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WIRELESSENGINEER

NUMBER 158 VOLUME XIII NOVEMBER 1936

A JOURNAL OF

RADIO RESEARCHAND

PROGRESS

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A4 THE WIRELESS ENGINEER November, 1936

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A6 THE WIRELESS ENGINEER November, 1936

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A7

The

WIRELESS ENGINEERA Journal of Radio Research & Progress

EditorHUGH S. POCOCK

Technical Editor,Prof. G. W. 0. HOWE, D.sc., M.I.E.E.

VOL. XIII. No. 158

NOVEMBER 1936

C 0 N T E N T SEDITORIAL

THE CHARACTERISTICS OF THERMIONIC RECTIFIERS. ByW. H. Aldous, B.Sc., D.I.C.

THE SUPER -REGENERATIVE RECEIVER. By M. G. Scroggie,B.Sc., A.M.I.E.E.

A CONTINUOUSLY VARIABLE PHASE -SHIFTING DEVICE.By 0. 0. Pulley, B.E., Ph.D.

CORRESPONDENCE

ABSTRACTS AND REFERENCES

SOME RECENT PATENTS

573

576

581

593

595

597

625

Published Monthly on the first of each MonthSUBSCRIPTIONS Home and Abroad: One Year, 321-, 8 Months, 16/-. Single Copies, 2/8 post free

Editorial, Advertising and Publishing OfficesDORSET HOUSE, STAMFORD STREET, LONDON, S.E.rTelegrams: " Experiwyr Sedist London " Telephone : Waterloo 3333 (5o lines)

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29 Hertford Street Guildhall Bldgs., Navigation St.. 2 26o Deansgate, 3 26a Renfield Street, C.2Telegrams Antiwar, Coventry'"' Telegrams Antoprees, Birmingham" Telegrams: "nitre, Manchester" Telegrams: Glasgow'Telephone: 5210 Coventry Telephone: Midland 2971 (4 lines) Telephone: Ellsokfriars 4412 (4 lines) Telephone: Central 4587

The Editor invites the submission of articles with a view to publication. Contributions which arenot exclusive should be so described when submitted. MSS. should be addressed to the Editor," The Wireless Engineer," Dorset House, Stamford Street, London, S.E.r. Especial care should be

taken as to the legibility of MSS. including mathematical work.

TIIG WINELFS; ENGINEER November, 1936

'V'

sidePersonally, we believe in stan-dardisation of condensers. But itisn't much good our believing init unless you agree with us.So we have planned our design and pro-duction sections to supply any specialtype, size or shape of condenser in theminimum of time. Actually, we alreadyproduce so many different standard EK

to 144,ea.4 14 re

D CONDENSERS 11UBILIEDUBILIER CONDENSER Co. (1925) Ltd., DUCON WORKS, VICTORIA ROAD, N. ACTON, LONDON, W.3

Kindly mention " The Wireless Engineer " when replying to advertiserC. R. Casson IC5

types that special cases go throughpractically as routine.Our laboratory, under the direction ofMr. Coursey, spend their lives workingwith set manufacturers on these individualproblems-perhaps you can make more

use of them than you do at present.But that still doesn't alter the factthat increased standardisation would

p be a good thing for everyone.

573

WIRELESSENGINEER

Vol_ XIII. NOVEMBER, 1936. No. 158

EditorialMagnetron Oscillators

IN the July editorial we discussed the recentdevelopments in the theory, constructionand use of the magnetron as a generator

of high -frequency currents. We feel com-pelled to return to the subject by a paper byG. R. Kilgore which was published in theAugust number of the Proceedings of theInstitute of Radio Engineers. The author,who is in the Radiatron Division of theR.C.A. Manufacturing Co. of New Jersey,has been working at the subject for someyears and had a paper on " MagnetostaticOscillators " in the Proceedings in 1932.The present paper is confined to the genera-tion of frequencies between 30o and 600megacycles (A = I metre to 0.5 metre) bymeans of the 2 -section split -anode magne-tron. There was for some years a tendencyto concentrate on the production of everhigher frequencies, even although the powermight be very small, but, as we pointed outin July, there has recently been a great dealof research directed to the development ofgreater power at a wavelength of about ametre, more especially since the introductionof the acorn type of valve has made it possibleto design a receiver to work satisfactorily atthese frequencies.

The author divides magnetrons into twoclasses. He defines those in which thefrequency depends essentially on the electron -transit time as electron magnetrons, whereasthose in which the frequency is controlled bythe constants of the circuit he calls negative

resistance magnetrons. In our opinion thisnomenclature is not very fortunate, since inboth cases the action is closely associatedwith the path of the electron and it ispossible to regard the effect of transit timeas equivalent to endowing the valve with anegative resistance. The former could becalled transit -time magnetrons and the latterdynatron magnetrons, since the word dyna-tron is now well known as a designation fora valve with a static characteristic over apart of which the slope is negative.

The author throws considerable light onthe action of this dynatron type of magne-tron, not only metaphorically but alsoliterally, for by introducing a little argoninto a specially constructed magnetron thepaths of the electrons were made visible andwere photographed.

In the study of the static characteristicscare must be taken to avoid oscillationsoccurring. If the two halves of the anodeare maintained at the same potential, say50o volts, the magnetic field can be increaseduntil no current flows, all the emittedelectrons returning to the filament. If nowthe potential of one half of the anode isincreased while that of, the other half isdecreased by an equal amount, current be-gins to flow, but, strange to say, not to theanode sector at the higher potential, but tothat at the lower. A decreasing voltage onthis sector leads thus to an increasingcurrent-the requisite for a negative differ-

574 THE WIRELE

ential resistance-and this goes on until theone voltage is three or four times the other.On a further increase in the voltage differ-ence, the conditions change very rapidly,the currents becoming equal and then the

+100

+100

Fig. 1.

ELECTRONPATH

SS ENGINEER November, 1936

cathode, where the electrons are movingslowly, they will tend to move at right anglesto the equipotential lines and thus to moveoff towards the more positive sector even ifthey were emitted in the opposite direction.

+150

+50

Fig. 2.

larger current going to the anode sector atthe higher potential. This latter portion ofthe characteristic is of no importance, sincethe magnetron can only operate as anoscillation generator over the negative re-sistance portion. When generating oscilla-tions of 600 megacycles per second theelectron stream switches over from the onehalf anode to the other 1,200 million times asecond, always flowing predominatingly tothe one which is momentarily at the lowerpotential.

The first of the figures which we reproducefrom Kilgore's paper shows the electron pathwhen the two sectors are at the samepotential and the electrostatic field thereforeapproximately radial ; the equipotentialelectric lines are concentric circles. Themagnetic field is 1.5 times the critical valueat which the electron only just missedstriking the anode. In Fig. 2 the meananode voltage is unchanged, but the uppersection has three times the voltage of thelower. The path shown in Fig. 2 is that ofan electron which leaves the cathode towardsthe middle of the more positive sector, whilstin Fig. 3 the electron is assumed to start offin the opposite direction. The magneticfield is without effect on a stationary electronand the force is proportional to the speed.In the immediate neighbourhood of the

ELECTRONPATH

+150

+50

Fig. 3.

ELECTRONPATH

This is clearly indicated in Fig. 3. Fig. 4 isa reproduction from a photograph of theelectron path in the special magnetron con-taining a little gas and having a specialcathode with an emitting spot. This cathodecould be rotated so that the electrons couldbe made to start in any direction. Bycoating the anode sectors with willemite thespot at 'which the electron stream impingedupon the anode could be made luminous.This photograph provides a very beautifulconfirmation of the general correctness of

Fig. 4.

Figs. 2 and 3. In Fig. 4, the magnetic fieldwas only 1.25 times the critical and theanode voltages were 300 and 25o volts. Itmust not be assumed that all the electrons

November, 1936 THE WIRELESS ENGINEER

reach the lower sector, but under suitableconditions the majority appear to do so.The potential distribution shown in Figs. 2and 3 will be modified in practice by thespace charge, which will limit the vu.Tnumber of the emitted electronswhich reach the anode.

The determination of the effi-ciency at frequencies of 30o to 600megacycles is no easy matter and theauthor claims no greater accuracythan plus or minus zo per cent.for the method finally adopted,which was that of absorbing the out-put power in a lamp filament which had beencalibrated photometrically on direct current.The skin effect can be calculated and allowedfor in determining the resistance, but thishas no appreciable effect on the relationbetween the power and the luminosity.

Experiments showed that for a givenfrequency the efficiency increased (a) as theanode diameter was reduced, (b) as the

INTERNAL CIRCUIT INSULATOR

it4

RADIATION -COOLED

Fig. 5.

anode voltage was increased and (c) as themagnetic field was increased. One of theprincipal factors which limits the efficiencyis the transit time of the electrons expressedas a fraction of the period. As this increasesthe efficiency falls.

