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Publication numberUS3716745 A
Publication typeGrant
Publication dateFeb 13, 1973
Filing dateJul 22, 1971
Priority dateJul 22, 1971
Also published asDE2229760A1, DE2229760B2, DE2229760C3
Publication numberUS 3716745 A, US 3716745A, US-A-3716745, US3716745 A, US3716745A
InventorsPhillips R
Original AssigneeLitton Systems Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Double octave broadband traveling wave tube
US 3716745 A
Abstract
An O-type traveling wave tube of the invention includes a slow wave structure having a broad band characteristic over a predetermined range of frequencies and in some cases is capable of broad band operation over a double octave of frequencies. In this the slow wave structure comprises a combination of a substantially nondispersive delay line, such as a helix, with a substantially dispersive delay line, such as a ring loop section. The ring loop sections complement in gain the gain drop off characteristic of the helix at the edges of the frequency band at which the gain from the helix circuit tapers off. In this the overall gain of the traveling wave tube is enhanced and extended over a frequency spectrum greater than that which is available solely with the helix construction.
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Description  (OCR text may contain errors)

United States Patent [191 Phillips [54] DOUBLE OCTAVE BROADBAND TRAVELING WAVE TUBE [75] lnventor: Robert Matthews Phillips, Redwood City,Calif.

[73] Assignee: Litton Systems, Inc., San Carlos,

Calif.

[22] Filed: July 22,1971

211 Appl. No.: 165,263

[ 1 Feb. 13,1973

3,123,735 3/1964 Hull ..3l5/3.6

Primary ExaminerEli Lieberman Assistant Examiner-Saxfield Chatmon, Jr.

Att0rneyRonald M. Goldman et al.

[57] ABSTRACT An O-type traveling wave tube of the invention includes a slow wave structure having a broad band characteristic over a predetermined range of frequencies and in some cases is capable of broad band operation over a double octave of frequencies. In this the slow wave structure comprises a combination of a substantially nondispersive delay line, such as a helix, with a substantially dispersive delay line, such as a ring loop section. The ring loop sections complement in gain the gain drop off characteristic of the helix at the edges of the frequency band at which the gain from the helix circuit tapers off. ln this the overall gain of the traveling wave tube is enhanced and extended over a frequency spectrum greater than that which is available solely with the helix construction.

18 Claims, 4 Drawing Figures PAIENTE FEB 1 3191s SHEEI 10F 2 [i This? M/VEA/TOA fade/1 Mall/1810: PAM/40f DOUBLE OCTAVE BROADBAND TRAVELING WAVE TUBE FIELD OE THE INVENTION This invention relates to a broad band traveling wave amplifier tube and, more particularly, to an O-type traveling wave tube that has a gain characteristic which is essentially flat over a broad band of frequencies.

BACKGROUND OF THE INVENTION The O-type traveling wave tube is a microwave vacuum tube amplifier which utilizes in operation electronic interaction" between traveling electrons and a microwave signal propagating along a slow wave structure, whereby, by means of such interaction, kinetic energy is transferred from the electrons to the microwave signal increasing the amplitude of the signal and lowering the velocity of the electrons.

In a conventional traveling wave tube the electrons are formed into an electron beam containing electrons of a predetermined average velocity for travel into the interaction region. Typically, this beam is formed with a source of electrons, the cathode, a focusing electrode for shaping the electron beam, and an accelerator electrode maintained at a high positive potential relative to the cathode for accelerating the electrons released at the cathode to a predetermined velocity at which the electrons enter the interaction region. In some instances a pervious grid electrode may be included in such a structure to be used to turn the beam on and off.

The slow wave structure is located within the interaction region and in present broad band tubes comprises an electrical conductor wound in the shape of an elongated helix. This helix includes an input terminal at the end of the helix most proximate the cathode and an output terminal at the end most remote from the cathode. A collector electrode is located at the remote end of the helix to collect electrons passing through the helix and a focusing solenoid or magnet system is provided surrounding the envelope or container housing the recited elements for maintaining the electron beam focusing in the interaction region. The electron beam is directed into the interaction region through the center of the helix. A microwave signal coupled to the helix input propagates along the wire at a predetermined velocity substantially equal to the velocity of light. However, the actual or effective lateral movement of the signal across the tube is only a fraction of that velocity because of its circuitous spiral path around the loops of the helix; hence, the signal is slowed." The pitch and diameter of the helix is designed so that the lateral propagation of the signal between input and output is slightly less than the velocity at which the electrons in the electron beam are traveling. And the helix diameter is also designed to permit the electromagnetic fields of the signal to extend into the electron beam to permit interaction. As a result of electromagnetic field interaction between the microwave signal and the electron beam over this interaction region, the electrons are slowed and thus give up energy to the microwave signal, which thereupon is evidenced by a growing amplitude of signal up to the output end of the helix, and the electron beam is finally collected at the collector.

In a more sophisticated traveling wave tube the helix is severed;" that is, the helix is broken up into two or LII more sections. This is accomplished by severing the helix at some point along its length and electrically grounding each of the thus formed ends, and by depositing sufficient microwave loss material at the sever" location to fully absorb the microwave signal at the end of the first helix section in addition to absorbing any reflected signals traveling from the second helix section toward the input end. This type of arrangement enjoys use in that it isolates the input and output of the tube.

As a result of interaction between the propagating microwave signal on the helix with the electron beam, the electrons are not only slowed down but they become bunched; that is, some electrons in the electron beam are slowed down more than other electrons and, as a result, the faster moving electrons catch up with the slower moving electrons so as to form bunches, regions along the electron beam having a high density of electrons, and, in like manner, voids or regions having low density of electrons. This bunching or series of nodes or antinodes of electron density corresponds on a sinusoidal basis with the frequency of the applied microwave signal and increases proportionate- Iy with the degree of amplification of that signal; thus, although the microwave signal applied to the input is terminated at the end of the first helix section by the sever, the bunched electron beam continues its travel into the second helix section. In entering the second helix section, the varying density electron beam induces the corresponding electromagnetic signal on the helix of the second section. This induced microwave signal thereupon proceeds to travel along the turns of the second helix section and by continued interaction with the same electron beam the signal grows and hence is amplified.

Thus, in the severed circuit arrangement the output circuit from the traveling wave tube is electrically isolated from the source of microwave energy applied to the input terminal, and the sole coupling of energy between the input and output ends of the slow wave circuit is due to the electron beam. These structures and principles are well described in greater detail in the literature.

