US2779891A - High frequency amplifier - Google Patents

Description

Jan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER Filed Jan. 27, 1951 JE; EEE. I

,HIFIII lll. Il Il I .Lrl.i|\|1| |l /lVl/E/VTO By J. A. MOE TON ATTORNEY Jan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER 4 Sheets-Sheet 2 Filed Jan. 27, 1951 /VVE/VTOR J. MORTO/V BV #mw ATTORNEY Jan. 29, 1957 J. A. MoRToN 2,779,891

HIGH FREQUENCY AMPLIFIER.

Filed Jan. 27, 1951 4 Sheets-Sheet 3 FIGZB /Nl/E/VTOR J. A. MORTO/v A TTORN-V Jan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER 4 Sheets-Sheet 4 Filed Jan. 27, 1951 FROM LOCAL OSC.

OUTPUT SIGNAL SOURCE /NPUT CAV/TY l L/05 s/GNA L SOURCE /Nl/EN To@ l A. MORTO/V FROM LOCAL OSC ATTO/@NE ,V

United States Patent O HIGH FREQUENCY AMPLIFIER Jack A. Morton, Neshanic Station, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 27, 1951, Serial No. 208,203

12 Claims. (Cl. 315--3.5)

tric eld of awhigh frequency wave transmitted along the path causing the high frequency wave 'to be amplified.

lt is an `object of this invention to increase the signalto-n'oise Yratio of such a device.

'it Ais also an object of the invention to launch an electron stream in a traveling-wave type tube with a very small transit angle between the gun rcathode and the helix.

Another object of the invention is toincre'ase the gain ot such an amplifier.

it is a further object of this invention to `maintain broad band widths `in such an amplier while decreasing the noise generated therein.

Another object 'of this invention is to ,prevent formation of noise producing 'plasma within such a device by eliminating the potential pockets along the electron beam.

Another object of the invention is to combine features of microwave triodes with thoseof traveling wave tubes so that beneits of both'devices may be realized in a single unit.

in devices of the type described above which are now known as traveling wave tubes, it is well `known that the phase velocity of the ,high frequency wave must be approximately equal to the average velocity of the electron stream in y'order that there will be `an appreciable energy transfer. The phase velocity `of a high 4frequency wave in airwill be of tlreorder of vthe vspeed of light while the velocityfot the 'electronsemitted by an `electron gun having a beam voltage on the order of 1500 volts will be about one-thirteenth `the speedof light.

In traveling wave tubes `of the type described in a copending application Serial No. 704,858, tiled October 22, 1946, by l. R. Pierce `(United States Patent No. 2,602,148y issuedluly l, 1952), synchronous velocities are approximated by causing the high frequency or travelingr waves to follow a helical transmission path of the proper pitch while the trajectory of the electron stream is a path adjacent and 'parallel to the longitudinal axis of the helical4 path. A series lofi` cylindrical support rods of any suitable insulator material which will not introduce excessive loss or capacitance in the `tube aroused to position the helix in the glass envelope in order to facilitate projection of the electron stream along the length of the coil without great loss of electrons to the coil conductor.

It has been found that traveling wave tubes tend to have relatively low signal-to-noise ratio due to sources of noise within the tube. ln attempting to determine what these sources are ithas been discovered that in a tube such as described `in the Pierce application the transit angle between the electron emitter and the helix and the random eaptureof current fromthe beam rissuch that the `electron `beam enters -the helix fasalm'ostfpure shot vnoise before `it receives'the signalimodulation.

Another shurceof noise has Vbeen A'discovered' to `be ion oscillations in plasma which form within the tube, `particularly `in the region of deep potential pockets,

A further source of noise has been traced by applicant to Ythe variablepressure and hence variable contact resis'tance and capacitance with which the loss coated ceramic rods of `a tube as disclosed in the Pierce application are held. in contact with the helix. Techniques are disclosed in my copenlding application Serial No. 208,204; tiled lanuary 27,V 1951,- for the construction of a unitary helix assembly whereby the 4ceramic rods are held in rigid contact 'with thehelix by glaze applied thereto.` Loss material is applied about the circumference of a 'center por-- tion `of the entire assembly. Since the glaze `insulates 'the rods from the helix, the effect of Contact resistance is substantially eliminated; capacitance between the wires and support rods is also made uniform. A higher loss per unit length over which loss material is applied is 'also obtained by using these techniques so that the loss-free interaction recien is increased, resulting in a higher igain for the same total length of helix. t

in traveling wave tubes it has been necessary to make specialprovisioin for preventing self-sustaining oscillations which .may result from waves reliected from the output section due tio an imperfect impedance match of the helix to the output section. One method as vdisclosed in my aforementioned copending application applies aV coating othigh loss conductive material such as aquadag over the center portion of the helix assembly. In addition to .attenuating reiiected waves this coating of loss material also promotes `isolati-on of the input and output sections ci the amplifier and therefore tends to `limit disturbances `due to reflection therebetween.