It is interesting to compare the magnetronsdeveloped by Kilgore with those described inour July number. The problem is not aneasy one. To get a high efficiency at thesefrequencies the anode diameter should notexceed 5 mm. for an anode voltage of 1,50o.The axial length cannot exceed a few centi-metres because of the difficulty of producingthe magnetic field. When several hundredwatts are supplied to a valve which has amaximum efficiency of zo or 3o per cent.,7o to 8o per cent. of the input is dissipatedat the anode and has to be got rid of. Figs. 5and 6 show two magnetrons described byKilgore. In both cases the anode sectors aresemicircular grooves in large masses of goodconducting metal which at the same timeconstitute the circuit around which the highfrequency current flows. In Fig. 5 the metal

575

is entirely within the valve and gets rid ofthe heat by radiation. The power is trans-ferred to an external circuit by bringing thelatter near the magnetron and thus coupling

ER JACKET INTERNAL CIRCUIT

A

INSULATOR

LOAD LEADS

WATER-COOLED

Fig. 6.

it by mutual inductance with the internalcircuit. This magnetron had an output ofabout 5o watts at 55o megacycles, theefficiency being about 3o per cent. Themagnetron shown in Fig. 6 is almost entirelyof metal and is water-cooled. Here anothermethod must be used to get the power out tothe external circuit ; this is done by meansof two leads passing through the glass in

addition to those connected to thecathode. This magnetron gives an

GLASS

output of about zoo watts at 600 mega-cycles with an efficiency of 25 per cent.

Kilgore mentions the phenomenonof filament bombardment to which wereferred in July and the fact that itcan so heat the filament that the valve

will Continue to oscillate with the normalfilament heating circuit open. We suspectthat the phenomenon causes the experi-menter some anxious moments. The authoris careful to point out that these specifictubes are to be regarded as laboratory modelsrather than commercial designs. They arecertainly of great scientific interest and, likethose which we described in July, mark agreat advance in the generation of an appre-ciable amount of power at these very highfrequencies. G. W. 0. H.

An ApologyIN the July editorial we mentioned the

researches on the magnetron carried out byGroszkowski and Ryzko and published in

the Proceedings of the Institute of RadioEngineers. We regret that we referred tothe authors as Russian and thus gave theimpression that the work had been done inRussia. The authors are Poles and the re-searches were carried out at the StateInstitute of Telecommunications in Warsaw.

T I 11-: WIRELESS ENGINEER November, 1936

The Characteristics of ThermionicRectifiers

By W. H. Aldous, B.Sc., D.W.(Communication from the Research Staff of the M.O. Valve Co., Ltd., at the G.E.C. Research Laboratories. Wembley. England)

SIIMMARY.-With the assumptions of infinite smoothing capacity and no emissionlimitation it is shown that a three halves power law characteristic leads to simple formulaewhich represent the anode wattage, output voltage, and peak current for thermionic rectifiersin terms of the input voltage and output current with an error of less than one per cent. overthe normal working range.

IntroductionMOST published work on small ther-

mionic rectifiers covers their use incircuits, and for this purpose some

ideal form for the rectifier D.C. characteristicis assumed, and relations between the D.C.output, A.C. input, and circuit capacityand resistance obtained. From the pointof view of the valve itself, however, the twothings which influence the design are thewattage dissipated at the anode, and thepeak current that is required from thecathode. These should be expressible interms of the input A.C. voltage and theoutput D.C. current, which are the twoindependent variables that are most easilymeasured, together with some easily deter-minable constant of the rectifier.

Fig. I. Basic singlephase half -wave rectifierEms° Ri circuit.

The necessary calculations are relativelysimple if a linear law is assumed for theanode current v anode voltage relation onthe positive sides, but a difficulty usuallyarises in practice in determining the mostsuitable straight line to take to representthe curved characteristic. In actual prac-tice a three halves power law much moreclosely represents this characteristic, aswould be expected from space charge re-lations. The calculations for this case havebeen carried out by Fortescue2, but theresults of the integrations concerned aregiven only as graphs, in which neither

1 W.E. and E.W., 8, 522, 1931.3 Proc. Phys. Soc., 39, 313, 1927.

ordinates nor abscissae represent direct func-tions of a single variable. The presentderivation gives the results in a much morereadily useable form.

Single Phase Half -Wave CircuitThe basic circuit considered (Fig. i) con-

sists of a rectifier feeding a parallel combina-tion of capacity and resistance. If thecapacity of the condenser is assumed to beinfinite, no fluctuation of the D.C. voltagedeveloped across it will occur. With aninput represented by the cosinusoidal waveE cos 8 (Fig. 2) the constant D.C. outputvoltage across the condenser will be givenby V = E cos a.

Let the rectifier static characteristic begiven by

i = Kv3/2 (I)where i = anode current

v = anode voltageK = constant.

Since anode current will flow only whenthe input voltage is greater than the con-denser voltage, and will depend on the

E cos -E cos a

cc it2

E cos 0

Fig. 2.-Time variation of voltage and current.

difference between these two voltages, themean anode current over the whole cycleis given by

I=I K(E cos 0 -E cos a)3/2 a (2)71. 0


The mean anode wattage is given by

W = IJ

a K(E cos 0 -E cos a)212 de . . (3)Tr 0

The peak current is given byK(E -E cos a)312 .. (4)

For the evaluation of I and W, the inte-gral is required of the expression

P = (cos 0 - cos a).+1/2 de

where ft is either 1 or 2.

(5)

Putting sin 2 = k this becomes

P =sin- 'A 9 n+1/2

(2k2 - 2 sins-) de . . (6)0 2

If now we let sin 0-= k sin 9S2

so that cos 0- d-e = k cos 04.2 2Equation (6) takes the form

26+3/2 I 42 [(I - k2 sins s6) - (I - k2)]6+ s 4(1 - ks sin s(6)'/2

By means of the reduction formula

(1 k2 sin20)m+ 1/2 cirk

k2

2m + I(r - k2 sins 95),6-1/2 sin 96 cos ck

(k2 2)I(I - k2sins s6),6- 1/2 dO2/X

2M + I

+ 2/X -I2M +I -1)J - k2sinschm-8/2 d4)

(7)

the required integrals may be set in terms ofthe complete elliptic integrals of the firstand second kinds denoted by

r/2E, f

o-

F(I-0l

respectively, giving

0

k2 sin2 0)1/2 d6

k2 sin2 0)-1/2 do

(cos 0 - cos a)2/2d0 =41/ 2 [( 2 + 4k2)E13

+ (2 - 5k2 3k4)F1] . . (8)

577

I 1/8 2- .., 23k2+23k4)E,(cos0- cos a)5/2d9=-- -- [(015

o

+ (- 8 -f- 27k2 - 34k4 + I5k6)F1] . . (9)The values of E1 and F1 may be obtainedfrom tables, and thence I and W computedaccurately for any given value of k.

If, however, E1 and F1 are expanded andintegrated term by term, substitution of theseries so obtained in equations (8) and (9),leads to the expressions

I.(cos 0 -cosa)3I2 de = 3N/27rk4+ 1/2 irk6-1- .. .o 4 16

. . (io)

fa(cos0- cos a)5/2d0= 5 Vi irk° + 5V, iwk2+...o 4 o4

.. (II)The error caused by neglecting the second 'terms in these two expressions is quitesmall. For example, at a =-- 45° i.e.,k = 0.38, which is rather larger than anyangle likely to occur in practice, the con-tributions of the second term in the twoexpressions are 1.3 per cent. and I.° percent. respectively.

Therefore, combining equations (2) and(10) we have

43V2 KE3/2

whence, from equation (ii)

Io(cos

0-cos a)5/2 de -L3-Vi'KE3/2]

5Vivr 4 I 12

(i3)

I

giving, from equation (3)W = 1.62 K-1/2 E1/1 /3/2

Also sinceV = E cos a E(i - 2k2)

E -V = 1.94K -1/2E1/411/2

and from (4) -L.. = KE3/2 21/2k3= 2.71 K1/4 E3/8 I3/4.. (16)

From (ILI) and (i5) we may obtain the anodewattage in a form independent of the rectifierconstant K.

.. (12)

(i4)

W = 0.83 (E - V)I . . (xi)

578 THE WIRELESS ENGINEER Novemberr, 1936

Eiphase Half -Wave CircuitThis will differ from the single phase case

only in that there are two conduction periodsper cycle. The equations correspondingto (2) and (3) are

2 aI = F K(E cos 0 -E cos a)3/2 dO . . (I8)..10

W = K(E cos 0 -E cos a)512 dO . .Tr o

whilst equation (4) is unchanged.It therefore follows that k4 has half of its

value in equation (12). Since W, E - V,and /,,,x. are proportional to k2, k2, and

(19)

, .7

5r/31.1.8

71

- 64

3

2 119

-61

51

41:).....--

51

4J10 i0 30 40 50

OUTPUT CURRENT (MILLIAMPERES)

respectively, they must be reduced to(i)I, and (DI of their values in equations

(14), (i5) and (i6), givingk -1/2E1/4 13/2W 1.14.. (co)

E -V = 1.37 K-1/2 E1/4 11/2 (21)

Imp = 1.61 10/4E3/8/314 . . . . (22)

It should be noted that in this case, K isthe constant in the current voltage relationfor one anode taken alone.

