US3576460A

US3576460A - Impedance match for periodic microwave circuits and tubes using same

Info

Publication number: US3576460A
Application number: US751258A
Authority: US
Inventors: Ward A Harman
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1968-08-08
Filing date: 1968-08-08
Publication date: 1971-04-27
Anticipated expiration: 1988-04-27
Also published as: DE1940241A1; DE1940241B2; FR2015320A1; GB1284455A; JPS509513B1

Abstract

A traveling wave tube amplifier is disclosed. The amplifier includes an electron gun for forming and projecting a stream of electrons over an elongated beam path to a beam collector electrode. A coupled cavity slow wave circuit is arranged along the electron beam path for electromagnetic interaction with the beam to produce an amplified output signal at the downstream end of the circuit. The slow wave circuit is made up of a number of severed slow wave circuit portions. Each of the severed circuit portions includes a plurality of substantially identical coupled cavity resonators forming a main periodic slow wave circuit portion having a passband with at least one bandedge frequency of interest which is capable of producing bandedge oscillations by interaction with the electron stream. Matching circuit portions are provided at opposite ends of the slow wave circuit portions for impedance matching the main periodic circuit section to the load and input transmission lines to prevent trapping of wave energy on the main circuit portion which could otherwise lead to bandedge oscillation. The matching circuit portions comprise at least two periods of the periodic slow wave circuit having essentially the same general physical configuration as that of the main circuit portion. The matching section has a cutoff frequency of interest substantially outside of the passband of the main circuit portion and is dimensioned to have an electrical length at the bandedge frequency of interest of the main periodic section to provide essentially a quarter wave transformer section.

Description

United States Patent [72] Inventor Ward A. Harman Los Altos Hills, Calif.

[21] Appl. No. 751,258

[22] Filed Aug. 8, 1968 [45] Patented Apr. 27,1971

[73] Assignee Varian Associates Palo Alto, Calif.

' [54] IMPEDANCE MATCH FOR PERIODIC MICROWAVE CIRCUITS AND TUBES USING SAME 5 Claims, 7 Drawing Figs.

[52] U.S.Cl 3l5/3.5, 3 15/36, 333/35 [51] lnt.Cl H0lj 25/34 [50] Field oiSearch.. 315/3.5, 3.6, 39.3; 333/35 [56] References Cited UNITED STATES PATENTS 2,575,383 11/1951 Field 315/3.5X 2,687,777 8/1954 Warnecke 3l5/3.6X 2,987,644 6/1961 Anderson 315/3.5 3,221,205 11/1965 Sensiper 3l5/3.5 3,414,756 12/1968 Famey 315/3.5

' FOREIGN PATENTS 860,451 2/1961 England 333/35 Primary Examiner-Herman Karl Saalbach Assistant ExaminerSaxfield Chatmon, Jr. Attorney-Stanley B. Cole ABSTRACT: A traveling wave tube amplifier is disclosed. The amplifier includes an electron gun for forming and projecting a stream of electrons over an elongated beam path to a beam collector electrode. A coupled cavity slow wave circuit is arranged along the electron beam path for electromagnetic interaction with the beam to produce an amplified output signal at the downstream end of the circuit. The slow wave circuit is made up of a number of severed slow wave circuit portions. Each of the severed circuit portions includes a plurality of substantially identical coupled cavity resonators forming a main periodic slow wave circuit portion having a passb'and with at least one bandedge frequency of interest which is capable of producing bandedge oscillations by interaction with the electron stream. Matching circuit portions are provided at opposite ends of the slow wave circuit portions for impedance matching the main periodic circuit section to the load and input transmission lines to prevent trapping of wave energy on the main circuit portion which could otherwise lead to bandedge oscillation. The matching circuit portions comprise at least two periods of the periodic slow wave circuit having essentially the same general physical configuration as that of the main circuit portion. The matching section has a cutoff frequency of interest substantially outside of the passband of the main circuit portion and is dimensioned to have an electrical length at the bandedge frequency of interest of the main periodic section to provide essentially a quarter wave transformer section.

