US2720610A - Noise reducing travelling-wave tube - Google Patents

Description

Oct. 11, 1955 B. KAZAN NOISE REDUCING TRAVELLING-WAVE TUBE Filed July 27. 1950 a 4 L 1 J m 5: 32 62 r r I rm TERMINATING FILTER IMPEDANCE ,/l3 l6 l||| FILTER R.F'. SIGNAL R.F. SIGNAL INPUT OUTPUT FIG. I

TERMINATING IMPEDANCE l4 [5 FIG. 2

INVENTOR. BENJAMIN KAZAN United States Patent Ofiice Patented Oct. 11, 1955 NOISE REDUCING TRAVELLING-WAVE TUBE Benjamin Kazan, Long Branch, N. J., assignor to the United States of America as represented by the Secretary of the Army Application July 27, 1950, Serial No. 176,241

Claims. (Cl. 3153.6)

(Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to a means for improving the signal-to-noise ratio in ultra-high frequency amplifiers wherein amplification is achieved by the interaction of an electron beam with a propagated electromagnetic field.

The primary object of the present invention is to provide an apparatus for improving the signal-to-noise ratio in travelling-wave tube amplifiers wherein an electron beam interacts with a circuit capable of propagating an electromagnetic wave in the direction of electron motion.

Another object is to provide an apparatus for reducing the thermal and shot noise effects in an electron beam as it travels through an appropriate section of a helix, or similar device.

Still another object is to provide an apparatus for reducing the noise in the electron beam before using it to achieve amplification in a travelling-wave tube.

Other features, objects, and advantages of the invention will be apparent from the following specification taken in connection with the accompanying drawing, in which: 7

Fig. l is an elevation, in part section, of a travelling- Wave tube embodying my invention; and

Fig. 2 is an elevation, in part section, of a klystron type tube embodying my invention.

In the conventional travelling-wave tube employing a helically wound conductor as a radio frequency signal propagating circuit, an electron beam is shot through the axis of the helix, or along the outer surfaces thereof, in a direction parallel to its axis. The axial phase velocity of the propagated signal, or travelling wave, is determined by the pitch and radius of the helix employed. An appropriate accelerating D. C. potential is provided so that the velocity of the electron beam before entering the helix is made substantially equal to the axial phase velocity of the input signal as it is propagated along said helix. Thus, interaction occurs between the electron beam and the electric component of the field established by the signal as it is propagated along said helix.

It is well known that such interaction may attenuate or amplify the propagated signal, depending on the relative velocity of the electron beam and the travelling wave. If the electron beam velocity is adjusted for attenuating the propagated, or input, signal, such attenuation may be considerably greater than the attenuation of the helix in the absence of an electron beam. When the velocity of the electron beam is adjusted so that amplification of an externally applied signal is achieved due to the interaction between the beam and the electric field established by the propagated signal, the input signal increases in amplitude, both on the helix and in the beam, as it travels through the helix. However, due to the thermal current and velocity fluctuations as the electrons are emitted from an electron gun or similar device, the electron beam, before interacting with the electromagnetic field of the applied signal, normally contains noise energy. This noise energy will induce noise currents in the helix which in turn will cause the noise energy of the beam to be amplified as well as the applied signal. Thus, because of the mutual coupling between the helix and the noise components in the electron beam, the noise energy is amplified together with the propagated signal. In other words, the interchange of energy between the electromagnetic field established along the axis of the helix and the electron beam will cause the existent noise in the beam to be amplified as well as the externally applied signal. Obviously, the amplified noise energy limits the minimum signal discernable in the output of a conventional travelling-wave tube. This, of course, limits the use which may otherwise be made of such travelling-wave tubes.

In accordance with my invention I propose a modification of the conventional travelling-wave tube in which the electron beam noise is substantially reduced before the beam passes through the signal propagating helix, thus increasing the signal-to-noise ratio of the travellingwave tube. In my invention, a second helix or similar device is inserted between the signal propagating helix, hereinafter called the amplifying helix, and the electron gun of the travelling-wave tube to reduce the noise energy in the electron beam before it interacts with the signal applied to said amplifying helix. In other words, the noise reducing helix is a non-Wave propagating circuit as distinguished from the amplifying helix which propagates the electromagnetic wave.

