US2638539A - Apparatus for converting electrical frequency variations into amplitude variations - Google Patents

Description

May 12,'1953 CUCCIA APPARATUS FOR CONVERTING ELECTRICAL FREQUENCY VARIATIONS INTO AMPLITUDE VARIATIAONS Filed May 28, 1949 Patented May 12., 1953 APPARATUS FOR CONVERTING ELECTRICAL FREQUENCYA VARIATIONS INTO AMPLI- TUnEfVARIArrIoNs Carmen L. Cuccia, Princeton,- N. J., assignor'to lRadio .Corporationof America, a corporation of Delaware Application May-28, 1949, Serial N0. 95,976

- 8 Qlaims. (Cl. .Z50-20) My invention relates toa-methodfandapparatus for converting :electrical frequency :variations into amplitude variations, .particularly latyultra high frequencies.

A---paper entitledA "Frequency Modulation .and Control .by-Electron lBeams, by Lloyd P; Smith and llCarl I.- Shulman, Proc. Iof I. -R.="E.,' July 21947, pages 644-57,describes a method of `frequency modulating :ai-resonant;electrical-.systembyuse of a-sp'iral velectron beam. A..beam.:of 'electrons is projected =at uniforml velocity along .'apath-lying between and parallel to-.two parallel .plates,"which are-connected to a source Aofoscillating-fvoltage of--frequency f toestablish an oscillatingelectric iieldA extending between. the plates. A fconstant magnetic eld is provided parallel to the'` beam. Due to the `crossed electric ande magnetic elds the: `electrons .revolve about". `the .beam axis. in spiral-paths. It .-.isshown'y that when the 4magnetic fiel-d strengthfH; is adjusted so that-the cyclotron. angular frequency, or; angular velocity of fthe electrons aboutjthe axis,

is equal to the angular-frequency w=21rf radians per second iof the :oscillating voltage source,where e4 and-.m are tthe `chargel and gmass,y respectively, of-.- an electron-,stiley electron:beamunctionsnas a purely resistiveload on -the sv stem. Whenqthe magnetic' iieldis so adjusted that:

new Patent-fue; anzuelo-ated February 2o,

1951,; andassignedf to the Asame assignee asl .the 'instant-.application The electron coupler comprisesessentially an-fevacuated envelope containing-a-irst-.pair ofparallel deflectingfplates; a secvond-painoipa-rallel plates-either aligned with. or

l-rotated.90wit-,h respect `to the .rst pair,..means connected to the vfirst pair of1 plates for. establishinga high .frequencyelectric lield ytherebetween-,means for supplyingand directing an. elec- .tron beam successivelythrough the.. spaceabetweenthe platesto collector,` meansforsubjecting -thelfelectron' .beam to aconstant-.mag-

netic eldparallel to'the beam path, and means connected tothe second pair of plates fon-extractingA electricalv :energy fromthe. coupler. Briefly, my 'invention comprises connectingfthe rst-pair ofparallel plates ofthe electron-coupler-to a source of variable frequencymicrowave energy, such as a frequency-modulated signal voltage, `adjusting the transit. time -r ofthe.- electrons :across-the plates by adjusting the-beam-accelerat-ing, voltage so that the Vtotal `frequency swing 2m of the signal multiplied-by the transit time r 'equals 21|'- .radians,y and adiustinglthe strength :of the magnetic field -so-that .He/m equals ythe maximum angular irequencyoffthe signal. Under these conditionsf-when-.the instantaneous-signal frequencyw -is-i ay maximum, that is,` when the signal modulation is maximum positive; @is zero, and hence, the net absorption off-energy by the beam from the input system is a maximum and the energy givenlupabylthe beam 'to the-output system is also a maximum. When the :instantaneous signal frequency. wais the minimumv frequency, that; is, Whenthey modulationis maximum negative,- 0 is 21r,4 and hence, the beam. absorbs noenergy-from the input -system and-therefore, 'delivers no energy totheA outputsystem. Thus,` the variation of vtheiinstantaneous4 -frequency-of the input signal from-.its maximumto its minimum value results in 100% amplitude modulationin the output system. The device-serves as a :converter to changefrequency modulation-to amplitude modulation.

-The object of the ypresent invention is,v therefore, to provide af novel method of converting frequency 'variations Yto amplitude variations. More specifically, the object is; to utilize .spiral beam devices such as-disclosed in my co-pending application referred to above as detectors for -irequency-modulated' signals.

