Linear Beam Tube Modulation System Using Modulation Of First Grid

Abstract

A system for operating a linear beam or an O-type microwave tube comprising a linear beam microwave tube having an electron gun with grid electrode for emitting electron beams, a microwave amplifying portion having input and output cavities so as to amplify television video signal carrier waves by the action of said electron beams, a collector for collecting said electron beams; means for supplying said input cavity with carrier waves modulated by television video signals; and means for subjecting said electron beams to density modulation by television video signals in the grid electrodes of the electron gun.

Description

The present invention relates to a system for operating a microwave tube for amplifying microwaves using linear electron beams and more particularly to a system adapted to modulate and amplify high frequency carrier waves by television signals or other similar telecommunication signals.

With a general television broadcasting apparatus, carrier waves are modulated in amplitude by video signals so as to be amplified up to a required energy level and broadcast to the air through an antenna together with speech carrier waves modulated in frequency by audio signals. Said carrier waves occupy VHF and UHF regions and the final electric power tube consists of a multielectrode vacuum tube or klystron. Where there is used a multielectrode tube at the final stage, there may be applied a system whereby the anode current will assume a maximum value with respect to the peak value of synchronizing signals by adoption of the AB class operation, so that the average anode current will have a value corresponding to the average value through the light and dark levels of a television image. Therefore, the anode current is used in practical operation with a value equal to 30 to 70 percent of its peak level. However, since the operation of the multielectrode tube is limited with respect to the UHF region, there is often adopted a klystron or linear beam tube for said UHF region.

Where a prior art operation system using a linear beam klystron, or other linear beam microwave tube is adopted the electron beam density corresponding to the anode current of a multielectrode tube was normally so designed as to be able to produce output corresponding to the peak value of synchronizing signals independently of the level of video signals. Accordingly, if video signals had a low level, electron beams would have an unnecessarily high density, so that the conventional operation system was found unsatisfactory from the standpoint of current efficiency, microwave tube life and collector electrode cooling device. Further disadvantage of the prior art was that input and output current presented a low linearity, preventing a microwave tube from operating over a broad frequency band with the resulting failure to obtain a large output.

The object of the present invention is to provide a system for operating a linear beam microwave tube which is capable of elevating the current efficiency of a microwave tube by modulating electron beams in density at the particular amplitude level of modulating signals thereby to render the construction of a collector compact, simplify a collector cooling device and power source equipment, prolong the life of a microwave tube, improve the linearity of high frequency output by said density modulation and in consequence produce a large output at a low strain using a compact microwave tube.

According to an aspect of the present invention, there is provided a system for operating a linear beam microwave tube comprising a linear beam tube having an electron gun with grid electrodes for emitting electron beams; a microwave amplifying portion provided with input and output sections so as to amplify microwaves; a collector for collecting electron beams; means for supplying the input section with microwave signals modulated, for example, by video signals; and means for modulating electron beams in density at the particular amplitude level of modulating signals or at a higher level in the grid electrodes disposed in the electron gun.

The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which:

FIGS. 1 and 2 are block diagrams illustrating the embodiments of the present invention;

FIG. 3 schematically indicates the properties of the grid electrode of klystron electron guns of FIGS. 1 and 2;

FIGS. 4A, 4B and 4C are diagrams of wave forms showing the operation of the embodiment of FIG. 2;

FIG. 5 is a diagram of the relationship of the voltage of high frequency input and that of the grid electrode;

FIG. 6 is a curve diagram of the characteristics of high frequency output from the present apparatus with a bunching parameter taken to represent the variables involved.

FIG. 7 is a diagram of the wave form of television video signals;

FIG. 8 is a diagram of the characteristics of high frequency output from the prior art apparatus with a bunching parameter taken to represent the variables involved; and

FIG. 9 is a block diagram of a modification of the present invention applied to a rectilinear wave tube.

