C. B. HOUSEASSOCIATE MEMBER AlEE

Flux Preset High-Speed MagneticAmplifiers

deviations from those indicated by Fig . 11for the doubler circuit.

References

1. A N INSTABILITY OF S EL F -SAT U R AT I N G M AG­NETIC AMPLIFIERS USING RECTANGULAR Loop COREMATERIALS. S . B. Batdorf, W . N . Johnson. AlEE

EARLY in 1951, R. A. Ramey pre­pared a paper! on magnetic ampli­

fiers. In this and succeeding reports-?on the same sub ject the theory and ap­plication of a new type of half-cycle re­sponse magnetic amplifier were developed.This amplifier depended upon square hys­teresis loop magnetic materials for itsproper operation, and the analysis in turnpostulated squa re-loop materials.

Further development in the Naval Re­search Laboratory has resulted in thecircuit being presented here. This circuitextends the benefits of the half-cycle reosponse to circuits utilizing magnetic ma­terials not possessing the square-loopcharacterist ics, i.e ., the ratio of remanentflux to saturation flux may possess anyvalue between 1.0 and 0.0. The circuittherefore, removes the restriction uponmagnetic materials heretofore existing,and broadens the range of application ofthe half-cycle response circuits to includesuch materials as grain-oriented siliconsteels and molybdenum-nickel-iron al ­loys, which have previously been deemedunsuitable. As an important corollarybenefit. the circuit will overcome the del­eterious effects suffered in the half-cycleresponse circuits with square-loop ma­terials when using rectifiers which do notpossess perfect rectifying characteristics.

Background

In reference 1, Ramey discussed in de­tail the theory of operation and circuitdetails of an amplifier which inherentlypossesses high response speeds (essentiallyone half-cycle of the power supply volt­age) and still permits large power gains.The complete development of that theoryis not within the province of this paperbut a brief resume will be given here to

Transactions, vol. 72 , pt. I. July 1953 , pp, 223­28 .

2. DYNAMIC HYST ERESIS Loops OF S EVERAL COREMATERIALS EMPLOYED IN MAGNETIC AMPLIFIERS,Harold W . Lord . AlEE Transactions , vol. 72,pt. I, March 1953 , pp. 85-88.

3 . A RECORDING FLUXMETER OF HIGU ACCURACYAND SENSITIVITY, P. P. Cioffi. Review of ScientificInstruments , New Y ork, N. Y ., vol. 21, no . 7,1950 , p p . 624-28.

provide a basis from which the reader mayfollow the further developments.

The basic half-wave form of the Rameyamplifier is shown in Fig. 1. The conven­tions used in this diagram are as given inthe following:

The polarities shown for the voltage sourceseacand eae' are instantaneous values .The polarities of the windings are as indi­cated.The rectangular symbol between the wind­ings indicates that the core is composed of amagnetic material with a rectangularhysteresis loop, as shown in idealized formin Fig. 2.The turns ratio may be any number desired,as long as the voltages eae and eae' possessthe same ratio in the same order.The voltage ee is an unidirectional signalvoltage .

In the first section of the analysis itwill be assumed that ee is zero and thatthe control terminals are short-circuitedtogether. The action of the circuit willthen be followed until a steady-state con­dition results.

In beginning a step-by-step analysis,first it may be assumed that at a timewhen the applied voltage wave eae ,eae' isgoing through zero (Fig. 3, point 1) thecore will possess a remanent flux of a po­larity and value determined by the pre­vious history of applied voltages. Tosimplify discussion it is further assumedthat the core had been previously satu-

Paper 53-337, re commended by the AlEE Mag­netic Amplifiers Committee and approved by theAlEE Committee on Technical Operations forpresenta t ion at the AlEE Pacific General Meeting,Vancouver, B . C., Canada, September 1-4, 1953.Manuscript submitted June I , 1953 ; made avail­able for printing July 9 , 1953 . ·

C. B . HOUSE is with the Naval Research Labora­tory, Washington, D . C .

The help and guidance of D . G . Scorgie in thedevelopment of this material is gr atefully acknowl­edged .

4. TUE TR ANSDUCTOR, Harold B. Rex. Ins/ru­ments, Pittsburgh, Pa. , vol. 20, 1947 , pp. 1102-09.

5. THEORY OF MAGNETIC AMPLIFIBRS WIT IISQUARE-Loop CORE MATERIALS, H. F. Storm.AIEE Transactions, vol. 72 , pt. I, Nov. 1953 ,pp .629-40.

- - - - +- - - -

No Discussion

rated in the plus direction and at time ofzero applied voltage the flux level is re­setting at point 1 in Fig. 2. (Furtherjustification of this assumption is con­tained in reference 1.)

As eaeand eae' increase with the polar­ities shown in Fig. 1, the current in theleft-hand or control circuit will produce amagnetomotive force equal to one-halfthe hysteresis loop width. The appliedvoltage will then react upon the core andchange its state (or demagnetize it) . Theamount of change will be a function of theapplied voltage and in usual design, 1/2cycle of applied eae' will carry the fluxlevel to just below saturation in the nega­tive direction (Fig. 2, point 2) . Duringthe period just discussed, the appliedvoltage eac on the right-hand, or load side,will be directed against the rectifier, andno current will flow on that side. Thishalf-cycle is commonly termed the resetcycle.

