US2676302A - Frequency modulation - Google Patents

Description

p l 20,1954 w. M. wees-ran, JR 2.676302 FREQUENCY MODULATION I Filed Dec. 1'4, 1950 William jwfi'im ATTORNEY Patented Apr. 20, 1954 2,676,302 FREQUENCY MODULATION William M. Webster,

J r., Princeton, N. J assignor to Radio Corporation of America, a corporationv of Delaware Application December 14, 1950, Serial No. 200,717

. 1. This invention relates to frequency modulation, and more particularly to arrangements employing a single gaseous discharge device as an oscillator and as a variable capacitance for the same.

Recently, there have been devised gaseous discharge devices of the type which utilize an auxj iliary discharge and in which a. grid can be used to controlor. modulate currents flowing from .a cathode to an anode. Devices of this type. are known. as .plasmatrons and their operation may be summarized in the following way. In conventional hot cathode gaseous devices the appliedanode potential serves a double function, the first being the provision of the ionization needed for. space-charge neutralization and the second being the provision of the. field needed fordrawing the required cathodev current. In the .plasmatronf? however, these functions are separated. As a result, the potentials needed for drawing .given cathode currents are greatly reduced and, in addition, grid control of the anode current becomes possible.

-This separation of function is achieved by utilizing what is termed an auxiliary discharge to provide the space-charge neutralizing ionization. The ionization from this auxiliary discharge manifests itself by creating a dense cloud of free ions and electrons in the region between the main operating electrodes, viz., the hot cathode, the anode, and the grid. This concentration of free ions and electrons is called a plasma. Because the ion and electron concentrations are approximately equal, the plasma is electrically neutral and acts as a good conductor for electron currents such as those from a, hot cathode. The higher the ion and electron concentration the better will be the conducting properties of the plasma. This property of good conduction is responsible for forcing practically all of an applied cathode-anode voltage to appearacross a thin electron sheath at the anode. Then, inefiect, the cathode is immediately adjacent-tothe anode.- This gives the plasmatron the high perveance associated with a vacuum tube having very closely-spaced electrodes.

.If a negativeielectrodeis immersed in the plasma, a field-absorbing layer called a "sheathforms around the electrode. layer-absorbs the electric field so that the plasma remains isolated andcan retain its field-free nature. .Since the negative field allows only positive ions to enterthis regiomthe sheath is called-a positive ion sheath. Y

"Ihe'thickness of a sheath such 'as described 7 Claims. (Cl. sea-1s) increases as the magnitude of the electrode potential increases. Thus, a negatively biased plasmatron grid will surround itself with a positive ion sheath that expands and contracts in accordance with an applied grid signal. Then'the plasma cross-section, which connects the anode with the cathode through the grid openings, will have its area modulated. This action, in turn, modulates the cathode-anode current by virtue of the fact that the conducting ability of the plasma, like that of other conductors, depends upon its cross-sectional area. This plasmatron" grid control can be pictured as a control of the area of the effective cathode which appears adjacent to the anode. -In some cases, .the grid sheath will control the plasma density, and hence its conducting ability, in regions close to the grid openings. This results in an additional control action whichmight be viewed as a control of the emission capabilities of the effective cathode.

The above action is explained somewhat more fully in the copending Johnson application, Serial No. 185,745, filed September 20, 1950.

Sincegaseous discharge devices of the abovedescribed .plasmatron type have. smooth voltage-current characteristics similar to thosev of vacuum tubes and since a control electrode may be used to continuously control the flow of current through such devices, it becomes possible to utilize such devices in oscillatory circuits similar-to those in which vacuum tubes have hitherto normally been employed. The present invention relates to oscillators utilizing such plasmatronsgly 7 An object of this invention is to devise an arrangement for frequency modulating plasmatron oscillators.

Another object is to provide an arrangement whereby plasmatron oscillators may be frequency modulated with a'high frequency deviation in an effective manner.

A further object is to devise an efiicient arrangement for frequency modulating discharge device oscillators having a low plate impedance.

An additional object is to provide an arrangement for frequency modulating an oscillator of the type which is particularly suited for lowvoltage, high-currentoutput.

The foregoing and other objects of thisin vention will be best understood'from the following description of some exemplifications thereof, reference being bad to the accompanyin; drawing. wherein: i

Fig. 1 is a circuit diagram or one arrangement also simultaneously nected. to. an. intermediate:

whereby this invention may be put into effect; and

Fig. 2 is a similar diagram of modified arrangement.

The objects of this invention are accomplished, briefly, in the following manner: A plasmatron gaseous discharge device has its three main electrodes, consisting of the main cathode, grid and anode, connected in a more or less conventional oscillatory circuit of a type which is commonly used for three-electrode vacuum tubes. potential on the auxiliary cathode ismodulated by a modulating input signal to vary the intensity of ionization of thezgas... variation in the thicknessof' the; positiveion sheath around the negative grid, thus varying the capacitance afforded by thissheath. Since.

this latter capacitance is connected directly across the external tuned circuit, variations of such capacitance produce corresponding variationsin the.oscillationfrequency.