Practical ApplicationThe equations (14) to, (i6) for the single

phase case and (20) to (22) for the biphasecase represent extremely useful new rela-tionships for the design of thermionicrectifiers. The constant K for any designmay be obtained either by calculation usingexact formulae for cylindrical or planeparallel electrodes' and approximate for -

1 Langmuir & Compton, Rev. Mod. Phys., 3,191, April 1931. 2 Y. Kusunose, Proc. IRE. 17, 1706, Oct. 1929.

6

mulae for filamentary cathodes2, or byactual measurement. Calculation of theanode wattage and peak anode current thenenables the valve to be designed with thecorrect anode radiating area and filamentemission.

The equations (14) and (15) have beenchecked experimentally for a particularsingle phase half wave rectifier, the resultsbeing shown in Fig. 3. The anode wattagewas determined by measuring the anodetemperature under running conditions bymeans of a thermocouple attached to theanode, a preliminary calibration underD.C. conditions showing the relationship

between temperature and wat-tage.

0The table shows the constant K

and the calculated wattage ando peak current for selected samples

o from the range of Marconi andO Osram rectifiers when run under0

their maximum conditions inbiphase half -wave circuits.

0

0Fig. 3. -Results of check on equationsfor anode wattage and output voltage.Points are measured values. Fulllines are calculated from formulae.

Limitations of EquationsThe initial assumptions in the derivation

of the above equations were that unlimitedemission was available, no resistance orinductance existed in the supply, and infinitesmoothing capacity was used across theload. If these assumptions are departedfrom, the equations for the mean outputcurrent and anode wattage corresponding to(2) and (3) become very complicated, and ingeneral can only be solved by graphical ornumerical methods, and are unlikely to leadto definite formulae such as have been

Valve

U.I0U.12U.I4MU.12MU.r4U.30

Kamps/(volts)s/$

E/VzVolts

Iamps. Watts

/max.amps.

4.2 X 10-4 250 o.o6 3.6 0.252.7 X I0-4 350 0.12 13.6 0.422.7 X 10-4 500 0.12 14.9 0.495.2 X 10-4 350 0.12 9.8 0.495.2 X 10-4 500 0.12 10.7 0.586.2 X 10-4 25o 0.12 8.2 0.46

November, 1936 THE WIRELESS ENGINEER 579

obtained for the simpler case. Only ageneral discussion of the effects on the anodewattage, peak current and output voltageof departure from the initial assumptionscan, therefore, be attempted. This dis-

Fig. 4.-Currentwave form withlimited emission.

cussion is on the basis of a constant outputcurrent, which can easily be effected byadjustment of the load resistance.

(a) Emission LimitationWith a sharply defined total emission

available, the form of the current v timecurve will be as shown by the solid linein Fig. 4. This will be approximatelythe case for a tungsten filament rectifier.With an oxide -coated cathode, which hasnot such a definite saturation value forthe emission, the dotted line will representthe working conditions more closely. Thegeneral effect of this form of curve is thatthe rectifier behaves approximately like onehaving unlimited emission but with a smallervalue for K. The anode wattage will, there-fore, be higher, the output voltage lower,and the peak current lower than with noemission limitation.

(b) Resistance of the supplyDuring the conducting part of the cycle the

rectifier behaves as a non-linear resistance tothe flow of current. The effect of adding re-sistance in the supply is, therefore, approxi-mately equivalent to reducing the value ofK.,,which can be regarded as the non-linearconductance of the rectifier. The peak

OUTPUTVOLTAGE

Fig. 5.-Time variation of voltage and currentwith finite smoothing capacity.

current and output voltage will, therefore,be decreased, whilst the total wattage isincreased. This total wattage is sharedbetween the rectifier and the added resist-

ance. Since, however, from (i4), a givenpercentage reduction in K only producesone-half that percentage increase in wattage,the effect is to produce a reduction of wat-tage in the rectifier itself.

(c) Inductance of the supplyThis will delay both the rise and fall of

anode current. The full difference betweenthe input voltage and the steady condenservoltage will not be applied to the valve, andthe anode current will flow for a longer time.Both the output voltage and peak currentwill, therefore, be reduced. With lower anodevoltage, but with a longer conduction period,it is not easy to determine whether the anodewattage will be increased or decreased.Practical experience, however, shows thatthe presence of inductance in series with the

6

WATTS .600

500

400

300

200

100

VOLTS

3

2

1

0 010 20 ' 30 40

SMOOTHING OAPAOITY (MIOROFARADS

Fig. 6.-Results of test on variation of anodewattage and output voltage with smoothing

capacity.

0

0

rectifier gives a considerable decrease in theanode wattage for the same output current.

(d) Finite smoothing capacityThe effect of not using infinite smoothing

capacity is to allow a fluctuation of the outputvoltage, which will rise during the conductingpart of the cycle and fall during the non-conducting part (Fig. 5). The relationshipsbetween output voltage, conduction periodand value of capacity, have been workedout for the case of a rectifier with a linearcurrent v, voltage law by Mariquel, and theresults will apply qualitatively to the presentcase. It follows that reduction in smoothingcapacity with constant mean output current

W .E. & E W.. 12.17.1935.

58o THE WIRELE

will reduce the output voltage and, byvirtue of the longer conduction period, willreduce the peak current. For this caseagain, it is difficult to predict the effect onthe anode wattage, but practical experienceshows that a reduction occurs. Fig. 6demonstrates this for a particular singlephase half -wave rectifier working with 450volts A.C. input and 6o mA mean D.C.output current.

These considerations show that the equa-tions derived for wattage and peak currentgive the maximum values which shouldoccur in practice, and for which the rectifiershould be designed.

Care should be taken in applying equation(ii) in practice, especially to publishedoutput v. input curves of rectifiers. Theseare usually taken under conditions involvingsome resistance and inductance in thesupply, and also a finite value of smoothingcapacity, which will tend to depress theavailable output voltage. The use of thisvoltage value in (iv) would give a fictitiouslyhigh value to the calculated wattage.

In conclusion, the author desires to tenderhis acknowledgments to The General ElectricCompany and the Marconiphone Company,on whose behalf the work was done whichhas led to this publication.

Service EquipmentPERHAPS the most generally useful of the

series of testing instruments recently intro-duced by Pye Radio for the benefit of agents

and their service engineers is the " All -WaveTrimeasy Signal Generator." This modulatedoscillator covers a wave range of from 12-3,o00metres (25 Mc/s-roo kc/s) infive steps controllable by aswitch, and thus covers all therequirements of service mendealing with broadcast recei-vers. An attenuator calibratedin decibels controls the radio -frequency output, which rangesfrom below r microvolt toabout o. r volt. The outputis modulated to a depth of3o per cent. at 40o c/s andfor special purposes an R.F.signal up to 0.5 volt may beobtained.

A multi -range meter iscombined with a signalgenerator in the Pye

All-purpose Tester.


The instrument also serves as an audio -frequencygenerator giving up to 2.3 volts maximum, con-trollable by an additional attenuator and deliveredthrough a separate socket.

This is a battery -operated instrument, and currentconsumption is stated to be extremely low. Thescale is hand calibrated and the equipment com-prises two dummy aerials. A somewhat similarinstrument without short wavebands is availableat a lower price.

The Pye Complete Tester comprises the signalgenerator described above combined in a portablecase with a Weston Selective Analyser. The PyeValve Tester gives direct readings on a 6 -inchscale of emission and mutual conductance of alltypes of valves and provides a check on inter -electrode insulation.

Among other Pye service equipment is an outputmeter calibrated in milliwatts and decibels, a setof trimming tools and an outfit comprises thenecessary materials-polishes, compounds, etc.-for the renovation and repair of broadcast receivercabinets.

Co-ordination of Radio andLand -Line Services

APUBLICATION of obvious importance totelephone engineers, and one that is ofinterest to radio engineers insofar as it

deals exhaustively with the technical problems ofco-ordinating radio telephonic and wire systems,is the recently published Proceedings of theInternational Telephone Consultative Committee(plenary meeting at Budapest) ; English Edition.The formulation of international standards forline characteristics and their measurement, whichhave an important influence on the technicalarrangements for the international exchange ofbroadcast programmes by wire, are dealt with.The book contains 66o pages, with many illus-trations and diagrams, and is published by TheInternational Standard Electric Corporation, Con-naught House, 63, Aldwych, London, W.C.2, at25s. nett.