The next half-cycle (which will be calledthe gating half-cycle) starts at point 2of Fig. 3. At this time, the voltagepolarities, shown in Fig. I, will reverse,so that the voltage applied to the controlside will be blocked by the rectifier. How­ever, on the load side , magnetizing cur­rent will start flowing and the appliedvoltage will now start magnetizing thecore in the plus direction. With the as­sumed values of voltage, the flux will becarried just to the knee in the positivedirection by the end of this half-cycle ofapplied voltage and thus return to point 1of Fig. 2. A steady-sta te condition hasnow been reached and this sequence ofoperation will continue as long as voltageis applied. Only magnetizing current willflow through the load, and the amplifierwill be in the state of minimum outputwith the flux swinging from knee to knee

CONTROL GATINGCIRCUIT CIRCUIT

ec~0r----?,LOAD

T~~ L~

Fig. 1. Basic half-wave form of half-cycleresponse magnetic amplifier

i28 House-Flux Preset High-Speed Magnetic Amplifiers JANUARY 1954

As a specific remedy for some of theills encountered in the Ramey high-speedresponse magnetic amplifier when usingsquare-loop materials and to extend thebenefits of that circuit to amplifiers usingthe nonsquare-loop material, the circuitshown in basic and general half-waveform in Fig . 6 was devised.

The notation in Fig. 6 is as follows:

Circuit Theory

The polarities indicated for Cae. cae', eae"(the alt ernat ing supply voltages) are in-

during the reset half-cycle will allow acurrent to flow through the gating wind­ing in such a direction as partially to resetthe core, giving the same detrimental re­sults at full output as low remanence.Again this result becomes most evidentat the full output end of the curve since atlow outputs the voltage in the controlcircuit is transformed into the load circuitin such a direction as to reduce the backvoltage across the load rectifier and inturn reduce the back leakage. This re­sult is also shown in Fig. 5. Of course, ifthe maximum leakage current is not aslarge as the magnetizing current require­ments in a square-loop material, then thisreset theoretically will not occur, accord­ing to this analysis.

However, during dynamic operationthere are other reverse current phenom­ena in rectifiers that occur in addition tothe rectifier leakage as measured usingreverse d-e voltage. Conrath of Vickers,Inc., has suggested that there is a capaci­tive component of reverse current. Intests on Vickers selenium rectifiers, he es­tablished at 60 cycles an equivalent capac­itance value of 0.02 microfarad persquare centimeter of rectifying area. Ob­servations at this laboratory suggest thatthese equivalent capacitance values mayvary with manufacturers of seleniumrectifiers.

Since leakage and capacity effectsshould increase with rectifier plate areaand frequency (as qualitatively checkedin experiments) it becomes evident thatcircuits which operate satisfactorily forsmall maximum load currents at 60 cyclesmay not give designed full output whenoperated at 400 cycles with large currentcapacity rectifiers.

At point 3, Fig . 3, when the reset cyclestarts, the core will again be at point 1,Fig . 2, and the cycle will repeat as before.The wave shape of the current throughthe load will be shown in Fig . 4.

Thus a magnetic amplifier has been de­scribed whose output power can respondto input signals within a half-cycle of thesupply voltage. The time in the gatingcycle at which the core saturates (or thefiring angle) will be determined only bythe net voltage time integral applied tothe core on the reset half-cycle. If..fr/ (eae' -ee)dt = 0, or a negative number,the core will not reset at all, and full out­put should be realized. It readily followsthat the control element need not be avoltage, but that any device which con­trols the net voltage applied to the core onthe reset half-cycle may be used."

The foregoing discussion has assumedperfect square-loop core materials andperfect rectifiers. However, complica­tions arise when magnetic materialswhich do not have a perfectly square hys­teresis loop, and rectifiers which haveback leakage, are used. L. J . Johnson hasdiscussed these effects. 3 In his attackupon the problem he established a theo­retically ideal transfer characteristic curveand compared this ideal curve with theactual curves obtained when using dif­ferent core materials in otherwise similarmagnetic amplifier circuits. In his dis­cussion of the suitability of the differentmagnetic materials, he made the compari­sons of curve shape and cited possiblecauses of the deviations from the idealcurve. Among the causes were the ef­fects of low remanence and rectifier backleakage.

Low remanence permits the core to re­set partially without regard to the controlvoltage. This uncontrolled reset will re­sult in the core-absorbing part of the volt­age integral during the gating half-cycle,and consequently the full half-cycle ofthe power voltage cannot be applied to theload when the control voltage dictatesthat it should. This effect will naturallybecome most prominent at the maximumoutput end of the transfer characteristiccurve, and it manifests itself as a reduc­tion in maximum load current available,as shown in Fig. 5.

Rectifier leakage in the load circuit

Fig. 2. Ideal­ized hysteresiscurve of a square­loop magnetic

material-

IV~

~t I

r-J.MMF

Fig. 3. Voltage wave of supply to magneticamplifier

2

with no current greater than magnetizingcurrent flowing.

Referring again to Fig . 1, it may be de­duced that, during the half-cycle wheneae'is applied through the control side, thetotal voltage applied to the core is thedifference between eae' and ee' If ee isequal to zero, as has been previously as ­sumed, the operation will be as explained.When e, equals some finite value less thaneae' the total voltage applied to the coreon reset will be less than the voltage ap­plied to the core in the previous half­cycle, and a new set of conditions willarise.

Under the new conditions the startingpoint for the analysis may remain thesame; i.e., the applied voltage wave hasgone through zero and the core flux is atposition 1, on the hysteresis curve, Fig. 2,and the polarity of the applied voltagewill become as shown in Fig. 1 in the nextinstant. The reset cycle is just begin­ning.

