PROCEEDINGS OF THE I.R.E.

Lp =equivalent parallel inductance of coil, henrys.Le= series inductance for permeability e..N= number of turns in winding.Q =ratio of reactance to resistance.Qc= oL/Rc.

QCr = WLm/Rc.Qe= wLs/Rm.Qm= wLm/Rm.R= dc resistance of copper winding.

Rm, Rt =equivalent series resistance due to core loss,ohms.

Rp= equivalent parallel resistance due to core loss,ohms.

p =resistivity, ohm-cm.Al ,e = effective permeability of core.Jm= permeability of magnetic core without air gap.A,u, = permeability at high frequencies.u'= real component of complex permeability.

Au" = imaginary component of complex permeability.w, w& =power loss, ergs per second, or watts.

x =hysteresis exponent.

BIBLIOGRAPHY(In addition to references given in text.)

1946-1948J. L. Snoek, "New Developments in Ferromagnetic Materials,"

Elsevier Publishing Co., Amsterdam, N. Y.; 1947.H. J. Lindenhovius and J. C. van der Bregen, "Measurement of per-

meability and magnetic losses of nonconducting ferromagneticmaterial at high frequencies," Philips Res. Rep., vol. 3, pp. 37-45;March, 1948.

1949F. G. Brockman, P. H. Dowling, and W. G. Steneck, "Magnetic

properties of ferromagnetic ferrite," Phys. Rev., vol. 75, p. 1298;April 15, 1949.

V. D. Landon, "Use of ferrite cored coils as converters, amplifiers,and oscillators," RCA Rev., vol. 10, pp. 387-396; September,

1949.C. L. Snyder, E. Albers-Schoenberg, and H. A. Goldsmith, "Mag-

netic ferrites, core materials for high frequencies," Elec. Mfg.,vol. 44, pp. 86-91; December, 1949.

1950K. E. Latimer and H. B. MacDonald, "A survey of the possible

applications of ferrites," Proc. IEE (London), vol. 97, part 11,pp. 257-267; April, 1950.

G. T. Rado, R. W. Wright, and W. H. Emerson, "Ferromagnetismat very high frequencies. III. Two mechanisms of dispersion ina ferrite," Phys. Rev., vol. 80, pp. 273-280; October 15, 1950.

E. Albers-Schoenberg, "Ferromagnetic oxide bodies-a counterpartto the ceramic dielectrics," Ceramic Age, vol. 56, pp. 14-16, 41;October, 1950.

C. Guilland and A. Barbezat, "Magnetic properties of MnZn ferritesin weak fields" (In French), Jour. Recherches, CNRS, No. 11,pp. 83-100; 1950.

C. Guilland, "Magnetic properties of ferrites" (In French), Jour.Recherches, CNRS, No. 12, pp. 113-122; 1950.

M. J. 0. Strutt, "Ferromagnetic materials and ferrites," WirelessEng., vol. 27, pp. 277-284; December, 1950.

1951M. Kornetzki, "Test results obtained with ferrite cores of high per-

meability" (In German), Z. Angew. Phys., vol. 3, pp. 5-9;January 1951.

H. A. Goldsmith, 'Ferromagnetic ceramics," Prod. Eng., vol. 22,pp. 97-102; April, 1951.

R. Herr, "Mixed ferrites for recording heads," Electronics, vol. 24,pp. 124-125; April, 1951.

V. E. Legg, "Ferrites: new magnetic material for communicationengineering," Bell. Lab. Rec., vol. 29, pp. 203-208; May, 1951.

F. Wagenknecht, "Dielectric and magnetic properties of ferrites athigh frequencies" (In German), Frequenz, vol. 5, pp. 145-155;June, 1951 and pp. 186-190; July, 1951.

E. Both, "Ferrite materials permit improved designs for magneticdevices," Mater. and Meth., vol. 34, pp. 76-79; July, 1951.

