magnitUde of oscillationsaf ter several oycles, but has l i t t l e effect on the f i r s t

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L.L-- I ~ _ DIODE Vfl~ l n n _ : - o r _ ~ ~ _ ~ _

--- (a)oirouithargingquivalent

F. atnwty voltage 1 I luppPFN voltage vN

(b)eformsoltage wav

DIODE CHARGING-CF1g. 4


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I,. II (b)Voltage Waveforms37 fE2 m

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few peaks. Neglecting damping, the successive voltagepeaks have the values itrEm, - 1 t ~ , ~ ' i i E m ' -21tEm, etc . , asshown in Figure 5(c}. I f good efficiency is to bemaintained, discharge must occur on the peaks of theresonant wave; however, the discharge should occur onlyon those peaks which are spaced an even number of halfwave lengths apart with respect to the supply voltagewave. Figure 6 i l lustra tes this for "full-cycle t f and"two-cycle" charging. I f discharge occurs on the oddhalf wave length peaks, the direction of t he pulse wil lalternate as shown in Figure 7. Therefore, the PRF isrestricted to the frequency of the supply voltage (fullcycle charging) or integral SUbmultiples thereof, unlesssome type of reversing switch is used to deliver a l lvoltage pulses to the pulse transformer with the samepolari ty. In this la t ter case the maximum PRF is twicethe supply frequency (half-cycle charging).4. Inductive Impulse Charging

Although this method of charging util izes a d-cpower supply, the modulator circui t is somewhat differentfrom those discussed previously, and for that reason isdescribed separately. The simplified circuit diagram isgiven in Figure 8, and Figure 9 shows the most importantwaveforms involved. The circui t employs a saturable-corereactor switch which presents a high impedance to low currents and a low impedance to high currents. Briefly,operation is as described below.

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A long preparation pulse (Figure g{e) from atiming multivibrator is applied to the grid of the pulsertUbe, Vl , causing i t to conduct. Plate current is drawnthrough three paths: through the charging inductor, Lctthrough the saturable reactor, Ls , and through the PFNand load. When the current through Ls (Figure 9(b)}reaches the positive l imit of the unsaturated region ofthe reactor (point A), the impedance SUddenly drops, 01discharges rapidly, and the current i L i s then limitedsby the value of Rl Concurrently, the PFN, which nec-essarily had been charged to Ebb prior to conduction ofVI' discharges and causes a small positive pip of current to flow in the load (Figure g(c)}. This could causean undesired r - f output pUlse, and therefore a small chokeLl is included to l imit the rate of discharge of the PFN.After 01 and the PFN discharge, the circuit remains in asteady state unt i l the input pulse to the grid of thepulser tube suddenly drops, thus commencing the actualmodulator operation. VI is cut off, but the current inLc cannot stop instantly, and therefore flows through thepm{ and the diode to ground. The only other availablepath would be through Ls and Cl , but th is path is deniedby the high impedance of the saturable reactor, iL havingssuddenly dropped to the upper l imit of the unsaturatedregion when VI ceased conduction (point B). The currentflowing into the PFN charges i t to a voltage that is verymuch higher than the power supply voltage due to the

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inductive impulse. 1Vhile the network is charging, thecurrent through the saturable-core reactor decreases slowly; when i t reaches the negative l imit of the unsatu ra tedregion (point e), Ls becomes effectively a short-circui tB.nd discharges the PFN. The discharge path includes themagnetron, and therefore an output r - f pulse is produced.

Reference to Figure 9(b} and 9(d) shows that theperiod for the PFN to charge to a maximum voltage must beequal to the period required for iLR to pass through theunsaturated region. The former is a function of andLc , and the la t te r depends upon the design of the saturablecore reactor. This critical" relationship l imits theusefulness of the circui t to cases where only one PRF i srequired 050 A Comparison of Charging Methods

Each of the charging methods described above havecertain inherent advantages and disadvantageso The mostimportant of these are l i s ted on the following page inoutline form.

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MethodD-c resonancecharging

Advantages DisadvantagesVery high efficienty Requires heavy high-voltage power s u ~ p l y

Inflexibil i ty ofPRFD-c resonancecharging wi tha diode

A-c diodecharging

A-c resonancecharging

Inductiveimpulsecharging

Continuously vari -able repetit ionrateHigh effioiency

Light and simple

Light and simpleGood effioiency

Relatively lowvoltage supply

20

Requires heavy high-voltage power supplySome loss of eff io-iency due to dampingof diodeInflexibil i ty of PRFRequires synoh-ronized switchingRequires synch-ronized switchingFlexibil i ty of PRFseverely restrictedCritical relat ion-ship between Lcand LsPoor pulse shapeLow efficiency forpulse widths greaterthan 1 microsecond


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CHAPTER I I ISWITCHIJG METHODS

1 . Requirements of the Switching DeviceThe characterist ics required of a switch for a l ine

type modulator are clearly defined. First , the switch mustbe non-conducting during the in te rpulse interval. Second,i t must be able to close very rapidly, and at a predetermined time. Again, i t must be capable of carryingthe fu l l pulse current and should have a very low res is t -ance during the pulse. The voltage across the switchfal ls to zero, or very nearly so, a t the end of the pulse.hence the switch is got required to be a current-inter-rupting device.