Patented April 27, 1971 2 Sheets-Sheet 2 PASSBAND 0F MAIN CIRCUIT CKT. TYPE FIG. 5

T... C R m E H T F llll I| I l I 1 II 0 4. D w R E P. 2 R I l l| E 2 w w -z I M m .l R m 0 M H 6 l H .H w m 2 I. E Y. S C A v N mm -BW o EL R F Aw J 7 G Z2E8 2265:;

N mM m N y .IA 0 R A .W M #0 W G F m r, M Z d n p 4 H b Q all g L.

ATTORNEY LntlEF DESCRIPTION OF THE PRIOR ART Heretofore, periodic slow wave circuits for microwave tubes have been impedance matched by means of terminating sections of the periodic circuit provided atboth ends of a section of the main periodic circuit. Such an impedance matched slow wave tube is described and claimed in copending US. application 516,939 filed Dec. 28, 1965, now US. Pat. No. 3,414,756 issued Dec. 3, 1968. In this prior art matching technique, a few of the periodic elements of the circuit, at both ends of the main periodic slow wave circuit, were dimensioned to have their cutoff frequencies of interest falling substantially outside of the passband of the main circuit portion.

In addition, these cutoff frequencies were tapered starting ata.

frequency just outside of the bandedge of interest and successively shifting in successive periodic sections to a terminal cutoff frequency still further outside of the passband of the-main circuit portion. While this technique is useful for preventing.

trapping of wave energyon the main periodic circuit section and, thus, for preventing bandedge oscillations, it introduces a substantial complexity. into the design of the circuit in that each'of the matching periodic elements must be dimensioned differently than its neighbor to provide the tapered cutoff frequency characteristic of the matching section. In addition, several of such matching periodic elementsmust be provided at both ends of the main periodic circuit, thus, substantially increasing the overall length of the matched slowwave circuit;

In another priorart tube, the periodicslow wave circuit is matched to the output transmission lineby means of a quarter electrical wavelength long section of periodic circuit which is dimensioned to have a characteristic impedance equal to the root means square of the characteristic impedance of the main periodic circuit and the characteristic impedance of the output transmission line. This quarter electrical length impedance transfomier is dimensioned to be a quarter wavelength long at the center of the passband of the main periodic circuit and no equivalent matching section was providedat the opposite end of the main circuit. Such a matching technique is describedin US. Pat. No. 3,346,766 issued Oct; 10, 1967. While such an impedance transformer provides a relatively simple structure for coupling energy from a slow wave circuit to atransmission line, there is no teaching nor suggestion in this reference that such an impedance transformer is suitable for inhibiting bandedge oscillation nor is there any teaching or suggestion that the transformer should be dimensioned to be a quarter wavelength long at a particular bandedge frequency of the main periodic circuit, which may be troubled by bandedge oscillations.

SUMMARY OF THE PRESENT INVENTION (2n+1)%fi10%)t where, n is any integer value excluding zero, and A, is the physical length of the matching circuit portion which will produce Z-rr'radians of phase shift of the wave energy traveling on the matching circuit at the bandedge frequency-of the main circuit portion, whereby the matching sections operate as quarter wave transformers for impedancexmatching the ends of the main periodic. circuit to inhibit the bandedge oscillations.

Another feature of the present invention is the same as the preceding'feature wherein the matching circuit portion has a characteristic impedance of a value intermediate the characteristic impedance of the main circuit portion and of the characteristic impedance of the transmission line coupled to and terminating the main periodic circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the matching circuit portion has essentially only one discrete cutoff frequency at the bandedge of interest, whereby design and construction of the matching circuit portion is facilitated.

Another feature of .the present invention is the same as any one or more of the preceding features wherein the slow wave circuit comprises an array of cavity resonators centrally apertured for passage of the electron beamtherethrough and said resonators being inductively coupled by means of inductive coupling slots communicating between adjacent cavity resonators and wherein the electron beam is projected through the coupled cavities with a velocity to predominately interact with a first space harmonic wave on the slow wave circuit within the passband of the slow wave circuit.