Fluctuations in an `electronstream as it leaves the electronemissive surface are largely velocity fluctuations but as is known, these fluctuations become density or convection current iiuctuations at some iinite transit angle from the cathode` andrat ysome point they approach pure shot noise. Applicant has utilized this concept to achievea `high signal-to-noise ratio by launching an electron beam down a helix with a small transit angle between gun cathrodenandlhelix vso thatthe noise eitect of the beam `issubstantially below the level of pure shot noise. Intone emhodiment, described below, the signal is impressed fon the stream in the region fpreceding the helix where the noise is greatly reduced by space charge suppression and asrnall transit angle. ThisV region is` just the cathode-grid input vof a microwave triode having a high transconductance so that both signal and noise are amplified bythe gain in the regionpreceding the helixbefore noise due to transit time o1- random capture `eect enterthe rstrearn. The helix then simply adds more gain and the original signal-to-noise ratio of the space charge region is preserved if the gain precedingthehelix is adequate.

A structure in accordance with the invention whereby anwelectron `stream may belaunched with asmall transit angle with `provisions for impressing a signaLto be ampliiied uponthe stream before itenters the helix is obtained by combining a microwave triode input section, i. e., the cathode and grid sections, with a traveling Wave tube. The helix :replaces the anode of the triodeso that the transit angle `of theelectron stream from the triode cathode as it enters the helix is lextremely small, particularly in comparison with conventional traveling 'wave tubes of the prior art. The transit angle is also small vrelative to the 'transit angle required for velocity fluctuations from the cathode to reach` the level ofv pure shot noise convec `tion `current*iluctuations and in `fact locates the helix Vinput 'in the region of or as close as possible tothe potential minimum just in lfront of the cathode. This region resulting from space charge is well `known `and is .illustrated for example 1in fElectronics"by l. Millmanjand iS. Seely, McGrawHill, 1941, `at page 223. The input signal modulates the beam convection current in the cathode-grid region and the output amplified signal is taken from the far end of the helix as from conventional traveling wave tubes. A collimating grid may be added to the helix input to insure a parallel sided beam so as to reduce noise due to plasma ion oscillation.

The combination of a triode and a traveling wave tube in accordance with the invention is herein called a hybrid tube. One feature of a hybrid tube is that the overall gain is greater than that of a conventional traveling wave tube of the same helix and beam dimensions due to the transconduct-ance of the region preceding the helix in which the signal is impressed on the electron beam. Further, the bandwidth of the device is greater than that of a similar microwave triode since in the latter, the bandwidth is limited by the output section and in the hybrid tube, this section is replaced by the broad band helix.

`Other features and objects of the invention may be better understood by reference to the following detailed description when read in accordance with the following drawings, in which:

Fig. 1 is a pictorial representation of a hybrid tube of the type described herein in detail;

Fig. 2A is a cross section view of the device including an input and an output circuit and showing particularly the general relation of the electrodes in the device;

Fig. 2B illustrates in diagram only the input section of a prior art traveling-wave tube;

Fig. 2C illustrates as modification of the input section of Fig. 2A to show the relation of the electrodes when the collimating grid is not included;

Fig. 3 is an enlarged cross section view of the input section of the device of Fig. 1, showing the detailed assembly when the collimating grid is included;

Figs. 4A and 4B compare, in a pictorial manner, electron beams with and without crossover;

Figs. 5A and 5B illustrate by diagram the collimating grid feature of the present invention;

Fig. 6 is an exploded perspective view of the collimating grid 95 of Fig. 3 and its immediately adjacent structure;

Fig. 7 is a pictorial view of the base of the device of Figs. l and 3; and

Figs. 8 and 9 illustrate, partly in diagram, features of the invention as applied to devices employed as modulators. Y

Gain

In general waves may be excited on a helix by any one or combinations of three mechanisms. These mechanisms are the following:

(a) By injecting an electron stream whose convection current is signal modulated;

(b) By injecting an electron stream whose velocity is modulated; and

(c) By applying an A. C. signal field to the beginning of the helix and injecting an unmodulated electron stream.

In any of these situations when the electron stream velocity is in synchronism with the helix wave Various amounts of four different waves are excited in the system comprised of the helix and electron stream. There is one backward traveling wave and three forward traveling waves. The three forward waves are as follows:

l. A growing wave traveling slightly slower than the beam direct current velocity;

2. A decaying wave traveling slightly slower than the beam velocity; and

3. A constant amplitude wave traveling slightly faster than the beam direct current velocity.

In the ensuing discussion attention will be focused only on the increasing wave, assuming the reflected backward wave is eliminated in actual practice by perfect output termination or by loss. The other two forward waves will be neglected on the basis that if useful gain is to be achieved, the tube will be suiciently long so that the growing wave will be many times greater than the other two in the output region of the helix.

