EP0040998B1 - Extended interaction oscillator - Google Patents

Description

The present invention relates to an extended interaction microwave oscillator.

Extended interaction oscillators are well known in the prior art. In English, refer to them as "extended interaction oscillators" or E.I.O.

These oscillators are mainly used towards millimeter wavelengths as measurement oscillators or as heterodyne radar transmitters and receivers. They consist of a relatively short periodical line section, since it generally only comprises around ten identical stages. This line generally comprises a succession of metal bars and slots or a series of metal valves, identical or not (case of the structure of the "rising sun" type). This line section is contained in a vacuum-tight housing.

A linear electron beam crosses the line or licks it, however, a microwave wave is created which propagates inside the housing.

There is interaction between the wave and the beam, and the whole of the line and of the case starts to resonate. The oscillation generally occurs in the ₇ r mode.

• - les tolérances mécaniques concernant la ligne à structure périodique sont très strictes. En effet, on peut considérer que l'oscillateur à interaction étendue est constitué par une suite de cavités à la résonance. Il est très. important, notamment pour éviter les oscillations parasites, que ces cavités aient exactement la même structure géométrique; ce qui impose des tolérances mécaniques très strictes surtout pour la ligne;
• - les oscillateurs à interaction étendue sont accordables mécaniquement dans une bande de fréquence relativement faible;
• - les divers modes d'oscillation sont très pro_- ches les uns des autres et des sauts de modes aléatoires se produisent. La qualité du spectre de fréquence engendré est donc médiocre d'autant plus que la surtension est faible. A cause de cette faible surtension, les pertes sont importantes et le rendement peu élevé.

The extended interaction oscillators according to the prior art have the following drawbacks:

• - the mechanical tolerances for the line with periodic structure are very strict. Indeed, we can consider that the oscillator with extended interaction is constituted by a series of cavities at resonance. He is very. important, especially to avoid parasitic oscillations, that these cavities have exactly the same geometric structure; which imposes very strict mechanical tolerances especially for the line;
• - the extended interaction oscillators are mechanically tunable in a relatively low frequency band;
• - various oscillation modes are very pro _- ches each other and random mode jumps occur. The quality of the frequency spectrum generated is therefore poor, all the more so when the overvoltage is low. Because of this low overvoltage, the losses are significant and the yield low.

The present invention relates to an extended interaction oscillator which does not have these drawbacks.

The extended interaction oscillator according to the present invention comprises a line with periodic structure constituted by a succession of valves, these valves being traversed or licked by a linear electron beam. This line surmounts a rectilinear cavity the dimensions of the cross section of which are determined so that it behaves like a waveguide at the cutoff frequency, along the longitudinal axis of your line and in a magnetic transverse mode, TM _mm , with m = 1, 3, 5 ... and n = 1, 2, 3, 4 ... Coupling orifices between the valves and the cavity are provided on the cavity, between two successive valves and at regular intervals. The beam anode voltage, the distances between two successive valves and between two successive coupling orifices are fixed so that the transit time of the electron beam from one coupling orifice to the next is close depending on the operating mode chosen (π, 2π, 3π), the period, or a multiple of the period, of the electric field whose wavelength is the cut-off wavelength of the cavity (Ac), the frequency of oscillations of the oscillator being equal to the cut-off frequency of the cavity. Finally, a coupling device makes it possible to draw from the cavity the energy output from the oscillator.

• - le fait que les tolérances mécaniques sur les dimensions des vannes de la ligne ne soient plus critiques comme c'était le cas pour la ligne à retard de l'oscillateur selon l'art antérieur; par contre, les tolérances mécaniques sur les dimensions de la cavité percée d'orifices de couplages sont assez strictes mais cela pose moins de problèmes que pour les vannes;
• - le fait qu'une grande gamme d'accord mécanique puisse être obtenue, particulièrement dans les modes de réalisation de l'oscillateur où la cavité est un parallélépipède;
• - enfin, le fait qu'on obtienne une résonance unique à très haute surtension, et donc une grande pureté spectrale de l'oscillation; ainsi les sauts de mode aléatoires sont inexistants et le rendement excellent.