Since the time integral of the voltageapplied to the core during the reset half­cycle will be less than the time integral ofeae' the flux level will not be carried topoint 2 on the hysteresis curve but will bechanged only to some point part waydown the loop (point 3) as determined bythe net voltage time integral.

On the gating half-cycle the core will beable to absorb a voltage integral equalonly to the net voltage integral appliedon the reset cycle before it saturates andits impedance drops to zero . The loadwill then absorb the remaining voltage inthe half-cycle, with current limited onlyby its impedance.

.../

/I

~-----';==---+------'c==--7f

Fig. 4 (left). Current in load ofhalf-wave magnetic amplifier

Fig. 5 (right). Transfer charac­teristics of half-cycle responsemagnetic amplifier using rectified

alternating voltage control

POWEROUT

~-- IDEAL CHARACTERISTICS,.,---- CHARACTERISTICS OfAN AMPLIfiER

WITH RECTIfiER LEAKACE

CHARACTERISTICS Of A MATERIALWITH LOW REMANENCE

CONTROL VOLTAGE ec

JANUARY 1954 House-Flux Preset High-Speed Magnetic Amplifiers 729

(2)

(3)

(4)

( 1)

During thi s stage the core will be partlydemagnetized. When e." = eae" the recti­fier in th e presetting circuit will unblockand st age three begins.

During th e third stage current flors inthe pre setting circuit and the load circuitas determined by the equations

Fig. 7 . Hysteresiscurve for intermedi­afe remanence mag-

ne tic material

where N, N ' , and Nil are the turns in thepower winding, reset winding and presetwinding respectively, and i m is the totalmagnetizing current referred to the loadcircuit coil. As this stage progresses, thecore will continue to demagnetize withthe magnetizing current shifting to thepresetting circuit until the supply volt ­ages have reversed polarity and eac rise toa value equal to ei. By this time, themagnetizing current has transferred to thepresetting circuit, so current cea ses toflow in the load circuit and that circuitbecomes inactive. Stage 3 now ends and

volt ages et, e,' and e/' at th e terminalsof the win dings on the core. These volt­ages will have positive polarities at theundotted ends of the windings. Thus thegenerate d voltages will be directed againstthe rectifier in the resetting circuit andwith the rectifier in the presetting circuit.However, as lon g as le/'I<Ieae" I the netvoltage in th e presetting circuit will beagain st the rect ifier . Since the net volt­ages in both control circuits are againsttheir respective rectifiers , the only activeelement is th e load circuit. The volt ageequation for the active element duringstage tw o will be

Fig. 6 ( le ft). Half -cycle responsemagnetic ampli fier using bo th re­setting and prese tting con trol

circuits

the most general case.In discussing the opera t ion , a specific

case using a core which possesses mag­netic hysteresis characteristics inter­mediate between square-loop materialand material possessing no remanentmagnetism (see Fig. 7) will be assumed .Reset will be resistance controlled (F ig.8A). The polarities of th e supply volt­ages ear, eae' and eae" are shown as theywill be during the ga t ing half-cycle. Inthe discussion, the Ramey control circuitwill be called the reset circuit and thecontrol circuit added will be called thepresetting circuit.

In the step-by -step an alysis of whatoccurs in this circuit, it mi ght assist theunderstanding of the operation if it isfirst emphasized th at the action may bedivided into five distinct stages of opera­tion . The first stage will be postulatedas the firing stage, and the next twostages merely cover the short tran sitionperiod into th e fourth stage where theimp ortant circuit reactions occur. Forunderstanding th e circuit, stages two andthree could be ignored if certain minoran omalies in sta te between stages 1 and 4were accepted . Stage 5 covers the pre­firing period in the gat ing half-cycle .For continuity the different modes will bediscussed in their order of occurrencerather than their order of importance.

In stage 1 the core has saturated andcurrent is limited on ly by the load im ­pedance. No change of the sta te of thecore is involved. As the instant aneousvoltage decreases in the ga t ing half-cycle,the current in the load circuit decreasesuntil it reaches a value equal to thesa turat ion ma gnetizing current, point 1,Fig. 7.

Stage 2 then begin s. As instantaneousa-c voltage continues to decrease there is achange of flux in the core which genera tes

GATING CIRCUIT~

D''''eoc

CONTROL CIRCUITA

stantancous polarities. These voltagespossess th e same relation to each other asthe turn s on their respective windings.The cont rol voltages, e.', and ec " , may bedirect current , rectified alterna t ing current,resistance drop , or any other proposedmethod of amplifier control.The polariti es of the windings are as indi­cate d.

It is apparent th at thi s circuit, with theexception of th e element contained withinth e dashed enclosure, is the circuit shownin Fig. I. The power delivered to theload on the ga t ing half-cycle is deter­mined by th e amount of reset dictated byth e contro l circuit on the reset half­cycle .

I t is also ev ide nt th at th e elementwithin th e dashed encl osur e is similar toth e Ramey contro l circuit in ap plied volt­age polarity and rectifier polarity , so thatit may assume functi ons of a control cir­cuit. It differs, however , in th at its wind­ing polarity is opp osite to th at of th eRam ey control circuit, i.e., currentthrough one control circuit changes fluxlevel in th e core in a directi on opposite toth at produ ced by current flow in theother circuit.