As discussed previously, one method of sWitching aline-type pulser is by the use of a non-linear inductance.Otherwise, however, the switch is usually an enclosedfixed spark gap, or a gaseous-discharge tube. Rotaryspark gaps are also employed, and are quite satisfactoryin some applications. The use of a grid-controlled highvacuum tube is obviated by the fair ly high resistance i tpresents during conduction, and also by i t s low cathodeefficiency. Each of the three main types of switch rotary gap, fixed gap, and hydrogen thyratron - will betreated with regard to their uses, characterist ics, andspecial requirementso2. The'Nature of the Spark Discharge

When the voltage between two electrodes is raised

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to , or above, the static-breakdown point, a spark willoccur. This spark is caused by the breakdown of the gasin the region between the electrodes, and is a resul t ofthe ionization of the gas molecules by the acceleratedfree electrons in the vicinitYa This acceleration i simparted to the electrons present by the electric fieldexisting between the terminals, and is directly proportional to the potential difference across the gap.The higher the applied voltage, or the greater thenumber of free electrons present, the shorter the elapsedtime between application of the voltage and ini t iat ionof the discharge. For example, i f the voltage be justequal to the static-breakdown potential , a "time lag" ofseveral minutes may occur. I f the voltage be raised to avalue two or three times the minimum required, the timelag i s reduced to the order of hundredths or eventhousandths of a microsecond. After the ionization com-mences, a short interval, of the order of .01 microsecond, transpires before the discharge attains the propert ies desired for use as a switch. At the end of thisvery short "breakdown time", the gas between the electodeshas changed from an insulating medium to one cape,ble ofcarrying quite high currents. Factors determining thecharacterist ics of discharge are the type of gas, gaspressure, gap geometry, and the shape of the appliedvoltage wave.

In order to ut i l ize the above properties of thespark, the breakdown must be controlled, and this can

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easily be done by heavily "overvo1ting" the gap. In thecase of a fixed gap, a high transient peak of voltage isapplied to one electrode. Another method is to ..maintaina high d ifference o f potential between electrodes, andvary the spacing between them at the desired rate - thisis the method employed in the rotary spark gap.3. Rotary Spark Gaps

Essentially, the rotary gap consists of an insul-ating disc c o n t ~ i n i n g a set of electrodes and rotatingin the vicinity of one or more fixed electrodes. Severalvariations are commonly used. The pins may be set paral lelto the axis of rotat ion, or they may extend radiallyoutward about the periphery of the disc. They may extendon either aide of the disc and discharge simultaneouslyto two fixed electrodes. The fixed and moving pins mayoverlap for a portion of their length, or pass one anotherwith end faces opposing.

Rotary gaps have a high power handling abi l i ty andare simple and rugged in construction. Furthermore, noexternal triggering voltage is necessary to in i t ia te theswitching action. To part ial ly offset these desirablefeatures, certain disadvantages are encountered;

(a) The P.RF depends directly upon the number ofelectrodes, both fixed and rotary, and upon thespeed or rotation. The spacing between adjacentelectrodes is l imited by electr ical considerations,such as breakdown voltage and deionization between

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pulses. The size of the rotor i s limited by mechanicaldesign. Hence, even wi th a high speed motor, thenumber of electrodes is limited, and so rotary gapsare not well suited for high recurrence ra tes . Thegreatest PRF used in practice is 800 p.p .s . Inaddition, f lexibi l i ty of PRF is restr icted; a changeof two to one may be effected by sWitching a secondfixed electrode in series with the one used for thelower rate , but in general each rotary spark gap isdesigned for one PRF only.(b) Rotary spark ga,s have an inherently large time" j i t te r" . This refers to the uncertainty in time ofthe init iat ion of breakdown, and results from bothelectrical and mechanical causes. Since the spacingbetween electrodes at the point of closest proximityi s such that a voltage of two or more times thestatic-breakdown potential exists, the spark willalways occur before the point of closest approachi s reached. The exact time of spark wil l wary fromdischarge to d ischarge. Increasing the relativevelocity of the electrodes, i . e . , increas ing thespeed of the rotor, tends to decrease the amountof this j i t t e r . Mechanical tolerances in assemblyof the rotating parts , in radial and peripheralspacing of the moving pins, and in r igidity mustbe extremely closeo As an example, an error inperipheral spacing of .02 inches can cause an increase of time j i t te r of 15 microseconds or more.