A linear beam klystron comprises an electron gun having anode and cathode, a microwave amplifying portion including cavities and a drift tube, and a collector for collecting electron beams. Such a klystron is operated on the basis of the aforesaid diode construction. In some of the conventional klystrons, the microwave amplifying portion includes input, output and intermediate cavities. For convenience of description, however, there will now be taken as an example a two-cavity klystron, that is, a type having only input and output cavities. If the effect of space charge is ignored, output microwave current I.sub.1 from the output cavity may be expressed as

I.sub.1 = 2I.sub.0 J.sub.1 (x) (1)

where:

J.sub.1 (x) = Bessel Function

I.sub.0 = direct beam current

x = bunching parameter

x = .beta..theta..sup.V 1/2V.sub.0 .alpha..sup. V 1/V.sub.0 (2)

where:

.beta. = constant of an input cavity gap

.theta..sub.0 = transit angle of electron beams in drift tube

V.sub.1 = high frequency voltage in the input cavity gap

V.sub.0 = beam voltage (D.C. anode voltage)

With the conventional operation system, the direct beam current I.sub.0 is primarily determined by the diode operation of the electron gun. As is apparent from the above equation (1), therefore, there is drawn out output current I.sub.1 modulated with a bunching parameter x alone taken to represent variables. To change the value of x expressed by the equation (2), it may, of course, be contemplated to vary V.sub.0 or V.sub.1. However, variation of the beam voltage V.sub.0 will lead to that of the beam current I.sub.0 as seen from the formula

I.sub.0 .alpha.V.sub.0.sup.3/2 (3)

further, any increase in V.sub.0 will result in a corresponding decline in the transit angle of electron beams. Thus input and output will present a complicated relationship, failing to assume linearity. Where, as in television operation, there is demanded a high degree of linearity in which differential gain and phase raise a significant problem, a circuit for compensating linearity will necessarily become extremely complicated. With a large capacity beam source, it will generally be difficult to carry out such compensation. At present, therefore, output control by change of V.sub.0 can only be applied in pulse operation which is unrelated to the linearity of input and output. Thus in general practice, there is utilized variation of high frequency voltage V.sub.1 in the input cavity gap, and not that of beam voltage V.sub.0. That is, increase in V.sub.1 will cause x to rise substantially in proportion. In this case, the primary Bessel function of the first kind J.sub.1 (x) varies in the form of a curve having a certain value, so that where there is taken into account the linearity of input and output, the region in which x varies will be unavoidably limited.

FIG. 8 presents the relationship of the bunching parameter x and the output current I.sub.1. Referring to FIG. 8, if the gradient between O and X.sub.0 is taken to be I.sub.1x /x.sub.0 when there are used particular values of x.sub.0 and I.sub.1x = 2I.sub.0.sup.. J.sub.1 (x), then the value of .DELTA.I.sub.1x = {2I.sub.0.sup.. J.sub.1 - x.sub.1.sup.. I.sub.1x /x } calculated at a particular point x.sub.1 may be used as a guide in determining the characteristics of the resulting linearity. There are now available various techniques for compensating the linearity between input and output. The NTSC television system of the present day can only effect about 20 percent compensation in practical application. The composition of television signals is for example, as shown in FIG. 7, namely, when the synchronizing signals thereof are taken to have a 100 percent amplitude level on the basis of voltage, the black level signals will have an amplitude level of 75 percent and the white level signals 12.5 percent. Assuming current I.sub.1x of FIG. 8 to have the same amplitude level as that of synchronizing signals, then a klystron will display an efficiency of only 30 percent at the position of synchronizing signals, 17 percent at the black level, and a far lower percentage on average. As used herein, the efficiency of the klystron is defined to mean percentage output with input voltage V.sub.0 I.sub.0 taken as 100 percent. It will be apparent, therefore, from the foregoing description that a substantial portion of input current is wasted in the collector in the form of released heat.

Accordingly, the klystron operating system now in common use which consists in varying x by changing only V.sub.0 or V.sub.1 offers a low current efficiency as described above and is unfavorable from the standpoint of a cooling device for the collector electrode and in consequence the life of the klystron itself. It is desired, therefore, that the primary Bessel function of the first kind J.sub.1 (x) given in the equation (1) be only affected by the high frequency voltage V.sub.1 in the input cavity gap and beam current I.sub.0 be modulated by video signals.