I t will become evident later that in mostcircuits there mu st be a resistance in serieswith hath control sources. The excep­tions are th ose cases where a fixed mini­mum or maximum ou tput might he ac­ceptable or where the ratios of rem anentto maximum flux is either 1.0 or 0.0. Theva lues of th e control source resist ance willbe influenced by the type of core, therecti fiers, a nd the typ e of control used.The circuit is drawn as shown to present

R'~cL_, deoc

1- - -

0--------'

I • II R" II C II _ + II II II eo l' II I

A

Fig. 8. Resistance con ­trolled half-c ycl e-respo nse

magnet ic ampli fie,r

A (left). During gating half­cycle

B (right). During co ntrolhalf-cycle

Hou se-s-Flux Preset High-Speed Magnetic Amplifiers JANUARY 1954

LOAD

•

R'c

abo ve zero flux level could use either orboth control circuits depending upon thedesired range of outpu t and the corecharacteristics.

If both circuits are used , the presettingcircuit can be adjusted to give maximumoutput power with no signal furnished,and then signa l voltage in the resettingcircuit can be supplied to change the out­put power. Or the resetting circuit can beadjusted to give minimum output with nosignal, and th e signal voltage in the pre ­setting circuit will increase output power .If desired, control can be inserted in bothcircuits. Control of the amplifier may beby an y of the commonl y used methodswith the restriction that under most cir­cumstances th e signa l source must ha veeither an ext ernal or internal series re­sistance.

Another source of benefits arising fromthe use of the circuit presented here is th ecompensation for rectifier back leakagewhen using square-loop material.

In man y circuits with small magnetiz­ing current requirements the back leak agethrough the power rectifier causes unc on­trolled reset when no reset is desired. Thiscondition is aggravated when larger poweroutputs are desired, as previously dis­cussed. This uncontrolled reset is quitesimilar to the effect observed when thecore material has a remanent flux lessthan maximum flux.

Fi g. 9 shows the magnetic amplifierduring the reset half-cycle . The d-e backleakage of the rectifier is portrayed as theresistor and th e a-c dynamic leakage isportrayed as the capacitor in parallel withthe power rectifier .

If it is assumed that the material issquare-loop, and full out put is desired,the first requ irement is that the signalvolt age, ee', be equ al and opposite to ap­plied ea-c ' (if voltage control of reset isused).

Indicated pola rit ies of the windings andapplied a-c show th at, if back leakagecurrent of a greater value than the mag­netizing current tries to flow in the loadcircuit, reset will occur which will not beunder control of the re set winding.

But in the pre setting circuit the gener­ated voltage, e-",will be with the rectifier,and current can flow in the circ uit , mini,mizing the amount of reset. In addition

(9)

(1 0)

(11 )

ec'Rc"Rc"+R/ ( 12)

Fig. 9 ( le ft). Ma gne tic ampl ifierwith leaking power rectifier

LOAD Fig. 10 (right). Single-windin ghalf-wave magne tic amplifier in­corpo rating both preset and reset

circuit functio ns

N"ie" - N 'ic' = Ni.;

eae' - ec'- e,' - it'R/ =0

This equation is simila r in form to thatpreviously given, except that there arenow four parameters which will influencethe final st a te of the core at the end of thecontrol half-cycle. This is now a magneticamplifier whose condit ion at the beginningof the ga ting half-cycle has been com­pletely predetermined in the previoushalf-cycle of applied voltage, and thi s con­trol may take place independently of thecore material used .

If the material is perfectly squa re-loop,the preset circuit may be open-circuited,and the reset will be controlled by the re­sistance (or voltage) in the reset circuit.If the material has zero remanence, the re ­set circuit may be omitted and the presetcontrolled by the re sistance or voltage inthe preset circuit. A material which has aremanence below maximum flux level and

zero, a transition period will be enteredwhich can be analyzed in a manner similarto that covered in stages 2 and 3 men­tioned previously .

During this transition interval , ma g­netizing current will be gradually trans­ferred to the load circuit. Howe ver, thisperiod is so short, and the net appliedvoltage in tegral is so small, that there willbe very little change of core state. At theend of this t ransiti on peri od the load cir­cuit voltage will be acting upon the core,and the flux will start toward positivesaturation from a point on the hysteresiscurve essentially established during stage4. When the integral of the volt age ap ­plied during the fifth stage is large enough ,the core will saturate and dr op out of thecircuit. This returns the core to stage l.For the more general case of resistanceand/ or voltage control, as shown in Fi g.6, the circuit equations developed in stage4 will become

So with 1: I : 1 turns ratio and ea-c= eae' =

eac"

(8 )

(5)

(6)

(7)

Rc" - Rc' . Rc"Rc'e/=ea- eRc" +Rc'+ZmRc"+Rc'

eae' - e,' - ic' Rc'= 0

It is in thi s stage that the state of th e coreat the end of the control cycle is deter­mined by the relative va lues of the re­sistances . If the presetting circuit is open(Re" is very large), e/= eac+imRc', and theresetting circuit will gain control to resetthe core . This implies that e, will notchange sign during the control cycle. Ifthe resetting circuit is open , e/= - eac+imR: ; and e, will go through zero andchange sign, as eae incre ases. Or , the corewill go towards saturation. At the timewhen e, goes through zero, the core statewill have regressed down the hysteresiscur ve to point 3, Fig. 7, after which timethe core will proceed towards sa turationto point 1, Fig. 7. For least ave rage cur­rent flow in the control circuit a t full out­put, Re" sho uld be chosen so that point 1,Fig. 7 is reached just at the end of thecontrol half-cycle. Between th ese limitsfor Re' and Rc" the direction and amountof core flux change will be dependentupon the relative values of the control re­sistances.

Near the end of the control half ­cycle,when the applied voltage approaches

From equa tions 5, 6, and 7, if 1: 1: 1turns ra tio exists and eae= eae'= eae"

th e true resetting-presetting period willbegin . At this time the core will be in acondition indicated by point 2, Fig. 7.

