US2220841A - High-frequency detector - Google Patents

Description

NOV 5, w40 'GQ F. METCALF 2,220,841 n HIGHJREQUENCY DETECTOR Filed March 50, 1940 zon-400V y if T? 66 ??Z? 400V Figlia. Figlie., O O O O d O O 0 O o 6 @by 59 O O Fl'gb. FQS.

In Venter:

20a -400 mix,

George F Metcalf;

Patented Nov. 5, 1940 j UNITE srATEs l 2,220,841 HIGH-FREQUENCY DETECTOR George F. Metcalf, Schenectady, NLY., assignor to General Electric Company, adcorporation of New York U Application March so, 194.0s'eria1Na-32a9es 4 claims. (c1. '25o-27) 'The invention described and claimed hereinl y By .charge density modulation I mean the' I ,controlled production of irregularities in the distribution of charges within a conduction-current stream. rThus, a charge density modulated electron beam is a beam in which at any given `time the electrondensity varies from point to point along they beam in accordance with some cohtrolled 4pattern of variation. l

-By velocity modulation? I meanV the controlled production of irregularities in charge ve- ..95

locities within a conduction current stream.

Thus,l a velocity modulated electron beam is aY beamin which at any given time `the electrons at` various4 points along the axis v`of the beam areV moving with -di'erent -velocities according to IKO y Icio somercontrolled,pattern ofvariation.

Y Quantitatively, any type lof modulation may be measured as the ratio of the magnitude of the .maximum departure of the modulated quantityl from its average valuetofthe magnitude of such averager value. Thus, a charge density modulated electron beam in which the electron-density along the beam axis varies from zero to twice .theaverage density may be said to possess one hundred per cent charge density modulation. y

In conventional electronicvacuum devices the .control member or grid is ordinarily so constructed and arranged as to aiect directly the electron emissionv from the cathode, thus producing a type of charge density modulation, as

hereinbefore defined. It may be shown that the current Variations Iso produced by the grid have the effect of inducing a similarly varying current in. the grid circuit. Under ordinary conditions and at low frequencies ,this induced current which is caused by instantaneouspdifferences in the lelectron charges approaching yand receding from thegrid is relatively small and is approxlimately 90 degrees out of phase with the. grid voltage, so that it produces no appreciable power` loss.v However, Yas the operating'wave length is decreased so that the electron transit time becomes appreciable with respect .to the reciprocal frequency (1 /f) lof the control .grid potential; Variations, `the induced current not only-increases butbecomes more nearly in phase with the grid voltage. These .two effects combine to produce .the result that the .apparent shunt' resistance vof the grid circuit varies inversely as the second power of thefrequency of the operating voltage. It is for this reason that at very high vfrequencies (i. e. very short Wave lengths) the conventional typeof grid attains such a-low shunt im` pedance and involves such a large'vpower loss las f to be practically unusable.

VIn the application Serial No. 153,602 previously referred to, novel discharge devices vand methods of operationA are described such that the shunt impedance of the control circuit maybe maintained ata very high value even `whencontrol potentials of very, short wave lengths are involved.V In one view of theinvention disclosedin the said application,v this is accomplished by providing control electrode structures which aresoy constructed and operated as to produce primarily velocity modulation `of :the discharge current without the occurrenceof appreciable charge density variations in v,the vicinity of the. control Y electrode. The veloci-ty modulation thus produced is subsequently converted into charge density modulationunde'r conditions which have --no cuit. f y t t The present invention is especially concerned with the provision of eiective kmeans for accom- .adverse reaction lon the control electrode' cirplishing detection or rectiiicationf of a received li signal by application of the principles of velocity modulation as described above. v

The features of novelty which I desire to protect hereinwill be pointed out with matic representations` illustrating-certain basic4 elements of `velocity modulation devices; Fig. `4 isa discharge device embodying one application t particularity in ther appended claims. The invention itself,=to-` gether with further objects and advantages of my presentv invention; Figs. 4a., 4b, and 4c are representations useful in explaining the op-` eration of the device 'of Fig. 4; Fig. 5 is a graphical representation 'illustrating certain of the characteristics of the device of Fig. 4;r and Fig. 6

illustrates a modification of the device of Fig. 4. l In the drawing above referred to I--haveindicated certain voltage ranges as being. suitable for the operation of the various electrodes. It

should be understood, however, that the values given are exemplary only and that they may be varied within wide limits even to the extent of 5 changing their order of magnitude.