Mention was made of a microwave signal without regard to its frequency or to the bandwidth of frequencies over which a given traveling wave tube effectively operates with uniform output. Broad band operation requires that signals of a predetermined level and of a predetermined frequency, (0,, applied to the input should be amplified and appear at the tube output with essentially the same level as any other frequency, (o of the same level within that range or band of frequencies. As a practical matter, the gain over a frequency range must be within about (6) decibles to be considered a flat or uniform gain over that frequency range. Obviously, in order to obtain broad bandwidth, one must use a broad band slow wave structure within the traveling wave tube. Frequently, this broad band characteristic of the slow wave structure is described in other terms, i.e., as nondispersive:" a slow wave structure in which the velocity of propagation of the signal along the structure is essentially independent of the frequency of the signal. Conversely, very narrow band slow wave structures are termed "dispersivez" the velocity of propagation of the input signal along the structure is highly dependent upon the signal frequency.

As is evident from the described mode of operation, for interaction to occur the propagation velocity of signals to be amplified along the slow wave structure must be slightly less than the velocity of the electrons in the electron beam. Thus, if the signal propagation velocity is substantially greater than the velocity of the electron beam, no interaction and, hence, no amplification of the signal occurs. Likewise, if the velocity of propagation of the signal is much less than the velocity of the electrons, there is no interaction and hence no amplification of the signal. Ideally, it is thus desired for the slow wave structure to permit all signals regardless of their frequency to travel along the slow wave structure at a given identical velocity. Unfortunately, this ideal is not possible. Hence the range of operation of any given traveling wave tube is limited to a certain bandwidth. The more dispersive the slow wave structure, the more narrow is the range of frequencies which will be amplified by that traveling wave tube structure. Conversely, as a general rule, the more nondispersive the slow wave structure, the wider the range of frequencies which can be amplified with uniform gain by the traveling wave tube.

The conductive wire helix is the most nondispersive slow wave structure known, so much so that it is used as a standard and ofttimes spoken of as nondispersive. Typically, the helix is capable of effective operation over but an octave of frequencies.

Basically, any O-type traveling wave tube constructed to have uniform gain over at least one octave of frequencies incorporates a substantially nondispersive structure, continuous or severed. The most commonly used of these is the helix. Even so, given a helix of predetermined diameter and pitch, a limiting factor at the lower frequencies is dispersiveness. This is due primarily to the nature of behavior of the electromagnetic fields at the low frequencies where the lines of force extend over physical distances large compared to the helix pitch. These lines or fields jump from turn to turn of the helix and signal propagation is not confined to travel spirally along the turns of the helix. At the low frequencies the velocity of propagation of the signals is faster than that which occurs when the signal travels solely along the turns of the helix. Because of this, the signal travels at a velocity exceeding the velocity of electrons in the electron beam and there is decreased interaction.

A second limiting characteristic to the helix occurs in operation at the upper edges of the frequency spectrum. While the helix, per se, is not truly dispersive at these frequencies in the sense defined previously, the interaction efficiency goes down again due to inherent limitations in the physical nature of the electromagnetic fields. For interaction between the propagating signalin the helix and the electrons in the electron beam, the fields must extend into the central area of the helix. At very high frequencies, however, the wavelengths of these fields become small in respect to the helix diameter and they do not fully extend into the electron beam. As the interaction efficiency thus goes down, signal amplification is reduced. As is commonly recognized, the most broad band or nondispersive slow wave structure is limited at the low frequency range by a dispersive characteristic and at the high frequency end of the range due to less efficient interaction with a uniform frequency gain characteristic between these two ends.

Heretofore, structures have been proposed for purposes of broadbanding microwave tubes. One prominent approach has been to use a series of dispersive circuit structures and to tune each to a different frequency so as to provide stagger tuning. This approach proves difficult because of the difficulties of matching together the numerous sections of slow wave structure necessary to obtain a smooth overall bandwidth characteristic. Moreover, inasmuch as successive sections require loss material for severing and isolation, excessi e losses or attenuation do not permit sufficient gain.

Other schemes for broadbanding microwave tubes usually involve essentially narrow band tubes for operation at a single or very narrow range of frequencies wherein it is desired to slightly extend the narrow bandwidth of operation of the tube. In traveling wave tubes this involves a highly dispersive slow wave structure for maximum amplification at a single frequency so as to maximize efficiency of the tube at that frequency and obtain highest gain. Such structures are broadbanded somewhat by modifications to the slow wave structure to make them slightly less dispersive. By way of example, slow wave structures have been produced consisting of a dispersive series of vanes which are coupled together on either side by elongated helixes. Hence, the broadband characteristic of the helix serves to broadband somewhat the very narrow band characteristic of the main portion of the slow wave structure, that being the highly dispersive vane assembly.

OBJECTS OF THE INVENTION Accordingly, it is an object of my invention to provide a new broad band traveling wave tube.

It is a further object of my invention to provide a traveling wave tube construction capable of providing essentially uniform gain over a double octave of frequencies without loss in efficiency.

It is a still further object of my invention to provide a broad band O-type traveling wave tube in which the bandwidth characteristic of a basic helix slow wave structure is enhanced at both the upper frequencies and the lower frequencies.

And it is a still further object of the invention to provide a new slow wave structure for an O-type traveling wave tube.

SUMMARY OF THE INVENTION Briefly, in accordance with my invention, a traveling wave tube is provided in which the helix slow wave structure is severed and consists of two helix portions spaced apart a predetermined distance within the electron interaction region. A dispersive slow wave structure is located within the interaction region and between the two helix sections to enhance the gain characteristic of the helix at one or more ranges of frequencies.

In accordance with a further object of my invention, the dispersive slow wave structure includes a first portion having its gain characteristic complementary over a predetermined range with the gain characteristics of the helix structure at the high end of the frequency band, and a second portion having its gain characteristic complementary with that of the helix structure at the lower end of the band.

Further, in accordance with the invention, said dispersive slow wave structure comprises a delay line of the contrawound helix variety, such as the ring loop or ring bar.

Those characteristics of my invention which I believe to be novel together with the objects and advantages of my invention and the relationship and cooperation of the elements comprising the invention, in addition to obvious substitutions and equivalents for those elements, become more apparent from a consideration of the following detailed description of the embodiments of my invention taken together with the illustrations thereof in the drawing.