1952J. J. Went and E. W. Gorter, "The magnetic and electrical properties

of ferroxcube materials," Philips Tech. Rev., vol. 13, pp. 181-193;January, 1952.

E. Gelbard, "Magnetic properties of ferrite materials," Tele-Tech.,vol. 11, pp. 50-52, 80-83; May, 1952.

The Effect of Impurity Migrations on ThermionicEmission from Oxide Cathodes*

IRVING E. LEVYt

Summary-A comparison of thermionic emission from oxidecathodes with different base alloys showed the dependence of thework function on migrating impurities from tube parts other than thecathode.

The effect of the base metal alone could be evaluated properlyonly by the use of a special diode structure which did not contributeany impurities toward the reduction of the oxide-coating.

In the test method used, saturated emission was measured butthe anode voltage was kept below the decomposition energies ofmost of the compounds apt to be found on the plate.

INTRODUCTIONWX yORK DONE over the past several years has re-

sulted in the conclusion that pure nickel is in-capable of reducing an oxide-coated cathode.

* Decimal classification: R138. Original manuscript received bythe Institute, May 14, 1952; revised manuscript received July 2, 1952.Sponsored by the Office of Naval Research, Contract N7onr-389.Chief Investigator, J. C. Cardell.

t Raytheon Manufacturing Co., Newton, Mass.

Whitei presented the thermodynamic data to substan-tiate this. Results reported on experimental diodes byNottingham, Cardell, and Levy2 in this country and byViolet and Riethmullerl in France showed the impor-tance of the base-metal impurities in influencing ther-mionic emission.Recent concepts dealing with the mechanism of get-

ting thermionic emission treat the oxide-coated cathodeas an excess impurity semiconductor.4 5 According to

1 A. H. White, "Applications of thermodynamics to chemicalproblems involving the oxide cathode," Jour. Appl. Phys., vol. 20, pp.856-60; September, 1949.

2 W. B. Nottingham, J. Cardell, and I. E. Levy, Summary Reportfor 0. N. R. Contract N8onr-389, Raytheon Mfg. Co., Newton,Mass.; July, 1950.

8 F. Violet and J. Riethmuller, "Contribution to the study of oxidecathodes" (in French), Ann. Radioelect., vol. 4, pp. 148-215; 1949.

4 A. S. Eisenstein, "Advances in Electronics," vol. 1, pp. 1-64,Academic Press, New York, N. Y.; 1948.

6 W. E. Danforth, "Elements of thermionics," PROC. I.R.E., vol.39, pp. 485-499; May, 1951.

1953 3I65

PROCEEDINGS OF THE I.R.E.

these ideas a cathode base metal free of reducing impuri-ties would be incapable of producing the stoichrometricexcess barium needed for good thermionic emission. Itwas an apparent experimental contradiction to this be-lief which led to the present work.

THE TUBE STRUCTUREThe tube structure used in this work is similar in de-

sign to the A.S.T.M. standard diode.6 This is a cylindri-cal diode with a conventional radio-tube oxide-coatedcathode. Actually, in this study, two structures werecompared to determine the effect of migratory impuri-ties. One structure will be called the "Standard" Diode,the other structure will be called the "Purified" Diode.The difference in the parts is shown in Table I. The

TABLE ICOMPARISON BETWEEN 'STANDARD" AND "PURIFIED" DIODE

Part Standard Diode Purified Diode

Getter KIC getter-heavy iron batalum contains Mo,with Si, Mg, Al, Ba, Cu Be, Pa, Ti, trace of Sipresent and Fe

Cathode tab silicon nickel (2.8-3.2% 499 alloy nickel (highSi) other constituents purity)the same as 'A" nickel

Plate grade 'A" ni-max. limits 499 alloy nickel (highin per cent: C 0.20, Cu purity) limits in per0.25, Fe 0.30, Mn 0.35, cent: C 0.10, Cu 0.04, FeS 0.008, Si 0.20, Mg 0.04 0.05, Mn 0.02, S 0.005,