Another feature of the present invention is the same as the preceding feature wherein the matching circuit portion includes only two coupled cavity resonators, whereby the physical length of the matching circuit portion is minimized.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional'line diagram of a traveling wave tube amplifier incorporating features of the present invention,

FIG. 2 is an enlarged schematic view of a portion of the structure of FIG. 1 delineated by line 2-2,

FIG. 3 is an enlarged sectional view of a portion of the structure of FIG. 2 taken along lines 3-3 in the direction of the arrows,

FIG. 4 is a sectional view of the portion of the structure of FIG. 3 taken along line 4-4 in the direction of the arrows,

FIG. 5 is a plotof frequency in gigahertz versus phase shift in radians per period of the periodic circuit and depicting the dispersion characteristics of the main periodic circuit and of the matching circuit portions,

FIG. 6 is a schematic line diagram depicting the impedance transforming function of the matching circuit portion of the present invention, and

FIG. 7 is a plot of reflection coefficient versus frequency in gigahertz depicting the improved match to themain periodic circuit by use of the terminal-matching circuit portions of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a microwave traveling wave tube amplifier 1 incorporating features of the present invention. Theamplifier tube 1 includes an elongated vacuum envelope structure 2 having an electron gun assembly 3 mounted at one end for forming and projecting a beam of electrons 4. over an elongated beam path to a collector structure 5 disposed at the. opposite end of the envelope 2. A periodic slow wave circuit 6 is contained within the envelope '2 intermediate the electron gun Sand the beam collector 5 for cumulative electromagnetic interaction with the electron stream 4 to produce an amplified output signal. The signal to be amplified is applied to the upstream endof the slow wave circuit 6 via an input waveguide 7 having a wave permeable vacuum tight window 8 sealed thereacross. The amplified output microwave signal is extracted from the downstream end of: the slow wave circuit 6 vie an output waveguide9having av wave permeable vacuum tight window structure. 11 sealed thereacross. Theoutput signal is fed to a suitable utilization device or load, not shown.

A power supply 12 supplies suitable operating potentials to the electron gun assembly 3 relative to the potential of the envelope 2, slow wave circuit 6, and beam collector structure 5 which electrodes other than the gun 3 are typically operated at ground potential. A beam focus solenoid l3 surrounds the envelope 2 for producing an axially directed beam-focusing magnetic field for confining the beam to the beam path 4 on the axis of the envelope 2.

The slow wave circuit 6 is preferably formed by a plurality of coupled cavity sections successively arranged along the beam path. The slow wave circuit is preferably severed by circuit severs 14, thereby dividing the circuit 6 into three severed slow wave circuit portions 6', 6", and 6". Resistive terminations are provided for terminating each of the severed slow wave circuit sections 6', 6", and 6 adjacent the circuit severs 14.

In operation, signals to be amplified are applied to the tube 1 via input waveguide 7. The input signals interact with the electron stream 4 in the first severed circuit portion 6 to produce bunching of the beam 4. The bunched beam couples energy into the second severed circuit portion 6" for exciting a wave on the second severed circuit portion 6". The wave in the second section 6" cumulatively interacts with the electron stream to produce further bunching and gain. The bunched beam is then passed into the output severed circuit portion 6" for exciting an amplified circuit wave on the output section 6". The output wave is extracted from the downstream end of the severed circuit portion 6" via output waveguide 9 and coupled to a suitable utilization device, not shown.

In a typical example of the tube of FIG. I, the tube produces approximately 12.5 kilowatts CW within a lower frequency band from 1.75 to 1.85 gigahertz and 7.5 kilowatts CW from 2.09 to 2.12 gigahertz with a gain at rated power of 39.5 db. and 37.3 db. in the respective frequency bands.

In a typical example, the first severed portion of the circuit 6 includes 10 coupled cavities, the second severed circuit portion 6" includes 10 coupled cavities and the output section includes 10 coupled cavities.