As explained in the aforesaid application Serial No. 153,602, an important feature of velocity modulation devices consists in the provision of a modulating space which is substantially shielded from the stream source or cathode. By virtue of such shielding, current or potential variations which occur in the modulating space have no tendency to effect changes in the cathode emission or to produce charge density variations in the space.

A modulating space as speciiied in the foregoing may comprise, for example, a space which is arranged to be traversed by an electron beam,

and the entrance and exit boundaries of whichV 20 are maintained at xed potentials with respect to one another. Thus, in the particular arrangement shown in Fig. 1, a modulating space is provided between two apertured conducting barriers or diaphragms I and II which diaphragms are electrically connected to one another and maintained at a denite potential with respect to ground. The chamber thus defined is, in use, traversed by an electron beam I3 which may enter through an opening I5 in the diaphragm I0 and leave through a corresponding opening I6 in the diaphragm I I.

In order to produce velocity modulation of an electron beam traversing a modulating space such as that bounded by the diaphragms I0 and II, the potential level of an intermediate region of the space may be cyclically raised and lowered with respect to the boundary potentials at such a rate that the velocity of any given electron in the beam is similarly affected as the electron ap- 40 proaches and recedes from such intermediate region. In connection with the particular structure of Fig. 1, this may be accomplished, for example, by the use of a control electrode in the form of a tube I9 which is positioned between the .45 diaphragms IIJ and II and spaced slightly from each of them. If the potential of this electrode is caused to vary above and below that of the diaphragms with the proper periodicity, as by connection to a source of cyclically varying con- '50 trol voltage 2I, effective velocity modulation of the transmitted beam may be obtained. The proper periodicity as specied in the last sentence is determined by the following considerations.

Consider an electron which has just passed through the diaphragm I0 at a moment when the potential of the electrode I9 is at a positive maximum with respect to that of the diaphragm. Such an electron is obviously accelerated in the `60 approach space between the diaphragm and the open end of the electrode tube. In the case illustrated, the interior of the tube I9 constitutes an essentially eld-free space, so that the velocity of the selected electron neither increases nor di- 65 minishes during its passage through the tube. If, however, the potential of the tube reverses so that the tube is at a potential minimum with respect to the diaphragm I I at the instant the electron leaves the tube, then the electron is again TO accelerated in the approach space between the tube and diaphragm. Similarily, an electron which ent-ers the modulating chamber at an instant when the control electrode is negative will be twice retarded as it successively traverses the 75 two approach spaces. These are effects which will occur most perfectly if the transit time of the electron through the tube I9 corresponds to a half cycle of the control potential applied to the tube, or to some odd number of such half cycles. I have so far considered the frequency of the control potential as the independently variable quantity. It will be obvious, however, that for any fixed frequency of control potential the electron transit time itself may be varied to fulll the condition emphasized in the foregoing paragraph. This may be done as a practical matter either by changing the length of the tube I9 or by varying the average velocity of the electron beam.

Assuming that the various elements have been properly adjusted as I have proposed, it will be seen that there is available an extremely eicient mechanism for producing velocity modulation of an electron beam. With a given peak value of the control potential, certain electrons in the beam will be accelerated by an amount corresponding to a potential variation approachingl twice that value, while others will be retarded by an equal amount. The ratio actually existing between the maximum electron acceleration or retardation (expressed in volts) and the peak value of the control potential may be called the velocity ratio of the control electrode. It will vary somewhat with the electrode geometry, but for the construction illustrated will be only slightly less than two.

It may also be noted that the production of velocity modulation by the method and means so far described is accomplished without appreciable power loss in the control circuit. This is due to the fact that the current variations produced in the vicinity of the control electrode structure are actually extremely small so that the resulting current induced in the control electrode is substantially negligible.

The precise form of control electrode structure shown in Fig. l is not essential, and in Fig. 2 there is shown another structure which may be alternatively used. In this gure the diaphragms 22 and 23 correspond generally to those already described in Fig. l. The control electrode structure, however, comprises a pair of wire grids 25 and 26 which are spaced to enclose a region of substantially the same axial extent as the r-egion enclosed by the tube I9 of Fig. 1. Since these members are electrically connected together, this enclosed region constitutes a substantially eld-free space; consequently, if the potential of the grids is cyclically raised and lowered with the proper periodicity as previously specied, velocity modulation of a passing electron beam will occur.