When all three methods of wave launching are used, the voltage amplitude squared of the growing wave at u0=direct current beam velocity;

where:

K1 is the helix impedance;

lo is the direct current beam current; Vo is the direct current beam voltage;

In the conventional traveling wave tube, in so far as signal modulations are concerned, vs=qs'=0, and the signal is applied only as a voltage Vs to the beginning of the helix.

In such a case, the maximum available gain becomes Where TT is the gain of a conventional traveling wave tube, 'and N is the length of the lossless helix-electron stream interaction space in wavelengths.

This reduces, in decibel, to

IT(db)=9.54+47.3 CN

In one manner of operating the hybrid tube, when the transit `angles in the spaces between grid and helix are very short, and when the helix is shorted at its input, then,

vs=l7s=0 and the maximum available gain becomes rH= (ymR/aKJ/a)(rl/CN) where IH is the gain of a hybrid tube; gm is the transconductance of the cathode-grid region;

and

and will be termed the beam impedance; or

/s i/a imdb) :10 10g,(g'" R"2-I-)9.54+ 47.3 CN

Hence:

The maximum available gain of the hybrid tube is thus greater than the maximum available gain of the conventional traveling wave tube by an amount which is a func tion of the transconductance, the beam impedance and armeni T the helix impedance. .In a typical experimental traveling wave tube, the lhelix .had thel'ollowing parameters: Kleine ohms Vo: 1600 volts 14T-'12X 1'0-3 amperes so that recount LEO-T Ohms Thus:

a infix 16W- 6i :2.98K 103 gm fl() gm For `a typical close spaced triode 'of the type used for an inputsection 'of the hybrid tube,

gm=50,000 l0*6=5 X 162 (for Iu=30 ma.)

Therefore,

Bandwidth Whenever one transmission system with a given field configuration is `matched into another of different field configuration, local distortions of the field in the region of transition must be set up. These distortions can be resolved into other higher order field modes than the principal modes of either of the two transmission systems and as `such they 'are primarily local in nature and do not propagate well into either transmission system. They do, however, represent stored energy and as such can be represented as lumped capacitances shunted across the principal modes of the two adjoining systems.

Let us represent the effect of such wave transforming o discontinuities vas etective capacitances lumped across the various signicant electron stream interaction spaces.

Because the helix-to-wave-guide transformation may be made as gradual aswe please and since the helix impedance is itself low, we may usually make the bandwidth of a helixto-wave-gnide transformation much broader than can be achieved in the input gap vof a hybrid tube where we vhave the sudden discontinuity of the cathode-grid gap capacitance and conductance.

It we define the band of an amplifier BMX) as that interval within which the voltage gain is Within a factor of times the maximum-gain, then:

defines BMX); where:

Fao is `the gain at the band center; and

Ilu is the gain `at a `frequency removed from the ban center.

Analogously, .if we `det`me-the band of a simple anti yresonant circuit as `Be('n then:

6 15%)) .denses .nieu

where:

Yc'(wu) is the admittance at the ibanlfcenter ofthe `circuit alone; and y Ycfw) is the admittance at a frequency removed from -the band center.

It follows, therefore, that where:

Gc is the circuit conductance; and Ce is the effective circuit capacitance.

Since amplifier gain is inversely proportional to the product of the input and output circuit admittances, ninazX. Two important cases are 'to be here considered.

(a) The band is equally shaped by the input and output as in the conventional traveling wave vtube in which case input 'and output Qs are equal and l GT= and is the helix conductance; and,

CT is the discontinuity capacitanceassociated with the field transformation from 'helix tow've guide.

Therefore and l G1 f Baco-2T CTvX t1 y where BT(A) is the bandwidth of the traveling vvave tube. For )5x2 at the 6 db down points:

(b) The band is shaped only by the. input; this is the case of the hybrid tube. Then, Qa Q1 and X=rt2 'so that where:

Gin is the input. conductance of the 'hybrid tube at the electron stream; and Cin=C11+Cp1 and is the total input capacitance -at the saine point Where: C11 is the gap capacitance; and, 1 l Cpi is the effective capacitance of the matching resonator; and BMA) is the bandwidth of the hybrid tube.

At the 6 ydecibel down points, X :2 so that 1 Grill! B A H( 2T 01H1/3 In a speciic hybrid tube of the type described below, using a close spaced triode input sectionyitivasnot possible at 4000 rnegacycles to tune on thelirst node since it exists inside the tube. The total effective input capacitance was thus very large and in an actual case lthe input bandwidth was measured to be about 500 megacycles corresponding to an input capacitance ofV about 58 micro microfarads for the case in which G1H2gm=2 son00 l'oemhes By designing the input section of 'the hybrid tubefso as to permit resonating at the rst node inside the envelope, the bandwidth may be raised tothe order of `1000 megacycles which is lalso a typical value for conventional traveling wave tube. l

In general, however, the straight traveling wave tube can achieve something of the order of twice the bandwidth of the best hybrid tube with about the same amount ofdesign effort. This sacrifice in bandwidth will in many applications be warranted in view of the increased gain and decreased noise and in further view of the fact that even the narrower bandwidth of 500 megacycles represents a very broad band.

