US2919374A - Improved traveling wave tube amplifier - Google Patents

Description

Dec. 29, 1959 R. l. HARRISON IMPROVED TRAVELING WAVE TUBE AMPLIFIER Filed July 5, 1955 5 Sheets-Sheet l #www A TTU/'PNE Y POWER GAIN (db) Filed July 5, 1955 R. l. HARRlsoN v2,919,374

IMPROVED TRAVELING WAVE TUBE AMPLIFIER 3 Sheets-Sheet 2 Flag E E :E O z m a o LLI n: ll.

E? 6 s D. OSCILLATION 4 OUTPUT 3.08 MAX. GAIN AND OSC. FREQUENCY 3.07

Iso

3.06 4 .6 .8 l0 L2 L4 1.6

BEAM CURRENT (millamperes) IN VEN TOR.

RICHARD l. HARRISON BY ma. Mw@

ATTORNEY Dec. 29, 1959 R. l. HARRISON 2,919,374

IMFROVED TRAVELING WAVE TUBE AMPLIFIER Filed July 5, 1955 5 Sheets-Sheet 3 F IG. 3

0 oI 0.2 o3

Iso -Io (ma) INVEN TOR. RICHARD I.` HARRISON ATTORNEY States TRAVELWG WAVE TUBE AMPLEMR Application July '5, 1955, Serial No. 519,806

4 Claims. (Cl. S15-3.5)

IMFRVED The present invention relates to improved traveling wave tube devices, and in particular to backward wave traveling wave tube amplifiers. Known traveling wave tubes are electron discharge devices in which a gun assembly produces and accelerates an electron beam along a predetermined path from an electron gun toward a collector electrode upon which the beam terminates. Along the predetermined beam path, there is provided a slow wave-guiding or interaction structure, generally in the form of a wire helix, through which the electron beam passes; the wave-guiding structure carries radio frequency energy in substantial juxtaposition to the electron beam, for interaction with the electron beam. Appropriate radio frequency input and output connections are provided to the slow-wave guide structure, and the entire traveling Wave tube is normally immersed in a uniform magnetic Ifield which serves to collimate the beam. In such traveling wave tube devices, one or more electrons moving in the electron beam interact with the same effective phase of the moving electric field and energy is transferred between the beam and field. Many structures are known in the art for utilizing this operating principle to obtain useful results in amplifiers and oscillators suitable for high frequency applications.

It is known in the art to operate such traveling wave tubes as backward wave amplifiers. In such backward wave amplifier operation, interaction takes place between the electron beam which travels along the predetermined beam path toward the collector electrode and the backward wave spatial harmonic, whose phase velocity moves in the forward direction, and whose energy travels in a direction opposite to the electron beam. It is characteristic of such backward wave amplifier tubes that the amplifica-tion or gain of the tube increases with increases in current flow of the electron beam adjacent to the interaction structure which carries the radio frequency energy. Such backward wave tubes exhibit narrow band regenerative type gain, the amount of gain being controlled by the beam current. It is further characteristic that the frequency of maximum amplification may be varied by changing the direct current voltageon the interaction Structure, that is the center frequency of the gain is controlled by the helix or active structure potential. As the voltage applied to the interaction structure is varied as a control over frequency of maximum amplification, the beam current is substantially independent and remains constant despite such voltage variation.

For a given helix voltage, there is a value of beam current, designated as the start-oscillation current, at which the backward wave 4tube yields infinite gain, and at which the tube will begin to produce oscillations at a single frequency determined by the voltage on the interaction structure. At such start-oscillation current, the backward wave tube becomes essentially an electronically tuned oscillator. In practice, the tube continues to oscillate for any value of beam current above the start-oscillation current, and the amplitude of oscillation at these arent increased beam currents is limited by non-linear effects. The value of the start-oscillation current is dependent upon the voltage applied to the interaction structure and the parameters of the tube. For example, as the length of the interaction structure is increased, the start-oscillation current is decreased; `and as the length of the interaction structure is decreased, the start-oscillation current is increased. When such conventional backward wave tube operates as an amplifier, the beam current is below the start-oscillation current and the gain of the tube increases with increases in the value of beam current up to the point -where t-he start-oscillation current is reached and the tube begins to oscillate. It has been reported that if a continuous wave input signal is applied while a tube is oscillating, with the frequency of the input signal substantially equal to or near the frequency of oscillationl as determined by the tube parameters, gain is realized at the same frequency as the frequency of oscillation.