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The accumulated j i t t e r due to mechanical and elect r ica l causes may be as much as . 50 microseconds,but with careful design this figure has been reduced to 20 microseconds in some instances.There are two direct consequences of this timej i t t e r that are important. Firs t . the entire system employing such a modulator must be synchronizedin time. For example, the sweeps on the indicato rtubes must be triggered by the power pulse. Second,i f d-c charging is used, a varia tion in time offir ing wil l resul t in a variat i on of pulse voltageamplitude, and consequently in output pulse power.While this effect is small in d-c resonance charging,i t is so serious in the case of l inear charging asto make the combination of l inear charging androtary gap switching unusable oIn addition to the two major limitationsof j i t t e r

and low PRF, there are a number of minor diff icul t iesencountered. Corrosive gases are generated by the breakdown of the air in the gap. These can be dispersed byforced ventilation in some instal lat ions, but where awide variation in ambient pressure is a factor , as inan airborne pulser, the gap must be pressurized in orderto maintain s tabi l i ty . In this case, the use of blowersi s not possible, and some absorbing medium, such asactivated carbon, must be enclosed in the chamber toremove the gases. Changes in gap geometry due to erosion

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and pitt ing of the electrodes will change the sparkcharacter ist ics. This is combated by the use of tungstenelectrodes, by making the fixed pin the high voltageterminal, and by the use of the overlapping construction.When used with a-c resonance charging, the use ofa rotary spark gap switch can resul t in a simple andcompact pulser. The charging reactor can be designedas the leakage indue,tance of the transformer secondary,and the rotor of the gap mounted directly on the shaftof the a-c machine that excites the circui t . The variation of pulse amplitude with time j i t te r is very muchless pronounced than in the case of d-c charging, andwith overall system synchronization the j i t te r can betolerated.4. Enclosed Fixed Spark GapsThe most commonly used switching circuit employing

fixed gaps is obtained by connecting two or three coldcathode gas-fil led diodes in series. These enclosedgaps are of two basic designs:(a) The oylindrical-electrode aluminum-cathodegap such as the Western Electric types IB22and lB34 and the Westinghouse type WX3240.(b) The iron-sponge mercury-cathode gap .... such asthe Western Electric type IB42.

Both types are operated in the same manner. Figure 10(a)

26


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POWERSUPPLY

N1


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and lO(b) show two methods of employmentD The voltagedivider is placed across the gaps in order to dividethe fu l l PFN voltage equally across each one. By impressing a rapid-risihg, high-voltage t r igger pulse,one of the gaps will break down. The ful l PFN voltagewill then appear across the remaining gap (s) , whichwill in turn rapidly break down, thereby completingthe closing cr the switch.

The cylindrical-electrode aluminum-cathode tubeconsists of a cylindrical aluminum cathode that almostcompletely encloses a long anode rod of the same materialoThis is i l lustrated in Figure 10(c)0 The enclosed gasi s a mixture of approximately 80% hydrogen and 20% argon.By providing a large cathode area the effects of cathodeerosion are minimized, and aluminum is used because ofi t s relatively low erosion rate in the hydrogen-argonmixture compared with other metalso For a giveripulsecurrent the erosion is directly proportional to pulseduration, and this property l imits tube l i fe when widepulses are used. The time j i t t e r when using th is typeof switch i s of the order of 6 microseconds.

The iron-sponge mercury-cathode gap consists of athin molybdenum anode, and a cathode of mercury immobilized by an iron "sponge tf o This sponge is made ofcompressed iron powder co ntaining about 60% void space.The sDonge holds approximately 9 cC o of mercury, thesurface tension of which maintains a film over the

28


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surface and prevents erosion of the sponge proper. Thegas in the tube i s pure hydrogen under relatively highpressure. The use of this gas, together with small gapspacing and a thin anode rod, makes time j i t t e r in thistype of switch very low - of the order of .02 microseconds. Low erosion makes for a longer tube l i fe anda wider range of operating conditions than in the caseof the aluminum-cathode gap.The choice of two or three gaps in series dependsupon the choice of tube and upon the PFN vo+tage. At

low power two-gap operation of aluminum-cathode gapsis satisfactory, but in general i t i s desirable to usethree gaps. When two only are used, a bi-directionaltr igger pulse should be used to insure satisfactorystar t ing. In the case of the iron-sponge type, two-gapoperation is generally satisfactory for a l l condi tions Iand a bi-directional pulse i s not needed GA third type of enclosed fixed spark gap is the

th ree elec trode type, of which the British "trigatron tti s the principal example. I t consists of a molybdenumcathode and anode, and a tungsten tr igger pin, as shownin Figure lO(d). The gas i s a mixture of 95% argon and5% oxygen at a nressure of about 50 psi . The tr iggervoltage must be very high - of the order of 6 KV - andl i fe is limited to about 200 hours by the rapid erosionof the tr igger pin. J i t t e r i s less than 0.1 microsecond.