Among the main advantages of the oscillator according to the invention, there may be mentioned:

• - the fact that the mechanical tolerances on the dimensions of the valves of the line are no longer critical as was the case for the delay line of the oscillator according to the prior art; on the other hand, the mechanical tolerances on the dimensions of the cavity pierced with coupling orifices are fairly strict but this poses less problems than for the valves;
• - the fact that a wide range of mechanical tuning can be obtained, particularly in the embodiments of the oscillator where the cavity is a parallelepiped;
• - finally, the fact that a unique resonance is obtained at very high overvoltage, and therefore a high spectral purity of the oscillation; thus the random mode jumps are nonexistent and the performance excellent.

• - La figure 1, une vue en perspective d'un oscillateur à interaction étendue selon l'art antérieur;
• - La figure 2, une vue en perspective d'un mode de réalisation d'un oscillateur à interaction étendue selon l'invention;
• - La figure 3, une vue en coupe transversale d'un autre mode de réalisation d'un oscillateur à interaction étendue selon l'invention.

Other objects, characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• - Figure 1, a perspective view of an extended interaction oscillator according to the prior art;
• - Figure 2, a perspective view of an embodiment of an extended interaction oscillator according to the invention;
• - Figure 3, a cross-sectional view of another embodiment of an extended interaction oscillator according to the invention.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and pro portions of the various elements are not respected.

Figure 1 relates to a perspective view of an extended interaction oscillator according to the prior art.

This oscillator comprises a delay line 1 which consists of two identical metal plates which face each other. Each of these plates comprises the succession at regular intervals of two types of slots of unequal lengths: a small slot 2 and a large slot 3; the slots of the same name of the two plates are opposite. This is therefore a delay line 1 which comprises a succession of metal bars and slots.

This delay line 1 is contained in a vacuum-tight housing 4.

A linear electron beam is produced by an electron gun, not shown in the figure and which is located at one end of the housing 4. This electron beam propagates between the two plates which constitute the delay line 1 according to an axis 00 'which is the longitudinal axis of the housing 4. At the other end of the housing 4, this electron beam is collected on a collector which is not shown. Finally, a magnetic focus, not shown and constituted in a completely conventional manner by a solenoid or a permanent magnet, guides the electron beam along the axis 00 '.

FIG. 2 relates to a perspective view of an embodiment of an extended interaction oscillator according to the invention and FIG. 3 relates to a cross-sectional view of another embodiment of the oscillator according to the invention .

The extended interaction oscillator according to the invention comprises a line with periodic structure 1 which is constituted by the succession at regular intervals of valves 5.

Each valve is pierced with an orifice 6, as shown in FIG. 2, or has a slot 11, as in FIG. 3. Through these orifices or these slots, a linear electron beam propagates along the axis 00 'which passes through the middle of the slots or holes. This electron beam is emitted by an electron gun, focused along the axis 00 'by a magnetic focuser and finally, received by a collector; all these elements, barrel, focusing and collector, are well known in the prior art and are not shown in the figures.

The electron beam can also be a flat beam which licks the upper edge of the valves 5 which then have neither orifice nor slot.

Line 1 surmounts an almost completely closed cavity 7 which is rectilinear. The section of this cavity can take various forms; it can be circular for example. However, the cavity is most often formed by a straight parallelepiped whose section is a rectangle or a square. This is the case in FIG. 3 where the section of the cavity has the dimensions a along the horizontal and b along the vertical.

The dimensions of the cavity are determined so that it behaves like a waveguide at the cutoff frequency, along the longitudinal axis 00 'of the line and in a transverse magnetic mode, TM _mm , with m = 1, 3, 5 ... and n = 1, 2, 3, 4 ...

By limiting oneself to the TM _mm modes with m = 1, 3, 5 ... and n = 1, 2, 3, 4 ..., one selects the modes for which the electric field is maximum according to the median plane of the cavity which contains the axis 00 '. It is in fact recalled that the indices m and n correspond to the number of half-periods of the electric field according to the dimensions a and b of the guide, in the case of a rectangular guide. By choosing m odd, we thus obtain a maximum field in the median plane with regard to the field according to the dimension a. Concerning the field according to dimension b, the fact that n is even or odd does not react on the value of the field in the indicated median plane.

In Figure 3, we have chosen m and n equal to 1 and the variations of the electric field, in the cross section are shown in thin lines.

The oscillator according to the invention comprises coupling orifices 8 between the valves and the cavity. These orifices are formed by slots drilled in the cavity between two successive valves and at regular intervals. In Figure 2, there is a coupling slot 8 in a gap between two valves.