It is broadly an object of the present invention to provide an improved backward wave amplifying device of the aforesaid character. Specifically, it is within the contemplation of the present invention to provide an electronically tunable amplifying device of the backwardwave, traveling-wave class which exhibits improved operating characteristics, prominently higher gains than achievedl with known backward wave tube amplifiers. Y

In accordance with an illustrative embodiment demonstrating features of the present invention, there is provided a backward wave amplifier tube including an electron gun and a collector electrode defining a beam therebetween, an interaction structure including transmission means adjacent to and along the beam path for propagating an electromagnetic wave -along the beam path from the collector electrode toward the gun, and means for applying a low frequency sinusoidal modulation to the beam current superposed upon the direct current beam current. It has been found that when the modulation is applied to the beam current, which modulation is of a relatively low frequency as compared to the microwave signal to be amplified, it is possible to obtain gain which is much larger than can be achieved in a similar tube operating in the conventional backward wave vmode as an amplifier. Specifically maximum gain is attained when the difference between the start-oscillation current of the tube as determined by the parameters of the tube and the direct current beam current is approximately equal to the modulated beam current. The term detectorgain is used in conjunction with a modulated beam backward wave amplifier of this type since the power gain for this mode of operation is varying at the beam modulation rate.

Numerous systems of applications are contemplated with backward wave amplifying devices having low frequency modulation of the beam current in accordance with the present invention, For example, such devices find use as electronically tunable microwave filters which exhibit high gain and narrow band width, as microwave receivers, :as tuned radio frequency circuits and as electronically tunable radio frequency amplifiers at the front ends of a superheterodyne circuit. Further, by technique well known in the art, it is possible to operate the present tube as a self-punching supergenerative amplifier.

The above brief description, as well as further objects, features, advantages and applications of the present invention will be best appreciated by reference to the following detailed description of a presently preferred embodiment, when taken in conjunction with the accompanying drawing, wherein:

Figure 1 is a schematic showing of a beam modulated backward wave amplifier embodying features of the present invention; f Figure 2 is a graph showing in solid lines the power gain for increasing values of beam current, in dot-dash lines the oscillator output power, and in dash-dash lines the frequency of maximum power gain;

Figure 3 is a graph of peak beam modulation current versus start-oscillation current less the average direct current beam current; and A Figure 4 shows curves of maximum detector gain as a function of input microwave power.

Referring now specifically to the drawings, there is shown in Figure 1 an envelope 1Q including an elongated, slender sleeve 12 and an enlarged bulb end or section 14. Disposed within the bulb end 14 is an electron gun 16 which is arranged to project a beam of electrons along a path axially of the sleeve 12 toward a collector electrode 18. The electron gun, which is well known per se, includes among other components a beam current controlling electrode 20 sometimes designated as the rst anode electrode, a focusing and accelerating electrode 22, sometimes designated as the second anode, a cathode 24 and a heater 26. Appropriate connections are provided for establishing operating potentials for the several electrodes of the electron gun 16 and for the collector electrode 18. Specifically, the heater coil 26 is connected by appropriate means to a heater supply (not shown); the focusing and accelerating electrode 22 is connected internally to the interaction structure, illustrated as the wire helix 2S, and both are connected via the lead 30 to the positive side of the vo-ltage source 32; and the collector electrode 18 is connected via lead 34 to an appropriate low voltage source 36, such that unconverted portions of the kinetic energy of the beam appear as heat at the collector electrode 18. The variable voltage source 32 applies direct current voltage between the interaction structure 28 and the cathode. The interaction structure 28 is provided with a downstream microwave input connection, illustrated as a coaxial coupling 38 at the end adjacent the collector electrode 18, and with an upstream microwave output connection, illustrated as a further coaxial coupling 40 at the end of the interaction structure adjacent the electron gun 16. Accordingly microwave energy may be supplied for backward wave operation. It will be appreciated that the foregoing description of the details of the backward wave tube are subject to a latitude of variations, as will occur to those skilled in the art.