29


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5. The Hydrogen ThyratronBy far the most successful switch available for use

in a l i n e ~ t y p e modulator i s the hydrogen thyratron. The3C45, 4C35, and 5022 are a t present on the market, ~ ~ la t te r being rated a t 16 KV peak anode voltage, and afourth tube designed for even higher voltages is underdevelopment. A diagram of the internal structure of thetube i s given in Figure 10(e). The gas in the envelope i shydrogen a t a pressure of about .005 rom. of Hg , and thegrid-anode spaoing i s very much smaller than in conventiona l mercury vapor thyratronse The tube has a positivecontrol-grid characteris t ic, and in order to s tar t conduction i t is necessary to drive the grid sufficientlypositive to draw grid cu rrent. This current producesions and electrons in the region external to the cathodeshield, and some of these reach the area of the gridbaffle. When the electron density there becomes highenough, the anode f ie ld, which i s present only in theregion above the grid baffle, will produce ionizationin that region and breakdown will occur. The ionizationtime i s .03 microseconds for the 3045 and .07 microseconds for the 50220

Due to the chemical activity of hydrogen, the metalparts must be very pure, and great care in manufadturei s necessary in order to prevent the inclusion of anycontaminating substanoe. Should combination of thehydrogen 'with any impurities take place, the phenomenonknown as "gas cleanup" occurs. This means tha t the

30


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hydrogen no longer exists in the gaseous form but basformed chemical compounds with the impurities, andthe ut i l i ty of the tube is lost . The cathode operatingtemperature is rather cr i t ical . The upper l imit isabout 850 00 due to the reducing action of the hydrogenon the oxide cathode above that temperature; the lowerl imit is about 8000 0 due to the rapid loss of cathodeemission below that temperature. For this reason indiredtheating is used to minimize temperature variations overthe cathode surface, and a fairly long filament warm-upperiod must be observed before applying plate voltage.Tube l i fe i s about 500 hours at maximum rating of pulsevoltage and pulse current, but i s considerably lengthenedby decrease of ei ther. Pulse rate also affects tube l i fe ,and a t maximum ratings should not be in excess of 1000p.p.s . for the 5022 and 2000 p.p.s . for the other tubesoAt reduced power output, however, recurrence rates up to40,000 p.p .s . have been achieved. Thyratrons may beoperated in series or in parallel in order to exceed thecurrent or voltage rat ings of a single tube. Specielcircuitry i s necessary in these cases to assure balancedoperation.

The hydrogen thyratron may be triggered very pre-cisely with very small time j i t t e r . The trigger voltageneed be only about 150 volts a t a rate of r ise of 200volts per microsecond. This means that a simple externaltr igger generator circui t can be employed. The j i t t e r

31


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obtained when usi ng such a t r i g g e r volta ge is l e s s than.05 microseoondso With a t r i g g e r puls e o f amplitude200 v o l t s and a r i s e r a t e o f 850 v o l t s p er mioroseoondthe j i t ter has been reduced to . 00 3 m ic ro se co nd s o6 0 A Comparison o f Switohing Methods

Each o f the methods fo r switohing discussed abovehas i t s l i m i t a t i o n s . A t r u l y i d e a l switoh s u i t a b l e toa ll a p p l i c a t i o n s has not been found. Lis te d on thefollowing page in o u t l i n e form a re the major advantagesand disadvantages of each ty p e.

32S2C J.k&SiRS2S4AtiAWi


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Method AdvantagesSaturable-core Long l i fereactor High PRF

High power levels

DisadvantagesPoor pulse shapeRequires specialcharging circuit

Rotary sparkgap

Aluminum-cathode gap

Iron-spongegap

Trigatron

Hydrogenthyratron

Simple and ruggedHigh power levelsHigh power levels

Low time j i t t e rLong tube l i fe

Very lowtime j i t te rVery lowtime j i t te rGood tube l i feLow t r iggervoltage

33

High time j i t te rLow PRF

Short tube l i t eRequires morethan one gapHigh tr iggervoltageRequires morethan one gapHigh triggervoltageRequires very hightrigger voltageShorter tube l i feat high power orat high PRF


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CHAPTER IVPULSE FORMING NETVlORKS

1 . Elementary TheoryIn the foregoing chapters i t has been taci t lyassumed that i f the pulse forming network were charged

to a voltage vN' and then placed directly across a loadequal to i t s characteristic impedance, a pulse ofvoltage of approximately i ~ would appear across theload. In order to verify this assumption, f i r s t considerthe PFN to be an ideal losaless transmission l ine , opencircuited a t the far end.* Assume the load resistance,RL, to be equal to the characteristic resis tance o f thel ine , Re, and that the l ine i s charged in i t ia l ly to avoltage vN. (Refer to Figure 11). At the instant theswitch 1s c losed, one-half of the voltage sto red on thel ine wil l immediately appear across RL, leaving thevoltage across the input terminals of the l ine reducedto ivN This i s equivalent to introducing a negativevoltage wavefront of amplitude ivN to the ihput terminals - this wavefront will trav el down the l ine a t thepropagation velocity to the open end, leaving the linecharged to iVN along i t s entire length. At the open end,the negative wavefront is reflected in the same phase,and returns to the input end, canceling the remainingvoltage on the l ine as i t travels . At the end of aperiod equal to twice the length of the l ine divided by* Shorted l ines are not used in p ractic e, due to the lackof a suitable switch to interrupt the high current.