Another `vobject is -to 4provide means for.-mini mizingdistortion in the output due to the.v frequency-changing properties ofthe beam during portions of the `modulation cycle. A further object Ais 'to-.provide` a mixer for a frequencyvmodulation. superheterodyne circuit.

These and-other objects and advantageszofimy invention will .be evident from the following .detailed description.

The novel features which I believeto be .characteristic of .my inventionfwill be set forthwith particularity inthe appended claims, but theinvention itself will best' be .understoodby reference. toi the following description taken in connection with the accompanying drawings.in

which Figure 1 is an axial sectional view of an electron discharge device with which my invention may be practised; Figure 2 is a transverse section view taken on the line 2-2 of Figure 1; Figure 3 is a schematic view of the device shown in Figure 1 showing a different mode of operation; Figure'4 is a graph used in describing my invention; Figure 5 is a schematic diagram of a circuit embodying my invention; and Figure 6 is a schematic circuit diagram illustrating a modification of my invention in which the electron coupler is used as a mixer.

Referring to Figures l and 2, the device shown comprises a pair of axially aligned, spaced cavity resonators I and 3 of cylindrical cross section. The resonator I comprises an outer cylindrical wall 5 and apertured end walls I and 9. The resonator 3 comprises an apertured end wall II and solid end wall I3. Two spaced parallel deflecting plates I5 are mounted by means of conducting stubs I I within resonator I as shown. Similarly, two spaced parallel output plates I9 positioned at right angles to plates I5 are mounted on conducting stubs 2I within resonator 3. ,The space between the resonators I and 3 is closed by suitable insulating sealing means 23. In order to provide means for exciting the input resonator I, a coupling loop 25 is mounted withinthe resonator as shown, the loop forming a continuation of the center conductor of an input coaxial transmission line 2T. A similar output coupling loop 25 and line 21 are provided in the output resonator 3.

An electron gun structure, including an indirectly-heated cathode 29, an apertured accelerating and beam-forming electrode 3I and an apertured beam-forming electrode 33, is suitably mounted at the entrance to the resonator I in position to project an electron beam coaxially through the two resonators and between the plates I5 and I 9 toward the wall I3, which serves as acollector.

Suitable means, such as an electromagnet surrounding the two resonators, is provided for establishing a constant magnetic field extending along the beam axis. The cathode 29 is connected'to the negative terminal and the accelerating electrodes 3I and 33 and resonators I and 3 yare connected to an adjustable relatively high positive point on a direct-current voltage source, such as a battery 31, in order to give the electron beam the desired axial velocity.

In operation, the input line 2'! is connected to a source of microwave energy which in my invention is a Variable frequency generator signal. The microwave current in the coupling loop 25 'induces an electromagnetic field within the resonator which establishes an oscillating electric field between the deection plates I5. Electrons entering this eld along the central axis are deected toward the plate having the higher instantaneous positive potential. However, due to the component of electron motion normal to the axial magnetic eld each electron is deflected by the magnetic field and caused to follow a curved path. As each electron absorbs energy from the fields its velocity tangent to the curved path increases which causes the radius of curvature to increase. Hence, electron follows `a spiral path of increasing radius, which at any instant is determined by a condition of equilibrium between the centrifugal force due to the instantaneous spiral energy of the electron, the deflecting force due to the constant magnetic field and the deflecting force due to the instantaneous electric field.

If the strength H of the magnetic eld and the angular frequency w of the deiiecting electric field are so related that He/m:w, the electrons of the beam will continuously extract energy from the electric field, and hence, the radius of the spiral path will increase uniformly as the electron traverses the input field region. All the electrons in the beam have the same angular velocity wo, and therefore, at any instant they lie along a straight line, as indicated in Figure l by the dotted lines 39, During a complete cycle of the input electric field the line beam 39 swings at angular velocity wo about the tube axis and generates a conical surface, as indicated by the dash line 4I in Figures l and 2. This portion of the beam is sometimes termed a cone-directrix beam. In their travel through the intermediate space between the deection plates I5 and output plates I9, the electrons experience no transverse electric field, and hence, the radius of the spiral path isunchanged and the beam is a cylinder-directrix line beam. Entering the space ben tween the output plates I9 the spiraling electrons induce an oscillating voltage on the plates and thusgive up energy to the output resonator 3. The angular velocity of the individual electrons andthe beam is constant throughout the intermediate and loutput regions and is equal to wo-He/m. Hence, energy is continuously eX- tracted from the beam by the output resonator at the angular frequency c:wo, causing vthe tangential velocity of the electrons to decrease with the result that the beam is a cone-directrix beam of decreasing radius. For best results, the two resonators should be constructed to be resonant at wo.