In accordance with the present invention, a sinusoidal low frequency modulation is applied in series with the direct current supply for the beam current controlling electrode 20. In the illustrative form of the invention, this is achieved by provision of a direct current supply 44 which is shunted by a resistor 46 having a variable tap A48 which is connected through a coil S0 to the beam current controlling electrode 20. Across the coil 50 there is provided the sinusoidal low frequency modulation derived from the beam modulating generator 52,

coupled via condensers 54, 56 and resistance 58 to the opposite terminals of the coil S which is connected in series with the variable voltage supply 44, 46, 48. The frequency for the modulation source 52 is determined in a relative sense by the frequency applied to the microwave input connection 38 and the terms low frequency and low frequency modulation should be construed as a frequency which is low as compared to the microwave frequency applied as signal input to the amplifier.

It is clear from the foregoing description that the present tube may take the form of any conventional backward wave amplifier which includes an electron gun, an active structure of the backward wave class, such as the helix 28, and a collector electrode, all contained within an appropriate sealed envelope. By applying the low frequency voltage to the electrode 20, the desired low frequency modulation of the beam is achieved. The direct voltage supply 32 for the interaction structure 28 is adjusted, as in a conventional backward wave amplier, for obtaining the center frequency as maximum gain is desired. Maximum detector gain is achieved in accordance with the present invention by adjusting the D.C. supply 44, 46, 48 and the low frequency beam modulating generator output 52 appearing across the coil 50 to meet the condition for maximum gain which is as follows:

Iso- 10:17u

where Iso=startoscillation current Io=direct current beam current Im=modulated beam current (peak) Details of experiments conducted with the beam modulated backward wave amplier tube operated according to the present invention may further contribute to a thorough understanding of the present invention. In the following experiments, the tube used was a modified tape helix backward wave oscillator. The interaction structure was a nine inch long helix made of molybdenum tape .020 inch by .075 inch and wound with eight turns per inch. The helix had an outer diameter of .420 inch and an inner diameter of .380 inch and was supported by a length of glass tubing of Corning 7052 glass. The electron gun was of the hollow beam type capable of delivering six milliamperes and the collector was cupshaped and fitted with heat radiating fins. This basic tube was modified by provision of both input and output signal connections capable of broad band transitions.

In Fig. 2, there is shown the static characteristics for this tube operated as a conventional backward wave ampliiier. Measurements were made at a helix voltage of approximately 925 volts, corresponding to a frequency of about 3000 megacycles. Power gain was measured for several values of beam current above and below startoscillation with the application of a continuous wave input signal of the order of 50 dbm. The frequency Was adjusted for maximum gain at each setting of the beam current. For higher input power it was noted that the gain was reduced. The frequency at which maximum gain was obtained at each Value of beam current was recorded. Oscillator power output was measured at each setting of beam current above the start-oscillation value. The solid line plot of Fig. 2 shows the power gain for increasing values of beam current. The dot-dash line plot is of the oscillator output power for increasing values of beam current above the start-oscillation current. The dash-dash line plot was for the frequency of maximum power gain for increasing values of beam current. The maximum power gain observed under any condition Was of the order of 30 db, and was obtained just below the start-oscillation current Iso. The discontinuity in the solid line power gain plot denotes a region just above the start-oscillation current where the current gain is unstable and undeterminable. The dot-dash curve shows that the oscillatoroutput increases as a linear function of the beam current for increasing values of beam current, starting from the start-oscillation current. The dashdash curve indicates that the frequency for which maximum power gain was obtained coincided with the oscillation frequency above start-oscillation.

Modulated beam current characteristics were measured for this tube with a sinusoidal low frequency radio frequency voltage source connected in series with the 1iirst anode direct current supply by a circuit of the type shown in Fig. l. The frequency of the beam current modulation was varied over a range from kilocycles to 1 megacycle and the peak amplitude of the beam current modulation from zero to .3 milliampere. The essential feature of operation under modulated beam current conditions is that with a continuous wave input signal the power gain and hence the amplied output power varied at the beam modulated frequency rate. Detector gain was measured under identical conditions and both with continuous wave and pulsed microwave input, it

was found that the detector gain Was the same. Tests were made of observed output under both continuous wave and pulse input conditions with a radio frequency carrier of 3000 megacycles, a modulation frequency of 100 kilocycles and a pulse width equal to approximately 40 microseconds. Comparison of the observed output of continuous wave and pulse input conditions showed a maximum gain of 60 db.

