EP0352961A1

EP0352961A1 - Klystrode frequency multiplier

Info

Publication number: EP0352961A1
Application number: EP89307345A
Authority: EP
Inventors: Louis T. Zitelli
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1988-07-25
Filing date: 1989-07-20
Publication date: 1990-01-31
Anticipated expiration: 2009-07-20
Also published as: DE68918021T2; DE68918021D1; JPH0279330A; EP0352961B1

Abstract

An electron tube frequency multiplier has a control grid (18) producing a density-modulated beam (14) which is further bunched by a velocity-modulating cavity (30) resonant near the signal frequency. Energy at a multiplied frequency is extracted by a second cavity resonant at a harmonic of the signal frequency. The frequency limit of grid-controlled tubes is thereby increased several fold.

Description

The invention pertains to linear-beam electron tubes in which the beam is density-modulated by a control grid. Such tubes have been found useful for generating amplitude-modulated ultra-high-frequency radio waves such as television broadcast transmission, with efficiency superior to klystrons.
Radio transmitters have generally used grid-controlled electron tubes such as tetrodes. When frequencies increased to the ultra-high frequency range (UHF), the gridded tubes reached their performance limits of power or frequency due to the transit times of electrons across the gaps between electrodes becoming comparable to the period of the generated wave. The first development to overcome these limits was the UHF klystron in which transit time is taken advantage of rather than being unwanted. However, the klystron is very inefficient for amplifying an amplitude-modulated wave such as the standard TV signal where amplitude corresponds to brightness. The klystron has to have enough power in the beam to generate the signal peaks, such as black and the still stronger synchronization pulses. The average power needed for an average signal is several times smaller, but the unused beam power is wasted as heat in the spent-beam collector.
Various proposals have been made to improve the average klystron efficiency. One is to modulate the beam current to follow the envelope of generated power. This has not been successful due to the complicated circuitry needed to correct problems with linearity and amplitude-to-phase distortion and to the power needed to modulate the beam.
The first successful attempt to improve efficiency is the "Klystrode®" described in U.S. Patent No. 4,480,210 issued October 30, 1984 to Donald H. Priest and Merrald B. Shrader. This followed the much older "Inductive Output Tube" described by A.V. Haeff in Electronics, February 1939, in that the beam of electrons was amplitude modulated by a control grid, and the output circuit was a non-intercepting resonator as in a klystron. Haeff's tube was a low-power device following the close-spaced grid art of that day. Priest and Shrader made a high-power tube using klystron beam technology and a much larger carbon grid to which the rf signal was applied in class B or class C modulation. The video-frequency power envelope of the beam is thus just what is needed to generate the instantaneous signal amplitude. The power efficiency in TV transmission was increased greatly.
A proposed improvement on the Priest-Shrader "Klystrode" is described in U.S. Patent No. 4,611,149 issued September 9, 1986 to Richard B. Nelson. One disadvantage of the "Klystrode" is that the rf bunches of current leaving the modulating grid as approximately half sine-waves in class B modulation are spread out somewhat by the time they reach the output resonator by the repulsive spare-charge forces between electrons. In the Nelson tube, a second resonator is inserted between the grid and the output. It is resonant at a frequency above the signal band to provide an inductive impedance to the beam which rebunches the beam current by velocity modulation as in a multi-cavity klystron amplifier. The bunches are even narrower than those leaving the grid, so the efficiency lost by space-charge spreading is more than recovered.
According to the invention there is provided a linear-beam frequency-multiplier electron vacuum tube as set out in Claim 1.
An example of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic axial section of an amplifier tube, and
Figure 2 is a graph of the harmonc content of the beam current.