On page 647 of the above-mentioned paper by Smith and Shulman, it was shown that when an electron beam is projected parallel to a constant magnetic iield and through the space between a pair of parallel plates connected to a source of oscillating voltage of frequency w, the electronic conductance introduced by the beam is L2 [loi l-cos 0 4 Ge(6) 4d2 V0 02 where L is the axial length of the plates, ld is the distance between the plates, [Io] is the absolute value of the beam current, Vo is the beam accelerating voltage, and 9:(wo-wf. It can be seen from Equation 2 that Gew), which is a measure of the net power absorbed by the beam from the electric eld, is a maximum when 0:0, and zero when 0:21r. When 6:0

L2 I0 l Ge(0) 4d2/To` In Figure 1, the dotted lines 39 and 39 illustrate the straight line' nature of the beams in the input and output regions when 6:0, that is, when ,:wa Theoretically, if the input and output electric field regions are similar, all of the energy absorbed by the beam in the input region will be given up in the output region, so that the beam collected at I3 will have no residual spiral energy. `In Figure?) is shown schematically the nature of the beam when 0:I -21r. From Equation Zit can be seen that the electronic conductance f the beam is zero. All of the spiral energy ab'- sorbed by the beam in the input cavity resonator is given back to that cavity resonator. The beam then enters the output cavity with no spiral energy, and hence, the output power is zero. The beamin the input cavity is not a straight line cone-directrixas in Figure l. If an electron Vbegins spiralling in phaseY with the ap# is lmegacycles,"the deflectirig plate'length` L during.; nev'eles -.1ef.ih'e.. ...eine Hence, at any instant the partpf .thebeam inithenputreeionhae th f f esili heiliger spiral'efnereeln Radwege adius during the@secr-J l first half iand decreasing r ond half, as indicated at 4Q in Fig. 3. The" beam 'l5' which itself rotates about thaxis at the angular velocity w of the input el'ctric field, generating e curved ,surface ofrevelutepif. f.

If thefrequencies 'of the" f f gnal ,use the input resonator 1 are .Suitablvrel magnetic fleldstrengthand electron transit the device vShown in Fieiire,l:3.ean;he '00 Convert.' .freqil'erlev iverietiens. .,nthe-. geel i0 amplitude 'varetonsipef.,eenstent .carrier frei angularhammer... a, eerrierreaiienev, es ,eed a sinewavefr'equency modulation o fuamplitude im. ,as a-.funcuono .the.phaseaneleeuef m0dulation.A C indicates one modulation No Thelmaxiinum, .frequency is w1=wc+^sv fendi minimumreeuenev isnzeefeeresentstheresiive 1.1 1f 'ei theeevelepe the..

ing. been.

microwave voltage. ndueedhyihe rei/olv w .of a signal, having...

"4de-v5 @r2 Comparing Equations A3 .and .alitfs Seerilei Ge fis. approximately .feu tenths efqgii as iiindicatediby point, b-'Leneurve 'B- Although the'wave form' ofthe ou 1v the Lsame e as that. ef.. the. input. euri/e; A, very similar theretdan S1 d iheveraiien 1S se, lent Sienalgwillzzb fnliQi duced A`.during lthe eonversiennfgge free. .ener modulated...signalr .to .empfinde meddle "fi output; i Thegbeam relies; .argued the; exis tne'touiput; resene.- r -attire .frequency Lefties?. signal. Hence, the output voltage varies in frequency .and amplitude in accordance with the signal modulation. However, the frequency variation can be ignored. The output voltage from the coupler can be fed into any suitable audio or video frequency detector to recover the modulation intelligence. If desired, the signal can be re-broadcast= after conversion from FM to AM, as an amplitude modulated signal.