A coupling device makes it possible to take the output energy from the oscillator: this device can be constituted by a rectangular guide 9 connected to the cavity via an iris and extended by a flange 10.

Finally, it is understood that the oscillator shown in Figure 2 is contained in a vacuum-tight housing which is not shown.

We will now examine the operation of the oscillator according to the invention. This operation presents analogies with that of a coaxial magnetron.

It will be recalled that the cavity behaving like a waveguide at the cutoff frequency along the axis 00 'and in a tm _min mode, the electric field which prevails inside the cavity is invariant along the longitudinal axis PP 'of the cavity which is parallel to 00'. The electric field E is symbolically represented in FIG. 2 by an arrow in broken line carried by the axis PP '.

The coupling orifices 8 are therefore excited in phase by the electric field

In the case of FIG. 2, where there is a coupling orifice 8 in an interval between two valves, it is possible to operate in modes ₇ r or 3 ₇ r. Beyond, that is to say for the modes 5π, 7 ₇ r ..., the impedance of the oscillator is no longer acceptable. We therefore do not go beyond the 3π mode.

Remember that for π mode, the electric field is out of phase with π from a valve to the other, while the phase shift is 3 ₇ r for the 3π mode.

In the case of FIG. 2, to operate according to the π mode, which is the most commonly used mode, the anode voltage which determines the speed of the electron beam and the distance between two successive valves are chosen so that the transit time of the electron beam from one coupling orifice to the next is close to the period of the electric field whose wavelength is λ _c .

There is thus a phase shift of π on the electric field from one valve to another.

The electron beam is thus braked by the electric field to which it transfers energy at the level of the coupling orifices, producing the useful microwave energy and maintaining oscillation.

A resonant regime is thus established in the cavity at the cutoff frequency of the waveguide to which the cavity can be assimilated.

In the case of FIG. 2, it is also possible to operate according to mode 3 ₇ r. The transit time of the electron beam from one coupling orifice to the next must then be close to three times the period of the electric field whose wavelength is λ _c . The anode voltage must be changed.

It is also possible to operate in 2π mode by providing a coupling orifice 8 in each interval between valves. The transit time of the electron beam from one coupling orifice to the next must then be close to the period of the electric field.

It can therefore be seen that the oscillation frequency of the oscillator according to the invention is the cutoff frequency of the waveguide to which the cavity 7 pierced with coupling orifices 8 can be assimilated. These are therefore the dimensions of the cavity which are important for setting the frequency of oscillations and not those of the valves as is the case for the oscillator of the prior art.

It is therefore understandable that a large gram of mechanical tuning of the oscillation frequency can be obtained very simply, particularly in the embodiments of the oscillator where the cavity is a straight parallelepiped.

It is indeed recalled that in the case of a rectangular waveguide, the dimensions a and b of the cross section of the guide are related to the indices m and n and to the cutoff wavelength λ _c by the relation: ''

By varying a or b (see Figure 3), we obtain a mechanical adjustment of the oscillation frequency.

où E_o est une constante.The variations of the electric field which are represented in FIG. 3 in thin lines are not however modified because the amplitude of the field related to horizontal and vertical axes whose origin is on the axis PP 'is written:

where E _o is a constant.

In Figure 3, there is shown schematically how it is possible to vary the horizontal dimension a of the base of the cavity formed by a right parallelepiped using a vertical piston 12. It would also be possible to vary the dimension b of the cavity.

The electric field E in the cavity and the current lines in its side walls are perpendicular to the plane of Figure 3. It is therefore not useful that the piston 12 is in contact with the horizontal walls 16 and 17 of the cavity. On the other hand, the piston must be in contact with the vertical walls which close the cavity and which are perpendicular to the axis PP 'because current lines pass through these walls.

