Amplifiers – With electron beam tube amplifying device
Patent
1994-05-17
1997-11-25
Lee, Benny T.
Amplifiers
With electron beam tube amplifying device
330 45, 315 533, 315 537, 315 539, 315 5, 333 81R, H01J 2354, H03F 354
Patent
active
056916671
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a radiation absorbing material and the use of this material to stop self oscillation in an amplifier, and particularly in an inductive output tube amplifier.
DESCRIPTION OF THE PRIOR ART
An inductive output tube amplifier, hereafter referred to as an IOT, is shown schematically in FIG. 1. This comprises a cathode 1 and an anode 2 separated by a grid 3, the anode 2 having a hole 2A passing through it.
In use, a high DC voltage, typically of the order of 30 to 40 KV is established between the cathode 1 and anode 2 and an RF input signal is applied between the cathode 1 and grid 3. The cathode 1, anode 2 and grid 3 are symmetrically arranged about an axis 6. Electrons are emitted by the cathode 1 and their movement towards the anode 2 is controlled by the relative voltages of the cathode 1 and the grid 3. Thus an electron beam coaxial with the axis 6 and modulated by the RF signal applied between the cathode 1 and the grid 3 is generated between the cathode 1 and the anode 2, most of the electrons in this modulated electron beam, often referred to as a bunched electron beam, pass through the hole 2A in the anode 2. This electron beam is then used to drive later stages in the amplifier.
The cathode 1 is linked to the grid 3 by a first resonant tuned circuit 4 while the anode 2 is linked to the grid 3 by a second resonant tuned circuit 5. As is conventional the tuned circuits 4 and 5 are each represented by an inductance and a capacitance in parallel in FIG. 1 although they will generally be far more complex than this in practice.
The first resonant tuned circuit includes variable components arranged so that by altering the value of the variable components its resonant frequency can be adjusted to the desired output frequency of the amplifier, the signal to be amplified is then coupled into this first resonant tuned circuit in order to apply it between the cathode 1 and the grid 3.
The second resonant tuned circuit 5 on the other hand is formed by the component parts of the amplifier and as a result resonates at a fixed frequency. RF baffles are provided to prevent interference between the first and second resonant tuned circuits 4 and 5 but it has been found that it is still possible under some conditions for RF radiation emitted by the second resonant tuned circuit 5 formed between the anode 2 and grid 3 to pass through the baffles and be picked up by the first resonant tuned circuit 4. This results in an RF voltage at the resonating frequency of the second resonant tuned circuit appearing between the cathode 1 and grid 3 causing the electron beam to be modulated at this frequency. This is self oscillation and the unwanted signal at the resonant frequency of the second tuned circuit 5 interferes with the signal it is desired to amplify and can generate high enough voltages to damage or disable the amplifier.
Self oscillation is generally a greater problem at higher operating frequencies and is often the limiting factor setting the maximum operating frequency of an IOT amplifier.
SUMMARY OF THE INVENTION
One way of reducing this problem would be to use an RF radiation absorbing material between the two resonant tuned circuits, but it was found that to achieve the best results such a material had to absorb RF radiation and be able to hold off the DC voltage between the anode and cathode.
This invention was intended to produce such a material and a method of using it to reduce self oscillation in an amplifier.
This invention provides an RF radiation absorbing material used to hold off a DC voltage difference and comprising a silicone rubber loaded with ferrite particles.
This material has been used in the past as a RF radiation absorber but it has now been realised that it can also be used to hold off a very high DC voltage, on the order of 30-40 KV.
In a second aspect this invention provides an amplifier including a cathode and an anode separated by a grid and enclosed within a ceramic envelope, where the ceramic envelope is surrounded by a layer of a sili
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"Submillimeter and Millimeter Wave Characterization of Absorbing Materials", Hamid Hemmati, John C. Mather and William L. Eichhorn; Applied Optics, 15 Dec. 1985, vol. 24, No. 24, pp. 4489-4492.
Patent Abstracts of Japan, vol. 10, No. 201 (E-419), Mar. 5, 1986 (JP A 61-045537).
Patent Abstracts of Japan, vol. 10, No. 192 (E-417), Feb. 25, 1986 (JP A 61-039435).
"The Characteristics of Electromagnetic Wave Absorber composed of Rubber, Carbon and Ferrite", by Mirtaheri et al, Transactions of the Institute of Electronics, Information and Communicaiton Engineers of Japan, vol. 72, No. 12, Dec. 1989, Tokyo, Japan, pp. 1447-1451.
Clayworth Geoffrey Thomas
Heppinstall Roy
Pickering Alan Hugh
Rowlands Geoffrey John
Sobieradzki Edward Stanley
Casey, Esq. Donald C.
English Electric Valve Co., Ltd.
Lee Benny T.
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