Further measurements were made with the aforesaid tube operated as a modulated beam backward Wave amplifier and as a conventional backward wave amplifier. With parameters adjusted for maximum detector gain, for modulated beam operation and maximum power gain for conventional operation, the maximum detector gain was found to be 60 db as compared to a maximum power gain of 30 db.

Further experimental studies were made to verify the conditions under which maximum detector gain could be obtained according to the present invention. The study indicated that maximum detector gain was obtained when the peak amplitude of beam current modulation (Im) was essentially equal to the difference between the start-oscillation current (Iso) and the D.C. beam current (I0). Another requirement found for maximum detector gain was that the level of the input signal should be of the order of -70 dbm. For higher levels of input signal, there resulted lower detector gains. The maximum detector gain was essentially constant at 60 db and was independent of the frequency of the beam current modulation used over the range of 100 kilocycles to l megacycle.

In Fig. 3 there is shown a plot of peak beam modulation current Versus start-oscillation current less the average direct current beam current. For this plot a fixed helix voltage coresponding to a signal input of 3000 megacycles was applied and `the modulating frequency was set at 100 kilocycles. The beam current was arbitrarily set at the values A, B, C, D below the startoscillation current, and for each value the beam current modulation was adjusted for maximum detector gain. The 45 slope of the curve substantiates the relationship for maximum detector gain at Im=IsO-I.

A further experimental study was made to ascertain the dependence of maximum detector gain upon input power. In Fig. 4, there are shown four curves of maximum detector gain as a function of input microwave power. The curves have been labelled by the letters A, B, C and D and correspond to similarly labelled operating conditions of Fig. 3. Saturation effects are quite evident from an inspection of these curves and the maximum sensitivity, which is defined as the power level of input signal that produces an output equal to the level of the output noise is of the order of 80 dbm.

Band width characteristics were measured under conditions for maximum detector gain, was found to be about l megacycle for a detector gain fall-off of 3 db.

Consideration of the foregoing detailed description and of the experimental results indicates that the present tube has properties suitable for many and varied microwave applications. Among the most interesting features of the present tube are the stable detector gain o'f the order of -60 db with a band Width of about one megacycle in the 3000 megacycle band, and maximum sensitivity of the order of -80 dbm.

Among the uses contemplated for amplifiers according to the present invention are that of an electronically tunable microwave filter which exhibits high gains and narrow band widths. Further, the present tubes find application in micro'waye receivers and as T.R.F. circuits. For superheterodyne circuits, the present tube may be used as the electronically tunable radio frequency arnplifier at the front end. By circuitry well known in the art, it is possible to operate the present tube as a selfquenching superregenerative amplifier. This is achieved by provision of means for the detection of some of the output, feeding the thus detected signal into suitable amplifiers which in turn feeds the beam current control electrode, thus achieving a feedback to provide oscillations at low frequencies. The thus produced oscillations in turn modulate the beam at a low frequency to obtain the superregenerative operation.

Numerous modifications may be made in the foregoing disclosure without departing from the spirit of the invention as set forth in the appended claims; and in certain instancessome features of the invention may be used without a corresponding use of other features.

What is claimed is:

l. An amplifying system comprising an electron emitting cathode; a collector electrode spaced therefrom; means causing a beam current in the form of an electron stream to iiow from the cathode to the collector electrode; an elongated wave propagating structure, between said cathode and collector electrode, coupledA in energy transfer relationship to the electron stream to absorb energy therefrom; an input circuit, carrying high frequency waves to be amplified, connected to the downstream end of said structure adjacent the collector electrode; a high frequency output circuit co'nnected to the upstream end of said structure nearest the cathode; a control electrode between said cathode and the upstream end of said structure; and, a source of relatively low frequency modulating waves, connected to the cathode and control electrode, acting to' increase the amplified high frequency output appearing in the output circuit connected to the upstream end of said structure, said amplifying system being adapted to break Vinto self oscillation generation and being so adjusted that the difference between the beam current of the amplifying system at the start of oscillations and the beam current without oscillations is slightly less than, equal to, or slightly greater than, the beam current when modulated by waves from said source of relatively low frequency modulating waves thereby causing said amplifying system to operate with periodically recurring high gain.