Since fluctuations in the beam current I.sub.0 directly affect high frequency output I.sub.1, it is required to stabilize beam voltage V.sub.0. Considering, however, that the recently developed klystron has an anode source capacity as large as several hundreds of KVA, its stabilization would require a large-scale apparatus. Since said stabilization is subject to a certain limitation, it is also desirable to improve an electron gun in such a manner that fluctuations in beam voltage, or anode voltage V.sub.0 exert substantially no effect or beam current I.sub.0.

There will now be described by reference to the appended drawings a system for operating a linear beam microwave tube according to an embodiment of the present invention. FIG. 1 presents a two-cavity klystron to which there is applied the present invention so as to modulate at the same time both beam current and high frequency voltage in the input cavity gap by video signals whose amplitude ranges from 0 to 100 percent. The body 11 of a klystron tube has an electron gun 12 disposed at one end, a microwave amplifying portion 13 at the center and a collector 14 at the other end. The electron gun 12 consists of a cathode 15, and an anode 17 bored with a hole 16 at the center, a first grid electrode 18 a second grid electrode 19 and focusing electrode 20, all these being so arranged as to face said cathode. The electron emitting surface of the cathode 15 assumes a partly depressed form. The first and second grid electrodes 18 and 19 are disposed close to the electron emitting surface of the cathode 15 and have a spherical surface of the same curvature. The meshes of either of said electrodes 18 and 19 are aligned in both size and position with those of the other. Gun elements including from the cathode 15 to the focusing electrode 20 are coaxially arranged with the axis of the klystron tube body 11.

The microwave amplifying portion 13 comprises an input cavity 21, drift tube 22 and output cavity 23 arranged in turn as viewed from the position of the electron gun and has the same D.C. potential as the anode 17.

The collector 14 is so disposed as to face the electron gun 12 with the amplifying portion 13 interposed therebetween and provided with a deep hole 24 to receive scattered electron beams. To the outer wall of the collector is attached a cooling device 25.

Under such arrangement, the electrodes are connected to D.C. sources 26, 27 and 28. Based on the potential of the cathode 15, the first grid electrode 18 is supplied with a negative potential, the second grid electrode 19 with a positive potential, the focusing electrode with a zero potential, and the anode 17 with a positive high voltage V.sub.0. The first grid electrode 18 of the klystron tube body is connected to the output terminal of a video signal amplifier 29, and is so controlled as to present a maximum displacement from the negative potential to the zero potential when the synchronizing signals of television signals have a peak value. A video signal generator 30 including a television camera and wave shaping circuit is connected to the video signal amplifier 29 and a video carrier wave amplifier and modulator 31 whose output terminal is connected to the input cavity 21 through a T-shaped branched circulator 32. Said video carrier wave amplifier and modulator 31 is connected to a video carrier wave oscillator 33. To one end of the circulator 32 is connected a swamping load 34. The output cavity 23 of the klystron tube body 11 is connected to the input terminal of a coupler 35, whose output terminal is connected to a transmission terminal to an antenna and also to the video signal amplifier 29 through an amplitude detector 36 and a feedback amplifier 37.

In the embodiment of the present invention arranged as described above, there are emitted electron beams from the cathode 15, the density of said beams being modulated in the first grid electrode 18. The video signal amplifier 29 is supplied with video signals amplified to a prescribed level in addition to the negative bias voltage already supplied to said amplifier 29. In this case, the first grid electrode 18 is so designed as to be biased at maximum from negative to zero potential when the synchronizing signals included in television signals assume a peak valve, so that electron beams are positively modulated in density by video signals in the first grid electrode 18.

With perveance in the first grid electrode 18 designated as G, its biased voltage as -Eg.sub.0 and its voltage biased in the direction of zero due to addition of video signals as -Eg.sub.1, then there results the following equation:

I.sub.0 = G (.vertline.Eg.sub.1 - Eg.sub.0 .vertline.).sup.3/2 (5)

Thus there is obtained the curve of FIG. 3, which shows that the beam current I.sub.0 increases when the voltage level of video signals rises and vice versa.