In the drawing:

FIG. 1 illustrates symbolically an O-type traveling wave tube embodying the slow wave structure of the invention;

FIG. 2 illustrates in greater detail mechanically the dispersive delay line;

FIG. 3 illustrates ideally the gain frequency characteristics of each portion of the slow wave structure together with the gain characteristic for the entire traveling wave tube; and

FIG. 4 illustrates the frequency versus normalized phase velocity characteristic of each portion of the slow wave structure.

DETAILED DESCRIPTION OF THE INVENTION The dashed line 1 in FIG. 1 represents the envelope of the traveling wave tube. As is conventional, envelope 1 comprises a vacuum-tight nonmagnetic stainless steel material which confines internally a region in vacuum. And, as is customary, the envelope is grounded. Cathode 3 provides a source of electrons. A filament or heater 4 is provided for heating cathode 3 to enhance emission of electrons. Both cathode 3 and heater 4 include electrical conductors which pass through a vacuum-tight terminal in the envelope to permit electrical connections to be made.

An accelerator electrode 5 is spaced from cathode 3 and electrically grounded to the tube envelope. Accelerator electrode 5 contains a central passage 7 for permitting passage of electrons emitted from the cathode 3.

A collector anode 9 is provided at the right hand end of the tube. The collector electrode is electrically grounded in the illustrated embodiment. Typically, the region within envelope 1 between the accelerator electrode and the collector electrode may be referred to as the electronic interaction" region, as hereinafter becomes more apparent. A slow wave structure" or delay line is provided in the interaction region. The slow wave structure of the invention as schematically illustrated in FIG. 1 includes a first helix portion or section 11 and a second helix portion or section 13 separated and spaced from the first helix portion. The input end 10 of helix 11 is connected to an RF input terminal 9. The right hand end 12 of helix 11 is electrically connected to the tube envelope and is therefore at an electrical ground potential. The left hand or input end of helix 13 is also connected to the tube envelope 1 and is therefore the electrical ground potential. The right hand or output end 15 of helix 13 is connected to the RF output terminal 16. These helix sections form characteristically as hereinafter explained in detail a nondispersive structure. The slow wave structure includes further a dispersive section of slow wave structure such as is available with the contrawound helix family of circuits and particularly a ring loop section located in the interaction region in between helix section 11 and 13 and is spaced therefrom. The ring loop section used here by way of illustration includes a first section 17 and a second section 19, which sections are illustrated ad being electrically and mechanically connected together at their respective right and left hand ends although they need not be. The input end 18 of section 17 is connected to the tube envelope 18 and the right hand end 20 of section 19 is electrically connected to the tube envelope.

As is conventional, these slow wave structures are physically supported in their respective locations within the tube envelope 1 by a series of three elongated ceramic support rods, which are simply illustrated in FIG. 1 by a single dashed line 23. The support rods are coated with a microwave loss or dissipative material, suitably carbon, in predetermined amounts and at predetermined locations thereon. Thus, as schematically illustrated, a tapered density carbon loss material appears on the support rods at adjacent the right hand end of helix 11, the left hand end of ring loop section 17, the right hand end of ring loop section 19, and the left hand end of helix 13, and are suitably labeled 25, 27, 29 and 31, respectively. The loss materials are conventionally provided to match the lines electromagnetically and prevent passage of microwave energy traveling in either direction past those points. Hence the carbon loss prevents coupling between sections of the slow wave structure and prevents electromagnetic energy from being reflected back from the tube output terminal to the source, which avoids undesired self-oscillation.

A source of voltage, V, represents the high voltage power supply. This power supply has its negative polarity terminal connected to the lead to cathode 3 and its positive polarity terminal connected to electrical ground. A source of filament current is applied to the filament leads. As is apparent such connection of the voltage source places accelerator electrode 5, the helix sections, the tube envelope 1, and anode 9 at a high positive voltage relative to cathode 3.

A magnetic field longitudinal of the slow wave structure and typically formed with permanent magnet rings or a solenoid is illustrated by the symbol B and the arrow.

FIG. 2 better illustrates the dispersive ring loop slow wave structure, schematically illustrated in FIG. 1 as elements 17 and 19, in mechanical perspective. For purposes of continuity and perspective, the bracketed sections of the ring loop are labeled l7 and 19' and the portions of the helix sections 11' and 13' are illustrated. It is apparent from the illustration that the ring loop slow wave structure is made up of a series of ring members or rings 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53 and 55. These rings are spaced apart a predetermined distance and are mechanically and electrically joined together by loop sections or loops 34, 36, 38, 40, 42, 44, 46, 48, 50, 52 and 54. In the preferred construction, alternate ones of these coupling loops join together adjacent rings on the upper side of the slow wave structure and the remaining ones are joined together on the bottom side.

Ring loop 17' has a highly dispersive velocityfrequency characteristic effective only at the higher range of frequencies of the helix, while ring loop section 19' is highly dispersive and effective only at the lower range of frequencies of the helix, as hereinafter discussed in greater detail.

For purposes of illustration, each of the sections of the ring loop is made up of six rings with section 17 comprising rings 33, 35, 37, 39, 41 and 43, whereas section 19' comprises rings 45, 47, 49, 51, 53 and S5. Coupling loop 44 joins together the two sections. Preferably, the rings of either section together with the coupling loops are'stamped out of a single strip of molybdenum material and then are bent at the coupling loop portions to form the series of joined rings.

Basically, the narrow range of frequencies at which a ring loop line is effective is determined primarily by the ring to ring spacing. Thus, section 19 is operative at the lower frequency range and the rings are spaced more closely together than the ring to ring spacing found in ring section 17' effective in a narrow range of higher frequencies.

As hereinafter explained, it is possible to tailor or adjust the electromagnetic characteristics of each section within the narrow frequency band by varying slightly the distance between individual rings of that section so as to have a tapered gain characteristic at that range of frequencies.

In a practical embodiment of my invention, I use a series of 40 rings spaced apart by an approximate distance of .100 inches for fulfilling the function of section 17 and using a series of 60 rings spaced apart by an approximate distance of .050 inches for operation of section 19. It is apparent and within the scope of my invention, however, for different numbers of rings to be employed in alternative embodiments. Moreover, as is also apparent, the number of rings in each separate section need not necessarily be equal to the number of rings in the other section. It is not necessary to couple together sections 17 and 19, moreover, although I prefer to do so. Should it be desired to separate these two sections of dispersive slow wave structure, one need only to remove coupling ring 44, by way of example, to ground the top end of each of rings 41 and 44, and apply dissipative loss to the support rods to terminate the wave.