Noise ligure In the conventional traveling wave tube, `assuming noise to be introduced only by fluctuating convection currents on the entering electron stream, the noise figure is given as r=1+2v2 2 T7" 0=1+s0v2v0c where 'y2 is the space charge noise reduction factor.

For the case in which the transit angle is very large or where there is temperature limitation, 72:1 `and we have pure shot noise. Then:

FT=1+80 Voc Substituting typical values, viz. Vo=1600 volts and C=.025, gives F=320l or about 35 decibel.

Actually, it is reasonable to suppose that 'y2 may be as small as 0.1 in which case the noise iigure calculates out to be about decibels and some traveling wave tubes have been measured that Iare almost this good.

In the hybrid tube, under the assumptions of:

a. very short transit angles in the cathode-grid and gridhelix regions;

b. no partition noise before the stream reaches the helix;

and

c. high transconductance so that we may neglect the effect of any velocity modulation in the input space,

we may write the theoretical minimum noise ligure FH for the hybrid tube as FFH-meng;

Tczcathode temperature in degrees Kelvin, and, T=temperature of input termination.

Typical values vare Tc: 1020, T=290,

giving 1:11220 or about 13 decibels or about one-half the number of decibels -to be expected from a good traveling wave tube.

Actually, this is essentially the same noise figure to be expected from `a close spaced triode under the same assumptions. Actual close spaced triodes have been measured with noise figures between 14 and 18 decibels. Any input passive loading would degrade the noise figure of both triode and hybrid alike so that 13 decibels actually represents a minimum figure.

For a detailed consideration of the equations relating to traveling wave tubes, reference may be had to a book entitled Traveling Wave Tubes by J. R. Pierce, Van Nostrand, 1950. For a consideration of the equations relating to close spaced triodes, reference may be had to an article entitled Design factors of the Bell Telephone Laboratories 1553 triode by R. M. Ryder and applicant, appearing in the Bell System Technical Journal for October 1950 at page 496.

I ilustrativa embodiments The major components of a hybrid tube embodying principles of the invention may be seen by referring to Fig. l. The tube there shown has an input section 11 `comprising the cathode and grid sections of a microwave triode, a wave interaction path comprising the helix 12 through which an electron stream is directed and along which a signal wave to be amplified is propagated, an output cor-.pier 13, and a collector 14 for the electron stream. The input section 11 is enclosed in a metallic housing and the helix 12 is surrounded by a glass envelope and the entire structure is evacuated. Direct current coupling With which to apply bias voltages to the collector 14 and helix 12 is provided by an inner metallic sleeve member 15 connected to the collector 14 by means of a lead 16, and an outer metallic sleeve member 17 insulated from the inner member 16 by the glaze material 18 and connected to the helix 12 by an insulated conductor 19 which may be seen by reference to Fig. 2A and the coupler 13. A few loops 20 are provided in the lead 16 to serve both as a radio frequency choke and as a spring to assist in holding the collector 14 in position.

Means by which an input signal may be applied to and an output signal taken from a tube of the type shown in Fig. 1 may be seen by reference to Fig. 2A. An electron emissive cathode 31 is housed in cylinder 32 and a heating coil 33 is located within the hollow portion of the cathode cylinder. Biasing potentials from batteries 34 and 35 are applied to the cathode 31 and heater 33 by the socket pins 36 which extend through the base of the cathode housing cylinder 32. An input cavity 37 is formed by the top of the cathode housing cylinder 32 and a plate 38 having a central aperture which houses the control grid 39. The circular side walls of the cavity 37 are composed of a vitreous material such as glass which is substantially transparent to microwaves. The cathode 31 and control grid 39, together with a collimating grid 40 whose unique utility will be described below, form an electron gun assembly. The helix 12 and collector 14 are biased highly positive, for example, on the order of 1500 volts, with respect to the cathode by battery 45, so that an electron stream flows from the cathode 31 to the collector centrally along the longitudinal axis of the helix. Focusing of the beam is aided by a magnetic coil 46 housed in a cylindrical nonmagnetic shield 47 which is concentric with the elongated portion of the tube which houses the helix 12. Direct current is supplied to the focusing coil 46 by battery 48 and is adjustable by means of a potentiometer 49.