A still further possible modication of the control electrode structure is shown in Fig. 3, in whichthe control electrode differs from those previously described in comprising a conducting member of negligible axial extent. This member is illustrated as a ring 30, but may alternatively comprise an apertured diaphragm or other equivalent structure.

With this arrangement the optimum conditions to be fulfilled are somewhat different from those previously postulated. Specifically, it is described that the electron transit time between either of the boundary diaphragms 3| and 32 and the plane of the control electrode correspond to a half cycle of the control potential. This being true, an electron which enters the modulating space at the instant the control electrode has assumed a positive potential is accelerated throughout the duration of its passage from the plane of the diaphragm 3I to the plane of the aandeel :cntrolgi electrode.v Also,I snceia vvoltage reversal occurs -jas :the electren Apasses thecbntrol electrode, .:itfxis again accelerated, during lthe enti-re .periodofitsapassage from the planea of thefcon- 5 trol-electrode toVV the plane'pi. the diaphragm 32'.

y ypiiovid'eda.focusing velectrode in the formy of a f :Ingltheisame Wayfan. electron whichpenters the modulatingespace one-half cycle later willrbe re,- ztarded: both :as it approaches andas. it recedes "-from the fring;3; vA similar result is obtained i1' :the transit time between Ieach ofthe Vbox-indary diaphragme and the controi electrode isl anodd multiple of one-halflfcycle rather-than' asingle half-cycle; as described .in the foregoing.

The mattersz-setiforthin the foregoingparafgraplisrelate primarily :to the productionof ve.- `lecity'modulation of :an electron beam. The ef- Iectivej utilization of the, Aprincple. requires the .prvision of meansl :for subsequently` causing. the

derstood by reference to a particular discharge devicev'suchasfthat shown inFig-4.v v

*Referring particularly to Fig. 47,.;there is shown .'ardis'charge device which comprises a sealed metal envelopeinclu'ding an elongated*vr tubular 'conducting tubular member 46. zThis latter ele- `=,40mentlis' supported directly by an insulating bushing 41" and indirectly by means of a second 'tubular member 49 which is rigidly secured to.

l all-transverse barrier 5I.y

i In the'use, .of theA device the =lamentary heater 5 4'3- is energized by means of va suitable energy source, such asa battery 524 connecting' with the heater 'throught bayonet contacts 54 and- 55 and leadinconncctions associatedl therewitns The focusing electrode 46 ismaintained at cathode potential or at 1a potential which, is slightly neg'- ative with respect to the cathode, and whenv so v-'Echarged''is eiiective to concentrate' the electrons Iemittedfrom the cathode surface into afibeam kkof ygenerally cylindricaloutline.4 Such a beam -m'aylb'e giyen the' desired' velocity by'impressing .San apr'n'opriate potential between theI cathode andthe transversey diaphragm `351. The magni- 'ti'idefof the potential t'o be 'applied'varies within wide' limits'depending onthe' conditions of operation. Eor a particular case it may be on the 'order rv'of vfrom 200 to 400 voltsandfmay be provide-d1 by means o f a battery 51V connected between one ofthe cathode terminals and the :nietal'structure ofthe discharge envelope.

Somewhat spaced from the conducting'memfberor' diaphragm 5l there is provided a second dia/lvhragmy 5i!y which in the case illustrated is maintained at 'van identical potentialwith the -'rst diaphragm by being electrically connected thereto. These diaphragms are'respectively provided With central apertures 6|' and 62 and in combination define a chamber which is shielded f. trom 'the 'beam source or cathode. iIn the contemplated use of the'devi'ce thisy shielded chamf-75 benconstitutesy a modulatingv space in which the 'velocitymoduiation to produce desired ,effects ,in i Jani:feinternal circuit.A The detecting vmeans which .'comprzsesmy presentinventionrrnay :best be unyelncity of th'econ'stituentelectrons of an electrorr beam "traversing4 the .space may be aiTected or modulated-ina desiredsmanner. Such velocity modulation may bey accomplished in anyv of the Waysv described in connection with Figs' 1v to 3,`

kand in. the particular arrangement illustrated I n have shown a tubular. control electrode '.65 of the. particular type illustratedin Fig. 1.