FIG. 3 illustrates ideally, by way of example, normalized dispersion characteristics of the various portions of the slow wave structure used in the traveling wave tube of FIG. 1. The dispersion curve is one of the conventional graphical tools used by tube designers for determining the operational characteristic of a traveling wave tube. Thus it is apparent that the helix characteristic is relatively flat between the frequencies 01A to (D. In terms of operation of the tube the phase velocity of propagation of a microwave signal applied to the helix is over that range of frequencies independent of frequency, and therefore for electron beam interaction in which the electron beam is set to a predetermined single velocity, interaction between the microwave signal and the electron beam can be obtained over the entire range of frequencies mA-wB. However, between the range of frequencies mE to wA in the lower frequency ranges the diagram illustrates that the phase velocity is somewhat dependent upon and a function of frequency of the signal and, in particular, in this lower range of frequencies the microwave signal of that frequency travels at a greater phase velocity. Physically this is explained because the electric fields at lower frequencies extend over greater physical distances than is true at the high frequencies, the low frequency signal has the ability to extend across, couple, or jump from turn to turn of the helix rather than ideally following only the spiral path of the conductor forming the turns of the helix. Hence, the low frequency signal for a given helix tends to travel therealong at a faster velocity than those microwave signals in the range of (DA to wB or midband signals. This, of course, precludes or reduces electronic interaction with an electron beam which, by design and as a general rule, is set to contain electrons which travel at a single velocity of the electrons must be slightly greater than the velocity of the signal in order for interaction to occur.

Ring loop section 19' has the dispersion characteristic in which the phase velocity of the applied signal along that section is highly dependent upon and a function of the signal frequency, as is indicated by the left hand curve in the figure. Likewise, ring loop section 17' has a dispersion characteristic which is highly variable and in which the phase velocity of an applied signal is highly a function of the frequency of the signal as appears in the right hand curve in the figure. Both dispersion characteristics are representative of high dispersiveness'," that is, they are effective only over a frequency range of 10 to 25 percent of the mid-band frequency. Thus, the dispersion characteristic for each of these loop sections is only effective over a predetermined narrow range of frequencies so that at the high end frequencies around wD only ring loop section 17' is effective, whereas around the lower end of frequencies mC only the ring loop section 19' is effective.

FIG. 4 illustrates the gain" characteristic of the entire slow wave structure and is represented as curve 60. By this graphical representation, the individual contributions to gain" or amplification of the traveling wave tube can be evaluated. Thus dispersion curves of FIG. 3 provide an understanding of whether or not interaction will occur and to what degree for a given electron beam velocity. The gain curve shows the result of such interaction in terms of amplification. The gain of the helix labeled as curve 61 is relatively flat (i 3 db) between frequencies (0A to (08. And either above mB or below wA the gain decreases.

As previously explained, the reduction in gain at the lower range of frequencies for the nondispersive" helix type of slow wave structure is due to loss of interaction between the low frequency signal and the electron beam because the helix in that frequency range is dispersive, as is apparent from the curve of FIG. 3. However, the departure of the helix gain from an ideal flat gain characteristic at the upper range of frequencies, those above (08, results because the signals at the high frequencies are of such short wavelength that the fields do not extend fully from the helix into the helix center where those signals must interact with the electron beam. Thus, in this way, a loss of interaction efficiency occurs and is responsible for the reduced and decreasing gain characteristic of the helix at the upper frequencies.

Curve illustrates the gain characteristic for ring loop section 17 and curve 63 illustrates the gain characteristic for ring loop section 19. As is apparent, each of the gain characteristics of the ring loop sections is highly peked" and dependent upon frequency. Moreover, it is apparent that at the upper range of frequencies ring loop section 19 has no gain and does not contribute any gain, whereas at the lower range of frequencies ring loop section 17 has no gain and does not contribute to the tube gain. The center frequency (highest gain) of ring loop section 19' is at the frequency wC, which corresponds to a point on curve 61 of the helix where the gain of the helix section is reduced approximately 20 db. Combining the ordinate of curves 61 and 63 in this region a combined gain characteristic is obtained represented by the dashed line 67 portion of curve 60. As is apparent, the gain characteristic is raised to the level corresponding with the gain of the helix in mid-frequency band and is relatively flat over a predetermined additional range of lower frequencies from mA down to mE (the 3 db point).

In this way, the bandwidth of the amplifier is increased over that bandwidth available solely with a nondispersive helix over an additional range of frequencies mE to (A.

In like manner, by design the peak of the gain characteristic of ring loop section 17 is set at the frequency mD which corresponds to a point on the gain curve 61 of the helix where the gain is substantially reduced. A resultant or combined gain characteristic is obtained by adding together the ordinator of the graph which is represented by the dashed line portion 69 of curve 60. As is apparent, the combination of gain from the helix and ring loop 17 raises the overall gain of the helix at the high frequency end and is relatively flat up to the frequency mF where gain commences to decline. Thus, given a flat gain characteristic for the helix effective over the frequency range of wA to (0B a resultant flat gain characteristic is obtained for the amplifier effective over a significantly larger frequency range wC to (0D by the addition of the highly dispersive ring-loop sections 17 and 19.

The operation of the traveling wave tube, schematically illustrated in FIG. 1 and containing the slow wave structure of the invention, is in great part conventional, well known, and described in the literature. Basically, electrons emitted by cathode 3 are attracted by the accelerator electrode at a high positive potential relative to the cathode and are accelerated up to a predetermined design velocity. The electrons are formed into a beam, pass through passage 7 and enter the interaction region. Obviously, the. velocities which the electrons attain up through passage 7 is dependent upon the power supply voltage, V, as well as the distance between cathode and electrode 5. Typically with a supply voltage 10 kilovolts these electrons are traveling at velocities of percent of the velocity of light upon entering the interaction region. These electrons travel ideally in a straight path through the helix section 11, ring loop sections 17 and 19, and helix section 13, and then to collector electrode 9 where the electrons return through electrical ground back to the positive polarity terminal of the power supply. As is well known, however, electrons in a beam because of their similar electrical charge repel one another which could cause the beam to spread. In addition, if any electrons enter the interaction region with a transverse velocity component they would collide with the slow wave structure. The magnetic field B, illustrated in the figure, axial with the beam path and created by a solenoid or magnet system outside the tube envelope 1 serves to focus those electrons to maintain them in a compact beam. The longitudinal or axial magnetic field forces any electrons traveling in a path away from the straight path defined are caused to spiral around the beam path and in this way prevents electrons from reaching the slow wave structure.