The Super -Regenerative Receiver*By M. G. Scroggie, B.Sc., A.M.I.E.E.

ABSTRACT.-The principles of the super -regenerative receiver are recapitulated togetherwith the more important theories and experimental results so far published. Divergencyamong these is ascribed to the number of variables involved and the impracticability of takingthem all into full account in any rigorous investigation. Some results of tests made by thewriter under representative working conditions are summarised, and the practical effect ofvarious conditions such as quenching frequency, operating voltages, point of injection, signalstrength, and r.f. circuit resistance are noted and discussed. The application of a number oftypes of multiple -electrode valve, intended as frequency -changers, is described. The practicaloutcome of the foregoing is summarised.

IntroductionAFTER the first flush of enthusiasm

aroused by Armstrong's disclosure in19221 of the remarkable results obtain-

able by employing the super -regenerativeprinciple, this device has been something ofa radio Cinderella. Its adoption has beenhindered by the fact that the majority ofthe serious papers on the subject are inGerman ; and they for the most part are soserious as to make only a restricted appealto the practical man who has no time toflounder in a morass of somewhat speculativemathematics.

Anticipating a little, it may be said thatthe factors controlling the performance of asuper -regenerative receiver are so numerousand complex that precise design based ontheoretical investigation, such as can beundertaken with other types of receiver, isimpracticable. For the same reason it isextraordinarily difficult to establish definiteprinciples of design on the basis of experi-mental results. In the writer's opinion thisaccounts for the measure of disagreementthat exists among both theoretical andpractical publications on the subject, andfor the unpopularity that the receiver hasendured.

On the other hand, both lines of attackcan be helpful if their limitations areappreciated ; and what follows is an attemptto sort out and possibly to supplement whatis most useful and generally applicableamong the material, and to present it innon -mathematical form. This may not beuntimely, in view of the growing interest inultra -high frequencies, for which (anticipatingagain) super -regeneration is particularlyadapted.

* MS. accepted by the Editor, March, 1936.

Regenerative AmplificationVarious theories of super -regeneration

have been developed to a greater or lessextent (see references at the end of thisarticle), and are by no means in perfectagreement in their details ; nor is it easy totest them experimentally. The main prin-ciple of working is clear enough. Explanationof it is usually approached by way of thewell-known regeneration or reaction device.It should be realised, however, that thecontrasts between the working of the twomethods are more significant than thecomparisons.

Both systems depend on control of theeffective resistance in an oscillatory circuit.Reaction is the most convenient method ofcancelling a proportion of the positiveresistance and thereby improving thesensitivity and selectivity of the circuit. Itis extremely easy by means of a back -coupled valve to introduce enough negativeresistance to cancel the damping of even avery inferior tuned circuit, and thus to gofar beyond what can be accomplished by themost meticulous and expensive " low -loss "construction. Amplification of a signal canbe pushed to any amount up to and includinginfinity, which is reached when the effectiveresistance is zero. An amplification ofinfinity sounds very attractive, but actuallyis undesirable, because it means that anysignal, however small, would give rise to acontinually increasing train of oscillations,which would continue at a constant ampli-tude after the signal had ceased (Fig. I).This " hangover " would be a very incon-venient adjunct to the output from the tunedcircuit. And, paradoxically enough, infiniteamplification does not mean that the signalnecessarily is amplified to an infinite or even

582 THE WIRELE

a large extent. It, is only so when the signalis active for an infinite length of time.

The oscillation due to the signal grows ata rate which is proportional to the signalvoltage, but also inversely proportional tothe inductance ; and, as inductance is oneof the two essentials of an oscillatory circuit,it is obvious that a signal of short durationmay not succeed in making much impressionon such a receiving system, even if it weregifted with infinite amplification.

And even if the goal were worth strivingfor it is not within practical reach, for itwould be necessary to adjust the reactioncontrol, and keep it adjusted, at one par -

SIGNAL

RESPONSE

Fig. i.

ticular setting, with no latitude for errors,or.variations due to temperature, etc. And,furthermore, the negative resistance obtainedby the means available-valves and theirassociated circuits-is a function not onlyof the adjustments of the apparatus, but alsoof the amplitude of the received signal ; sothat whenever the signal is injected it eithersends the resistance negative, causinginstability, or positive, causing finiteamplification.

This is the reason why it is fallacious toassume that it does not matter how " bad "the coils and condensers may be, on theground that the loss resistance can alwaysbe nullified by reaction. The lower theintrinsic resistance, and hence the less onedepends on reaction, the less in proportionare the inevitable variations in the amountof reaction, and the nearer one can approachto zero resistance with consistency ofperformance.

Limitations of RegenerativeAmplification

When the radio frequency of the signal islow, the time limitation is the most seriousone, because there are then relatively fewoscillations to each " unit of intelligence "(as a dot in morse or a single modulationwave in telephony may loosely be termed),


and the inductance in the circuit is high, sothat before the limit in stability of adjust-ment has been reached the growth or decayof the oscillations is so slow as to distortthe " intelligence " seriously.

There are ways of compensating for such dis-tortion-tone-correction or the " Stenode "-but there are also alternative effectivemethods of amplifying low radio frequencies ;so we turn to consider the high frequencies,and more particularly the " ultra -high "frequencies (defined arbitrarily as those above3o Mc/s, or below 10 metres in wavelength).Here there are very many oscillations perunit of intelligence, and the inductance issmall, so the distortion due to inertia is lessserious a limitation than the precision andstability of reaction control. Reaction is animportant aid to the amplification of ultra-high frequency signals, because other methodstend to lose their effectiveness as the fre-quency is raised. But the extreme selectivityaccompanying its use may not be an unmixedblessing, for " searching " becomes so difficultand tedious that much or all of the trans-mission may be over before it is tuned in.Searching over a band of only 5-6 metreswavelength is equivalent to covering thewhole of both medium and long broadcastbands nine times. Coupled with thenecessity for very precise adjustment ofreaction all the time, this renders simpletypes of ultra -high frequency receiversextremely troublesome to operate, or alterna-tively (by using only moderate reaction)extremely insensitive.

Super -Regeneration : QuenchingThe principle of the super -regenerative

receiver is to vary the effective resistance ofthe circuit periodically at some frequencyintermediate between the radio frequencyof the signal (which we shall call r.f.) and the" intelligence " or modulation frequency(m.f.). This intermediate frequency is oftencalled the modulation frequency, but toavoid confusion we shall refer to it as thequenching frequency (q.f.).

The q.f. must be lower than the r.f. inorder to allow the oscillation time, duringthe portion of the cycle when the resistanceis low or negative, to build up to a usefulamplitude. In fact, viewed from thisstandpoint alone, it would seem that the q.f.ought to be as low as possible in order to

November, 1936 THE WIRELE

achieve maximum amplification. On theother hand, it must be greater than the m.f.,because it is only once during each cycle ofq.f., that the modulation amplitude of thesignal has any appreciable control over thefree oscillations set up in the tuned circuit.Moreover, suppression of the q.f. from thefinal output ought not to carry away with itpart of the m.f. band.

The q.f. is usually chosen, then, to be onlyjust above the m.f. band-about 5 to io kc/sin the case of telephony. There are, however,other considerations that ought to be takeninto account in selecting the q.f. This iswhere there appears to be some divergencyof result among the authorities. During thenegative resistance period of the q.f. cycle,theoretically oscillations need not start atall unless some impulse is applied. In theabsence of a signal an impulse always existsdue to " shot " effect in valves, similareffects in conductors, and stray interference.These being of an irregular character, andthe ultimate amplitude to which oscillationsbuild up being dependent on the intensityof the impulse and the time of its occurrenceduring the cycle, the amplitudes of successiveq.f. cycles are also irregular and give riseto a characteristic hissing or rushing sound.

When a signal carrier wave is injected, ifit is at least comparable in intensity withthe irregularities, it tends to regularise theresponse by virtue of its continuity and itsagreement with the natural frequency ofthe r.f. circuit. Consequently the hiss tendsto be suppressed if the carrier wave is ofconstant amplitude, or replaced by modula-tion frequencies if the carrier is modulated.

During the positive resistance period of theq.f. cycle any oscillations that have beenbuilt up during the preceding period arewholly or partly quenched .by the more orless large decrement introduced ; if wholly,then the apparatus starts the next q.f. cyclewith a clean sheet.