Microwave signals to be amplied from a source 50 are launched by means of the antenna-like extension 51 of the central conductor of the coaxial cable 52 in the rectangular wave guide 53. The tube 11 extends through the guide 53 at a point substantially a quarter of a wavelength from the lower closed end with the tube axis normal to the broad faces of the rectangular guide. A pair of annular anges S4 and 5S extend inward from either face of the guide flush with the outer surface of the cathode housing cylinder 32 and with the circular edge of the apertured plate 38 which mounts the control grid. At their farthermost extent into the guide 53, the ends of flanges 54 and 55 are flush with the top of cylinder 3:2 and the left face of plate 33, respectively. The electromagnetic waves in the guide 53 induce standing waves in the input cavity 37 which modulate the electron stream convection current passing therethrough. As the stream enters the region within the helix, the variations in the stream cause a radio frequency wave corresponding to the input signal from the source 50 to be launched and impressed upon the helix. The signal wave travels along the helix with a longitudinal phase velocity component substantially equal to the average electron velocity, the pitch of the helix being proper, and interaction between the stream and the longitudinal electric field components of the wave causes the wave to grow in amplitude until it reaches the far end of the helix 12. At this point, the antcnna-like extension 21 of coupler 13 couples the amplied wave into an output rectangular wave guide 56 and thence to a load 57.

By way of comparing the transit angles and methods of signal input with conventional traveling wave tubes there are shown in Fig. 2B the major components of a traveling wave tube of the type disclosed in the book and Y'arrastrar applications of J. R. Pierce mentioned above. These comprise the electron gun assembly 56,. the input coupler- 57 havingan antenna-like extension 58 to which is welded the end of the helix 59 an input wave guide-63, and helix support rods 41. With such a tube the transit angle is of necessity suliiciently large so` that the beam enters the helix as substantially pure shot noise.

Referring now to Fig. 3, the input section 11 of the device of Fig. l utilizes techniques disclosed in Patent 2,502,530, dated April 4, 1950, of which I am a joint inventor together with R. L. Vance to effect a small transit angle between the electron source and the helical transmission line. Therein is disclosed a space discharge device comprising a cathode, one or more grids, and an anode, capable of operating at ultra-high frequencies. For example, such a device employing one grid, and known as a close spaced triode, has been found at an operating frequency on the order of -4000 megacycles to have a bandwidth of from 100 to 200 megacycles and a gain of approximately 9 decibels. In this patent it is pointed out that extremely small electrode spacings, for example of the order of 1/2 `to l mil between cathode and grid and of the order of to l0 mils between grid and anode, are achieved whereby electron transit times are minimized and high transconductance and good signalto-noise ratio and band width are obtained. A close spaced triode is also described in the Bell System Technifcal journal article mentioned above and one of such devices is known commercially as the Western Electric 416A.

The device disclosed in the latter articles is characterized by input and output sections of relatively low and high Qs respectively so that the bandwidth is limited chiefly in the output section. The present invention replaces the anode and output section of a close spaced triode with a helical transmission line of a traveling wave tube. Helices have relatively unlimited bandwidths at ultra-high frequencies so that t-he bandwidth of the combination or hybrid tube is that of the input section which in specific embodiments tested was found to be 1 on the order of SGO megacycles.

The cathode-gridsection illustrated in Fig. 3 is similar to a` great extent to the close spaced triode disclosed in Patents 2,455,381, dated December 7, 1948, to J. A. Morton and L. l. Speck, 2,502,531, dated April 4, 1950, to LA. Morton and R. M. Ryder, and also 24,527,127, dated October 24, 1950, to R. S. Gormley, C. Maggs and L. F. Moose. The detailed description in these patents', as well as the techniques and methods disclosed by them archereby incorporated the present disclosure and will be described but briefly herein by way of illustration.

The container of the inputsection comprises a cylindrical metallic shell 61 provided with an outer lange 62 at one end and an inner lflange 63` at the other. A control grid terminal ring 64 is mounted on the flange 63 byV a vitreous spacer 65 hermeticallys'ele'd between the parallel facing surfaces ofthe ring 64 and the flange 63 of the shell. A conductive path to the planar control grid 66 is by way of the circular gr'idconnector 6 7.

The cathode assembly is precision` fabricated to insure positive parallelism with the wire's of the grid an'dmaintain close spacing thereto on the order of 1/2 mil. The cathode assembly comprises a metallic'sleeve` member 68 both to enclose the cathode heater' element 69 and to support the rigid metallic 'discs 70V and `71. The disc 71 provides a stable flat surface for 'the electron einis'siv'c coating applied thereto.

About the sleevel member 63 and 'coaxial therewith is an annular spacer ring 72 of ceramic material. The spaer ring is maintained in substantially coplanar relation with the @missive surface of the cathode 71 by support members, not shown, which are attached about the periphery of the sleeve member 68 `and extend to slots `provided :in thespacer ring72.

The stem closure 4is a dished-metallic member 75 with aplurality ofl'apertures inthe. base: portionA andi aperipheral flange 76 to correspond to the ange" 62 o'f the: shell 61.. The flanges 62 and 76 are ring sealed to form a strong tightV joint between the Shelli and stem. Terminal pins 7'7 and 77' extend through the apertures inthe base member and are supported therein by glass beads 78 which are hermetically sealed to the pins and to the metallic eyelets 79. Two ofthe pins 77" support a heater shield 80 coaxially mounted about the shield member 68 by means of the support members 81 joined to and insulated from the pins 77 by the glass beads 82. These two pins 77f also provide a conductive'path for the heater element 69 to its supply 35, indicated schematically in Fig. 2, by means of the conductor strips 83.