A microwave signal of a given frequency, w, assumed to be in the band of frequencies between mA and m8, is applied to the RF input terminal and proceeds to travel or propagate up lead 10 into and along the turns of the helix section 1 1, spiraling around therealong to the end of helix 11. Since electromagnetic energy, such as this signal, travels, essentially, at the speed of light, a known constant, c, along the turns of the helix, the actual longitudinal travel of that signal is less than the speed of light because of the circuitous path around the spirals of the helix taken by the signal. Essentially, this longitudinal travel is slowed down by a factor in direct proportion to the diameter of the helix and the pitch or distance between turns of the helix, normally about 20 percent the speed of light and approximately 10 percent less than the electron velocity of the electron beam. Hence, the designation slow wave structure. In propagating along the slow wave structure, such as helix 11, the microwave signal possesses fields which extend from the structure into the region of the electron beam and creates periodically alternating fields which act to accelerate or decelerate relative to the electrons traveling through the helix. Under the influence of the decelerating field the electrons in the electron beam are slowed down. Conversely, under the influence of the accelerating fields the electrons in the electron beam are speeded up. These two groups of accelerating and decelerating electrons form together into regions along the electron beam path where the electrons are said to be bunched. The microwave signal applied to the slow wave structure changes the electron beam into a beam comprising spaced bunches of electrons. These bunches tend to form at the locations of nodes along the slow wave structure; that is, the position between the accelerating and the decelerating fields, or zero field.

The phenomenon of interaction occurs in that the electron beam velocity by design is made to be on the average slightly greater than the velocity of longitudinal travel of the microwave signal along the slow wave structure. Typically, as was stated the electron beam velocity is from 1 to 10 percent greater than that of the electromagnetic signal on the slow wave structure. The traveling bunches of electrons hence are attempting to travel at a greater speed than that of the microwave signal and the decelerating fields generated thereby so that the electron bunches are increasingly under the influence, during travel, of a decelerating electromagnetic field. The microwave field, hence, tends to slow down the bunch of electrons. And in slowing down the electrons some kinetic energy is given up by the electrons which is, in turn, transferred to the microwave signal. Hence, by continued interaction" over the length of a slow wave circuit, the average velocity of the electrons in the beam diminishes,

bunching becomes more pronounced, and the average amplitude of the microwave signal applied to the slow wave structure input tends to increase. A more detailed and scientific explanation of these phenomena is available in the literature.

In itself, the foregoing phenomenon describes basically the operation of an O-type traveling wave tube. Generally, however, it is desirable to isolate the input signal source applied to the microwave tube from the output or RF load end for reasons, including the prevention of internal reflections in the tube from causing self-oscillation and in preventing any mismatch in the load from affecting the operation of the input signal source. To do so the input and output circuits are severed, typically by the use of microwave attenuation material, combined with or without physical separation of the parts of the helix. This is described in the further description in the mode of operation of the traveling wave tube of FIG. 1.

At a position near the output end of helix 11 the input signal, a), has been amplified to a high level and the underlying electron beam is correspondingly bunched. The amplified signal travels to attenuator 25 where it is absorbed and dissipated. The attenuator thus prevents the microwave signal continuing along the slow wave structure. However, the electron beam has been bunched inproportional intensity and these bunches continue to travel in a course toward helix 13, omitting for the present the description of any interaction in ring loop sections 17 and 19. In entering helix 13, the bunches of electrons induce within the first turn of the helix the microwave signal of frequency, (0, corresponding to the input signal which originally bunched the electrons beam, somewhat analogously to the manner in which the grooves of a phonograph record reproduces the signals which originally were used to cut those grooves. Subsequently, this induced microwave signal, (0, travels along the helix section 13 and continues electronic interaction with the electron beam, further bunching the beam, essentially in the same manner as was discussed in connection with the helix operation of helix l1, and the microwave signal, to, continues to grow in amplitude until the output of the helix is reached. The signal travels via lead 15 to the RF output'terminal where it is coupled to an electrical load, not illustrated.

The electrons in the beam which have given up much of their energy at this point continue to travel to collec' tor 9 with which they collide, dissipate most of their remaining energy in heat at the collector, return to ground and back to the original D. C. power supply. Attenuator 31 coupled to the front end of helix section 13 dissipates any reflected microwave energy, i.e., any microwave energy which might travel from the RF output terminal into the helix in a reverse direction toward the front'end of the helix. Such reflections could cause oscillations and are thus desirably eliminated in this manner.

The electron bunches formed during interaction contain the information representative of the original microwave input signal applied to helix 11, much as a fingerprint is representative of the fingertip that made the impression, and it is this information that is induced upon helix section 13 as the bunched electrons enter the second helix section.

As was previously assumed, the microwave signal, f, applied to the RF input terminal was in the mid-range of frequencies for which the embodiment of FIG. 1 was designed to operate. Reference to FIGS. 3 and 4 show that the microwave frequency signals within this range are uninfluenced by ring loops 17 and 19 which are effective only at the higher or lower frequencies, respectively. Thus any signal induced on structures 17 and 19 during passage of the electron beam bunches is uneventful in that it is quickly dissipated without interaction.

During passage through ring loop sections 17 and 19, there is no change either to the bunching of the beam or to the slow wave structure, as is apparent from FIG. 3 and from FIG. 4. Ring loop section 17, represented by curves and 17, are ineffective or lacking in gain, as may be variously termed, to signals in the mid-band range of frequencies. In like manner, as the electron beam passes through the second ring loop section 19, and it too is ineffective to change the characteristic of the bunching, there is no interaction inasmuch as the characteristics of the ring loop section 19, as illustrated in curve 19' of FIG. 3 and 63 of FIG. 4, have no influence upon signals in the mid-band range and may be considered neutral or inert.