Fig. l is a circuit diagram whereby this invention may be carried intoeffect. Referring to this figure, numeral. I. denotes a gaseous. dise charge device of the so-called plasmatron type, such as described in .the aforementioned copendingapplication; it will be seen that. the. representation oithedischargedevice. l is quitesimh lar to that in:Fig. 1 of, such application. Inside: the envelope of device; I amain cathode 2 par tially surrounded; by a U-shaped control grid 3 and a. U-shapedanode. 4,..grid3 being located between cathode 2-.and anode 4. Opposite the. openends. of grid 3..and anodel is mounted a cylindrical focusing. electrode. 5. An.v ionization.

or. auxiliary. cathodev 6: is. mounted .coaxially and. concentric-with. respect to :cylindrical; electrode 5.

Asindicated. in: Fig. 1,,electrode. 5. is. provided with an: opening I the. center of which; is an a, planev passing. throughthe axes of; auxiliary and main. cathodes. Band 2;-

For. further. details of; the constructioniof. gaseous. discharge device l,.reference may be had to the. aforementioned; Johnson application.

Device. I .is proyidedwitha .gaseous.atmosphere prior. to. sealingtofi; .as .indicatedzimFig; 1..

In; the operation. of fplasmatroni" l. as .an; 05. cillatorv according. to; this. inventiom. focusinge electrode .5; is normally-:leit...free? or: floating? although it. is. to. be; understood; that; this. elemtrode -may be; connected to auxiliary.- cathode; 6 cathode 6.. is; connected.

if. desired. Auxiliary. through the secondary. 8 of; an inputaor; modulartionztransformer 9,-and. through=.a-..current limit-.- ing resistance 10 to the I l of L unidirectional.- potential, the positiveside ofwhich'is grounded. 2, 3 and 4 are connected as a Hartley oscillator. To .efiect: this-result, the :-lower end i of. the parallel L..C.. oscillatory circuita l2; consistingzof induc-- llgris groundedtance- I 3 .and. variable :condenser:

The

This. causes ;a..

negative side of a sources.

4 herently negative with respect to cathode 2 or ground. The output of the oscillator may be taken off by means of a winding l1 inductively coupled to inductance [3. The source H has been indicated as a simple battery but it is, of course, to be understood that any suitable source may be utilized.

The potential of source; H, whichlis .the D. C. potential between-auxiliary cathode B and main cathode 2, is so selected or adjusted that it is sufficient to cause an ionizing discharge to appear between these two cathodes, cathode 2 being positive with respect to cathode 6. For example, thispotential. may be in the range of ap- The potential defierence between themain cathode 2 and anode 4 is forexamplelS volts and is too low to produce The .three .main electrodes and the anode 4 is also:grounded'.for oscillatory currents by capacitance l5. Cathode. 2. is-conl3.and.operates at D. ode 4. is connectedv to point.. on inductance: An-

unidirectional source, oi the magntitude of 15- volts, for. example. Grid 3*is connected t'd the upper end of inductance l3 through ai -parallel R. C.,cir cuit [6... which functions toestablish: on saidgrid, during operation, abias which-is negative with, respect tocathode 2 onground. Al.--

ternatively,. this. biasing; arrangement-maybe omitted and grid 3 left free.toseekits-own.pee;

tential .during.operation, such potential being-duanode region as well. Thus,=.there. is a highly COD? ductive: path. from. the..- main: cathode .L 2 ithrouglr; the grid 3 to;-the ..main;anode:4;;

As ,the grid swings :aoout itSZbiZJSiEEDOtEIltifil the: main current is controlled. over a wide. range of. approximately. zerdto; 200;oramorermilliamperes; in. the mannerspreviouslyx described imthe sum. mary of operation: of? the: plasmatronf Since this grid control of theinain .anodezcurrent; does. takeplace, the devicel maybe connected-Lin. an. oscillatoryr circuit; similar to. vacuum tube =..oscil-.- latory circuits, .and.- such. device williunction: to: produce oscillatory energy in: acsimilar: 'manner: InIig. 1', the devicexzl isconnected innaiilartley oscillator. circuit, such: device then functioning to produce. oscillatory energy in tank or tuned circuit l2, which; energy: may: bewabstracted. coil-J1 asoutputc According: to this i ventiony.the frequency of operation of a-.plasmatronoscillator such as that previously-described may be varied or modu lated' electronically-by varying-*thedntensity oi the ionization. or auxiliary-- discharge current.