Figure 1 illustrates a tube which has a thermionic cathode 10 with preferably concave emitting surface 11, heated by a radiant wire coil 12. A convergent beam of electrons 14 is drawn from emitter 11 by a hollow anode 16. Directly in front of emitter 11 is an electron-pereable grid, preferably of pyrolytic graphite bars 18 bounding apertures 20.
Beam 14 is converged toward anode 16 by the convergent electrostatic field. It passes through anode 16 and an annular ferro-magnetic polepiece 22 which forms one terminus of a strong axial magnetic focusing field generated by a surrounding solenoid coil (not shown). Beam 14 then passes through a hollow metallic drift tube 24 and crosses an interaction gap 26 between input drift tube 24 and an exit drift tube 28. Drift tubes 24 and 28 form the center conductor of a coaxial cavity 30, resonant at preferably a frequency just above the frequency band of the tube's input signal.
After passing through cavity 30, beam 14 traverses a second cavity 32 having an axial drift tube 34 divided by an interaction gap 36. Cavity 32 is resonant at a harmonic of the band-center input frequency and is excited by the harmonic component of the modulated beam current.
After leaving harmonic output cavity 32, beam 14 passes through a second annular polepiece 37 which terminates most of the axial field. Beam 14 then expands under its own space-charge repulsion and is collected on the hollow, inner surface of a beam collector 38. The heat energy dissipated is removed by a coolant 40 (such as water) circulating from a coolant pipe 42.
In operation an input signal to be amplified and frequency-multiplied is fed in from a coaxial transmission line 46 through a coaxial dielectric vacuum window 48 to the space between the gird support 50 (usually at rf ground) and cathode support 52. This space may be partially blocked from input line 46 to form a resonant cavity to properly match impedances.
Drift tube 34 of harmonic cavity 32 is smaller in diameter than drift tube 24 of fundamental cavity 30, to provide good interactive coupling between beam and cavity at the higher frequency. The beam size is tapered down by a gradual increase in strength of the focussing magnetic field by increasing the wire turns per unit length of the solenoid. Shaping of polepieces 22, 37 to concave-convex shapes may also be used to generate the tapered field. In the strong "confined flow" focussing, the electrons follow the magnetic flux lines.
Useful harmonic energy is extracted from output cavity 32 via a coupling orifice 54 into an output waveguide 56 which is sealed off by a dielectric vacuum window 57.
The distinct advantage of the "Klystrode" frequency multiplier arises from the surprisingly high harmonic content of the beam current which is obtainable. The harmonic current increases with the shortness of the electron bunches. In a klystron frequency multiplier, it is impossible to get all the electrons into a short bunch by simple velocity modulation. This is illustrated in U.S. Patent No. 3,622,834 issued November 23, 1971 and U.S. Patent No. 3,811,065 issued May 14, 1974, both to E.L. Lien. In both patents, FIG. 4 shows calculated trajectories (in rf phase) of sample electrons where harmonic content of beam current is enhanced by bunching at a harmonic frequency. On the other hand, in the present invention with class B or class C grid modulation, there is no current at all in the antibunch. The Lien patents show that getting current out of the antibunch regions is a distinct limitation to klystron bunching. When we start out with vacant antibunches followed by velocity modulation compression, the bunches can be made remarkably tight, and thus the harmonic content is surprisingly high.
FIG. 2 is a graph of calculated harmonic components of beam current in the inventive frequency multiplier, plotted as functions of distance Z from the amplitude-modulating grid. Graph 60 is the fundamental component having a decreasing value 62 after leaving grid 18 due to space-charge debunching. In gap 26 of floating cavity 30 beam 14 receives velocity modulation which in following drift tube 28 increases the A.C. component 64. At the position of output gap 36, the A.C. component reaches a maximum value 66. In the calculated design an output circuit with a gap at the position of harmonic gap 36 but resonant at the fundamental frequency gives a conversion efficiency of 87%.
The second graph shows the second harmonic component 70 of beam current. The space-charge debunching 72 is more severe than for the fundamental current 60 due to the shorter wavelength. After velocity modulation in gap 26, the second harmonic current also increases faster due to the increased number of wavelengths traversed. The peak value 76 is reached at about the same distance as that of fundamental 60. At this point output gap 36 is located. The conversion efficiency for second harmonic power was calculated as 75%, a value completely out of reach in klystrons or simple grid-controlled tubes.
In a practical case, the second or third harmonic would be used. With the high fundamental current available at the lower driving frequency, the limits of power and frequency available from the multiplier are greatly extended.
The above-described tube is a single preferred embodiment of the invention. Other embodiments will occur to those skilled in the art. The tube could be used without the floating cavity, but with degraded performance. Additional cavities could be added for still higher efficiency. The scope of the invention is intended to be limited only by the following claims and their legal equivalents.

Claims

1. A linear-beam frequency-multiplier electron vacuum tube comprising:
an electron emissive cathode;
an electron-permeable control grid closely spaced from the emissive surface of said cathode;
means for supplying a high-frequency signal voltage between said cathode and said grid;
an anode spaced from said grid and facing said emissive surface, apertured for passage of an electron beam from said cathode;
a hollow conductive drift tube beyond said anode for transmitting said beam;
a first hollow cavity joining said drift tube at ends spaced from a first gap in said drift tube to form a reentrant cavity resonant at a frequency near said signal frequency;
a second hollow cavity joining said drift tube at ends spaced from a second gap in said drift tube removed from said first gap away from said cathode to form a reentrant cavity resonant at a frequency near a harmonic of said signal frequency;
means for extracting wave energy at said harmonic frequency from said second cavity; and
means for collecting said beam downstream of said second cavity.

2. The tube of claim 1 wherein said first cavity is resonant at a frequency higher than the band of said signal and lower than the band of said harmonic.

3. The tube of claim 1 wherein said second cavity is resonant at a frequency approximating the center of the band of said harmonic.

4. The tube of claim 1 wherein said grid is of pyrolytic graphite.

5. The tube of claim 1 wherein said means for supplying signal voltage comprises a resonant circuit connecting said cathode and said grid.

6. The tube of claim 1 wherein said drift tube is smaller at said second gap than at said first gap.

7. The tube of claim 1 wherein said second gap is shorter than said first gap.

8. The tube of claim 1 comprising means for generating a steady magnetic field in the direction of said beam between said anode and said second gap.

9. The tube of claim 8 wherein said magnetic field means comprises ferromagnetic polepieces surrounding said drift tube.

10. The tube of claim 8 wherein said magnetic field is stronger at said second gap than at said first gap whereby said beam is compressed between said gaps.