• - les modes de cavité. Ces modes sont des modes TE et des modes TM présentant une variation longitudinale, c'est-à-dire des modes TM_mno avec p≠O. Tous ces modes présentent des composantes de courant transversales. Il est donc aisé de les atténuer en disposant au niveau des arêtes longitudinales de la cavité une substance atténuante 13 protégée par un cache métallique 14 comme cela est représenté sur la figure 3 pour deux arêtes. En effet, dans les modes TM_mno qui sont utilisés dans l'oscillateur selon l'invention, même la composante longitudinale du courant est nulle sur ces arêtes. On peut également disposer de la substance atténuante 13 dans l'épaisseur du piston mobile;
• - les modes dus aux orifices de couplage. Les fentes 8 qui constituent les orifices de couplage présentent des fréquences de résonance que l'on atténue en disposant une substance atténuante 13 protégée par un cache métallique 15 aux extrémités de ces fentes de part et d'autre des vannes.

In addition, thanks to this particular distribution of the current lines, it is possible to eliminate all the parasitic modes. There are essentially two types of parasitic modes:

• - cavity modes. These modes are TE modes and TM modes presenting a longitudinal variation, that is to say TM _mno modes with p ≠ O. All these modes have transverse current components. It is therefore easy to attenuate them by placing at the level of the longitudinal edges of the cavity an attenuating substance 13 protected by a metal cover 14 as shown in FIG. 3 for two edges. In fact, in the TM _mno modes which are used in the oscillator according to the invention, even the longitudinal component of the current is zero on these edges. It is also possible to have the attenuating substance 13 in the thickness of the movable piston;
• - the modes due to the coupling orifices. The slots 8 which constitute the coupling orifices have resonant frequencies which are attenuated by placing an attenuating substance 13 protected by a metal cover 15 at the ends of these slots on either side of the valves.

Finally, one can have the attenuating substance inside the vacuum-tight housing which contains the oscillator to dampen the parasitic modes which could propagate there.

This elimination of the parasitic modes makes it possible to obtain a unique resonance with very high overvoltage and a high spectral purity of the oscillation. So the fashion jumps random are practically nonexistent and the performance excellent.

Claims (10)

1. Hyperfrequency oscillator having an extended interaction, comprising a line having a periodic structure and composed of a succession of vanes (5) traversed or wiped by linear electron beams, characterized in that:

- this line is disposed above a rectilinear cavity (7) the cross-sectional dimensions of which are so determined that it behaves as a waveguide at the cut-off frequency along the longitudinal axis (00') of the line and with a magnetic transverse mode TM_mn, with m=1, 3, 5 ... and n=1, 2, 3, 4 ...;

- coupling openings (8) between the vanes and the cavity are provided on the cavity between two successive vanes and at regular intervals, the anode voltage of the beam, the distances between two successive vanes and between two successive coupling openings being determined in such a manner that the transit time of the electron beam from one coupling opening (8) to the following is close to the period or to a multiple of the period of the electric field, according to the selected mode of operation (π, 2π, 3₇r), respectively, the wavelength of the electric field being the wavelength (À_c) for cut-off of the cavity, the oscillation frequency of the oscillator being the cut-off frequency of the cavity;

-a coupling device (9) allowing the output energy of the oscillator to be derived from the cavity.

2. Oscillator according to claim 1, characterized in that each vane (5) is pierced by an opening (6) or has a slot (11) wherein the electron beam propagates.

3. Oscillator according to any of claims 1 and 2, characterized in that the oscillation is of π type or 3π type and in that the distance between two successive coupling openings (8) is twice that between two successive vanes (55).

4. Oscillator according to any of claims 1 and 2, characterized in that the oscillation is of 2π type and in that the distance between two successive coupling openings (8) is the same as that between two successive vanes (5).

5. Oscillator according to any of claims 1 to 4, characterized in that the cavity (7) is a straight parallelepiped the base of which is a rectangle or square having the dimensions a and b, these dimensions a and b being related to the cut-off wavelength of the cavity λ_c and to the indexes m and n by the relation:

6. Oscillator according to claim 5, characterized in that it comprises a damping substance (13) protected by a metallic mask at the longitudinal edges of the cavity (7).

7. Oscillator according to any of claims 1 to 6, characterized in that it comprises a damping substance (13) protected by a metallic mask (15) at the ends of the coupling openings (8) on both sides of the vanes (5).

8. Oscillator according to any of claims 5 to 7, characterized in that it comprises a piston (12) permitting to modify the dimensions a or b of the cavity, this piston contacting only the two walls closing the cavity and which are perpendicular to the longitudinal axis (PP') of the cavity (7).

9. Oscillator according to claim 8, characterized in that the damping substance (13) is disposed within the thickness of the movable piston (12).

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