34


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VNT

"7 - R....0 - .., C

., .

o

o

o

I ! - --+------------- . . ; . t

-

b 2.S . . ~ 4S ..I0

tF1.g. 1135

t

.' i


44/64

the velocity of propagation, 'the l ine is completelydischarged. At this time the voltage across RL dropsto zero, and the energy stored in the distributedcapacitance of the l ine has been completely transferredto the load. Considering th is capacitance to be lumped.and denoting i t as the. 1fstorage capacity" of the l ine,~ ~ the following important relat ion is developed,{MIT ( 7 ) .

Energy stored on l ine : Energy transferred to loadi ~ V N t . VL1Lt

This relation i s widely used in the design of the pulsercircui t . particularly when resonance charging i s to beemployed.

I f RL is not equal to ReJ then the voltage acrossthe load wil l accordingly be either greater or smallerthan iVN. and the effective wavefront wil l similarly bedifferent. In addition, since the l ine is not terminatedin i t s characteristic impedance, a series of reflectionswil l occur, in accordance with elementary transmissionl ine theory (assuming the switch remains closed). Thevoltage waveforms appeari ng on the load for the threecondi t ions. RL RC. RL -== 2RC, and RL= ' iRC are i l lus tratedin Figure ll{a)(b)&{c). respectively. In practice, RL i s

36


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made equal to RC in order to get a single pulse, andin order to completely discharge the PFN on each cycleo2. Line-simulating Networks.

In order to obtain a pulse of one microsecondduration from an actual transmission l ine having apropagation velocity of 500 feet per microsecond (arepresentat ive value), a length of 250 feet would berequired. Furthermore, this l ine would have to be capableof withstanding the high voltages used in radar pulsers.That such a long high-voltage cable would be much toomassive and heavy for practical application i s obviousoAs a resul t , pulse forming networks that simulate actualtransmission l ines were developed.

The PFN is essential ly a two-terminal network,since one end i s open-circuited. I t i s a characteristicof two-terminal networks that their behavior is completelydetermined by the driving-point impedance function, i . e . ,the input impedance, expressed as a function of (angular)frequency, Glasoe (2) and Guillemin (3). The problem offinding a network which is equivalent to the ideal transmission l ine then becomes one of mathematical synthesis;given the impedance function, determine the network whichsat isf ies that function. The input impedance of theideal l ine i s given by the equation,

3?


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where Zo i s the character ist ic impedance of the l ine ,and t i s the one way transmission time. The Laplace-transform impedance function is then,

z( s) = Zo coth( st)where s is the Laplace-transform operator.

Figure l2(a) shows the well known "L-section"ar t i f ic ia l l ine . I t can be shown mathematically,Glasoe (2), that the input impedance function, Z(s), ofthis line will be of identical form to equation (3) inthe l imit as the number of L-sections approaches inf in i ty . I f a f ini te numbers o f section s are used, therewill be an upper l imit to the frequenc ies present in thepulse, and hence the square shape wil l be only approximated. The g reater the number of sections, the betterwil l be the approximation, but a network of. ten or moresections wil l generally be necessary in pulser app lica-t ions.

Figure l2(b)&{c) shows two more line-simuiatingnetworks, obtained by the synthesis method mentionedabove. The f i r s t of these i s obtained by expanding Z(s)as a r at ional f ract ion , Glasoe (2), and the second byexpanding in the same way the admittance function,yes) = l / z (s ) J

y{s)=Z1 tanh{st)o

38


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; ,

,;.", '

, , ', :.- .... '

L L

(a)

~ , . ', , Co,, '(b) 'Y

8e0--0 - J gc.T9-iT'- __

(0)

F1g. 12


48/64

The rational-fraction expansion of either of thesefunctions will result in an infini te series . Eaoh termof the series i s identified as the operational impedancefunction of a capacitance or inductance, or moregenerally a combination of the two . Comparison of thecoefficients of the terms yields recursion formulaewhich are used to determine the values of the reactiveelements in terms of 20 and t . In these cases, again,an infinite number of sections must be employed toobtain a square pUlse.In general, i t may be said of each of the threenetworks discussed above, and of any other that may bederived similarly, that a large number of physical

,elements are necessary i f a reasonably square pUlse isto be formed. In addition, analysis of these circuitsusing a f ini te number of sec tions reveals two propertiesthat are undesirable in radar pulsers. Firs t , overshootsw ill exist at the beginning of the pUlse, and second,excessive oscil lat ions occur along the top.