During stage 4, the polarities of theapplied voltages will be as shown in Fig.S(B). At the beginning of st age 4, et' =eae', and eae' will continue to increasein ab solute value .faster than e,' so therectifi er in the reset ting circuit unblocksand starts to take part in the control ofthe core. Current is now flowing in bothcontrol circuits and the circuit equationsbecome

I

e'~cW~

ericr------------I

I ~" e'I " c II ec II L-.;- I: e" IL.. ~ J

J ANUARY 1954 House-Flux Preset High-Speed Magnetic Amplifiers 731

Fig. 13. Magnetic amplifier circuit used for obtainingtransfer characteristics

~e~T=-~_,J

eoci---~---------'

I R" • II e" - c II C f+..L II ~- II e" II OC:-------------

Fig. 11 (left). Full­wave magnetic am­

plifier

A . Using squere­loop magnetic me­

terielsB. Using zero-reme­nence mdgnetic me­

terlels

LOAD

LOAD

0 • + -

e~c

0eac

• •A

I+

e'~

e~ +

B

MMF

x3u,

..Ju,

MMF

A B

As the control voltage, ee", increases, itwill force the core to reset an amountequal to the voltage integral of the signal ,until at full signal the output currentwould be equal to the magnetizing cur­rent.

A more dramatic illustration of the re­quired difference in approach is shown inFigs. l2(A) and l2(B) for full-wave opera­tion with doc control. Although the cir­cuits appear similar, examination showsthat Fig. l2(A) will not operate becauseboth cores are being acted upon simul­taneously; and with element polaritiesas shown, there would be no alternation ofreset between the two . However, withthe circuit in Fig . l2(B) , the flux col­lapse in one core during the time that coreis being preset generates a voltage equalee" (which voltage is blocked by therectifier in the other core circuit) . Thusfull wave operation may be obtained withthe zero remanence material with the cir­cuit in Fig. l2(B), whereas a differentcircuit must be used for the square-loopmaterial.

Reduction to Practice

As a check of the operation of this cir­cuit, two toroidal cores of different hys­teresis characteristics were wound andsuccessively placed in the circuit shownin Fig. 13. Transfer characteristic curveswere then obtained for these half-waveamplifiers.

Fig. 14. Hysteresis curve

A . For core no. 1B. For core no . 2

operation ; i.e., if the material is one withzero remanence, the resetting circuitwould be omitted and the presettingpolarity would be in such a direction as tobuck the voltage generated by the fluxcollapse. If the material is square-loopwith rectifier leakage, the polarity of thevoltage ec" would be reversed to provide apositive presetting of the core to thesaturated condition.

The resetting-presetting circuits pre­sented so far have been in the half-waveform . They may also be used in full­wave form with the stipulation that sometypes of control may require changes inthe circuit configuration because of thepossibility of disturbing voltages from oneelement acting upon the other element.It is emphasized that among other factorsthe approach to design of full wave cir­cuits depends upon whether the materialhas square-loop or zero remanence charac­teristics. For example, the circuit givenin Fig. Il(A) for square-loop materialwith doc control appears similar to thatgiven in Fig . 11(B) for zero remanencematerial, but closer examination showsdifferent functions for the various volt­ages .

In Fig . Il(A) , eao' will reset the core inabsence of control voltage, ec', giving zerooutput. However, in Fig. Il(B), eae"has only the purpose of supplying a slightboost voltage to overcome I R losses in thepresetting circuit. If zero resistance inthe control circuit is assumed, eae" shouldbe zero.

In the absence of voltages ee" andeat". in the control circuit, the zero rem­anence core in Fig . Il(B), which is onthe presetting cycle, will generate a volt­age causing the magnetizing current tocirculate through the control winding.The core is now acting as an inductancewith infinite time constant, and refusingto reset from the saturated condition.Maximum output current would beavailable to the load on the gating cycle.

LOADB~·· +

A

8 1 •- + +

e; • IB

Fig. 12. Full-wsve magnetic amplifier circuit

A . Using square-loop mdgnetic materialsB. Using zero remanence mdgnetic meteriels

the applied voltage, e.", can give a posi­tive current flow in this winding in such adirection as to resist demagnetization ofthe core . If some reset of the cores doesoccur during the first part of the resetcycle from the capacitive componentpulse of rectifier leakage, the current flowin the preset circuit, as forced by eac",can reset the core to the saturation pointand cause the core to be fully saturatedwhen the power cycle begins. The fullhalf-wave of voltage in the gating half­cycle will thus be made available to theload .

Although, to add clarity to the dis­cussion, the circuit heretofore has beenpresented as though three separate wind­ings were placed on the core, the circuitwill operate in the same way if only onewinding is used and the control circuitsadded to that winding as shown in Fig.10. The circuit in Fig. 10 is not strictlyanalogous to that presented in Fig. 6,since eat is not placed in the presettingcircuit, but its functions are taken over bye-", The relative polarity of the signalvoltage in the presetting circuit, e.",appears ambiguous because it will dependupon core material and desired circuit

732 House-Flux Preset High-Speed Magnetic Amplifiers JANUARY 1954

75eo

WITH POOR RECTIFIER­CONTROL VOLTAGE INPRESCTTING CIRCUITAND SUPPLYING RESETPOWER

ec (VOLTS)

2~

751-----'---- ....~d=====t

Fig. 18. Transfer characteristics of core no. 2with control voltage in presetting circuit end

supplying power

.::25>1---II---t-----t------l

The next concern was the operation ofthe circuit when the control voltage wasinserted in the presetting circuit. Thecircuit is initially adjusted by varyingvalues of Rc' and Re" so that no resetoccurs when the control voltage ec" iszero and full reset occurs when controlvoltage is equal to ea/'. With this methodfull output occurs with zero control volt­age, as is shown by transfer characteristiccurves in Fig. 17. Again it may be notedthat little difference is evident in thecurve regardless of whether the poor orthe good rectifier is used in the power cir­cuit.