It is obvious that there is room for a gooddeal of variety in the final result of thisprocess, depending on such factors asfrequency, intensity, and waveform ofquenching, radio, and modulation fre-quencies ; constituents of the r.f. oscillatorycircuit ; and characteristic of the valve orvalves associated with the circuit. Con-sequently any theoretical or experimentalinvestigation, even if well founded, can hardly

SS ENGINEER 583

take account of all these comprehensively ina simple manner, and are likely to showdivergencies according to the particularconditions selected for investigation. ThusArmstrong' came to the conclusion that thelower the q.f.-subject only to the m.f.limitation-the better. Atakas, on the otherhand, adduces theoretical and experimentalresults pointing to a much higher optimumq.f. ; 200 kc/s for example. According tohim, only the initial part of the train ofoscillations during a q.f. cycle is subject toinfluence by the signal. After that, anyprolongation of the period of negativeresistance merely sustains an amplitudewhich would have been reached by that timeeven without the stimulus of a signal.

Fig. 2 shows the growth and decay ofoscillation during one quenching cycle,derived from oscillograms by Ataka. Theshaded area represents the increase due tothe injection of the signal, and it is seen thatits effect is to advance the growth of oscilla-

RESISTANCE OFOSCILLATORYCIRCUIT

AMPLITUDE OFOSCILLATION

Fig. 2.

tion, but not to increase the rate of growth.The maximum amplification is attained whenthe number of shaded areas per second is asgreat as possible, short of encroaching ontheir duration ; which leads to quite ahigh q.f.

At the same time, Ataka states that theamount of advance due to the signal is greaterwhen the q.f. is low, and therefore an ex-cessively high q.f. results in a falling off insensitivity. The optimum q.f. varies tosome extent with circuital conditions.

The manner of growth described byArmstrong and some subsequent writerss ismore like that illustrated in Fig. 3. Here theoscillations grow exponentially at a ratecontrolled from the start by the strength ofsignal, and therefore so long as the amplitudeis not restricted by valve saturation theamplification increases as the q.f. is reduced.It should be pointed out that a rectangularquenching waveform is assumed.

B 2

584

The oscillograms shown by David2 weretaken under rather artificial conditions anddo not show oscillations building up in theabsence of a signal. They agree with thoseof Ataka, however, to this extent, that it isthe place in the quenching cycle at which

oscillations start thatRESISTANCE varies with the signal

+I strength, while thefinishing point is in-dependent of thesignal. They agree,also, in showing a con-

siderable angle of lag between the start of thenegative resistance half -cycle and the startof oscillations, even with a very strong signal.

Characteristics of Super -RegenerativeReception

Whatever the precise form the growth ofoscillations assumes (and undoubtedly itvaries considerably with conditions) theaction is very different from that of simplereaction, which results in extreme selectivityas it is pressed to its limit where the decre-ment tends to zero. As super -regenerationis increased, on the contrary, the incrementsand decrements become greater and theoperating point is carried farther away,above and below, from the zero resistanceline. Selectivity, then, is comparativelyvery poor under the usual conditions ofsuper -regeneration. This has ruled it outfrom the medium broadcast band (to whichit was originally applied), and with thegrowing congestion of the higher frequencybands its usefulness has now becomequestionable up to about io-zo Mc/s. Butin the ultra -high frequency bands inselectivitymay be, as we have seen, a considerableadvantage.

The actual degree of selectivity obtaineddepends very largely on operating conditions,as will be discussed in detail later. In themeantime it may be noted that David hasproposed the use of a quenching waveformof the type shown in Fig. 4 to give greaterselectivity, by making the resistance nearlyzero during the greater part of the quenchingcycle. High selectivity in association withthe characteristics of the super -regenerativereceiver is obtained by combining it withthe superheterodyne principle.13 This devicealso gets rid of another objectionable featureof the super-regenerator-its radiation.

AMPLITUDE

OSCIOFLLATION

Fig. 3.

THE WIRELESS ENGINEER November, 1936

As regards sensitivity the super -regenera-tive receiver far exceeds the best of whichsimple reaction is practically capable. Buttoo much importance should not be laid onan unqualified comparison of sensitivity.Sensitivity being almost unlimited, whatreally counts most is signal/noise ratio ; forif one strives after mere sensitivity it mayhappen that the result is a signal/noise ratiowhich is much worse than that given bycritical reaction, the product of which canalways be amplified at audio frequency tobring it up to the volume given by super -regeneration. The real advantage lies inthis high sensitivity in conjunction withnon -critical and stable adjustment of verysimple apparatus. In the design of receivers,then, sensitivity should be considered in itsproper relation to signal/noise ratio and easeof control.

Those who have any experience of thesuper -regenerative receiver will agree thatfor variety of unexpected noises and effectsit is probably unrivalled. It seems to belargely a matter of luck whether a circuitthat is tried performs in a useful and con-trollable fashion, or behaves with exasperat-ing perversity. It is possible, for example,to provide all the conditions for operationand yet to obtain no super -regenerative effectat all. Or reception is accompanied by allsorts of whistles ; ornoise is less of a merebackground than anall - pervading con-fusion. Or the re-ceiver respondssharply to the signal at numerous pointsover a wide band of tuning.

In examination of these effects severalstates or zones of super -regeneration havebeen distinguished by investigators. Daviddescribes three of these (for all details seehis paper2) :-

A . This is the normal super -regenerationdescribed by Armstrong. The negativeresistance is small enough for the growth ofoscillations to fall short of valve saturation ;the positive resistance is great enough forthe free oscillations to be completely ex-tinguished at each quenching period ; andthe maxima attained during each periodare proportional to signal e.m.f. Receptionof telephony is unaccompanied by self -generated whistles.

RESISTANCE


B. In this state whistles are produced byan incoming carrier wave beating with thepartially extinguished free oscillations leftover from the previous period. These beatsare themselves of inaudible frequency, butaudible beats are produced by a stroboscopiceffect due to the phase relationship betweenthe two sets of oscillations varying from onequenching cycle to another. Obviously inthis case the positive resistance must beinsufficient to cause complete extinctioneach cycle. This state is unsuitable forreception of telephony, but David pointsout its advantages over simple reaction forreceiving high frequency c.w. signals : inaddition to the sensitivity being higher,adjustment, both of regeneration and fre-quency controls is far less critical. (Thesefeatures, together with the large outputpower released by a signal, make the super -regenerative receiver particularly suitable foroperating automatic call devices). It isalmost impossible to hold an audible beatnote for long periods with simple apparatusdepending on ordinary reaction. Butobviously the method is open to interference,and if inapplicable for this reason it isnecessary to adopt very refined methods formaintaining frequency stability at bothtransmitter and receiver.

C. This is described as the " anti -jamming " state, and is obtained when thenegative resistance is great enough to causevalve saturation in the presence of a c.w.signal. In these circumstances there is acondition of immunity from interferenceby damped impulses-atmospherics, spark,etc.-a property that is utilised, for example,in police car receivers.13

The quenching cycles in each of thesecases are compared in Fig. 5.

The present writer's experiments, 'detailedlater, confirm the above divisions-or atleast the first two of them-but suggestthat the classification is not entirelycomprehensive.

Another important factor in super -regenerative reception is what might becalled the A.V.C. effect. When a modulatedsignal from a standard generator is appliedto a sensitive superheterodyne receiver ofthe usual delayed A.V.C. type, the back-ground noise is at first large when the signalis zero. The modulation frequency outputthen grows as the signal is brought in, and,

SS ENGINEER 585

when it reaches full volume, any furtherincrease in signal strength causes the back-ground gradually to disappear.

Exactly the same is characteristic of thesuper -regenerative receiver. It should benoted that, although the output may remainconstant within a few db. while the input isvaried from io2 to toe pN, over the wholeof this range the output is reasonablyproportional to the percentage modulation.This distinguishes the effect from the

RESISTANCE

(a) (b) (C)

Fig. 5.

saturation obtained with the simple regenera-tive detector, in which the modulationproportionality is much upset. The effectis the subject of a paper' of which an abstractappears on page 35 of The Wireless Engineerfor January, 1934, and is also dealt with byAtaka3 (p. 874) and Lewis & Milner.12

An extremely novel and ingenious applica-tion of this effect is a cathode-ray voltmeterhaving logarithmic characteristics over avery large range (10-105 µV), forming partof a resonance curve indicator such as isused in the examination of highly selectivereceivers.11

Means for Obtaining Super -RegenerationHaving considered the characteristic

features of super -regeneration, we now goon to the means for obtaining it, and theinfluence of operating conditions on results.So far reference has - been made to thequenching process as a cycle of alternatelynegative and positive resistance. In practicethis is obtained by providing some form ofregeneration to set off against the intrinsicresistance of the r.f. oscillatory circuit, andthen varying it periodically by means of avalve oscillating at the q.f. Sometimes thisis so arranged that a normally stable circuitis brought into the oscillating condition bythe q.f. oscillator, and sometimes the circuitis normally in the oscillating condition andthe effect of the q.f. oscillator is to bringpositive resistance into it. Althoughtheoretically it does not matter in whichmanner a given periodic resistance charac-

586 THE WIRELE

teristic is obtained, in practice the types ofcharacteristic actually possible and theamenability to control depend on how thequenching is applied.