YIn the use .of the device as a detector the potential. of the electrode 65 may be alternately raised and. .loweredfby impressing thereon a cyclicallyyarying signal voltage. This may be derived, f or example; from a radio frequency input circuit shown as including an antenna 66 and a tuned circuit comprising a condenser 61 andan-inductancev68. A battery 'Hl is also em7` i ployed; to'- impress on the control' electrode-structure a direct current potential .of the sameorder of magnitude .as that impressed on the diaphragms 5l and 59.V f f u Assuming :the device to be in normal operation, the electron Vbeam issuing from the opening v62 is velocity modulated; that is, itis charac- -terize'ct "by successive variations in electron vvelocity fromr'pointto' point along thebeam.

The :degree r`of,' 'in-odulation may vbe extremely sliglrtxifon'ly weak control :potentials areavail able; butit-may' bezchanged'into charge density modulation ofr a considerably higher order of themodul'ating space. by the-diaphragm .59, may

'bebiasedgasfbya battery 16, `to such a lowvoltage that only .'aplgircixii'natel'yI one-half lof the electron fbeam is collected', the other half being'reflec'ted @am :back alongthebeam axis. If the beam 'is ve-' l'lccity'v modulated asdescribed above,` the vfaster :lectrons ht-thecollector while the slower ones 'are repulsed by'it. Since thefaster and slower electrons are respectively hunched iin alternatelyY spaced 'groups along the axis of the beam; it will beseen that bothfthe collectedk currentand the reflected current will be charge dens-itymod'ulated.' I f methodffof separating fasty and slow electron-s described in the I-i'oregoingwillgperhaps',

itet-ter understood byl referring to the" diagrarrrmatic'representations of Figs. 4a, 4b, and 44c. In 4a,for example, I have shown a stream {ofelectrons which assumed t'crbe 've'- Iocity modulatedso has" to yconriiprlise alternate y 4gro1i1')s'o'f` fast yandsl'ow electrons. f The fast electrons are represented by the groups a, and

thelislow electrons by 'the lgroups b.v The line vd m represents theplane fof Y.the collecting anode hereinbefore described.A In"Figs. lib and 4c, I

haveshown the components ofthe electronbeam whichzh-ave A-been respectiyelyvcollected and. re,-l

flectedby the anode d. It will be seenithatleach lef lthese components is characterized by succese sive variations in charge density and isftherevfore charge density modulated. within the dei-inition vof that term previouslygivenherein. l

Fromr a consideration oi'a-theidealized situation represented. in the foregoing it might seem that yeven the. slightest degree kofffvel'ocity modulation ofthe electron beam wouldfproduce 100 vpercent charge density modulationfof:` both the collected and'reflected components vof.'cur-rent'.v That thisr I 201,953, filed April 14, 1938.)

isnot sois due in partto the random electron velocity variations inevitably present in the'beam even before-modulation takes place. Because of such variations the ideal grouping of fast and slow electrons shown in Fig. 4a cannot actually occur, some slow electrons being necessarily mixed with the fast groups and vice versa. This being so, some'random fast electrons are collected from even the slow electron groups and some random slow electrons' are reflected from the fast groups. As a result, a given amount of velocity modulation produces through the mechanism described an amount of charge density modulation which may be considerably lower than 100 per cent and which is variable with the degree of velocity modulation involved. The actual relation between velocity modulation and chargeV density modulation is determined by the ep--z'p static curve of the collector as illustrated in Fig. 5. This curve represents the variation of the collector current ip which is observed as the collector potentialep is varied in a continuous manner.

The slope of the curve of Fig. 5 is xed by random electron velocity variations occurring in the discharge stream, and by certain other considerations including particularly the inclination of the collector With respect to the electron path. It may be very steep for practical tube constructions, so that if such a bias is applied that the collector operates, say, at the point w, substantial amplication may be accomplished. (This mode of operation is more fully described and claimed in my copending application S. N. Alternatively, by operating on a curved portion of the characteristic, say, at point y or z, the relative magnitude of the reversed and collected portions of the beam varies non-linearly with the instantane- Gril()A ous beam velocity, anddetection or rectification eifects may obviously be obtained. To this end, Fig. 4 illustrates diagrammatically a high impedance detector output circuit comprising (in addition to the battery 16, which is assumed to be of such voltage as to cause operation at point y), a resistor 'I8 and a blocking .condenser 80. As-a result of the rectifying effect of the apparatus described, the voltage developed across the terminals of this circuit may be used for the "550 production of audible sounds corresponding to f160 tive oscillations. VIn the use of this feature,

which is the separate invention of Harry L. Thorson, being claimed by him in his application S. N. 264,877, iiled March 30, 1939, the returned electrons are collected on the imperforate por- -'65 tion of the header 59.