As another example of the versatility ofthis circuit, it was decided to insert thecontrol voltage in the presetting circuitso as to give zero output with zero con­trol voltage. eae" was omitted in the pre­setting circuit and the reset circuit re­sistance value adjusted to give full resetwith zero control voltage. Then, as thecontrol voltage was increased, the outputincreased (see Fig. 18). These curveshave the same general slope as those inFig. 17 with reversed sign .

As may not be surprising, these im­provements have not been realized with­out some penalties. In this case it isevident, both from the equations and fromintuition, that the input current on the re­set cycle must be increased since fromequation 7 N"i"c-N'i'c=Nim and therelative values of ic' and ie" will be deter­mined by Rc' and Re" whose values inturn will depend upon core material andrectifiers used. Practically, however, theincreased input power requirement is notdetrimental in many cases . This is es­pecially true where resistance control isused. If Rc' and Rc" are caused to varyindividually, the circuit requirements aresuch that either the reset or preset circuit

50

15

WITH RESfTTlNG CIRCUITONLY-GOOD RECTIFIER

25ec (VOLTS)

WITH POOR RECTifiER - ,CONTROL VOLTAGE IN ,PRESETTING CIRCUIT '

o

V>~ 501-----\--,-------1u.JQ..

""-<~....J2:

Fig. 17. Transfer characteristics of core no. 2with control voltage in presetting circuit

75F"_---'------!

.....~ 251-------'-'\----1

Fig. 15 shows the results obtained whileusing core no. 1. It may be observed thatthe original transfer characteristic for thiscore approximated quite closely the idealcurve as shown in Fig. 5. However, whenthe poor rectifier was substituted in thepower circuit, the maximum output de­creased considerably and a curve was in­troduced at the knee. When the preset­ting circuit was added to the core , thecurve returned to its former shape andthe maximum output increased to its pre­vious value.

Figure 16 shows the results obtainedwhile using core no. 2. It is seen thatwhile using the resetting circuit only thetransfer characteristic does not approachthe ideal curve even with good rectifiers,and it is also seen that the curve is seri­ously affected by the type of rectifier.When the presetting winding is added.the transfer characteristic curve againduplicates the transfer characteristic ob­tained using a square-loop material andgood rectifiers, exclusive of the rectifiertype. This is also shown in Fig . 16.

Fig. 16. Transfercharacteristics for core no. 2

25~----,'I-----l-

75 WITH PRESETTINGCIRCUIT ADDEO ­POOR RE CTIf iER

1525

2S

75 WIT' PREEfflNGCiRCUrT .,'ADDE O- COOl) OR POOR . WITH RESETTING CIRCUITRE C TiFIE~ _ /-DNlY-GDOD RECTIfiER

/ ... --------------J..£.- WITH RES£TT1NG CIRCUIT

'I ONlY- POOR RECTIfiER

- ----.-li,'l---- - --,-- - ---- --

I r; I/~/ ~ !

I

, ,/-~- -t------I . j :'/ i~. I..-

50ec (VOLTS)

Fig.15. Transfer characteristics for core no 1

Core no. 1 was a grain-oriented silicon­iron material which has a relatively broadsquare-loop, with a published coerciveforce of 0.2 to 0.4 oersteds and a maxi­mum flux density of 14,000-15,000 gauss.Core no.2 was an alloy of 5per centmolyb­denum and 79 per cent nickel, with a rel­atively narrow hysteresis loop of 0.02 to0.04 oersteds, a maximum flux density ofapproximately 6,600 gauss and a rema­nent flux density of approximately 5,000gauss . The relative size and shape of thehysteresis loops are shown in Figs. 14(A)and 14(B).

Rectifier" were of a selenium 4-platehalf-wave type. rated at 0.25 ampere.However. during the tests involving theeffects of poor rectifiers a selenium recti­fier of the same rating from a differentmanufacturer was placed in the load cir­cuit. In this discussion "poor rectifier"means only that back leakage effects aregreater, with deleterious effects on theaction of the magnetic amplifier .

Each core was wound so as to saturateat 40 volts, 60 cycles alternating current.The circuits were arranged as half-waveamplifiers with rectified alternating volt­age control, since this type of controlgives a straight-line transfer characteristiccurve when the amplifier is operatingproperly. Thus any deviations from theideal curve may be easily noticed. Thistype of control was used in reference 3.

Tests were first run using the resettingcircuit only (presetting winding open) .Then the presetting circuit was closedwith a resistance in series of a proper valueto give full output when the net resettingvoltage in the resetting circuit was zero,and the test runs repeated. The next stepwas to take readings using the resettingcircuit only, after inserting a rectifierwhich was not as suitable for the circuit.This run was then repeated after the pre­setting winding was placed in the cir­cuits.

JANUARY 1954 House-Flux Preset High-Speed Magnetic Amplifiers 733

10't---+-- -t-''t--+- ---i

~-e~

~ 10zl---t- -+- -1\----f

10' 1---+---1f---r-L-+---"c'rl

Fig. 19 (left). Power relationships inmagnetic amplifier circuit

Fig. 20 (right) . Full-wave magneticamplifier circuit for exciter

will be effect ively open when th e othereleme nt is contro lling the amplifier.Conseq uently th e input power require­ments have not been increased. How­eve r, if it is desi red to leave one resist an cefixed, and to va ry th e resistance in theother eleme nt , power requirement in­creases will still be well within pra cticallimi ts. For outp uts of many watts theinpu t current ca n st ill be only a few mil­liam peres.