The variety of operating conditions to beconsidered, and the impossibility of takingfull account of all these factors mathe-matically so as to be able to predict theperformance of any actual design, havealready been pointed out. A completeexperimental investigation would also betoo laborious. And it is curiously difficultto derive any really general conclusions aboutsuper -regeneration by trying to isolate andtackle individual problems experimentally.In attempting to do so the difficulty is tosimplify the conditions without renderingthem highly artificial. The experimentalwork to be described makes no claim torigour or strict generality, but is rather an

R.C.

O

100 un F

1MU

MODULATEDU.H.F.

OSCILLATOR

X

R.F.C.

oi)

R.F.C.n nR.F.C.

601) 6.0

SS ENGINEER Noveniber, 1936

XL valve was used unless otherwise noted,being a midget valve with very small inter -electrode capacitances. The reaction con-denser RC was of the trimmer type, having amaximum of about ioopµ F, but wasgenerally set very low. R.f. chokes woundwith about 5o turns on half -inch ebonitetubes, one inch long, separated the r.f. circuitfrom the q.f. oscillator and m.f. circuits,which were also kept at a distance of about2 feet to allow the orientation of the receiverportion to be controlled. The q.f. wasgenerated by a separate oscillator andintroduced into the anode lead to the r.f.valve, and V, (quenching voltage) measuredby a cathode-ray oscillograph and expressedin peak volts. The r.f. signal was derivedfrom a modulated oscillator with variablescreening (biscuit -tin type) and the m.f.output from the receiver coupled to an

AC/P1

9-400 k cisJOAO°, -II- 100-200 V.

0'02 F

25,000 0

0-5mA

0 -100 V.

9 0 ' 2 5 H .0-1µF

0-10,000wAssce--

r

Fig. 6.

attempt to examine the performance ofreceivers under typical working conditions.

Test CircuitThe circuit used for many of the tests is

shown in Fig. 6. The r.f. receiver portionwas of the Hartley type, with a 9 -turn coilof 12 s.w.g. bare copper 2 cm. dia. by 3.5 cm.long. The tuning range with this coil wasabout 40-60 Mc/s (7.5-5 metres). A Hivac

OUTPUTMETER

amplifier (" Gram " terminals of an ordinaryreceiver) and output meter.

Signal strengths on this u.h.f. band wereonly roughly comparative, but below io Mc/swere derived from a standard signal generatorand were therefore under more exact control.

Results of TestsThe performance of this receiver was

examined at a signal frequency of 43 MO


(7 metres), and at a wide selection of q.f.s,with variation of V, and actual anodevoltage (Va) of the receiver valve. Undertypical conditions the threshold of oscillationwas reached with V. = 35, at which /.was 0.5. mA. As it would be tedious to givethe results in full detail the following arethe principal conclusions drawn :

(I) As q.f. is raised, a higher V, is neces-sary. Thus while at 20 kc/s the optimum V,is of the order of 12 volts, at ioo kc/s it ismore like 7o volts. This is what theorywould lead one to expect.

(2) As q.f. is raised, the optimum V. islower.

(3) As q.f. is raised up to 30o kc/s, super -regeneration becomes less certain, controlis more tricky, sensitivity is lower, butsignal/noise ratio tends to improve.

(4) Over certain bands of q.f., super -regeneration fails altogether. This is inagreement with theory set forth in a paperby Barrows ; see also 3.

(5) From about 40 kc/s upwards in q.f.(depending on the signal strength) theresponse divides into a number of more orless separate resonances (Fig. 7). Measure-ment shows the intervals between theseresonances to be equal to q.f. The freeoscillations tend to synchronise with amultiple of the q.f. This spreading into anumber of separate responses is anotherobjection to a high q.f.

(6) Optimum q.f. is difficult to determinewith certainty, but although V, was variedover a wide range the comparatively highoptimum q.f. of Ataka definitely was notconfirmed, but rather a continual increase inamplification as q.f. is lowered, and a choice

LOW Q F MEDIUM Q.F.HIGH Q.F.(OR STRONG (OR WEAK

SIGNAL) SIGNAL)

Fig. 7.

probably in the region of 15 kc/s on thegrounds of background noise and ease inseparation of q.f. from m.f.

(7) At any one q.f. there is a tendency forthe optimum V, for any value of V. to besuch that V, V. is constant, with a very

Vflat optimum for -g. As q.f. is increased,V.

V, + V. rises.

SS ENGINEER 587

(8) Under certain conditions-notably,high q.f.-behaviour is similar to that withsimple reaction, except for increased sensi-tivity ; the output increases at a rapidlygrowing rate as V. or V. or both are in-creased, until a " threshold of oscillation "(? change from " A " to " B " states dis-tinguished by David) is reached, at whichadjustment is very critical, and beyondwhich the modulation loses its characteristictone and becomes " mushy."

(9) With V. substantially higher thanthat required for self -oscillation in theabsence of V5, the noise level is very highand increases with V5, the sensitivity is low,and there is a tendency towards David's" B " state.

(io) Other things being equal, sharperresponses are obtained when V, is low andV. high (approximating to that necessary tostart self -oscillation). This agrees withtheory, for it corresponds to a condition ofnumerically low average oscillatory circuitresistance.

(II) The signal/noise ratio sometimesV, .

im-

proves as -- increased ; sometimes theV.

reverse. The controlling factor is obscure.(12) Increasing the receiver tuning from

43 Mc/s to 6o Mc/s, the q.f. required to givesimilar results is greater, but it is not possibleon the evidence available to say definitelythat it is proportionately greater.

(i3) If it is desired to cover a wide bandof receiver frequency, the most importantpractical aid is a closely constant reactionsetting. This corresponds to a constantlevel of the base -line in Fig. 5. It hasalready been noted what a wide variationof results depends on the nature of thecharacteristics exemplified in Fig. 5. Whileother factors-V5, q.f., etc.-are ordinarilyunaffected by receiver tuning, the base -linelevel may not be, and this is a prominentcause of difficulty in handling the receiver.

Anode versus Grid InjectionEase of obtaining and maintaining favour-

able working adjustments is of outstandingpractical importance, and is the feature whichthe super -regenerative type of receiver hasnotoriously lacked. Working with a lowq.f., and a V. well below the threshold ofoscillation, the circuit of Fig. 6 has beenfound reliable and stable (except in so far

588 THE WIRELE

as no special provision in this respect ismade for tuning over a wide waveband). Butmost of the super -regenerative circuits thathave been published show grid injection ofthe quenching oscillation. Next, therefore,a comparison was made between anode andgrid injection.

The preceding circuit (Fig. 6) was supple-mented by a grid coupling coil as shown inFig. 8.

Grid injection introduces an extra com-plication-the depression in grid voltage dueto rectification of V0. This makes thereceiver considerably more difficult to control,because a readjustment of V, alters the gridbias of the valve, and hence the " lumpedvoltage " (Va + p.V0) of the valve, on whichr.f. reaction depends. It is still worse in asingle valve super -regenerator, in which thequenching oscillation is set up by the same

Fig. 8.

valve responsible for the r.f. reaction ; forthen the two adjustments are interlocked.What happens, for example, is that when V,is reduced the resulting increase in gridvoltage causes " squegging " due to excessiver.f. oscillation. If then q.f. oscillation isincreased, r.f. reaction is inadequate.

After having made allowance for depres-sion of mean grid potential by grid injection,all tests showed the behaviour to correspondwith that resulting from anode injection.Thus, Va must of necessity be greater thanthat at which r.f. oscillation starts ; but, ifonly slightly more, and V, is large, resultscorrespond to those with anode injectionwhen V. is low and V, large. When at thesame time the q.f. is high, it is possible toduplicate the " oscillation threshold " typeof tuning previously noted in the case ofanode injection (par. 8 above). And withvery low V, the extremely noisy and un-favourable condition described in par. 9 is


obtained. The values of V, are, of course,lower-tests showed them to be lower, asone would expect, at least approximatelyin the ratio of Z. It is difficult to confirm this

with exactness, owing to the difficulty inmeasuring /.1. under equivalent working con-ditions for the valve.

Careful quantitative comparisons of thetwo methods at a fixed q.f. of 20 kc/s,employing a valve voltmeter for moreaccurate measurement of low values of V0,were (like most results of our investigationof super -regenerative receivers) singularlybarren of clear conclusions, in spite ofstrenuous attack by graphical analysis.The following are of interest however :

(I) With anode injection the m.f. outputfor a given r.f. input rises with both V,and Va, but signal/noise ratio tends to fall.