3. Guillemin LinesI t was fe l t by Dr. Guillemin that the above l imit-

ations arose from attempting to generate, by means of alumped-parameter circui t , a pUlse having an infini terate of rise and fa l l . That is , the pulse displayeddiscontinuities, and such a function cannot be produced by a f ini te series . Therefore, he proceededon the premise that the pulse to be generated should

40


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have a f ini te rise and fa l l time. Mathematioally, thediscontinuities in the wave no longer exist , and theFourier series is uniformly convergent. The propertyof uniform convergence signifies that the overshootsand oscillations may be reduced to any desired degreeas more sections are added. In order to write the Fourierseries of a function, the function must be periodic.Therefore, a further assuption was made: the networkderived using an a-c wave Will, when used in the pulser,produce a pulse similar to a section of that wave. Thisassumption has been just if ied by actual application.Two of the alternating-current waves used are shown inFigure 13(a)&(b}. The Fourier series of the trapezoidalwave contains only sine terms, since i t i s an oddfunction. The series is given by

wherek1rtsin _dtS

Co}I i is the amplitude of the wave; i ( t ) is the equation ofthe a-c wave in terms of t , 8. and the r i se time J as. Thisequation i s represented by three functions, correspondingto the three regions, r i se , top, and fa l l . i ( t ) i s then

41


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." j

- ..............--.....-....;,.t,.,-+-- - - - - - f - - - - - - - . . A - - - ~ t\\\\'. .,(a.,), Trape;oldilla-o WB.ve o

.., ..

IIL - l, Ir I. . ._ . . . . . . Q.S, I

- - + - - ......_.--..I..-_......... I-- -.a.__ {;J, ...-- ----illflI, ,\\

." ~

v.',

\ .(b) A ~ .wave with f la t top 'and

: P a r a b o ~ ; o .rise ,and fa. l l . ,.

., ....

Flgo 1342


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defined by

o - tasa :S t:o:: S -a b (7 )

SUbsti tu ting the above in the equation for bk and performing the integration gives the resul t

sin k -n- ak1l"'a (k = 1 , 3, 5, ee) (8)

Each term in the series is a sine wave of frequency k/2 S ,and of amplitude bk as defined above. Such a current, i k ,can be produced by a series Lk-Ok circui t , excited by ad-c voltage# Vo - The characteristic impedance of thiscircu it is

z =.:!.9.o I (9)

and the values of Lk and Ok are determined by comparisonwith the Fourier ser ies coef fi ci en ts , giving the recursionformulae

Zo b(10)-k1l' bk

Ok bkb= k1l' Zo

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Since each term is generated by such a circui tdescribed above, a pulse of n Fourier components canbe obtained by adding n Lk-Ck circui ts in paral le l .The resulting network i s of the type shown in Figure14(a), for n =5.

Such a network is not well suited to practical89plication, but is very valuable from the mathematicalpoint of view, for it serves as a start ing point indetermining equivalent networks. In other words, theimpedance function necessary to produce the desiredpulse i s now known, and i t is therefore possible tosynthesize other networks which wil l produce the samepulse shape.

The f i r s t equivalent form i s found by applyingFoster 's reactance theorem, Guillemin (3). The math-ematical procedure i s tedious and involved. In brief , i t cons is ts of determining a l l the roots and poles of the func-t ion Z(s) of the pEevious network, and then expanding thefunction in par t ia l fractions about i t s poles. The result ing expression is a Laurent series of n terms, involving the operator s . As before, the various termsmay be identified as the Laplace-transform impedancefunctions of reactive circui ts , and the network of Figure14(b) resul ts .

Cauer's extension of Foster 's reactance theorem

44


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"

__T eg

, .

. '

.2nd Foster Form

o I C ~ i .I c r ~ l t . _ ' _......._c_B_.....'............. c/o(0) 1st Oauer Form"

"

(0) 2ndCauer Form


54/64

yields two additional canonic forms, Guillemin (3). Inthis case, the procedure is to expand Z(s}, or yes), asa continued fraction, and then identifying the terms ob-tained with network elements. The two Cauer networks arei l lustrated in Figure 14(c)&(d) .

By various mathematical operations on the impedanceand admittance functions, a large number of otherequivalent networks m be determined. St i l l othersmay be found by combining the four canonic forms ofFigure 14 in various ways. Of importance is the networkhaving equal capacitances per section, since this resul tsin much greater ease of manufacture. Start ing with thef i r s t Cauer form, in which the capacitances are closeto being equal, the network of Figure 15(a} may befound. When the values of capacitance a re altered, aseries inductance must be inserted in the shunt armsto compensate for the change, and th is inductance isnegative when the value of the capacitance is increased.Figure 15(b) is called the "type E" network, and is theequivalent of the one above, being obtained by theproper use of mutual inductance to give the effect ofnegative inductanc e. The type "Eff network is the onemost extensively used in practice.4. Pract ical Considerations.