When a negatively charged electrode such asgrid 3 is placed: in a strongly-ionized gas -at' low pressure itlbecomes surrounded by a sheath of" positiveions. This sheath:lia's-'a= certain oapaci-- tance, and by: analogy I with "a condenser of ordi' nary construction the platesaraconstitutedby the electrode3 itself and the-outer surfaceartha positive ion-sheath: Sinoe 'elctrodesaZ, 3' and 4 are connected to-t'uned circuit I 2-; it may-be seenthat this sheatli-capacitance-iscoupled to 'the' oscillator tuned I circuit. When-this capacitance is I increased, the oscillator frequency isdecreased." while when: this capacitance is descreased, the oscillator frequency is increased; since the sheath-capacitance is essentially-' across tank 1 circuit i 2.

The positive ionsheath about grid il lias'aden-- nite'thickness, its'thickness-depending on, among-'- other things, the:intensity of ionization of the gas. Thus,:. about: the negatively charged: trode 3 there is an electric: fieldi -Tl'liscfieldf:ex.-:-- tends out-- ta-isuchr-aidistance thatritsisrexactly connected elece 1 aczacoe since electrons cannot enter it and thepositive ion current is small. This region extends between two conductive surfaces, one being the electrode 3 carryingv a negative electron charge and the. other being the surface of the, sheath, whichis.

at the potential of the gas within the envelope, thatis, with gases at low pressure, at approximately the potential of. the cathode 2. This gas is conductive since negative electrons are moving freely about in it. As such, this sheath is in reality a condenser. Sincethe distance betweenv the plates of the condenser, that is, the thickness of the sheath, varies with the intensity of ionization of, the gas, as already explained, the capacitance of the condenser variesin accordance with the intensity of ionization of the gas. The capacitance of the condenser, of course, varies in an inverse direction with the distance between the condenser plates (the thickness of the sheath).

An input modulating signal is applied to the primary I 8 of transformer 9. Such a signal, appearing in secondary 8, which is in circuit between auxiliary cathode 6 and main cathode 2, changes the current flowing between these two cathodes. This change of'current varies the intensity of ionization of' the gasin device I, it being remembered that main cathode 2 acts as the anode for auxiliary cathode 6. This variation in intensity of ionization causes a variation in the thickness of the positive ion sheath around grid 3, as previously explained, and a variation in the capacitance afiorded or provided by this sheath. This capacitance variation produces a corresponding variation in the frequency of the oscillator. Thus, input signals applied to [8 result in frequency modulation of the plasmatron" oscillator disclosed.

To particularize, an input signal of such polarity as to increase the current flowing between cathodes 6 and 2 increases the ionization intensity in device I, decreasing the thickness of the ion sheath around grid 3 and thereby increasing the capacitance provided thereby; this decreases the oscillator frequency. Conversely, an input signal of the opposite polarity decreases the ionization intensity, resulting in an increase of oscillator frequency.

In a circuit according to the present invention which was actually built and tested, it was found that the plasmatron oscillator was easily frequency modulated; in other words, the frequency deviation was high. This may be seen from the following two examples. At 17 megacycles, increasing the discharge or ionization current from one to two milliamperes caused a frequency decrease of 500 kilocycles, with no change in amplitude. At 70 kilocycles, increasing the discharge current from one to six milliamperes caused a frequency decrease of 50 kilocycles.

It is desirable to point out that the oscillator of this invention is particularly adapted for modulation at audio frequency rates, up to about kilocycles. The upper frequency limit of the the. field of the positive. ions in "plasmatron determined.

vFig. 2 discloses a modified arrangement. In this figure, elements the same as those of Fig. 1 are denoted by the same reference numerals. In Fig. 2, instead of feeding the input modulating signal to auxiliary cathode 6 by way of a transformer. as in Fig. 1, the input signal is fed to such cathode by way of a vacuum tube 1 9 which acts in effect as a series variable impedance between source II. and auxiliary cathode 6. The negative side of source ,H is connected to the cathode 20 of tube I9 and the positive side of this source is grounded. The input modulating signal is applied between the control grid 2| of tube l9 and ground, while the anode of tube !9 is connected to auxiliary cathode 6. Thus, ,tube 19 pro: vides a series variable impedance between source H and cathode ii, and by theapplicationof a signal to control grid 2! this impedance is variedto thereby vary the yoltage between cathodes 6 and 2 to vary thev intensity of ionization of the gas. This results in a variation of the oscillator frequency, as previously described in connection with Fig. 1.

What I claim to be my invention is: r 1. A frequency modulation arrangement, comprising a gaseous discharge device, means for ionizing the gas in said device, said device having a plurality of electrodes including anode and cathode electrodes and also acontrol electrode operating at a potential negative with respect to .the potential of said gas, therebyforming a positive ion sheath about said control electrode, meansfor supplying said electrodes with operating'potentials, a tuned circuit; means coupling said anode, cathode and control electrodes to spaced points on said circuit, thereby to develop oscillations in said device and said circuit, and means for controlling said ionizing means by a modulating signal to modulate the intensity of ionization of said gas and thereby also the thiCk-.

ness of said sheath and the provided thereby.