In practice, the conditions set forth in the preceding paragraph may be obtained by first adjusting the beam voltage to adjust the transit time 1- so that 2Aw.r=21r, and then adjusting the xmodulat and io, ne 1,120 @anna Yle. tnefiiesijrianc ch tv power will bwikvlan dei?? 0 Mase fronrthecoi'pl may compensate "for the Sieilillifil,@euere e, th... 0.14 fee' ie zere "before ifre modulatl wing re 'chesan 'w corresponding circuits, v' as'A f shovvn 99.11. tron beam current den`s1 theft Y Willgeedliee"yeiiieliliv.

perrecnepeiesra,Hel aridiee neef The'geeeriereejeleetr 'eee bef'us d'as'th control' 'gdfwitha suit negal ivei'lbi a pot i I'applied'the'r'etof-l E W fiile =I have' "indicated" the' "prferr'ed 'ernbodi ments of my invention of which I now am aware and have illustrated only one specific structural embodiment it will be apparent that my invention is by no means limited to the exact embodiments or structure disclosed or the uses indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of the invention as set forth in the appended claims. For example, although the invention is best suited for use with cavity resonators at microwave or ultra high frequencies,

about '.1430 ass, "andl'the @heter Figuresglre ritfiesistens ie made less' lerlmay also be used asa'A in inaugure-ine* ed 'signalA f refused. cp

ollf,

a frequency modulation' As indicated schematically, a 'frequecy?"L modulated Output von; j

the resonators may be replaced byconventional tuned circuits at lower frequencies. Various kinds of beam-forming and focussing electrodes may be used instead of or in addition to those disclosed. The magnetic iield'may be adjusted to correspond to some frequency other than the maximum frequency of the signal. The electron beam may be collected by a separate collector held at a, positive potential somewhat below that of the accelerating electrodes and resonators.

The resonators at microwaves may be in the form of half-wave length coaxial line resonators. shorted at either end, appropriate lengths of short-circuited rectangular wave guide excited to yield a proper transverse iield, or even cavity resonators cf the type shown in Fig. 3 or Fig. 14 (magnetron cavities) in my above-mentioned copending application.

I claim:

1. A system comprising: an electron discharge device including means for supplying and directing a, beam of electrons along a predetermined beam path, a pair of deflecting electrodes adjacent said means and disposed on opposite sides of said path, a pair of inductive output electrodes spaced from said first pair and disposed on opposite sides of said path, and means for collecting said beam of electrons; a cavity resonator coupled to said deilecting electrodes; a source of variable frequency energy coupled to said resonator to establish a, variable frequency electric eld between said defiecting electrodes; means for establishing a constant magnetic eld parallel to said path; means for adjusting the strength of said magnetic eld; and an output cavity resonator coupled to said pair of inductive output electrodes.

2. A system according to claim 1, further comprising a control electrode in said path between said electron supplying means and said defiecting electrodes, and an oscillator coupled to said electron supplying means and said control electrode, for modulating said beam.

3. A system according to claim 1, further comprising a resistor connected in parallel with said deflecting plates and having a resistance somewhat lower than the beam resistance of said electron discharge device.

4. An electrical system comprising: means for generating an electron beam along a predetermined initial path; means for establishing a constant magnetic eld substantially parallel with said path; means, including first electrode means adjacent said path and a source of variable frequency voltage coupled to said electrode means,

for establishing an alternating electric eld of.

variable frequency transverse to said path and said magnetic eld, for causing the electrons of said beam to traverse spiral paths having radii dependent upon the net energy absorbed by said electrons from said electric field; and second electrode means, adjacent and spaced laterally from said spiral paths and separate from said rst electrode means, for inductively extracting spiral energy from said beam.

5. A system according to claim 4, wherein the strength of said magnetic field is adjusted so that is equal to a predetermined value of said variable frequency, where H is the magnetic field strength and e and m are the charge and mass, respectively, of an electron.

6. A system according to claim 5, wherein said variable frequency voltage is a frequency modulated signal,

er m

is made substantially equal to the maximum frequency of said signal, and thev electron transit time through said electric'iield is adjusted to a value substantially equal to 2W divided by the total angular frequency swing of said signal.

7. A system according to claim 6, further comprising means for pre-modulating said beam at a predetermined frequency prior to the passage thereof through said electric field.

8. An electrical system comprising: means for generating an electron beam along a predetermined initial path; means for establishing a constant magnetic eld substantially parallel with said path; means, including a pair of deflectingelectrodes adjacent said first-named means and disposed on opposite sides of said path and a source of variable frequency voltage coupled to said electrodes, for establishing an alternating electric eld of variable frequency transverse to said path and said magnetic field, for causing the electrons of said beam to traverse spiral paths `having radii dependent upon the net energy absorbed by said electrons from said electric field;

and means, including a pair of inductive output electrodes spaced from said rst pair and disposedy on opposite sides of said path, for inductively extracting spiral energy from said beam.

CARMEN L. CUCCIA.

References Cited in the file 0f this patent UNITED STATES PATENTS

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