Ra ther th an discuss th e amplifier interms of power gains (which can becomea difficult subject when examining thehalf -cycle response circui t) the curves inFig. I9 prese nt the in put power and out ­put power for increasing signal voltageas ob ta ined on a 5-79-molybdenum nickelalloy core with resistance load and recti­fied alternat ing voltage control in a half ­wav e circuit. It is realized th at th ere areman y ways of presenting power gain inamplifiers, but in this discussion inputpower is presented as the ave rage currentthrough th e signal source, multiplied bythe voltage genera ted by th e signal source.Outp ut power is calcula ted as ave rageload current squared, multiplied by th eload resist an ce. Average reading meters(d-e instruments) were used for instru­menta tion. T he ordinate, it will be noted ,is a logari thmic scale . The odd shape ofthe power in put eurve resul t s from themeth od of control and the circuitry em­ployed . The circuit was so arranged th atmaximum out put was obtained at zerocontrol voltage. Under the condition formaximum outp u t th e average currentthrough th e signal source was flowing inone directi on. As the signal voltage wasincreased, th e average current wentthrough zero and reversed direction. Itthen increased in the opp osite directi onfor a whil e before it started decreasing

again to zero. No claim is m ade that cir­cuit elements used were the optimum forhigh power gain. The results are merelypresented as an example.

The circuit pre sen ted herein has pr ovedits practica bili ty when incor porated in aresistance controlled exciter using mag­netic amp lifiers with nominall y squa re­loop materi als. The circuit used is shownin Fig . 20. Originally maximum out pu tcurrent in the power circui t could not beobtained because un controlled reset oc­curred before ad dition of the preset ti ngcircuit. Maximum availa ble current inthe load circuit was increased by 70 percent with its addition. In fur therance ofth e previous contention that, although in ­put current requirements are increased ,the current will remain within pr acticallimits, it was noted in this case that whenthe available output cur rent wa s in­creased to 3.4 amperes maximum, themaximum current in the control circuitwas still less than 9 milli amperes at zerooutput . This conditio n entailed maxi­mum input current requ irements.

Conclusions

The circuit here presented is a generalcircuit for half-cycl e response time mag­netic amplifiers . It permits the use ofmagnetic materials which possess ratios ofremanent flux to maximum flux of va luesbetween 1.0 and 0.0 in half-cycle responseamp lifiers . It may be said tha t this cir­cuit pre sents a genera l class of amplifiersof which th e half-cycl e response amplifiersusing square-loop materi al prev iouslyreported up on represent a special ca se.Another specia l case would be the use ofmagnetic materials with zero remanencein half-cycle response amplifiers. Thiscircuit encomp asses both cases, and pro­vides for the use of materi als possessingintermediate characteri sti cs.

In addition, compensation for some de­fects enc ountered in the speci al case am­plifiers using practical circuit eleme ntsmay be achieve d with this circuit .Among th ese defects are those ca used bysome loss in rem an ent magnetism insquare-loop materials, or some residualmagneti sm in sa turable induc tance ma­teri als. Rela ted to these effects andsimilar in nature are those caused by recti­fier back leak age. These two effects maybe simult aneously compensated for inthis amplifier .

In general, th e input power is an in­verse fun ction of the maximum resistanceswhich may be placed in the control cir ­cuits. As a consequence, the input powerrequirements will decrease as the rerna­nent flux level approaches either zero ormaximum flux level , all other factors re­maining cons ta nt.

The foregoing factor leads to the con­clu sion th at if narrow hysteresis loop ma­terial s could be manufactured with zerorem an ent flux level, the half-cycle re­sponse circuits could be designed with amuch lower input power requirement thanth e present square-loop materials. Pre­liminary work is being conducted on asystem which will use magnetic materialswith low coercive force requirements suchas are csmmercially available. The zeroreman ence conditi on which is desirable forlow input power will be simulated by in ­trodu cing a cons tant current bias.

However , it must be reali zed th at thiscircuit is not an ab solute pan acea . Al­though it s use will enable full power out ­put in amp lifiers employing non-square­loop magnetic materials, and leak y powercircuit recti fiers, the input power require­ments may be raised .

The circuit responds best as far aspower gain is concerned when resi st ancecontrol is used but it is perfectly fea sibleto use voltage control or any of the meth-

73-1 House-Flux Preset High-Speed Magnetic Amplifiers JANUARY 1954

The Measurement of Random

Monochrome Video InterFerence

ods of control th at were previously cited .The circuit is versatile in that th e con ­

trol voltage may be inserted in either thepresetting circuit or the resetting circuit,with full ou tpu t at either zero or full con­trol voltage, as arranged by the designer'schoice of comp onents. The circuit mayeven be arranged so that different control

J. M. BARSTOWMEMBER AlEE

O N E OF the import ant fact ors in thedesign and maintenan ce of t elevi­

sion transmission circuits is random inter­ference, sometimes referred to as ra ndomnoise. Much of this type of interferenceoriginat es in the input st ages of ampli­fiers but it may also originat e in the lineor in other parts of th e transmission path.These noise com ponents are greatly modi­fied by circuit equalizati on which mayvary from a few decibels (db) to possibly27 or more db per oct ave. Thus thetra nsmission engineer is confronted withthe problem of eva lua ting random inter­ference having different energy levels, percycle, over the tel evision bandwidth .Before noise eva lua t ions ca n be made, in­form ation mu st be obt ained by subjec­tive tests on the interfering effect ofbroad , narrow, and mixed bands of ran­dom noise distributed throughout thetelevision band.