I t should be noted that a l l the networks shown inFigures 14 and 15 consist of five sections. The resultsof experiments show that the improvement in pulse shape

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~ l ~ ~ -,Ie .. ~ , e: I---,--c1c Je0011= ' ....... _- .__ -

(a) PFN having equal .capacltancee. . , t,r' ., "

l , - L,Z

c c c

(b) Type "E" Network"

.F1g_ 1547'


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as the number of sections are increased is sl ight formore than five secti ons. The network used in theseexperiments was derived on the basis of a trapezoidalwave shape, with a fractional rise time of 8%, i . e . ,as: : : .08 S Use of only four sections resulted in anappreciably poorer pulse shape, and i t was concludedtba. t. five secti ons was optimum. In applicati ons wherea steeper wave front is essential , however, a largernumber of sections are necessary.

In the type "En network the value s of the inductanc-es in the three center sections are very nearly equaloAlso the values of the various mutual inductances areclosely the same. For ease in manufacture i t would bedesirable to make them precisely equal, and this canbe done without appreciably altering the pulse shape,by the following means. A continuous solenoid is woundhaving a to ta l inductance of, Glasoe (2):

T__ ::: l.-Z S- l 2 0The storage capacity, ~ is equally divided, and eachcondenser is tapped to the s o l e n o i d ~ These taps are solocated that the center inductances are a l l equal, whilethe end inductances are from 20% to 30% greater . The Ridrat io of the coil is such as to give a mutual inductancebetween adjacent sections of a value equal to 15% ofthe self-inductance of a center section.

48


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'.. - "THE LOAD

1. The Pulse TransformerSinc:e the pulse is generated in a PFN' having, almost

invariably, an impedance level of 50 ohms, and since thestat ic impedance of the magnetron during oscil lat ion isin the vicini ty of 1000 o ~ ~ s , some impedance matchingdevice is necessary. The pulse transformer nicely performsthis function, at the same time stepping up the pulsevoltage and thereby making possi ble the use of a lowervoltage network.

Although i t is possible to conceive of an idealtransformer, such as shown in Figure 16(a), i t i s notpossible in practice to construct one that will passundistorted a ll the frequency components in a squarepulse. In Figure l6(b).the equivalent circui t of apractical pulse transformer i s given, together withan explanation of the symbols used. Since the shuntinductance, Le , consti tutes a load on the source, gooddesign procedure dictates that it be made as large aspossible. At the same time, the l e ~ ~ a g e inductancemoo t be minimized, and as Le is increased, this becomesincreasingly diff icul t to accomplish. As a resul t ,pulse transformers differ from more conventional typesin the method of construe t iod. Primary and secondarywindings are wound as close together as possible on thesame core. The core i t sel f is fabricated of very thinlaminations of special n ~ t e r i a l s o In order to r ~ r o d u c e

50


59/64

v 's ,

(a) Ideai TransformerIn ' , R (n ')2. 't"l . L sv--- --",-. - - -.1._ - J Z .,ns 0 ~ p ..

-.l----,R .L .

l

o.0II

.f

(b) Equivalent 'oircuitof a pulse tra.nsformer. .. 'I '. Li= lealw.ge inductance -due to flux 'from primary' ourrent, vibich f a ~ l s to l:1.nk ~ th secondary .'

O ~ ' ' ; . effec t of electrostat ic energy stored in tbe primary- 'secondary dIstr ibuted capacIty ., .,

"squirted lndu,ctance" a.rising from non-unif.orm currentd i s t ~ 1 b u t l o n ' d u e t h e c ~ a r g i n g ef 0Def fc to fe lec t ros ta t1c ,energy stored.between pr1maryand core. '

L0': effect of magnetic energy' stored in "squirt;ed flux"which comes from non-uniform current in p,rirnary co i la-r1s1ng from the chargluf, of Co .

. .Le ef fec t tva shunt or E}elf-1nduqtance.Re resIsta,n'ce due to eddy ourrent in t ron a,nd to hysteres isI'

Fig. 1651

4


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a rapid-rising pulse. these special materials mustpossess a very high permeability a t frequencies rangingfrom a few cycles up to several megacycles. The effectof the series resonant circui ts . Lc-C c and Ln-CD isto introduce oscillations along the top of the pulse.and possibly after the pulse i t se l f has passed as welloSiuce even relat ively small variat ions at the magnetroninput can callS e faulty operation, the se top-of-the-pulseoscillations must be minimized by careful transformerdesign. Small oscilIa ti ons subsequent to the main pulsemight cause the magnetron to oscil late weakly. Evenvery low r - f output would block the sensi tive receiversused in radar applications, thereby swamping out echosreturning from nearby objects. For this reason a dampingresistor or diode is sometimes placed across the pulsetransformer secondaryo

Proper impedance matching is vi ta l , for i f the PFNdoes not see a load equal to i t s own characteristicimpedance, a charge, either positive or negative, mayremain on the network af ter the switch opens. (SeeFigure 11(b ).( c)) 0 This effect can be cumulative overseveral charging cycles, and eventually break down theswitch at an undesired time, or cause other faultyoperation.