A low frequency coupling connection is provided from the other pin 77 to the cathode by means of a conducting strip d5, a cylindrical' condenser can 86, and a cathode connector ring 87 which is in contact with the sleeve member 68. A high frequency coupling for the active cathode surface is provided by they condenser can 86 and shell 61 to `an external circuit. In Fig. 2 the low frequency connection to the cathode is' indicated by the lead 43 and the high frequency connection by the members 44.

To insure accurate parallel relation between the Wires of the control grid 66 and the active surface of the cathode by a definite spacing on the order of 1/2 mil, a spacer shim comp-rising a ring-like wafer 3S is inserted between the ceramic spacer ring '72 and the frame of the control grid 66. (As previously explained, the ceramic ring 72 is in a substantially coplanar relation with the emissive surface of the cathode element 71.)

Holding the cathode connector ring S7 in proper position and coaxial with the spacer ring 72 is a second ceramic ring 91. A helical spring 92 is seated in the metallic spring support 93 which is welded to the shell 61 and the stern closure 7S. The `spring 'presses against the ring 9i, forcing all components towards the grid connector ring 67 and the dished ceramic ring 94 which mounts the collimating grid 95 and the helix support rods 96. All elements are thus held in accurate relation by the spring 92 during the operating life of the device.

The collimating grid 955, which will be described hereinafter in detail with particular reference to Fig. 6 is spaced from and maintained in parallel relation with the control grid 66 by means of the inner flange 97 of the control grid connector 67 `and the ceramic piece 94. The helix 12 -is welded directly to the frame of the collimating grid 9S and Iis matched thereto by a distortion 98 of the helix. Referring to Figs. l and `2, the helical transmission line 12 is matched to the output wave guide by means of the conductive coupler `ring 13 which has an antenna portion 21 welded to the helix. rlhe conical collector 14 for the electrons of the electron beam is insulated from the coupler 13 by a ceramic ring 22.

It has previously been mentioned that ion oscillations in plasma within the tube are a source of noise. Oscillations of this type are described in detail in an article entitled Oscillations in ionized gases by Tonks and Langmuir in the Physical Review, volume 33, second series, 1929. Brieily, molecules of gas which remain in the tube after evacuation or which enter later due to an imperfect seal or which are subsequently liberated from the parts by electron bombardment and heating drift into the region of the electron beam where they are ionized. These ions are trapped in the depressed potential along the beam due to the electron space charge. A beam subject to cross-over will have maximum current density at a cross-over point. As shown by the cross-sectional potential vdiagrams of Fig. 4A, the deepest potential pocket will therefore exist at the cross-over points due to the more intense space `charge in these regions. Hence the density of positive ions in the .region 'of the electron beam will befgreatest `at the cross-overA points.

lf there are venough molecules inthe. tube, the'beam -Vwill eventually become a plasma in which densities of ions and electrons are equal and the total space charge will be zero. `As described in greater detail by Tonks and Langmuir, such a medium is capable of supporting oscillations of the ions due to random displacement. These oscillations have been found to appear as sidebands of the signal wave and as little as decibel down from the signal. Such sidebands will not only distort the signal but will also interfere with close space frequency multiplexing.

There are several Ways to prevent such oscillations. One way, largely theoretical, is to obtain a perfect vacuum, or at least one yso hard that the few molecules of gas remaining in the tube will not be able to ionize and form a plasma capable of supporting ion oscillations. Another is to reduce the voltages and the length of the electron beam so that the ionization rate will be decreased. A further method, as disclosed in my copending application Serial No. 136,206, filed December 31, 1949 (United States Patent 2,692,351, issued October 19, 1954), is to sweep the positive ions away from the region of the electron stream so that plasma will not form.

Another method and a method with which the present invention is concerned is to prevent the formation of deep potential pockets along the electron beam by using a parallel-sided beam. In the absence of density modula* tion such a beam if perfectly collimated as shown lin Fig. 4B will have a substantially uniform electron density along its length. The beam therefore Will not give rise to the potential pockets.

It is, of course, possible to launch a collimated beam by the use of a carefully designed electron gun. The present invention, however, is concerned with the use of an auxiliary grid to collimate the beam.

With only a cathode 10i, control grid 102, and helix 103 as represented diagrammatically in Fig. 5A and which, for example, are at 0, -l and +1700 volts potential respectively, the electric eld will have a configuration as illustrated by the dashed field lines. Due to the steep potential gradient between the planar control grid and the cylindrical helix, the field is distorted about the helix input, and a beam launched through such a field configuration will be subject to cross-over.