(2) With grid injection, output also riseswith Va, but falls as V, is increased. Withconstant V0, varying V. has, little or noeffect on Ia. Because of this it is difficultto deduce how sensitivity depends onlumped voltage, and hence how much theraising of V, in grid injection affects resultsby depressing the grid and how much byincreasing quenching, but there appears tobe no doubt that the fall in output as V, israised is due to grid depression, and that ifVa were raised sufficiently to compensatefor this the output would rise above theoriginal level.

(3) With either method of injection,increasing V, (Va constant) decreases'a.

From a practical point of view the lowerV, required for grid injection is an advantage,and the possibility of doing everything witha single triode may appear to some workersin that light also ; but in the author's opinionthese considerations are outweighed bythe greater battery voltage required, and(more especially with the single -valve circuit)the much lower stability and ease of control.

Mention must be made of what used to becalled the Flewelling circuit. This is asingle -valve super -regenerator in which noquenching components appear. The quench-ing action is obtained by suitable choice ofgrid leak and condenser and tightening thereaction coupling in such a way as to causesquegging at a workable frequency forquenching. Although it is possible to obtain

Norember, 1936 THE WIRELE

very good sensitivity in this way, adjustmentis very tricky, especially when tuning overa r.f. band ; it is sometimes quite usefulwhen an extremely simple compact receiveris required for working on a more or lessfixed frequency.

Signal Generator TestsOwing to the technical difficulties of

measurements at u.h.f., further investigation

bSIGNALGENERATOR

DUMMYAERIAL

SS ENGINEER 589

was kept constant at the point marked(I000 1.4.11) and the modulation percentagevaried. The proportionality of output maybe judged from Fig. io (b).

It is also interesting to note the effect ofvarying r.f. inputs on the shape of theresponse curve. Some typical results aresketched in Fig. ii. The effect of increasingthe signal strength beyond that necessary toovercome what may be referred to as the

100 00F 14-'\

na1 M11

0'065 H

O'065H

cP 0'013 H.

1,000 -5,000,0F

25,000 0

1,000µµ F

2,5000 t F

was made at lower signal frequencies between6 and io Mc/s, with a standard signalgenerator as the source.

The previous test circuit, although itworked quite well after substitution of anappropriate r.f. coil, was unsuitable formeasurement purposes owing to the largeamount of interference picked up by leads.A more compact arrangement was thereforeadopted (Fig. 9). To minimise the load ofthe receiver valve on the q.f. oscillator, a5 : z step-down was adopted. Three q.f.swere available : 20, 15 and 9 kc/s. The r.f.was kept at 7.5 Mc/s.

Fig. io (a) shows the input/output charac-teristic. The curves shown were actuallytaken with a q.f. of 9 kc/s, but those with15 and 20 are similar. The output scale isarbitrary, being variable by means of them.f. amplifier gain control. The rise inoutput above zoo µV input was due to ashift in tuning caused by the increasingsignal intensity. It is interesting to comparethe slope of these curves with that relatingto an ordinary receiver without A.V.C. Toshow that this super -regenerative " pseudo-A.V.C.," like the genuine article, affects ther.f. but not the m.f. amplitude, the r.f. input

g. 9.

TO M.F.AMPLIFIER

imaginary A.V.C. delay voltage is to broadenthe response. The shape of the responsecurve is, in general, irregular ; and there areusually multiple side responses. Someresponse curves with ordinary critical re-action instead of super -regeneration areincluded for comparison.

Influence of R.F. Circuit ResistanceCertain writers (e.g., 2 and 9) have sug-

gested that it is advantageous to use a highlydamped r.f. circuit, in conjunction with alarge amount of reaction. This point was

10

-NORMALOF A

SLOPERECEIVERy/ Vq .30

Vq =15-

G F9-

, 1 I10 10 10 1

p.V SIGNAL INPUT

(a)

Fig. io.

10 0 25 50 75PERCENTAGEMODULATION

(b)

therefore given special attention, and carefultests were made to determine what sort of

590 THE WIRELE

influence circuit damping has on perform-ance. The tests were made at approximately9 Mc/s r.f., using a coil of 14. /LH selected forlow r.f. resistance, giving a fairly highdynamic resistance (of the order of o.r MS2).Artificial damping was introduced by con-necting non -reactive resistors, of Io,000 SZupwards, across the whole coil, which wascentre -tapped in the usual Hartley circuit.

The results obtained with this circuitunder a great variety of operating con-ditions consistently led to the conclusionthat the most favourable conditions are :

(r) No added damping.(2) Full normal Va for the type of valve

(60-80 volts).(3) Adequate V (say, 20 volts ; but not

critical) all three allowing of(4) Minimum reaction (capacity control).The most favourable all-round conditions,

for sensitivity, signal/noise ratio, and m.f.output, appeared to coincide with thethreshold of r.f. oscillation (in the absence

6

E

a.a.I- 20

4

M.Q.F. =20 kc/sV =40 V.9 MEM-

----WITHWITHOUT

SUPER -REGENERATION..

111 PRI 11111111!A. EaltralriiiiiRapr6-1\

0..),AN 10.0ablida noc)`) I :AIDAIMEN EPI-AMalai ,ERA V Ea11a Fillaffillaaraa i ooc; 'Ma

71 72 73 74 75 7'6 77 78

FREQUENCY- Mcis

Fig.

of quenching oscillation). It is quite possibleto obtain super -regeneration with zero, oreven negative Va., by liberal use of reactionand V4,, but these conditions are relativelyunfavourable. Any adjustment necessitatingan increase in reaction led to inferior results,whether tested by signal generator or actualreception.

Application of Multiple -Electrode ValvesUp to this point the circuits illustrated

have employed triode valves, with separatequenching oscillators. There is virtuallyno limit to the variety of valve and circuitalarrangements ; it is not practicable to devotespa,ce to even a representative selection of

79 80


them. But one class is considered by thewriter to be worthy of special notice, on thegrounds of theoretical interest, practicalvalue, and paucity of published information ;namely, the use of valves of types now incommon use as superheterodyne frequency -changers.

The first valve of this class to be appliedthus by the author was the Mullard FC4

1 *f

100 ttuF

-,ran1 M

(\F.C.4

41

01) 05-0.Et 0'4H.

5. tegF

M

9 I1

500 tut

Fig. 12.

0'065 H.

c2

LL

0

0

TO M.F.AMPLIFIER

octode (Fig. 12). The two innermost grids,forming the usual oscillator section, found acongenial task as the quenching oscillator,with about 4o volts on G2, which was alsosupplied to G3 and G5 (which are normallyrun at about double this voltage). The r.f.input was taken to the control grid (G4). Theoutput was taken from the anode, run at100-200 volts ; the actual voltage being ofminor importance.

At first sight it may not be clear how r.f.reaction is obtained, as no provision appearsto be made for it. Remembering that thevirtual cathode, so far as the anode andcontrol grid are concerned, is somewherebetween the physical cathode and G4, ther.f. circuit is effectively a variety of theColpitts or capacitance potential dividercircuit (Fig. 13). The dotted condensersrepresent the capacitances from the virtualcathode to physical cathode and control grid.

A curious feature-with this valve atleast-is that, contrary to the usual experi-ence, oscillation is less free at lower radiofrequencies. At medium broadcasting fre-quencies, for example, oscillation could notbe obtained at all. No difficulty was experi-enced in obtaining vigorous oscillation at


75 Mc/s (4 metres). Even on the 6 Mc/sband, a high ratio was essential-the series

capacity from the dummy aerial had to belimited to 2 or 3 µµF-and a shunt ofoa MS2 was nearly enough to kill it. On the4o -6o Mc/s band, however, greater libertiescould be taken, and a wide band covered bytuning condenser.

Oscillation appears to be freest with G1 andG2 at zero (cathode) potential, but whereasG2 may be varied considerably withoutimportant effect, G2 controls I. and /,,,,very steeply, and oscillation also. As mightbe expected, therefore, the quench coilcoupling influences the r.f. regeneration aswell as the q.f., and, in fact, is the onlycontrol apart from r.f. tuning. Contiary towhat might be expected, control is notablyeasy and stable. Depending to some extenton the q.f. chosen, and on other conditions,sensitivity reaches a maximum at or near thepoint where coupling is loosened sufficientlyto bring q.f. oscillation to the verge ofstopping. Alternatively, the coupling maybe fixed, and V varied by a suitablepotential divider.

Quantitative comparison with a triodereceiver showed results not unlike thoseshown in Fig. xi, but sensitivity was higher.The octode also " handles " better ; and itis an obvious advantage to work with anuntapped uncoupled r.f. coil.