Out of the work associated with suchtest s comes a reasonable ques t ion, " Ca n­not random interference be measured insuch a way that equa l measurement s willmean approxima te ly equal interferingeffect, regardless of the frequency com­position of the noise?"

This que sti on may be reasonablydivided into tw o parts :

1. What is the weighting or relative im­portance of random interference in differentpart s of the video spectrum?

2. Does the human visual mechanism sumup the interference effects of random noisein various par ts of the video spectrum insuch a way that the over-all effect can beuniquely relat ed to an over-all measure­ment . made with equipment that is not toocomplicated?

Preliminary an swers to these que stionsare given in th e following sections of this

voltages are inserted in the two circuitsproviding the restricti ons as to source im­peda nce are followed.

References1. O N THE M ECH ANICS OF MAGN ETI C A MPLIF IE ROPERATION. Robert A. Ramey. A lEE Transac­tio ns. vo l. 70 . pt. II. 1951 . p p . 1214 - 23 .

H. N. CHRISTOPHERNONMEMBER AlEE

paper. The answe rs are called prelimi­nary because it is obv ious that t hey dependt o a large extent on th e equipment used inmaki ng th e judgment te st s, and thisequip me nt is undergoing const ant cha ngeas the television art advances. At pres­ent, it is believed that a stage has beenreached at which results of th e type in­volved will be useful over a sufficient lylong period to make publication worthwhil e.

Others have considered this problemand have deduced approximate result s byra tionalizing a relatively small number ofsubjective measurements with relatedwork in volvin g photographic reprodu c­tion processes. t-" T he test results givenhere, however , are purely experiment aland seem sufficient in scope and numbersto warrant conclusions . Although theyconfirm, in part , the genera l cha rac­teri sti cs of random noise weighting pre­viously estimated, they are indepe nde ntof any thus far pub lished.

Summary of Results

T he frequency weighting derived fromthe judgment t est s is give n in Fig. 1.This weighti ng is for use with a simp lepower -summing measuring device.

T he general principles deri ved from thetest s may be summarized as follows :

I. Low-frequency noise is judged muchmore interfering than high-frequency noiseof equal power.2. A given amount of noise power is judgedmore objectionable if it is concentrated in anarrow band than if it is spread out over awider band in the same frequency region.3. Human vision in combination with thepresent television monitors does not pre­cisely sum weighted noise powers in arrivingat an over-all assessment of th e interfering

2 . O N T H E CONT RO L OP MAGNETIC A MPL IFIERS.R . A. Ramey. I bid. , pp. 2124-28.

3 . TUE EFFECT O F C OR E MATE RIALS ON MAG­NETIC AMPLIFIER C IRCUITS, Leo J . Johnson.A lEE T ran sacti ons . vol. 71, p t . I , J an . 1952 , pp .26-31.

----+----

No Discussion

effect of random noise bands. However. areasonable compromise can be obtainedwith weighting applied to a power meter.In the region of 7 db above threshold,average errors of the order of I db withmaximum errors of 2 db will obtain .4. At frequencies above 4.5 megacycles(me) an unexpected effect was observedcalled sparkle effect. When th e tot al noisepower is cont ained in th e region above 4 me,sharp points of light of very brief durationappear on the raster. It is believed thatth is effect is the result of the random occur­rence of high peak poten tia ls in the randomnoise which produce the sharp points oflight on the raster before the extremelyfine-grain noise effect becomes visible.The sparkle effect tends to flatten theweighting above 4 me.

Subjective Test Conditions

T he object ive in setting up the testconditions was to simulate somewhatmore severe viewing condit ions tha n areusually encountered in the home. T hetest results were obtained by a jury ofobservers viewing the raster from a dis­t an ce of four times the picture height.T he rast er was adjuste d t o 6 by 8 inch esand was viewed in almost total darkness(ambient about O.0025-foot lambert) .The jurors were subjecte d one at a timeto the test condit ions , and for most t est sthree juries of 10 observers each wereemploye d. Th e three juries had memb ersin common, th ere being 14 differentjuro rs used in all.

T he tes t set up is illu strated schemat­ica lly in Fig. 2. A te st consisted of hav­ing t he juror comp are t he judgme nt cri­terion (7.22 me low-pass band of flatnoise at ab out 7 db above the thresholdof visibility) with a te st noise obtained by

Paper 53-356, recomm ended by the Al E E Tele ­vision and Aural Broadc as tin g Systems Committeea nd approved by t h e AlEE Com mi t t ee on T ech­nical Ope rations for presenta tion at the AlEEP aci fic General Meeti ng, Va nco uv er, B .C., Canada,September 1-4, 1953. Manuscript submitted Jun e3 , 1953 ; made available (or printing J uly 13 , 1953.

J . M . BARSTOW and H . N . C HRISTOPHER a re wit hthe Be ll Telep hone Laboratories, Inc. , N e w Yo rk,N . Y .

T he authors wish to ac know ledge the very ableassis tance given by A. D . Fowler of the Bell Tele­phone Laboratories , I nc . , in rev iewing th e testresults and pro vi ding va luable comments du ringt he preparation of the mater ia l (or this paper.

J AN U ARY 1954 Barstow, Christopher-Measurement of Monochrome Video Interf erence 735