It is not necessary to the functioning of the invention that the electrode which acts to reverse the low velocity electrons shall also serve to collect the others, and in some cases the alternative 70 arrangement of Fig. 6, is found to be advantageous. Since all the elements in the figure except the output electrodes are identical with those shown in Fig. 4, they are similarly numbered. The output system includes an electron-perme- 75 able reversing electrode such as a'tubular member 88. 'I'his is biased to such a voltage that the eld established thereby reverses the lower velocity electrons in a manner which varies nonlinearly with the instantaneous beam velocity. The electrons of higher velocity pass through the electrode 88 and are collected by the anode 89 (which may, in this case, be biased to a positive potential). A detect-ion circuit is connected to this anode as previously described in connection with Fig. 4. The expedient of tilting the electrode 88 avoids returning the reversed electrons to the discharge space.

In connection with both the constructions which have been described above, it will be understood that the variations which exist in the unreversed portion of the electron beam also occur in `equal measure in the reversed portion. Consequently, in either construction detection effects may alternatively be realized by connecting the output circuit to an electrode which is appropriately positioned to collect the reversed electrons.

While I have in the foregoing referred to particular structural embodiments, various modcations may be made by those skilled in the art without departing from my invention. I therefore aim to cover in the appended claims all such variations of form and use as come within the true scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A detector comprising means for generating a concentrated beam of moving charges, a modulating element having anopening therein to permit passage of the beam therethrough, means including the said element for subjecting the said beam to cyclically varying potential gradients effective to produce variations in the axial velocity of the beam as it traverses the element, electrode structure providing a retarding field acting on the beam after its traversal of the said element for separating the high and low velocity components ofthe beam by selective reversal of the latter, the strength of the retarding iield being such that the relative magnitude of the reversed and unreversed components varies non-linearly with the instantaneous beam velocity, means for separately collecting the said components, and circuit means connecting with one of the said collecting means for utilizing the variations which exist in the collected current.

2. A detector comprising means for generating a concentrated beam of moving charges, a modulating element having an opening therein to permit passage of the beam therethrough, means including the said element for subjecting the said beam to cyclically varying potential gradients effective to produce variations in the axial velocity of the beam as it traverses the element, electrode structure providing a retarding field acting on the beam after its traversal of the said element for separating the high and low velocity components of the beam by selective reversal of the latter, the strength of the said iield being such that the relative magnitude of the reversed and unreversed components varies non-linearly with the instantaneous beam velocity, means forming a part of the said electrode structure for collecting the unreversed component of the beam, and circuit' means connecting with said collecting means for utilizing the variations which exist in the collected beam current.

3. A detector comprising means for generating a concentrated beam of moving charges, an elongated hollow element arranged to be axially traversed by the beam, meansincluding said element A for Vsubjecting the beam-to. cyclically varying potential gradients .elective to produce lhigh frequency variations in the axial velocity of the beam as it traverses the element, electrode structure providing a retarding field acting on the beam aftervts traversal of the said element for separating the high and lowvelocity components relative magnitude of *the reversed and unreof the beam by selective reversal of the latter,

the strength of the said field being such that the versed components varies non-linearly with the instantaneous beam'velocity, means forming a l part of the said electrode structure for'collecting the unreversed components of the beam, and cirvcuit means connecting withsaid collecting means for utilizingthe variations Which exist in the said collected components. f i' k4.V A detectorcomprising means including an emissive cathode .for generating afstreamof electrons, a modulating electrode traversedfbyA the stream but suiiiciently shielded from the cathode a signal voltage so as to produce corresponding variations in electron velocity, meansfor reversing the lower velocity components ofthe stream after its traversal ofthe said element'andvfor collecting the vhigher velocity components thereof, said vlast-named means includingmeans for producing a retarding field of such strength'that ythe number of electronsreversed thereby'varies non-linearly with thev instantaneous velocity oi the stream, and a circuit for utilizing the varia-- tions which exist in the said 'collected components.

GEORGE F. METCALF.

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