Si 0.01, Mg - .d0Mica selected class 1 fair stained or better

Plate supports grade 'A" nickel 499 alloy nickeland welds

Stops and con- grade "A" nickel 499 alloy nickelnetors

heaters which were conventional aluminum oxide-coatedtungsten were the same for both structures. The bulb inboth lots was standard lime glass.Table I shows that the "standard" diode consisted of

parts commonly used in commercial radio tubes. The"purified" diode parts, on the other hand, were fabricatedout of the purest available materials.

METHOD OF EVALUATING THERMIONICEMISSION

To evaluate thermionic emission correctly it is neces-sary to have some means of observing the total emissioncurrent not limited by space charge. At the usual op-erating cathode temperatures (720 degrees C-800 de-grees C), this saturation emission measurement involvesthe use of pulse techniques in order to keep the plate dis-sipation down to a practical maximum. However, thereare several disadvantages to pulse testing. There is someexperience to indicate that the drawing of current underpulse conditions changes the state of the cathode. It isalso known that bombardment of the anode with high-energy electrons will bring about decomposition of ox-

* R. L. McCormack, 'A standard diode for electron tube oxide-coated cathode core material approval tests," PRoC. I.R.E., vol. 37,pp. 683-687; June, 1949.

ides, chlorides, and other compounds likely to be foundthere, and cause a significant reduction in electron emis-sion. Metson,7 and Metson and Holmes8 have studiedthis phenomenon and have reached the conclusion thatdecomposition of anode impurities will occur even atelectron energies as low as 6 volts.

Consequently, a test was established here which arbi-trarily used 4 volts applied potential between anode andcathode. In order to insure temperature-limited emis-sion, at this plate voltage it was necessary to drop theheater voltage to 1.75 volts (335 degrees C). This is truetemperature as measured with a thermocouple.

This low field test was adopted as the standard methodof evaluating thermionic emission in the experimentsdescribed below. In addition, from 1947 to date, severalthousand diodes have been tested under ONR spon-sored research with this low field technique,9 and it hasproven itself to be a simple, satisfactory method whichshould have general utility and value.

TESTS CONDUCTED ON STANDARD DIODEFor the first test lot six "standard" diodes were as-

sembled and given optimum processing, three with vac-uum-melted "D" cathodes and three with regular 220alloy cathodes-melt 66. A comparison between thechemical analysis of these two cathode melts is madebelow in Table II.

TABLE IICOmPARISON BETWEEN VACUUM MELT ID" AND MELT 66

CATHODES AS RECEIVED BEFORE ASSEMBLY

It was anticipated that the decreased availability ofsilicon, magnesum, and titanium in the vacuum-meltednickel cathodes would significantly reduce the ther-mionic emission from diodes made with these cathodesleeves. The surprising results are shown in Fig. 1, wherethe low field emission for each tube is plotted againstlife. The life-test conditions were as follows: Ef = 6.3volts, Ep=40 volts, Ip=100 ma/sq cm. It is apparentfrom the results that there is no significant difference inemission between the normal alloy cathodes-melt 66,and the exceptionally pure nickel cathodes-vacuummelt D, both initially as well as during life. The cathodeswere taken from these tubes after 500 hours' life and ana-lyzed spectrochemically.'0 The results of the analysisfollow in Table III:

T G. H. Metson, "Note on volt dependent poisoning effects in ox-ide cathode valves," Proc. Phys. Soc. (London), vol. 62B, p. 589;September, 1949.

8 G. H. Metson and W. B. Holmes, "Poisoning in high-vacuumoxide-cathode valves," Naturc, vol. 163, p. 61; June, 1949.

9 W. B. Nottingham, J. Cardell, and I. E. Levy, Summary Reportof,O.N.R. Contract N7onr-389, Raytheon Mfg. Co., Newton, Mass.;July, 1950.