2. A frequency'modulation arrangement, comprising a gaseous discharge device having a plurality of electrodes including anode, cathode and control electrodes, means for ionizing the gas in said device, connections for supplying said electrodes with operating potentials, a tuned circuit, means coupling said anode, cathode and control electrodes to spaced points on said circuit, thereby to develop oscillations in said device and said circuit, and means effective on said first-named means for varying the intensity of ionization of the gas in said device, to thereby vary the effective capacitance provided by said control electrode.

3. A frequency modulation arrangement, comprising a gaseous discharge device having a plurality of electrodes including anode and cathode electrodes and also a control electrode operating at a potential negative with respect to the potential of the gas in said device, thereby forming a positive ion sheath about said control electrode, connections for supplying said electrodes with operating potentials, a tuned circuit, means coupling said anode, cathode and control electrodes to spaced points on said circuit, thereby to develop oscillations in said device and said circuit, and means for varying the intensity of ionization of said gas when ionized, to thereby vary the thickness of said sheath and the effective capacitance provided thereby.

4. A frequency modulation arrangement, coineffective capacitance as an oscillator has not as yet been agewaca:

p i ing a: gaseous disch'argeadevicemaving: a plurrality of electrodes including anode, cathode and.

control' electrodesameans for ionizingvtheigas'in saidedevice; connections for supplying said; elec trodes with operating potentials, atunedzcircuit,

means coupling. said: anodcycathode and. control. electrodes .to spaced" pointspn'said-circuit; there. by to developoscillations in. saidi device andsa-id'circuit; and -means-i'or controllingsaidvionizeatmodulating signal: to: modulate. the :intensity of ionizatiomofsaid-gas and there-- by alsothe eire'oti vecapacitance provided by ing meansiby said control electrode.-

5. A-frequency modulation arrangement comrdischarge device; means 1 f or prising: a gaseous establishing an ionizing discharge in the gas in-said device, said: means including "an electrom emitting cathode,-

trode-and a control electrode, in said device; means for supplying said electrodes with operating potentials, a tuned circuit;

to spaced points on said circuit,-.thereby to develop oscillations insaid device-and" said circuit; and means forvary the intensity of J ionization :of said gas.

62 A frequency modulation arrangement-com prising: a gaseous-dischargedevice; means for: ionizing discharge in the gas establishing anin said device, said meansincluding anelectron emitting cathode, another-r cathode, and means for establishing an ionizingxcurrent between said cathodes; a -plurality of electrodesimsaid device, said plurality including: saidiv anodeelectrode, and: also" including respect to the potential of said gas, thereby forming a. positive trode;-- meansfor. supplying said i electrodes 1 with operating potentials, a tuned circuit means'coupling said anode, other cathode and control .elec-;

trodes to" spaced points-om said circuit, thereby anothercathode, and" means for establishing .an -ionizing current between said' cathodes a plurality-oi electrodes; including said other-"cathode and also including an anode e1ec+ meanscoupling said anode, other cathode and 'controlelectrodes varying said ionizing currentv to other" cathode, an. a control. electrode operating at a potential: negative with:

ion sheathabout said. control. elecin said device,

a: coupleasaith'iomsheathrtossaid' circuit'land tm device" and'l said cir cult; and: means forwvaryingi. said ionizing cur rent to vary. theiintensitys ofnionization-ofsaid gas and thereby; alsozthetthiclmess of-saidrsheath and the 1 effective capacit'anceeprovided; thereby;-

1; A :.frequency.= modulation arrangement coma prising: a gaseous? discharge'z'device; means for. establishing: ion'iz'mg discharge the: gas said means including anelect'rom emitting cathode; another? cathode; and: means for: establishingzanzionizing current thetweenisaid' said cathodesg: a. plurality with respect to thecpotential .of'sald gas; thereby forming a positive. ion sheath about said control electrode; means forfisupplyinglsaidi electrodes with operating potentials, 1a. tunedv circuit; means" coupling said anode; other cathode. and.- controlv electrodes to spaced points on saidcircuit ,5 thereby tocouplesaid ion sheath to said circuit: andrto develop oscillations in: said 1 device. and' said: sir-- cult; and means for controlling said. ionizing: current by a modulating signal tomodulateuthe intensity of ionizationzof: said gas and. thereby also the thickness ofi saidisheathcand the: ei.-- iective capacitance providediithereby;

References. Citedin; the file-of. this; patent.

UNITED STATES- PA-TENTS of: electrodes in said device; said:plurality?includingisaid other." oath--- I ode, an anode-electrode; andialso' including acon:-

trol electrode: operating: at a: potential negative:

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