Electron beams modulated in density pass through the second grid electrode 19. Since the second electrode 19 is already supplied with a certain degree of positive potential, the anode voltage V.sub.0 can vary substantially without any effect on the beam current I.sub.0 due to the stabilization of a bias source 27 associated therewith. After being accelerated by the anode 17, the density-modulated electron beams pass through the gap of the input cavity 21, which is supplied through the circulator 32 with video carrier waves generated by the carrier wave generator 33, amplified by the video carrier wave amplitude modulator 31 and further amplitude modulated by video signals, that is, high frequency voltage V.sub.1 in the input cavity gap. While, therefore, passing through the aforementioned units, the accelerated electron beams are modulated in velocity. Namely, electron beams are subjected to density modulation in the first grid electrode within the full range of amplitude of video signals and to velocity modulation during passage through the input cavity gap by carrier waves which are already amplitude modulated by video signals. Thus electron beams are processed twice by video signals within the full range of amplitude thereof, that is, when said beams pass through the first grid electrode 18 and when they pass through the input cavity gap. Accordingly electron beams which are prominently accelerated in velocity with respect to the frequency band occupied by video signals are modulated to the same phase by video signals while travelling through the grid electrodes and input cavity.

After passing through the input cavity 21, drift tube 22 and output cavity 23, electron beams are collected by the collector 14. However, the velocity modulated portions of the electron beams are further density modulated during transit through the drift tube 22 to supply the output cavity 23 with output current I.sub.1, which is conducted to an antenna through the coupler 35.

As apparent from the aforementioned equations (1) and (2), output microwave current I.sub.1 bears a primary proportionate relationship with beam current I.sub.0 and, with respect to high frequency voltage V.sub.1 in the input cavity gap or briefly referred to as input cavity voltage, an interrelationship which may be indicated by the primary Bessel function of the first kind J.sub.1 (x). Accordingly, output microwave current I.sub.1 will have a value

I.sub.1 = 2G.sup.. J.sub.1 (x) (.vertline.Eg.sub.1 - Eg.sub.0 .vertline.).sup.3/2 (6)

derived from the equations (1) and (5).

Part of said output microwave current I.sub.1 is supplied from the coupler 35 to the amplitude detector 36 to have its amplitude detected thereby, and then fed back to the video signal amplifier 29 through the feedback amplifier 37 so as to control video signals to a required linearity.

The aforementioned embodiment of the present invention can control the density of electron beam in the klystron tube body 11 according to all amplitude levels of video signals involved, reducing the requirements of electron beams for the white level. The system of the present invention can be operated with electron beams averaging one half to two-thirds those required for the prior art. This permits 1.5 - or 2 - fold elevation of current efficiency of the klystron tube 11, decreasing power consumption, prolongation of the tube life and simplification of a collector electrode cooling device.

Further, the aforesaid embodiment of the present invention allows the term J.sub.1 (x) of the Bessel function of the first kind given in the equation (1) to be affected only by high frequency voltage V.sub.1 in the input cavity gap and beam current I.sub.0 to be modulated by video signals, so that output current I.sub.1 becomes proportionate to the beam current I.sub.0, thus enabling the linearity of the input and output characteristics of the klystron tube elevated simply by increasing the beam current I.sub.0. Accordingly, the present invention can obtain a large power gain with low strains, prominently simplifying the wave shaping process of original signals obtained during television image pickup, for example, .gamma. compensation and compensation of differential gains and phases.

Further, the stabilization of the bias source 27 for supplying the second grid electrode 19 with a positive potential prevents variation of the anode voltage V.sub.0 from affecting that of the beam current I.sub.0. Therefore, the beam current I.sub.0 is only modulated by video signals in the first grid electrode 18, preventing input and output from assuming a complicated relationship.

In the embodiment of FIG. 1, the beam current I.sub.0 is modulated over the entire amplitude range of video signals as described above, causing the microwave tube to be rather complicated in design.

Therefore, the embodiment of FIG. 2 permits the beam current I.sub.0 to be modulated only at a higher amplitude level than the particular amplitude level of video signals in order to realize by simple means the technical concept displayed in the embodiment of FIG. 1.