If another planar grid idd is inserted between the helix and the control grid and maintained at approximately the same potential as the helix, the field will not be distorted but will appear as shown in Fig. 5B. The steep potential gradient is now between the coplanar grids where the electric field line will be substantially straight, and, since the helix 103 and the collimating grid 104 are at the same potential there will be no distorting field at the helix input so that the electrons emitted from `the cathode will tend to be propagated in a parallel-sided beam.

With reference now to Figs. 3 and 6, a grid 95 to collimate the electron lstream is interposed between the control grid 66 and the helix l2. The grid 95 comprises a tungsten mesh, lil held between two discs 112 of conducting material. The conducting discs are welded together, the welding being aided by a third disc 113 of suitabile material, and the helix l2 is welded to the completed grid assembly as is more fully disclosed in the first of my aforementioned applica-tions. The grid 95 and helix 1.2 are thus maintained at the same potential so as to produce the field configuration of Fig. 5B and corn sequently a parallel-sided beam.

The completed grid assembly 9S fits about the support rods 96 by means of the slots .1.14 provided in the grid frame and is seated in the ceramic cup 94 which insulates the colflimating grid from the control grid connector 67.

It may be seen that the hybrid tube structure is especially adapted to the use of a collimating grid and that its compact assembly is not disturbed by the addition of such a grid, if desired. As has been indicated, the collimating grid is not essential to the operation of the tube as an amplifier. Therefore, if the additional noise reduction to be had with use ofthe collimating grid is not necessary or desired, that grid may be omitted from the structure and the input section of Fig. 2A would then be as illustrated in Fig. 2C. it may be noted that the collimating grid di! of Fig. 2A does not appear in Fig. 2C so that the turns of the helix 12 in Fig. 2C are immediately adjacent to the control grid 39 rather than to an interposed collimating grid as shown in Fig. 2A. Also, without av collimating grid the assembly of Figs. 3 and 6 wil-l not be required.

Fig. illustrates a suggested modification of a hybrid tube when used as a modulator. The modulating signal is applied to the control grid 162 from a source 105 as previously described, but the helix 12 is moved away from the electron source so that the beating oscillator signal may be launched down the helix as in the traveling wave tubes of the prior art (see Fig. 2B). Such an arrangement, of course, entails a sacrifice of signal-to-noise ratio due to the increased transit angle between the cathode 101 and helix l1.

The modulator suggested in Fig. 9 makes no sacrifice in signal-to-noise ratio, but, due 'to the added capacitance 166 in the input circuit, the bandwidth is decreased. The capacitance 106 isolates the local oscillator from the signal source 105 and is resonated at the signal frequency by the inductance 167.

These and many other modifications will readily occur to one skilled in the art and the fact that the invention has been described as relating to a specific embodiment should not limit the appended claims.

Wherever in the description or claims the expression immediately adjacent is used it is to be understood to mean that two objects so described have nothing comparable to either between (or separating) them. For instance, when two electrodes are immediately adjacent, the space between them is electrode free.

What is claimed is:

l. A high frequency space discharge device comprising a cathode having a plane face, a planar grid member, means for spacing the plane face of said cathode and said planar grid in aligned parallel manner, means defining an input cavity which includes the gap between said cathode face and said grid member, a wave transmission path comprising an elongated helix having one end immediately adjacent said grid member and coaxially aligned with said cathode and grid member and capable of producing an alternating electromagnetic field in an interaction space substantially contiguous with said grid member for interacting with an electron stream from said cathode, and an electron collector at the end of the helix remote from said cathode.

2. A high frequency electron discharge device cornprising a wave transmission path, said path comprising an elongated helix, means to propagate a stream of electrons along a path adjacent and parallel to the longitudinal axis of said helix comprising an electron emissive cathode at one end of said helix and an electron collector at the other end of said helix, a first planar grid member interposed between and coaxially aligned with said cathode and said helix, a second planar grid member interposed between and aligned with said first grid member and said helix to collimate the electron stream entering the helix, said stream characterized by a region of potential minimum resulting from the effect of space charge in the vicinity of said cathode, and supporting means holding the end of said helix nearer said cathode immediately adjacent to said second grid member and as close as possible to said potential minimum.

3. An electron discharge device comprising a source of electrons having a plane face, a first planar grid member, a second planar grid member, farther removed from said source than said first grid member, means spacing said face of the source and said planar grid members in aligned parallel manner, `means definining an elongated wave transmission interaction path having one end located immediately adjacent to said second grid member whereby the electrons from said source are collimated immediately before reaching the wave interaction path and the electron transit angle from said source to said path end is substantially less than the minimum transit angle necessary for random fluctuations in an electron stream proceeding from said source to said wave interaction path to reach pure shot noise, and a collector of electrons located at the end of said wave interaction path remote from said source.