I t has been experimentally determined that in apulse transformer having an efficiency >'It' and a turns

52(.-.. t


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ratio n, the following relations wil l hold:

I '::: '1ts n

that i s , the voltage transformation is conserved, andthe losses appear as shunt losses a ffec ting the currentand impedance ra t ios . Pulse transformer efficienciesrun from 75% to 90%0

In general, it may be stated that the pulse transformer must be especially designed for each application.I t s function is to match source and load impedance, andat the same time preserve the pulse shape.

Vlhen used in l ine-type modulators, pulse transformers are commonly wound with bif i lar secondaries.I t is formed by laying two insulated wires side byside, so that the sarne secondary voltage wil l be induced in each one. The magnetron heater curreht issupplied from a filament transformer through the bif i larwindings; in this way no high-voltage insulation is required in the filament transformer.

J

20 The M'a.gnetronAlthough the osci l lator is not a part of the pulser

proper, certain inherent characteristics of the magnetronaffect the design of the pulser circui ts preceding i t .

53JE_ , @


62/64

The magnetron is a non-linear load; i t has a fa ir lyhigh static impedance and a low dynamic impedance. Thestat ic impedance determines the power drawn from thepulser, and is given by the ratio V m / ~ at the magnetronoperating point . The dynamic impedance AVm/AIm is theslope of the Vm-Im curve a t this point. Since thisrat io is small, a small voltage drop may decrease thecurrent to such an extent t ha t o sc il la tion s cease. Forthis reason i t i s important that the modulator furnisha pulse with low top variation (Figure 1 ) .

The r ise time of the mpplied voltage pulse is ofOJ nssiderable importance. I f the rate of ris e i s toofast , the oscillations in the magnetron may f a i l tobuild up a t a l l . I f the r ise rate is too slow, theoscil lat ions may build up in an undesired mode, or else.the phenomenon of "mode-skipping" may occur. In thisla t te r case the magnetron oscil lat ions wil l jumperrat ical ly from one mode to another. Mode-shifting,wherein the operating mode changes during the pulse,may also occur, but l i t t l e can be done in pulser designto correct for th is .

Magne tron "sparking" is anothe r aspect to considerin pulser des ign. All magnetrons will spark occasionally,particularly when new. This i s a gaseous discharge insidethe magnetron, and is usually a resul t of occiliuded gasesinside the metal being released by heet. Sparkingmagnetrons present virtually a short circui t to the

54


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modulator, and care must be taken in the design, bythe use of p ro tective devices i f necessary, to insureagainst failure of the circui t .

Magnetron plate current variation, in additionto distortion of the r - f output pulse and possiblecessation of oscillations, causes frequency modulationof the o u t p u ~ . A small change in anode current canquite possib ly change the frequency of the r - f outputto such an extent that i t will be largely outside thebandwidth of the receiver, and hence a loss of receiveds igna l s treng th wil l result . This i s another reasonwhy the pulser must supply a fair ly flat-topped pulse.

Finally, the allowable plate dissipation of themagnetron must be considered. This wil l determinethe maximum duty ratio of the pulser, hence themaximum PRF for a desired pulse width. In general,duty ratios in excess of .0025 are not used in pUlsedradar, and for output powers in excess of 100 KVf, themaximum d U ratio is usually considered to be .001.

550'


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BIBLIOGRAPHY

1 . ' Fundingsland, O.T., and Anna Walter. Analysis ofl ine modulator behavior with a sparking magnetronload. Radiation laboratory report no. 765,August 10, 1945.

2. Glasoe, G.N., and J.V. Lebacqz. Pulse generators.New York, McGraw-Hill, 1948. (MassachusettsInst i tute of Technology. Radiation Laboratoryseries. No.5) .3. Guil1emin, E.A. Communication networks, vol. I I .

New York, Wiley, 1935.4. Kru1ikoski, S.J . Technical data and opera ting notesfor the 5C22 hydrogen thyratron. Radiationlaboratory report no. 828, November 14, 1945 05. Millman, J., and S. Seely, Electronics. New YorkMcGraw-Hill, 1941.6 0 Ridenour, Louis N. Radar system engineeringo NewYork, McGraw-Hill, 1947. (Massachusetts Inst i tu teof Technology. Radiation laboratory series.No.l) .7. Staff, MIT Radar School. Principles of radarNew York, McGraw-Hill, 1946.

line type modulator

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