There will now be descried the embodiment of FIG. 2 which uses a linear beam four-cavity klystron. The same parts of FIG. 2 as those of FIG. 1 are denoted by the same numerals and description thereof is omitted. The linear beam four-cavity klystron has intermediate cavities 40 and 41 provided between the input cavity 21 and output cavity 23, all these cavities being joined together by drift tubes 42, 43 and 44 which are so arranged as to have the same potential as the anode. Further, according to the embodiment of FIG. 2, beam current I.sub.0 is modulated at a higher amplitude level than the particular amplitude level of video signals, so that there is additinally provided a video signal distributing circuit 50.

The video signal distributing circuit 50 supplies the video signal amplifier 29 with those video signals which are drawn out at said particular amplitude level and also conducts said video signals themselves to the video carrier wave amplitude modulator 31 as modulating signals. The video signal distributing circuit 50 which includes the video signal generator 30, level distributor 51 and gate 52, causes part of output signals (indicated by S in FIG. 4) to be sheared at a prescribed distribution level, for example, the black level and the video signal amplifier 31 to be supplied with level-exceeding signals S.sub.1 obtained by said shearing (shown in FIG. 4B), namely, synchronizing signals. On the other hand, part of video signals S is distributed to the carrier wave amplitude modulator 31 for modulation of carrier waves, so that the amplitude modulator 31 supplies the input cavity 21 with modulated carrier waves S.sub.2 shown in FIG. 4C. The video signal amplifier 29 is supplied with signals S.sub.3 at the shearing level. During this time, the first grid electrode 18 is supplied with a certain D.C. bias voltage corresponding to said shearing level. Upon arrival of synchronizing signals S.sub.1 at the video signals amplifier 29, there are obtained signals corresponding to the amplitude of said synchronizing signals S.sub.1 to raise the bias voltage of the first grid electrode 18 to zero voltage, namely, to increase beam current. The gate 52 is positioned between the video signal amplifier 29 and feedback amplifier 37 and so designed as to open the circuit while the video signal distributing circuit 50 is in operation. In this case, feedback signals and video signals may also be aligned in phase by output synchronizing signals. The voltage Eg of the first grid electrode 18 and beam current I.sub.0 have an interrelationship shown in FIG. 3. Accordingly, the voltage Eg of the first grid electrode is kept at - Eg.sub.11 until the voltage V.sub.1 in the input cavity gap shown in FIG. 5 exceeds a certain value of V.sub.10, and when said input cavity voltage V.sub.1 increases over V.sub.10, the first grid electrode 18 is made to present variation corresponding to (V.sub.1 - V.sub.10). In this case, the absolute value Eg of the voltage of the first grid electrode 18, output microwave current I.sub.1 and beam current I.sub.0 assume a relationship given in a table below. ##SPC1##

where:

B = ratio constant, which may be arbitrarily selected in setting an operating condition

C,d }= constants determined according to B

c = b/(-eg.sub.11 + Eg.sub.0) .sup.. (2V.sub.0 /.beta..theta..sub.0)

d = bv.sub.10 /(- eg.sub.11 + Eg.sub.0) - 1

Fig. 6 presents the relationship of input and output when a value of B' = (G/I.sub. 01).sup.2/3 . B is selected to be 0.1797 and 0.1437 with V.sub.10 set at (2V.sub.0 /.beta..theta..sub.0). FIG. 6 indicates variations in the ratio I.sub.1 2I.sub.01 with respect to a bunching parameter x, providing that the system of the present invention gives a prominently improved linearity as compared with the J.sub.1 (x) characteristics of the prior art.

For briefness, there has been described a simplified theory of signals ignoring space charge. Though there has not yet been established any complete theory in connection with the operation of a klystron, said simplified theory presents no practical problem.

As mentioned above, the system of the present invention causes beam current I.sub.0 to vary with the particular value V.sub.10 of the input cavity voltage V.sub.1 used as a boundary. In the level distributor 51 of the video signal distributor 50 the value of V.sub.10 is chosen to represent the black level. Accordingly, the baem current I.sub.0 is kept constant until video signals rise beyond the distribution level of FIG. 4A, and is increased when said distribution level is exceeded. Further, if the value of V.sub.10 is so selected as to denote other levels than the black level of video signals, then there may be used any arbitrary distribution level.