Referring now to FIGS. 2-6 the structure and mode of operation of the output severed circuit portions 6" will be described in greater detail. The severed slow wave circuit section 6" includes a hollow cylindrical conductive barrel 21, as of copper, having a plurality of conductive discs 22 transverse- .ly mounted therewithin to define a plurality of cavity resonator sections 23 in the interior spaces between the discs 22. The discs 22 are centrally apertured in axial alignment with the beam path 4 to allow passage of the beam through the output slow wave section 6". Axially directed reentrant drift tube sections 24, as of copper, project into each of the cavity resonator sections 23 from the discs 22 to define interaction gaps 25 in the spaces between the mutually opposed reentrant drift tube sections 24. Each of the transverse discs 22 includes an inductive coupling iris 26 communicating therethrough between adjacent resonators 23. The coupling irises 26 are disposed on alternate sides of the beam in adjacent conductive discs 22. A centrally apertured collector pole piece 27 as of soft iron is disposed at the downstream end of the severed output slow wave circuit section 6" for collecting the beam focus magnetic field and permitting the beam to expand into the collector aperture 5.

This type of slow wave circuit 6 is described in greater detail in a book titled, Traveling-Wave Tubes" by .I. R. Pierce, Van Nostrand (1950), see page 61, FIG. 4.11. Briefly, the slow wave circuit 6' has a fundamental backward wave dispersion characteristic and is operated on the first forward wave space hamionic with aphase shift per period of the slow wave circuit falling within the range of 1r to 211- radians. (See FIG. 5).

The slow wave circuit section 6' provides a gain per cavity section 23 within the range of 2 to 3 db. The upstream cavity 23' of the output section 6" has a section of rectangular waveguide 28 coupled thereto via a capacitive coupling iris 29. A hollow cylindrical dielectric window member 31 is sealed at its ends to the broad walls of the waveguide 28 and a lossy fluid, as of water, is piped axially through the center of the hollow cylindrical window 31 for absorbing wave energy coupled from the upstream cavity 23 via the iris 29 and waveguide 28 into the lossy fluid. The lossy fluid within the waveguide section 28 forms the resistive termination 15 and is substantially nonreflective for wave energy within the passband of the slow wave circuit section 6".

It has been found that when the output section 6" is terminated in a nonreflective load at its output end, i.e., matched by a waveguide 9 to a resistive load, not shown, and terminated by a nonreflective load 15 at its severed end, assuming each of the cavities 23 is essentially identical throughout the length of the circuit section 6", that the bandwidth of the circuit is essentially as shown in FIG. 7'by the curve identified 0-100. In such a circuit, the dispersion characteristic for the circuit is shown in FIG. 5 by the line identified as circuit type I. In such a circuit, bandedge oscillations are encountered near the upper cutoff frequency f l. These bandedge oscillations are caused by the impedance mismatch at the terminal ends of the circuit between the circuit and the waveguide sections 9 and 28, respectively. It is desired to inhibit bandedge oscillations in order to permit use of higher gain periodic circuit sections for improved efliciency and gain.

Accordingly, matching circuit sections 35 are provided at both ends of the main periodic circuit section 36. The matching periodic circuit portion 35 is dimensioned to have a cutoff frequency f,II substantially outside of the passband of the main circuit portion 36. In a specific example, the main circuit portion 36 had an upper cutoff frequency f,l of approximately 2.25 gigahertz and the matching circuit portion was constructed of periodic elements having an upper cutoff frequency f ll of approximately 2.45 gigahertz. Bandedge oscillations were not encountered at the lower bandedge of the main circuit because the beam potential applied to the tube was not sufl'rcient to bring the velocity of the electrons into synchronism with the circuit wave near the 1r mode of operation. Therefore, the lower bandedge of the main periodic circuit 36 was not of particular concern. However, if oscillations had been encountered at this frequency the type II circuit elements would be dimensioned to have a lower cutoff frequency substantially below the lower cutoff frequency of the main circuit 36.