Considering now the operation of the tube with a high frequency band edge signal, wD, applied to input lead 10, the mD signal interacts with and causes bunching of the electron beam in helix section 11 in the same manner as discussed with respect to the mid-band section (0. As the bunched electron beam proceeds into ring loop section 17 it induces a corresponding microwave signal on the ring loop. It is within this frequency region, as represented graphically by FIGS. 3 and 4, that ring loop section 17 is effective. Accordingly, the signal induced on ring loop section 17 thereupon interacts further with the electron beam and is amplified. Conversely, the amplified signal causes greater bunching of the electron beam. Much in the same manner as the original microwave input signal is dissipated in a terminating attenuator 25, previously discussed, the signal generated upon and amplified in ring loop section 17 proceeds along the loop through ring loop section 19 to a similar terminating attenuator 29, where it is dissipated. The increasingly bunched electron beam travels through section 19 where it undergoes no further change inasmuch as ring loop section 19 is ineffective as evidenced by the gain curve 63 of ring loop section 19, and the bunched electron beam proceeds to enter the second helix section 13 where, in the normal manner previously described, a signal is induced upon the initial turns of the helix and through interaction that signal is amplified and taken at the output.

The converse situation occurs with respect to a microwave signal in the low frequency range, such as mC,-applied to the RF input terminal. In this operation the ring loop section 17 is ineffective and the second ring loop section 19 is effective to cause further bunching and amplification of he electron beam. As before, the amplified signal is dissipated in attenuator 29 at the end of ring loop section 19 and the increasingly bunched electron beam proceeds into the final helix section 13 where it induces a corresponding signal upon the initial turns of this helix, which thereupon ina constant .10 inches, the peaking is found to be smoothed out. Thus, by adjusting the amplitude of the gain characteristic of the dispersive ring loop the overall gain of the traveling wave tube can be maintained at the desired level and is accordingly enhanced.

The foregoing embodiments are intended to be illustrative of the invention and not as a limitation to my invention.

As is apparent, many alternative embodiments and substitutions for the element become apparent to one skilled in the art from this specification, all of which come within the spirit of my invention. Accordingly, it is expressly understood that my invention is to be broadly construed within the breadth and scope of the appended claims.

What I claim is:

1. An O-type traveling wave tube having a uniform level of gain over at least a one octave frequency range which comprises:

a cathode for providing a source of electrons,

a collector electrode spaced from said cathode for collecting electrons,

an accelerator electrode spaced in between said cathode and collector for accelerating electrons from said cathode for travel through an interaction region defined in the space between said accelerator electrode and said collector,

and slow wave structure means located within said interaction region, said slow wave structure including an input for receiving signals to be amplified and an output for passing signals amplified as a result of interaction in said interaction region to a load; and said slow wave structure including:

first and second substantially nondispersive delay lines spaced apart in said interaction region, said first and second delay lines possessing an overall gain characteristic which is of a substantially uniform level over a first predetermined range of frequencies and which decreases to lesser levels of gain at other frequencies above and below said first range;

said input being coupled to the end of said first nondispersive delay line most proximate said cathode and said output being coupled to an end of said second nondispersive delay line most proximate said collector;

a first substantially dispersive delay line section spaced from and located in the space between said first and second delay lines, said dispersive delay line section having a second predetermined gain characteristic effective to provide gain substantially only in an increment of frequencies adjacent and outside said first predetermined range for complementing the gain characteristic of said first and second delay lines in an increment of frequencies adjacent and outside said first predetermined range without providing substantially any gain within said predetermined frequency range to thereby extend said uniform level of gain over an additional increment of frequencies adjacent and outside said first predetermined range;

support means for supporting all said delay lines in said interaction region;

and microwave loss material located on said support means at those locations thereon corresponding to the locations of the opposed ends of said nondispersive and dispersive delay lines for absorbing microwave energy propagating to said opposedends.

2. The invention as defined in claim 1 wherein each of said first and second delay lines comprises further: elongated helixes of electrically conductive material.

3. The invention as defined in claim 1 wherein said substantially dispersive delay line comprises an equivalent contrawound helix assembly.

4. The invention as defined in claim 3 wherein said equivalent contrawound helix assembly comprises further a ring loop line.

5. The invention as defined in claim 2 wherein said substantially dispersive delay line comprises a ring loop assembly.

6. A broad band double octave O-type traveling wave tube of the type having a metal envelope containing a cathode and accelerator means for generating an electron beam of a predetermined velocity for travel into an interaction region and a slow wave structure located within said interaction region; said slow wave structure comprising a first section of helix having an end most proximate said cathode connnected to a microwave input coupling means and an end most remote from said cathode connected electrically in common with said envelope; a second section of helix located in said interaction region spaced from said first section of helix; said second helix having its end most remote from said cathode connected to a microwave output coupling means and having its end most proximate said cathode connected electrically in common with said envelope; a ring-loop type slow wave circuit of a predetermined length; said ring-loop structure comprising a plurality of spaced rings of metallic material in which adjacent rings are coupled together electrically by short metal loops; said ring-loop section being located within said interaction region in between said first and second helix sections and having at least one end thereof connected electrically in common with said envelope; said first and second helix sections capable of providing a relatively fiat gain characteristic over a predetermined range of frequencies, designated (0 to (0 and having a decreasing gain characteristic in an adjacent upper range of frequencies, (0 to w and in a lower range of frequencies, w} to (.01, where w w and w, w and said ring-loop structure including a first plurality of rings spaced apart a first predetermined distance and a second plurality of rings spaced apart a second predetermined distance different from said first predetermined distance and having a gain characteristic effective substantially only in the region of frequencies m to m and a), to m, and which gain characteristic complements that of said helix structure substantially only within the region of frequencies of m to w and an to w, to extend said relatively flat gain characteristic for said traveling wave tube over the larger frequency range of an to run.

7. An O-type traveling wave tube of the type containing a source of electrons; means for forming said electrons into an electron beam for travel into an interaction region; a collector electrode for collecting electrons traveling from said interaction region, input coupling means, output coupling means, a slow wave structure located within said interaction region for teracts with the electron beam, is amplified progressively along the helix section 13 and is taken from the RF output terminal.

As is evident from FIG. 4, the gain of the helix sections, taken together, is substantially flat over a predetermined frequency band A to (0B and drops off the frequencies above 00B and similarly drops off at frequencies below 01A.