As mentioned above, the strain ratio of the equation (4) should be limited to 20 percent maximum. If, therefore, the output current I.sub.1x of FIG. 8 is made to correspond to the black level, then the klystron efficiency will rise to 30 percent on average, a prominent improvement over that of the prior art device wherein I.sub.1x is modulated at the level of synchronizing signals. The embodiment of FIG. 2 ensures the increased klystron efficiency and the elevated linearity of the input and output characteristics of a microwave tube as in the embodiment of FIG. 1.

FIG. 9 illustrates a travelling wave tube embodying the concept of the present invention. In a travelling wave tube body 60, input signals are carried through a helix 61 and drawn out in a form amplified by the action of separate output beam current. The same parts of FIG. 9 as those of FIG. 1 are denoted by the same numerals and description thereof is omitted. In the electron gun 12 of the travelling wave tube body 60 according to the embodiment of FIG. 9 as in that of FIG. 1, electron beams are modulated in density by the first grid electrode 18 according to the amplitude level of video signals, and the beam current is protected by the second grid electrode 19 from the effect of variation in the D.C. anode voltage, thus offering the same advantage as in the embodiment of FIG. 1. Referring to the operation of the travelling wave tube, there is provided, needless to say, a video signal distributing circuit 50 shown in FIG. 2 to subject beam current to density modulation at a higher amplitude level than the particular amplitude levelof video signals, thus permitting the easy design of the travelling wave tube. Signals used in modulating electron beams are not necessarily limited to television video signals as described above, but may consist of similar microwave telecommunication signals.

As mentioned in the foregoing embodiments, the present invention provides a system for operating a linear beam tube wherein electron beams are modulated in the electron gun of a klystron or travelling wave tube over the full range of amplitude of video signals or at a higher amplitude level than the particular amplitude level thereof, thus displaying a noticeable effect of elevating the efficiency of a microwave tube and improving the linearity of the input and output characteristics thereof.

Claims

What is claimed is:

1. A system for operating a linear beam tube comprising:

a linear beam tube which includes in turn: an electron gun for emitting electron beams, the electron gun having a first grid electrode; a microwave amplifying portion provided with at least an input section receiving the electron beams and a microwave signal, and a microwave signal output section coupled to an output utilization device and amplifying microwaves; and a collector for collecting the electron beams passing out of the microwave amplifying portion;

first means for conducting microwaves modulated by modulating signals of a first amplitude level to the input section of the microwave amplifying portion of the linear beam tube, said first means including a video signal generator; a level distributor for distributing video signals into video signals having a higher amplitude level than said first amplitude level and into those signals having a second lower amplitude level; a video signal amplifier for amplifying the video signals having said higher amplitude level and supplying said amplified higher level video signals to the grid electrode of the electron gun; a video carrier signal oscillator for generating video carrier signals; a video carrier amplifier and modulator for modulating said video carrier signals by those of the video signals from the level distributor which have said second lower amplitude level; and a circulator for supplying said input section of the microwave tube with said video carrier signals modulated by said video signals having said second lower amplitude level; and

second means responsive to said modulating signals and connected to said first grid electrode of the electron gun for density modulating the electron beams by said modulating signals in said first grid electrode of the electron gun said second means modulating said electron beam by a higher amplitude level than said first amplitude level.

2. A system according to claim 1 wherein said first amplitude level of said modulating signals represents the black level of television video signals.

3. A system according to claim 1 wherein said electron gun comprises:

an electron emitting cathode maintained at a predetermined potential;

said first grid electrode which is a mesh-shaped grid electrode disposed adjacent to the cathode and operated with a negative potential relative to said predetermined potential and coupled to the output of said second means;

a second mesh-shaped grid electrode positioned close to said first grid electrode and maintained at a positive potential relative to said predetermined potential, the meshes of said second grid electrode being aligned in size and position with those of said first grid electrode;

a focusing electrode actuated with said predetermined potential so as to focus electron beams emitted from said cathode; and

an anode having a high positive potential so as to accelerate the electron beams emitted from said cathode.

4. A system according to claim 1 wherein said modulating signals are television video signals and said microwave signals are television video signal carrier waves.

5. A system according to claim 1 wherein said microwave tube is a klystron.

6. A system according to claim 1 wherein said microwave tube is a rectilinear microwave tube.