The matching circuit portions 35 are dimensioned to have an overall physical length defined by equation 1, above. In this manner, the matching circuit portion 35 serves as a quarter wave impedance transformer for transforming the relatively low characteristic impedance Z, of the main periodic circuit section 36 t0 the higher characteristic impedance Z, of the terminating waveguide sections 9 and 28 respectively. (See FIG. 6). Alternatively, under certain conditions of cavity parameters the relative magnitude of the characteristic impedances Z and Z may be reversed. In such a case, the matching transformer effect is equally advantageous.

A particularly advantageous arrangement, for the matching circuit portion 35, includes the use of only two coupled cavity resonators 23 in each of the matching circuit portions 35. Each of these matching circuit portions 35 provides approximately 45 of relative phase shift per periodic element or cavity 23 relative to the phase shift per cavity in the terminal cavity of the main circuit section 36. This is shown in FIG. 5 by the relative phase shift of approximately 45 per period of the circuit measured between

points

37 and 38 at the upper cutoff frequency f l of the main circuit section 36.

The upper cutoff frequency f ll of the matching circuit portion 35 is conveniently raised relative to the cutoff frequency of the main circuit portion 36, f l, (See FIG. 4) by decreasing either d or 1 or both while maintaining the same gap spacing g or by increasing the gap spacing g while leaving the other dimensions constant.

The principal advantage to use of the quarter wave transformer matching circuit portion 35 of the present invention, is that the matching circuit portion 35 can be reduced to a minimum physical length, as compared to the prior art frequency tapered design described in the aforecited U.S, Pat. No. 3,414,756. ln addition. the design of the matching circuit portion is facilitated since each of the periodic elements in the matching circuit section can be dimensioned to have nearly identical dimensions to provide the same upper cutoff frequency for the cavities or periodic elements of the matching section 35. Thus, machining or formation of the parts is facilitated. The improved bandpass characteristics of the main periodic section, when utilizing the matching circuit portion 35, is shown in FIG. 7 by the line identified 26-2. By utilizing the matching circuit portion 35 at either end of the main circuit portion 36 bandedge oscillations are substantially inhibited.

Although the matching circuit portions 35 have been described asdesigned for use in a coupled cavity slow wave circuit 6, this is not a requirement and such matching circuit portions may be utilized with slow wave circuits in general, for example, interdigital lines, ring-and-bar circuits, resonant bar circuits, resonant vane circuits, and many other conventional slow wave circuits. Furthermore, the main periodic circuit need not be a fundamentally backward wave circuit but may also include fundamental forward wave circuits operating within the frequency range corresponding to 11/3 to 1r phase shifts per period of the microwave circuit.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. In a high frequency electron tube apparatus, means forming a periodic slow wave circuit having a main circuit portion and at least one matching circuit portion at one end of said main circuit portion, means for causing a stream of electrons to pass adjacent said main and matching portions of said slow wave circuit for cumulative electronic interaction with high frequency wave energy traveling on said slow wave circuit to produce a wave of increasing intensity with time traveling on said slow wave circuit, said main portion of said slow wave circuit having a passband with at least one bandedge frequency of interest which is capable of producing bandedge oscillations by interaction with the electron stream, means for extracting output high-frequency wave energy from the tube apparatus at a frequency within the passband of said main circuit portion, the improvement wherein, said matching circuit portion comprises at least two periods of said periodic slow wave circuit, said matching circuit portion having the same general physical configuration as that of said main circuit portion, said matching circuit portion having at the bandedge of interest essentially only one cutoff frequency substantially outside of the passband of said main circuit portion, said matching circuit portion having an overall physical length which is defined by the relationship (2n-l-1)%i10% (1) where, n is any integer value excluding zero, and Ag is the physical length of the matching circuit portion which will produce 2n radians of phase shift of the wave energy traveling on said matching circuit at the bandedge frequency of said main circuit portion, whereby bandedge oscillations are inhibited.

2. The apparatus of claim 1 wherein said matching circuit portion has a characteristic impedance of a value intermediate the characteristic impedance of said main circuit portion and the characteristic impedance of said means for extracting the high frequency energy from the tube.