It was while examining this arrangementthat the world's simplest super -regenerative

Fig. 13.

circuit was accidentally arrived at (Fig. 14).This clearly works on the squegger principle ;the q.f. is controlled by the grid condenserand leak. The lead from G1 must be longenough to have appreciable inductance ;otherwise a high potential point in theelectron stream is short-circuited and oscilla-

tion is weakened below squegg point. Unlikemost circuits of this type, it is able to coverquite a wide band (e.g., 4-6 Mc/s) withoutadjustment of q.f. But the best resultswere inferior to those obtainable with thequench coils. It is interesting to note thatthe " auto -super -regeneration " would onlytake place with V below about 3 volts ;above this, it ceased, while quench coilaction then became possible.

The corresponding battery octode, theFC2, quite expectedly oscillates less readily,

OUTPUT 150V.+

30V.+

Fig 14.

and the loading on the r.f. circuit must bekept very small. The electrode voltagesshould also be selected carefully by trial ;the writer found it advantageous as regardsnoise level and sensitivity to work withquite a low anode voltage-of the orderof 50-and slightly higher voltages on grids2, 3, and 5 ; 7o for example.

A battery heptode was also tried-theOsram X21. Results are similar to thosewith the FC2, and satisfactory r.f. oscillationobtained, with suitable lay -out of wiring, byapplying 15 volts to G2, 6o to G3+5, andIoo to the anode. The optimum voltages,however, seem to depend very largely onother circumstances.

With the untapped r.f. circuit illustrated,the virtual tapping, and hence the extent towhich reaction approaches the optimum,depends for the most part fortuitously onthe internal arrangement of the valve. Whenthe approach is not sufficiently close, betterresults can be obtained by tapping the coil.With the X21, for example, the virtualtapping appears to be too near the grid end,for stronger oscillation was obtained with thefilament approximately one turn furtheraway from the grid end (Fig. 15). A similarcircuit can be used embodying the Osram X41triode-hexode frequency -changer valve.

592 THE WIRELESS ENGINEER November, 1936

An attempt to use a tetrode, with theanode providing q.f. dynatron oscillation,and the outer grid giving r.f. oscillation bymeans of an ordinary reaction circuit, wassuccessful up to the point of setting up thenecessary oscillations, but very little further(Fig. i6). The circuit handles badly andresults are poor.

Summing UpIn the super -regenerative type of receiver

sensitivity is very high ; selectivity andsignal/noise ratio (if one considers internallygenerated noise only), in general, low ;and there is considerable radiation. Forthese reasons it is unsuitable for long ormedium wave reception, doubtful for shortwaves, but useful for ultra -short wave work-more especially for compact portable setsor for simple apparatus.

When suitably adjusted, discriminationagainst interference is markedly better thanwith other types of receiver.

A useful feature, particularly for mobilereceivers or transmitters, is that, in spiteof being much less elaborate than the super-heterodyne, it shares the benefits of A.V.C.as commonly provided in the latter.

Selectivity can be improved by arrangingthe amplitude and waveform of the quenchingoscillation, so that the mean resistance(regardless of sign) of the oscillatory circuit

Fig. 13.

is reduced ; and the signal/noise ratio byavoiding a very low quenching frequency ormuch reaction, and by adjustment ofquenchffig voltage.

When high selectivity and absence ofradiation are necessary, a superheterodynepre -selector can be added.

A high quenching frequency is, in general,undesirable ; it necessitates greater quench-ing power, reduces sensitivity, causes multipleresponses, and is associated with uncertainty

of operation. On the other hand, an ex-cessively low quenching frequency increasesnoise, distortion, and filtering difficulties.Approximately r5 kc/s is suggested as auseful compromise.

Fig. 16.

It is preferable to obtain a low radio -frequency circuit resistance by " low -loss "construction rather than by much reaction.

It is much easier to achieve satisfactoryoperation and ease of control by providingseparate quenching and regenerating valves;and anode injection is preferable to gridinjection. Alternatively a frequency -changer type of valve may be used, whichavoids having to provide the appreciablepower needed for anode injection, and alsodispenses with radio frequency reactioncomponents.

REFERENCES(Only a selection of the more important literature on the subject

is included.)" Some Recent Developments of Regenerative Circuits "

E. H. Armstrong, Proc. I.R.E., Aug., 1922, Vol. 10, pp. 244-260.2 " Les Super -Reactions " : P. David, L'Onde Elearsque, June,

1928, Vol. 7, pp. 217-259. Also correspondence, June, 1933, Vol. 12,pp. 326-328.

3 " On Super -regeneration of an Ultra -Short -Wave Receiver " :H. Ataka, Proc. I.R.E., Aug., 1935, Vol. 23, pp. 841-884.

H. 0. Roosenstein, Hochf:fech. u. Ekktakus., Sept., 1933,Vol. 42, No. 3, pp. 85-89.

" On Super -regeneration " : E. 0. Hulbert, Proc. I.R.E.,Aug., 1923, Vol. 11, pp. 391-394.

" Tests on Five Ultra -short Wave Receivers " : R. L. Smith -Rose and H. A. Thomas, The Wireless Engineer, April, 1932, Vol. 9,pp. 186-194.

" A Balanced Modulator Super -regenerative Circuit " :W. Van B. Roberts, Q.S.T., July, 1932, Vol. 16, pp. 19-20.

" Frequency Modulation and the Effects of a Periodic CapacityVariation in a Non -dissipative Oscillatory Circuit " : W. L. Barrow,Proc. I.R.E., Aug., 1933, Vol. 21, pp. 1182.-1202.

" A Study of Super -regeneration " . D. Grimes and W. S.Barden, Electronics, Feb., 1934, pp. 42-44.

10 " Oscillating Circuits with Slowly Pulsating Attenuation " :A. Erdelvi, Ann. der Physik, Series 5, No. 1, Vol. 23, pp. 21-43.

" " A Logarithmic Cathode -Ray Resonance -Curve Indicator "S. Bagno and M. Posner, Radio Engineering, Jan., 1936, pp. 15-17.

A Portable Duplex Radio -Telephone": W. B. Lewis, Ph.D.,and C. J. Milner, B.A., The Wireless Engineer, Sept., 1936, Vol. 13,pp. 475-482.

" " A Modern Two -Way Radio System " : S. Becker and L. M.Leeds, Proc. I.R.E., Sept. 1936, Vol. 24, pp. 1183-1206.


A Continuously Variable Phase -ShiftingDevice*

By 0. 0. Pulley, B.E., Ph.D.(Walter and Eliza Hall Fellow, Sydney University)

present day cathode ray oscillographicequipment, it is usual to provide a timebase synchronous with the power supply

frequency as a general purpose and stand-by unit, because of the essential simplicityof the apparatus. In the investigation ofpower supply problems, e.g., switchingtransients and the like, such a synchronoustime base is essential. It is frequentlydesirable to be able to alter the phase of thetime base so that the portion of the cycleunder observation occurs on the usefulpart of the oscillograph screen and it iswith a device for effecting the necessaryphase shift that this article is concerned.

The usual phase shifting transformer (orvoltage regulating transformer) as used inpower engineering is usually far too bulky foruse as an oscillograph adjunct and its in-complete iron circuit gives rise to largeleakage fields so that it must be placed somedistance from the oscillograph itself. Also,if a multi -phase supply is not already avail-able, special phase -splitting apparatus mustbe incorporated. Another type in commonuse is the resistance -capacity combination,either single circuit or bridge connection, inwhich phase changes are effected by varyingthe resistance ; the apparatus operates froma single phase supply but the phase variationis limited to considerably less than 180° andthe input impedance varies over wide limits.The use of potentiometers in place of variableresistances maintains a constant input im-pedance but the phase shift is then limitedto 90°. The complete shift of 360° can beeffected by switching devices but the lackof continuity and the frequently concomitant" blind " spots are a disadvantage.

Fig. is gives a schematic diagram of theapparatus to be described ; one arm of thecircuit consists of capacitances and a re-sistance in series, while the other has in-ductances in series with a resistance. The

* MS. accepted by the Editor, March, 1936.

.593

elements are so proportioned that r/c0C1 =I/c0C2 = R0/2 = RL/2 = wL1 = 01/.2, where(.0 is the angular frequency of the supply(in this case, 5o c/s). The vector diagramfor the circuit has been drawn in Fig. ib,

(a) (b)

Fig. I.

where Vol is the voltage across the con-denser C1, or, more precisely, Vav is thepotential of the point c with respect to that ofpoint a=and so on. If the resistances havea sliding contact, i.e., if they are potentio-meters, then it is possible to select potentialsalong be and fe and by suitable choice therelative 'potentials of the two points mayhave any required phase difference. Supposethe two potentiometer arms are at thelowest points of their travel, i.e., at the pointsb and f ; then the potential of b with respectto f is the vector fb. If the tapping on theresistance R, is moved to a point such asx then the end b of the vector fb is movedto the point x and the relative potential isthe vector fx ; if the potentiometer is movedto the upper limit of its travel (to the pointc), the potential difference becomes fc andthe phase has been moved through +90°.If then

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