4. A high frequency space discharge device having an input section comprising a cathode member having a planar emissive face, a first planar grid member adjacent the face of said cathode, a. second planar grid member adjacent said grst grid member, means to insulate said second grid member from said first grid member, means spacing said cathode face and said grid members in aligned parallel relation, and means defining an input cavity which includes the gap between said cathode face and said first grid, an electrical wave transmission path comprising an elongated helix having one end immediately adjacent to and connected to said second grid member, means coaxially aligning said cathode, said grid members and said helix and an electron collector at the end of said helix remote from said cathode.

5. The combination in accordance with claim 4 and a plurality of support rods longitudinally disposed about the outer surface of said helix and seated at one end in said second grid member.

6. An electron discharge device comprising a source of electrons having a plane face, a rst planar grid member adjacent said face, a second planar grid member adjacent said rst grid member, means to insulate said second grid member from said first grid member, an elongated wave transmission path connected at one end to said second grid member, a collector of electrons located at the other end of said path, means spacing said face and said grids in aligned parallel relation, said transmission path end being located immediately adjacent to said second grid member whereby the electron transit angle between said source and said transmission path end is substantially less than the transit angle required for convection current liuctuations in the electron stream emanating from said source to reach the level of pure shot noise, and means to modulate said stream before it reaches said wave transmission path.

7. The combination in accordance with claim 6 Wherein said last-named means comprise means defining a cavity resonator which includes at least a portion of the gap between said source and said first grid member, and means responsive to a signal to be amplified to induce standing waves in said cavity.

8. A high frequency space discharge device comprising a helical transmission line, means to cause a stream of electrons to be propagated along a path parallel and adjacent to the longitudinal axis of said helical line, said means comprising a cathode, a control grid, and a collimating grid, said collimating grid positioned immediately adjacent to one end of said helical line and transverse to said axis, said helical line connected directly to said collimating grid, and means comprising an input cavity resonator which includes the gap between said cathode and said control grid to modulate said stream.

9. A space discharge device for amplifying high frequency signal waves comprising a transmission line at least several wavelengths long, said line comprising a helix of uniform pitch along the greater portion of its length, means to cause a stream of electrons to be propagated along a path adjacent and parallel to the longitudinal axis of said helix, said means comprising a source of electrons, a control grid adjacent thereto, and a collimatng grid interposed between said control grid and one end of said helix immediately adjacent to said end of helix, said control grid and said collimating grid in aligned parallel relation and coaxial with said helix, and means to modulate said stream as it traverses the space between said source and said control grid.

l0. An electron discharge device comprising a first support member having a plane seating surface, a cathode mounted by said support member and having a face coplanar with said surface, a second support member having a plane seating surface, a first planar grid member, a second planar grid member seated in said second support member, means spacing said cathode face, said first grid member and said second grid member from each other in aligned parallel relation comprising spacer means between said grid member and the seating surfaces of said first and said second support members, means separate from said first and second support members locking said support members against said spacer means, a transmission line at least several wavelengths long immediately adjacent to and connected directly to said second planar grid, said line comprising a helix which has a substantially uniform pitch along a greater portion of its length, support means to mount said helix coaxially with said cathode and said grids, said support means comprising a plurality of rods of insulating material spaced symmetrically about said helix in a longitudinal manner and held in firm contact therewith, and electron collector means adjacent to the end of said helix remote from said second grid.

11. An electron discharge device comprising a helical transmission line at least several wavelengths long rigidly supported by a plurality of support rods spaced about said helix in a longitudinal manner, means to cause a stream of electrons to be propagated along a path adjacent and parallel to the longitudinal axis of said helix, said means comprising a cathode, a first plane grid, and a second plane grid each axially displaced from said helix, with said second grid immediately adjacent to said helix, said cathode having a plane face, said face and said first and second grids in aligned parallel relation and means to space said cathode and said grids from each other in accurate relative positions, and electron collector means at the other end of said helix from said cathode and said grids.

l2. A high frequency space discharge device comprising a cathode having a plane face, a planar grid member, means for spacing the plane face of said cathode and said planar grid in aligned parallel manner, means defining an input cavity which includes the gap between said cathode face and said grid member, said cavity being the sole means for impressing signal wave energy from an external signal source on an electron beam from said cathode, an elongated helix coaxially aligned with said cathode and grid member and having one end adjacent the` grid member for defining a beam interaction space whlch 1s substantially contiguous with said grid member,

and an electron collector at the end of the helix remote from said cathode.

References Cited inthe file of this patent UNITED STATES PATENTS 2,064,469 Haeff Dec. 15, 1936 2,300,052 Lindenblad Oct. 27, 1942 2,502,530 Morton et al Apr. 4, 1950 2,521,760 Starr Sept. 12, 1950 2,575,383 Field Nov. 20, 1951 2,595,698 Peter May 6, 1952 FOREIGN PATENTS 934,220 France Ian. 7, 1948 OTHER REFERENCES Article by Kompfner in Proceedin s of I. R. E. 124421. February 1947. g PP

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