Considering first the characteristics of ring loop section 19' as represented by curve 63 in FIG. 4, the ring loop section has a predetermined and peaked gain characteristic which by design centers around a frequency wC and drops off at a predetermined rate at frequencies either above or below mC. The center frequency of gain for the ring loop section 19 is designed so that it occurs where the gain of the helixes alone is down about 20 db relative to the gain characteristic of the helix section. By adding together the relative gain illustrated in curves 61 and 63 in this region, the curve representing the summation of such gain is derived and is indicated by the dashed lines 67. By this combination of the relatively nondispersive helix and the dispersive ring loop section 19' the bandwidth of the traveling wave tube is seen to have been extended relatively flat from frequency 00A to a lower frequency (0C.

In like manner, the design characteristic of ring loop section 17 is highly dispersive and is peaked at a frequency wD within the region above frequency wB at which the gain characteristic of the helix section is decreasing. At frequencies either above or below mD the gain characteristic of ring loop section 17' decreases. The center frequency 0D, for which the ring loop is designed to have peak gain, is selected so as to fall at a point along the gain characteristic of the helix section as illustrated in curve 61, so that the gain of the helixes alone is again down about 20 db. By combining the contributions of signal gain from each of the helix section and ring loop section 17 a gain curve represented by the dashed lines 69 is obtained and is seen to be of the same highly level over a predetermined range of frequencies as the helix gain at the midband frequencies. The gain for the tube is seen to have increased from the high frequency (.08 to a higher frequency mF.

In this way, the tube so constructed has a broad band or flat gain characteristic, however termed, over a greater frequency range of mE to wF. This is substantially greater than the bandwidth wA to 013 available solely with a nondispersive helix-type structure.

As was previously noted, the helix alone as a slow wave structure has uniformly, because of its substantially nondispersive characteristics, been chosen as the slow wave structure for broad band operation. With the addition of dispersive slow wave structures at the band edges of the helix it is seen that a new structure is obtained that has a broad band characteristic greater than that possible with a series of highly dispersive slow wave structures or with the helix alone.

It is further apparent that the broad banding is accomplish without any reduction in the tube efficiency in that no attenuation is used and that the contribution of the dispersive lines'is either a contribution to signal gain or none at all.

In the foregoing description of the preferred embodiment of the invention, a traveling wave tube structure was described which provided the maximum possible gain enhancement at both the higher and lower frequency ranges of an ordinary helix by the inclusion of two individual highly dispersive ring loop lines operative and effective at the higher and lower frequency band edges respectively. It is apparent that further modifications of this invention can provide improved bandwidth characteristics in a traveling wave tube by simply enhancing the gain at either the lower range of frequencies or, alternatively, at only the high range of frequencies. Such embodiments, however, do not provide the same extent of broadbandedness as is found in the preferred embodiment, but do provide a substantial increase in bandwidth over those traveling wave tubes which use a helix slow wave structure without more.

Thus, a traveling wave tube which has a broad band gain characteristic but in which the gain is enhanced solely at the low frequency band edge in an embodiment of my invention requires a modification to the traveling wave tube, schematically presented in FIG. 1, to omit ring loop section 17. In that embodiment the end of ring loop section 19 shown connected to ring loop section 17 is instead connected to the tube envelope 1 where it is electrically grounded. In addition, a microwave attenuative material on support rod 23, illustrated as 29 in FIG. 1, would instead be located proximate the left end of ring loop section 19.

In like manner,'a broad band traveling wave tube of the invention is provided in another embodiment in which the gain of the traveling wave tube is enhanced solely at the higher or upper band edges. That embodiment requires a modification to the embodiment illustrated schematically in FIG. 1 in which ring loop section 19 is omitted and in which the right end of ring loop section 17, illustrated connected to ring loop section 19, is connected instead to the tube envelope 1 where it is placed at an electrical ground potential. Additionally, the microwave loss material 27 located on support rods 23 adjacent the left end of ring loop section 19 is located in such an embodiment at the left hand edge of ring loop section 17.

In a practical construction of one embodiment of the invention in which solely the upper frequencies were enhanced with the inclusion of a single ring loop section, the helix had a gain characteristic which was flat essentially over the frequency range of 1.6 GHz to 4.0 GHz. A ring loop section having 40 rings and of the same diameter as the helix was constructed in which the rings were spaced apart by a uniform pitch of approximately .100 inches. In this way the gain of the traveling wave tube was enhanced over an additional range of frequencies of 4.0 GHz to 5.4 GHZ. It is noted however that in such construction that the gain was slightly peaked at the high frequency end by a factor of 3 db which suggests that the ring loop provided slightly larger than the desired gain. However, as is apparent,

- this is easily corrected by tapering or adjusting the ring to ring spacing. In this way the dispersion characteristics of this highly dispersive iine, and accordingly the gain characteristic, is changed. For example, by adjusting the ring to ring spacing so that it varies from .08 inches to .10 inches with at least one half of the rings at receiving signals, to, to be amplified at one end and providing amplified signals at an output end; and said tube having a gain characteristic of a predetermined uniform level over a predetermined range of frequencies, between m and m where m is a predetermined first frequency and w,, is a predetermined second frequency in said range of frequencies; the improvement thereto wherein said slow wave structure comprises: first substantially nondispersive slow wave structure means substantially nondispersive over the frequency range of (9,, to (9,, where w, is a third predetermined frequency within the range of w to (o and having a substantially uniform flat gain characteristic over said frequency range m to w, and a gain drop off at frequencies between w, and a) said first nondispersive slow wave structure comprising two spaced helix sections and said input means coupled to the input end of said first helix section and said output means coupled to the output end of said second helix section; and second substantially dispersive slow wave structure means disposed between said two spaced helix sections; said dispersive slow wave structure means having a gain characteristic limited to a narrow range of frequencies substantially less than the difference |w -m and said gain characteristic of said second slow wave structure being substantially complementary with said gain characteristic of said first slow wave structure means in the frequency range between w, and w to provide in combination with said nondispersive slow wave structure a uniform gain characteristic over the frequency range of w to w 8. The invention as defined in claim 7 wherein (1),,