2. An amplifying system for high frequency wafves comprising an electron emitting cathode; a control electrode; a focussing anode electrode; an elongated helical conductor; a collector electrode; said cathode helical conductor and electrodes being arranged coaxially in the order named; means to energize said cathode and electrodes so that electrons are projected as a current beam from said cathode along the axis of said arrangement in the direction towards said collecting electrode; means to apply high frequency waves to be amplified to the end of said helical conductor adjacent the collector electrode; means to remove amplified high frequency waves from a connection to the end of said helical conductor nearest said electron emitting cathode; and a source of relatively low frequency, modulating voltage waves of the order of kilocycles to one megacycle in frequency connected to said cathode and control electrode for subjecting said control electrode to a variable vvoltage with respect to said catho'de thereby increasing the amplified high frequency output of said amplifying system, said amplifying system being adapted to break into self oscillation generation and being so adjusted that the difference between the beam current of the amplifying system at the start of oscillations and the beam current without oscillations is slightly less than, equal to, or slightly greater than, the beam current when modulated by waves fro'm said source of relatively low frequency modulating Wa'ves thereby causing said amplifying system to operate with periodically recurring high gain.

'3. In combination, in a wave amplifying system, a conductor in the form of an elongated helix; means including an electron emitting cathode for producing a stream of electrons in the form of a beam of current traveling lengthwise of and in the field region of said helix; collector means for receiving electrons at the downstream or far end of said helix; a high frequency input circuit feeding waves to be amplified to' the downstream, input end of said helix; a high frequency output circuit connected to the output, upstream end of said helix; a control electrode for said stream of electrons; said control electrode being between the electron emitting cathode and the upstream, output end of said helix; and, a source of relatively low frequency modulating waves, means for applying said relatively low frequency modulating waves to said control electrode for increasing the amplified output of high frequency waves appearing in said high frequency output circuit at the upstream end of said helical conductor nearest the electron emitting cathode, said amplifying system being adapted to break into' self oscillation generation and being so adjusted that the difference between the beam current of the amplifying system at the start of oscillations and the beam current without oscillations is slightly less than, equal to, or slightly, greater than, the beam current when modulated by waives from said source of relatively low frequency modulating waves thereby causing said amplifying system to o'perate with periodically recurring high gain.

4. In an amplifying system for very high frequency waves, a discharge device comprising an evacuated envelope, means within the envelope, including an electron emitting cathode, for producing an electron stream in the form of a current beam wave transmission means disposed along the path of the stream arranged to permit propagation o'f high frequency electrical waves thereover in a direction opposite to the direction of propagation of the stream of electrons emitted from the electron emitting cathode; means to impress high frequency waves to be amplified upon the input end o'f said wave transmission means, said input end being relatively remote from said cathode; output means to remove amplied high frequency waves from said transmission means at a point on said wave transmission means upstream of said Stream and relatively adjacent to said electron emitting cathode; a control electrode at an upstream point in said stream adjacent said electron emitting cathode; and, means for applying a relatively low frequency modulating wave having a frequency of the order of magnitude of to 1,000 kilocycles to said control electrode to increase the amplified wave output of said amplifying system, said amplifying system being adapted to break into self oscillation generation and being so adjusted that the difference between the beam current of the amplifying system at the start of oscillations and the beam current withoutoscillations is slightly less than, equal to, or slightly greater than, the beam current when modulated by waves fro'm said source of relatively low frequency modulating waves thereby causing said amplifying system to operate with periodically recurring high gain.

References Cited in the le of this patent UNITED STATES PATENTS 2,603,772

OTHER REFERENCES Hetner, Backward-Wave Tube, Electronics, October 1953, pages -137.

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