Referring now to Fig. 1 of the drawing, there is shown a travelling-wave tube designated generally at 1 comprising an electron gun section 2 which includes cathode 3 and accelerating beam forming anode 4, noise reducing conductive helix 5, amplifju'ng conductive helix 6 and a collector anode 7. Noise reducing helix 5 and amplifying helix 6 are positioned adjacent to each other intermediate electron gun section 2 and collector anode 7, helix 5 being anterior to amplifying helix 6. Both helices are axially aligned with electron beam 8 which is projected from electron gun 2. A D. C. potential is applied between helix 5 and cathode 3 by battery 9, or any other suitable source, the positive terminal of said battery being connected to lead-in conductor 10 of noise reducing helix 5. As explained above, this applied potential is of such a magnitude that the velocity of the electron beam, as it passes through or along noise reducing helix 5, is adjusted to attenuate any signals propagated on said helix. Filter section 11 is inserted between lead-in conductor 10 and battery 9 to prevent radio frequency leakage through the D. C. source. To prevent reflection, noise reducing helix 5 may be terminated by an impedance 15 having a value equal to the characteristic impedance of said helix.

As shown in Fig. 1, amplifying helix 6 is provided with input lead 12 and output lead 13. Portions of waveguide or coaxial cable, not shown, may be coupled to said input and output leads in the conventional manner to serve as the radio frequency signal input and output circuits. A D. C. potential is applied between input lead 12 and cathode 3 by battery 14, or any other suitable source. This potential is of such a magnitude that the velocity of the electron beam, as it passes through or along amplifying helix 6, is adjusted so that amplification of the externally applied radio frequency signal is achieved as it is propagated along said helix. Filter section 16 is inserted between input lead 12 and battery 14 to prevent radio frequency leakage through battery 14. A suitable source of positive potential may be applied to anode 7 in the usual manner.

In operation, the electron beam projected from electron gun 2 passes sequentially along or through the axis of each helix in the direction of theircornmon axis before impinging on anode 7. As a result, interaction occurs between the beam and said "helices for the entire length of each helix. 'As the electron beam passes through noise reducing helix 5, any noise signals which exist in the electron beam are induced on the helix so that, in

, asense, helix may be considered as a propagating ciron helix Sand the electron beam passing therethrough,"

will tend to attenuate the noise existing in the beam. In other words, the amount of'noise in the beam as it passes through noise reducing helix '5 will be progressively'lessened so that when the electron beam enters amplifying helix 6 it contains less noise than when it entered helix 5.

It is to be understood that instead of a helix, other devices such as filter type propagating circuits may be employed for abstracting the noise energy from an electron beam before said beam enters the amplifying section of a travelling-wave tube.

While the invention has been described by reference to'a travelling-wave tube, it is to be understoodthat my invention may be applied to conventional ultra-high frequency tubes employing current or velocity modulation, such as the klystron. Such tubes depend upon resonant cavities for coupling between the electron stream and the electromagnetic-circuit in order to obtain a high field strength and hence efficient coupling,

In'Fig. '2, there is shown a klystron tube 20 having a noise reducing helix 21 axially aligned with electron beam 7 22 which is projected from electron :gun 23 along a predetermined path at a predetermined velocity. Helix 21 is positioned between electron gun 23andresonant cavity 24. As, explained above,.noiseenergy'is abstracted from beam 22 by noise reducing helix 21' before the electrons enter resonant cavity 24. Other details of the device What is claimed is:

1. In an electron discharge device comprising a vac-,

uum tube having an electron source at one end thereof and an anode at the opposite end thereof, means for projecting an electron'beam along a path from said source to said anode, a first coil conductorg'a second coil conductor spaced from said .first coil and intermediate said first coil and said electron source, said coils being in coupling relation with said electron path, means for-propagating an electromagnetic wave only on 'said firstcoil, a first source of direct-current potential connected between said first coil and said electron source for adjustingthevelocity of the electron beam as it enters said first coil whereby said wave interacts'with said beam to amplify said electromagnetic wave, and a second source of direct-current potential connected between said second coil and said electron source to adjust the velocity of the electron beam through said second coil whereby noise energy is abstracted from said beam as it passes through said second coil. 7

2. An electron. discharge device in accordance with claim 1 wherein said first and second coils are helices.

3. An electron discharge device in accordance with claim 1 wherein said second coil surrounds said electron beam. a

4. An electron discharge device in accordance with claim '1 wherein said first and second coils are axially aligned with respect to said beam. V I

5. In an electron discharge device having an electron source at one end thereof, an anode at the other end 7 i and a source of direct-.current potential connected be-.

shown in Fig. 2' may be similar to those described in confnection with Fig. 1.

tween said coil and said electron source to adjust the velocity of the electron beam through the coil whereby said noise energy in said beam is progressively attenuated as said beam passes throughv said coil. v 7

References Cited in thefile of patentv UNITED STATES PATENTS 7 Pierce Apr. 28, 1953

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