1 Although the cathodes were 27 mm long, only the center por-tion, 12mm long, which contained the coating, was taken for analysis

366 Marci't

Levy: Effect of Impurity on Emission

z

0 10 LOT 372 MELT 66 CATHODESLOLO x LOT 368 VACUUM MELT 0D CATHODES

LJ

0

0 100 200 300 400 500

HOURS LIFE

Fig. 1-Vacuum melt 'D" cathodes versus melt 66 cathodes inthe "standard" diode.

Comparison of the analyses of the cathode melts as

received (Table II) with Table III on concentrationsclearly shows that migrations of impurities, especiallysilicon, iron, and magnesium do occur, and that theseimpurities are in evidence beyond the concentrationsthat were originally contained in the cathodes under test.

TABLE IIISPECTROCHEMICAL ANALYSIS OF COATED CATHODES REMOVED

FROM STANDARD DIODES AFTER 500 HOURS LIFE

MNt %Si %Fe %Mn % Mg %Cu %Ti.~ ~No.

"D:" 0.04 0.06 0.02 0.01 .0.02 0.01

66 0.04 0.12 0.08 0.03 0.02 0.04

It was concluded that with reference to the standarddiode structure used the results were not a true indica-tion of cathode-emitting properties. It was rather feltthat the emission current measured especially in thecase of vacuum melt "D" was largely a function of mi-grating reducing impurities which become available tothe cathode during processing and life.

TESTS CONDUCTED ON PURIFIED DIODE

To separate the true cathode thermionic emission

from the influence of migrating impurities as much as

possible, six tubes were made up as before except thatthe purified diode structure was used (see Table I). The

HOURS LIFE

Fig. 2-Vacuum melt "D" cathodes versus melt 66 cathodesin 'purified" diode.

results are shown in Fig. 2. This conclusively shows thatvacuum melt "DI cathodes do result in exceptionallylow emission when the effect of migrating impurities, as

far as possible, is eliminated.At the end of 500 hours' life the cathodes were re-

moved from these tubes and spectroscopically analyzedas before. The results are shown in Table IV.

TABLE IVSPECrROCHEMIcAL ANALYSIS OF COATED CATHODES REMOVED FROM

'PURIFIED" DIODES AFTER 500 HOURS LIFE

These results show no significant increase in cathodereducing impurities over the original concentrationsshown in Table II.One can conclude from these data that the use of

"purified" structures similar to the one used in theseexperiments is essential in order to minimize impuritymigration and to properly evaluate the emission proper-

ties of cathodes."

11 Although this report deals with only a small quantity of tubes,the same consistent data was obtained from several hundred tubesrun over a period of years. Details are reported by W. B. Notting-ham, J. Cardell, and I. E. Levy, "Summary Report for O.N.R. Con-tract N7onr-379," July, 1950.

I

1953 367

PROCEEDINGS OF THE I.R.E.

THE EFFECT ON THE WORK FUNCTIONThe ratio of emission in the "purified" structure to

emission in the "standard" structure at 605°K is seento be about 1:100 at the end of life for the case of melt"D" cathodes. It is interesting to apply Richardson'sequation to these results.

If we call Is, the emission density from melt "D" cath-odes in the purified diode, and Is2 the emission densityfrom melt "D" cathodes in the standard diode, then

Is, = A T2eE41IKT (1)

IS2 = A T2e f2/KT (2)

whereA = Richardson's constant= the Richardson work function in volts

T= temperature in degrees Ke = electron charge 1.6 X 10-1' coulombsK = Boltzman's constant 1.38X10-23 joules per de-

gree,

and

IS2-= 100 = e7e(¢,2q01)/KT

Is, (3)

From (3) the difference in work function is

1- 02 = 0. 24 volt.

This work function difference, however, is due to noth-ing more than the migrating impurities resulting fromthe "standard" structure. This indicates that extremecaution is advised in oxide-coated cathode research andproduction to insure that the cathode itself and not mi-grating impurities are being evaluated.