9. The invention as defined in claim 7 wherein (0,,

10. The invention as defined in claim 8 wherein said second slow wave structure comprises a ring loop line.

11. An O-type traveling wave tube of the type which includes: a source of electrons, means for forming electrons from said source into an electron beam for travel through an interaction region, a collector electrode for collecting electrons traveling from said interaction region, and a slow wave structure in said interaction region for receiving signals to be amplified by interaction with said electron beam in said interaction region, said tube having a gain characteristic of a predetermined flat level over the range of frequencies w, to where an is the lower frequency of the range and mu is the upper frequency of the range, 'the improvement wherein said slow wave structure comprises:

first nondispersive slow wave structure means substantially nondispersive over the frequency range of m, to w and having a substantially flat gain characteristic over the frequency range m, to a) and a gain drop off at frequencies greater than to, and lesser than ai where w w w wu; said first nondispersive slow wave structure including: a first helix section, and a second helix section, said second helix section being spaced from said first helix section; and second dispersive slow wave structure means having a relative peaked gain characteristic and effective to provide gain substantially only within the range of frequencies between an and w, and

between to and (o said gain characteristic of said second slow wave structure means being substantially complementary with said gain characteristic of said first slow wave structure means in the range of frequencies between 0),, and w, and in the range of frequencies between an and ou said second dispersive slow wave structure means being located in between and spaced from said first and second helix sections; and

microwave energy input coupling means coupled to one end of said first helix section and microwave energy output coupling means coupled to one end of said second helix section.

12. The invention as defined in claim 11 wherein said second dispersive slow wave structure comprises a ringloop line.

13. The invention as defined in claim 11 wherein said second slow wave structure comprises a ring-loop line.

14. The invention as defined in claim 11 wherein said second slow wave structure means comprises a member of the contra-wound helix family of circuits.

15. The invention as defined in claim '14 wherein said member of said contrawound helix family of circuits comprises further a ring loop line.

16. An O-type traveling wave tube which comprises:

a cathode for providing a source of electrons;

a collector electrode spaced from said cathode for collecting electrons;

an accelerator electrode spaced in between said cathode and collector for accelerating electrons from said cathode for travel through an interaction region defined in the space between said accelerator electrode and said collector; and

slow wave structure means located within said interaction region, said slow wave structure includ ing an input at one end for receiving signals to be amplified and an output at its other end for passing signals amplified as a result of interaction in said interaction region to a load, said slow wave structure including:

first and second substantially nondispersive delay lines spaced apart in said interaction region, each of which comprises an elongated helix of electrically conductive material, said first and second delay lines possessing an overall gain characteristic which is of a substantially uniform level over a first predetermined range of frequencies and which decreases to lesser levels of gain at other frequencies above and below said first range;

a first substantially dispersive delay line section spaced from and located in the space between said first and second delay lines, said substantially dispersive delay line comprising a ring-loop assembly having a first portion with a relatively wide spacing between rings for providing gain at high frequencies, and a second portion with a relatively small spacing between rings for providing gain at low frequencies, said dispersive delay line section having a gain characteristic which complements the gain characteristic of said first and second delay lines in an increment of frequencies adjacent and outside said first range to extend said uniform level of gain for an additional increment of frequencies adjacent and outside said first predetermined range;

support means for supporting all said delay lines in said interaction region; and

microwave loss material located on said support means at those locations thereon corresponding to that of the opposed ends of each said delay lines for absorbing microwave energy propagating to said ends.

17. The invention as defined in claim 16 wherein the pitch of each of said first and second of said ring loop structures are tapered to provide a tailored gain characteristic.

18. An O-type traveling wave tube of the type which includes:

a source of electrons;

means for forming electrons from said source into an electron beam for traveling through an interaction region;

a collector electrode for collecting electrons traveling from said interaction region; a slow wave structure in said interaction region for receiving signals to be amplified by interaction with said electron beam in said interaction region;

input coupling means coupled to one end of said slow wave structure and output coupling means connected to the other end of said slow wave structure;

said tube having a gain characteristic of a predetermined flat level over the range of frequencies on to m where an is the lower frequency of the range and w is the upper frequency of the range, the improvement wherein said slow wave structure com prises:

first nondispersive slow wave structure means substantially nondispersive over the frequency range of w, to m, and having a substantially flat gain characteristic over the frequency range m, to w and a gain drop off at frequencies greater than gu, and lesser than (or, where m w w m and comprising a first helix section and a second helix section, with said second helix section being spaced from said first helix section; and

second dispersive slow wave structure means having a .relative peaked gain characteristic, said gain characteristic of said second slow wave structure means being substantially complementary with said gain characteristic of said first slow wave structure means in the range of frequencies between p and m and inthe range of frequencies between on and m said second dispersWe 516w wave structure being located in between and spaced from said first and second helix sections and comprising a ring-loop line having a first section operative to provide gain only over a limited increment of frequencies in the high range of frequencies and a second section operative to provide gain only over a limited increment of frequencies in the lower range of frequencies.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4118671 *Feb 15, 1977Oct 3, 1978The United States Of America As Represented By The Administrator Of The National Aeronautics And Space AdministrationTraveling wave tube circuit
US4147956 *Mar 4, 1977Apr 3, 1979Nippon Electric Co., Ltd.Wide-band coupled-cavity type traveling-wave tube
US4178533 *Sep 15, 1977Dec 11, 1979Thomson-CsfMicrowave delay line for travelling wave tube
US4263532 *Dec 18, 1979Apr 21, 1981Thomson-CsfMicrowave delay line
US4549112 *Dec 17, 1982Oct 22, 1985Thomson-CsfDelay line for a travelling wave tube
US5162697 *Aug 6, 1990Nov 10, 1992Hughes Aircraft CompanyTraveling wave tube with gain flattening slow wave structure
US6049249 *Sep 8, 1997Apr 11, 2000Hughes Electronics CorporationTWT with mismatched section for controlled gain variation with frequency
US6285254 *Jan 14, 2000Sep 4, 2001Teledyne Technologies IncorporatedSystem and method for linearizing vacuum electronic amplification
US6498532Jan 12, 2001Dec 24, 2002Teledyne Technologies IncorporatedSystem and method for linearizing vacuum electronic amplification
US6590450 *Aug 31, 2001Jul 8, 2003Teledyne Technologies IncorporatedSystem and method for linearizing vacuum electronic amplification
US6734734Jul 24, 2002May 11, 2004Teledyne Technologies IncorporatedAmplifier phase droop and phase noise systems and methods
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Classifications
U.S. Classification315/3.6, 315/3.5, 330/43
International ClassificationH01J23/16, H01J23/27, H01J25/38, H01J25/00
Cooperative ClassificationH01J23/27, H01J25/38
European ClassificationH01J25/38, H01J23/27