3. The apparatus of claim 1 wherein said slow wave circuit comprises an array of cavity resonators electromagnetically coupled together to form a slow wave circuit, said cavity resonators being centrally apertured for passage of said stream of electrons therethrotgh.

4. The apparatus 0 claim 3 including inductive coupling means forming the predominant electromagnetic coupling between said cavity resonators to form a fundamental backward wave slow wave circuit, said means for causing a stream of electrons to pass adjacent said slow wave circuit including means for projecting the electrons through said apertured coupled cavity array with sufficient velocity to predominately interact with the first space harmonic wave on said slow wave circuit within the passband of said slow wave circuit, whereby forward wave interaction is obtained between wave energy on said slow wave circuit and the electron stream.

5. The apparatus of claim 4 wherein said matching portion of said slow wave circuit includes only two coupled cavity resonators, said resonators having a phase shift per cavity at the upper bandedge frequency of the passband of said main circuit portion falling within the range of 3/211 to 211' radians, and said matching cavities each having a phase shift relative to the phase shift at the same frequency in each of said coupled cavities of said main circuit portion of approximately 45, whereby the composite relative phase differential between the phase of wave energy in the terminal cavity of the main circuit section and the terminal cavity at the matching circuit portion is approximately to form a quarter wave impedance transformer between said main circuit portion and said wave energy extracting means.

Claims

1. In a high frequency electron tube apparatus, means forming a periodic slow wave circuit having a main circuit portion and at least one matching circuit portion at one end of said main circuit portion, means for causing a stream of electrons to pass adjacent said main and matching portions of said slow wave circuit for cumulative electronic interaction with high frequency wave energy traveling on said slow wave circuit to produce a wave of increasing intensity with time traveling on said slow wave circuit, said main portion of said slow wave circuit having a passband with at least one bandedge frequency of interest which is capable of producing bandedge oscillations by interaction with the electron stream, means for extracting output high-frequency wave energy from the tube apparatus at a frequency within the passband of said main circuit portion, the improvement wherein, said matching circuit portion comprises at least two periods of said periodic slow wave circuit, said matching circuit portion having the same general physical configuration as that of said main circuit portion, said matching circuit portion having at the bandedge of interest essentially only one cutoff frequency substantially outside of the passband of said main circuit portion, said matching circuit portion having an overall physical length which is defined by the relationship where, n is any integer value excluding zero, and lambda g is the physical length of the matching circuit portion which will produce 2 pi radians of phase shift of the wave energy traveling on said matching circuit at the bandedge frequency of said main circuit portion, whereby bandedge oscillations are inhibited.

3. The apparatus of claim 1 wherein said slow wave circuit comprises an array of cavity resonators electromagnetically coupled together to form a slow wave circuit, said cavity resonators being centrally apertured for passage of said stream of electrons therethrough.

4. The apparatus of claim 3 including inductive coupling means forming the predominant electromagnetic coupling between said cavity resonators to form a fundamental backward wave slow wave circuit, said means for causing a stream of electrons to pass adjacent said slow wave circuit including means for projecting the electrons through said apertured coupled cavity array with sufficient velocity to predominately interact with the first space harmonic wave on said slow wave circuit within the passband of said slow wave circuit, whereby forward wave interaction is obtained between wave energy on said slow wave circuit and the electron stReam.

5. The apparatus of claim 4 wherein said matching portion of said slow wave circuit includes only two coupled cavity resonators, said resonators having a phase shift per cavity at the upper bandedge frequency of the passband of said main circuit portion falling within the range of 3/2 pi to 2 pi radians, and said matching cavities each having a phase shift relative to the phase shift at the same frequency in each of said coupled cavities of said main circuit portion of approximately 45*, whereby the composite relative phase differential between the phase of wave energy in the terminal cavity of the main circuit section and the terminal cavity at the matching circuit portion is approximately 90* to form a quarter wave impedance transformer between said main circuit portion and said wave energy extracting means.