ACKNOWLEDGMENTThe author wishes to express his gratitude to Dr.

W. B. Nottingham of Massachusetts Institute of Tech-nology for his interest and guidance in this work, and toJ. Cardell, Chief Investigator in this research project,under whose administration this work was carried out.

Electrically Tuned RC Oscillator or Amplifier*OSWALD G. VILLARD, JR.t, SENIOR MEMBER, IRE AND FRANK S. HOLMANt, STUDENT, IRE

Summary-Two RC circuits based upon all-pass phase-shiftnetworks are described. They are useful as electronically tunableaudio oscillators, selective amplifiers, or bridges. Change of reso-nant frequency is accomplished by varying amplitude of transmissionin one or more circuit branches by means of vacuum-tube modulators.

In theory, both circuits may be electronically tuned from zero toinfinite frequency, and their feedback-loop gain at resonance shouldbe independent of the frequency to which they are tuned. However,the frequency ratio conveniently obtainable in practice, before ap-preciable changes in gain occur, is about four to one. This limitationis in part a consequence of the decrease in effective Q of the fre-quency-controlling portion of the circuit when they are tuned farfrom center frequency.

INTRODUCTION

J\ /iANY APPLICATIONS exist for electronicallytunable oscillators and amplifiers capable ofoperation at the lower audio frequencies where

RC circuits become preferable to LC. The best oscillatorcircuits disclosed so far appear to be those of Mc-Guaghan and Leslie,' Ames,2 and Anderson.' Electroni-cally tuned amplifiers do not seem to have receivedmuch attention.

* Decimal classification: R355.914.3XR363.2. Original manu-script received by the Institute, October 15, 1951; revised manuscriptreceived August 11, 1952.

t Radio Propagation Laboratory, Stanford University, Stanford,Calif.

t Electronics Research Laboratory, Stanford University, Stan-ford, Calif.

1 H. S. McGuaghan and C. B. Leslie, "A resistance-tuned fre-quency-modulated oscillator for audio frequency applications,"PROC. I.R.E., vol. 35, pp. 974-978; September, 1947.

2 M. E. Ames, "Wide range deviable oscillator," Electronics, pp.96-100; May, 1949.

3 F. B. Anderson, "Seven league oscillator," PROC. I.R.E., vol. 39,pp. 881-890; August, 1951.

There will be described two circuits which have aninteresting property, that in theory it should be possibleto tune their frequency of resonance, electronically,from zero to infinite frequency. Furthermore, the mag-nitude of their positive feedback voltage should be inde-pendent of the frequency of resonance. However, the ef-fective Q of the feedback loop falls off when the devia-tion from center frequency becomes large, so that thepractical operating frequency ratio is about four to one.Over this range the feedback loop transmission at reso-nance is found to be constant enough to make the cir-cuits useful as electronically tuned selective amplifiers.The curve of frequency versus modulating voltage has

a point of inflection; therefore, modulation about thispoint is highly linear. Furthermore, the nature of thecurve is such that if a maximum departure from linear-ity of the order of five per cent is permissible, a fre-quency ratio of the order of two to one may be obtained.Tuning is accomplished by variation of voltage ampli-

tude in a vacuum-tube modulator rather than by varia-tion of an effective circuit impedance. The desired per-formance is obtained when the transfer characteristic ofthe modulator is linear. In one of the circuits the modu-lator is of the conventional balanced type, whose trans-fer-characteristic linearity is inherently high.There is a resemblance to the phase-shifter approach

of De Lange4 to the extent that frequency modulationmay be considered obtainable in these circuits by varia-tion of transmission. Other differences are considerable.

40. E. DeLange, "A variable phase-shift frequency-modulatedoscillator," PROC. I.R.E., vol. 37, pp. 1328-1330; November, 1949.

368 March