EP0228947B1 - Superheterodyne receiver for two microwave signals with two opposite circular polarizations - Google Patents

Description

The invention relates to a device for the simultaneous reception of two waves with circular or elliptical polarization in opposite directions.

An international agreement has provided that television broadcasts transmitted by geostationary satellite could be carried out on 40 channels of carrier frequencies between 11.7 and 12.5 GHz, each channel occupying a band spanning 27 MHz. Thus two consecutive channels can have overlapping frequencies. To avoid interference, provision is made for two successive channels to be transmitted by waves with reverse circular polarizations, one being with right circular polarization (dextrorotatory) and the other with left circular polarization (levorotoric).

In an area receiving two adjacent frequency channels, it is therefore preferable that the reception devices are capable of receiving circularly polarized waves in opposite directions. To minimize costs, provision is made in general to use only a single antenna for receiving these signals, the latter therefore being associated with a separation device.

Such a device is located in the vicinity of the antenna. It comprises a waveguide means transforming the waves with circular polarization into waves with rectilinear polarization. Each wave with rectilinear polarization is introduced at the input of a circuit which comprises an amplifier, in particular at low noise, a filter, a local oscillator, a mixer and an amplifier at intermediate frequency IF.

It has been found that the television images obtained from the signals originating from these circuits have defects, in particular in the form of vertical lines.

The inventors have discovered that these defects arise from the fact that, on the one hand, the frequencies of the local oscillators cannot be strictly equal and, on the other hand, despite the care that can be taken in the construction of the separation device, the signals of each channel act on the other channel. This results in interference or beats which cause visible defects on a television picture.

A reception device according to the preamble of claim 1 is known from document US-A-3,092,828 (see FIG. 9). The device according to the invention is characterized by the characteristics of the characterizing part.

• la figure 1 est un schéma d'une antenne et d'un dispositif de réception associé,
• la figure 2 est une vue en coupe axiale à échelle agrandie du dispositif de réception de la figure 1,
• les figures 2_a à 2_j sont des schémas montrant le fonctionnement d'un dispositif de séparation,
• la figure 3 est une coupe selon la ligne 3-3 de la figure 2,
• la figure 4 est un schéma de circuit du dispositif de réception de la figure 1,
• la figure 5 est une coupe selon la ligne 5-5 de la figure 2,
• la figure 6 est une vue schématique en perspective correspondant à une partie du dispositif de réception de la figure 2,
• la figure 7 est une vue en perspective et en coupe d'une partie de dispositif de réception pour une variante, et
• la figure 8 est une autre vue en perspective du dispositif de la figure 7.

Other characteristics and advantages of the invention will appear with the description of some of its embodiments, this being carried out with reference to the attached drawings in which:

• FIG. 1 is a diagram of an antenna and an associated reception device,
• FIG. 2 is a view in axial section on an enlarged scale of the reception device of FIG. 1,
• FIGS. 2 _a to 2 _j are diagrams showing the operation of a separation device,
• FIG. 3 is a section along line 3-3 in FIG. 2,
• FIG. 4 is a circuit diagram of the reception device of FIG. 1,
• FIG. 5 is a section along line 5-5 of FIG. 2,
• FIG. 6 is a schematic perspective view corresponding to a part of the reception device in FIG. 2,
• FIG. 7 is a perspective view in section of a part of the receiving device for a variant, and
• FIG. 8 is another perspective view of the device in FIG. 7.

The example which will be described in relation to the figures relates to the reception of television broadcasts transmitted via a geostationary satellite, the television signals being carried by microwave waves with circular gyroplane or dextrorotatory polarization .

To receive such waves, a parabolic antenna 10 (FIG. 1) is provided, the axis of which is directed towards the satellite and which thus receives the waves emitted by the latter to reflect them on its focus. A device for receiving microwave waves 11 is therefore placed at the focal point of the parabolic antenna 10.

Such a device 11 comprises a receiving horn 12 (FIG. 2) constituting the inlet of the device 11. This horn 12 has the shape of a truncated cone. It is extended, at the rear, by a waveguide 13 of circular section which may contain a bandpass or highpass filter. This waveguide 13 is connected to a waveguide 15 of rectangular section via a waveguide 14 allowing the transition from the circular section to the rectangular section.

The waveguide 15 of rectangular section has, over part of its length, a depolarizing metal strip 16 parallel to two walls 17 and 18 (FIG. 3) of the rectangular parallelepiped that constitutes the waveguide 15 and at equal distance from these walls. The front edge 19 of the blade 16 has a general bevel shape (Figures 2 and 6) which allows depolarization: polarized waves circular to the right are transformed into waves with rectilinear polarization remaining on one side of the blade 16, the side being a function of the direction (right) of the circular polarization. As a variant, the front edge 19 has a staircase shape.

FIGS. 2 _a to 2 _j are diagrams which allow a good understanding of the depolarizing role of the blade 16 and of its entry edge 19.

dont les directions forment entre eux un angle de $\frac{\text{π}}{\text{2}}$

radians. Le sens de rotation (à droite ou à gauche) de l'onde circulaire dépend du signe du déphasage entre ces deux vecteurs

Ainsi la propagation d'une onde à polarisation circulaire dans le guide d'onde 15 correspond à la propagation de deux ondes à polarisation rectiligne. Les figures 2_a à 2_d montrent la propagation de la composante de vecteur

et les figures 2_e à 2_h montrent la propagation de la composante de vecteur

A circularly polarized wave (right or left) can be broken down into two linearly polarized waves (Figure 2 _j ,) represented by the two electric field vectors

whose directions form an angle of $\frac{\text{<figure-callout id="2" label="π" filenames="imgf0005.png" state="{{state}}">π</figure-callout>}}{\text{2}}$

radians. The direction of rotation (right or left) of the circular wave depends on the sign of the phase shift between these two vectors

Thus, the propagation of a wave with circular polarization in the waveguide 15 corresponds to the propagation of two waves with rectilinear polarization. Figures 2 _a to 2 _d show the propagation of the vector component

and Figures 2 _e to 2 _h show the propagation of the vector component

Figures _2b and _2f are sections of the guide 15 in a cross section corresponding to the front part of the bevel 19; the diagrams in FIGS. 2 _c and 2 _g correspond to straight sections in the posterior zone of the bevel 19 while FIGS. 2 _d , 2 _h and 2 _i correspond to the waveguide at the rear of the bevel 19.

est de le séparer en deux vecteurs

de mêmes sens de part et d'autre de la lame 16. Autrement dit la composante de l'onde d'entrée correspondant au vecteur

est divisée en deux ondes à polarisation rectiligne en phase.As can be seen in Figures 2 _a to 2 _d the effect of the blade 16 on the horizontal vector

is to separate it into two vectors

in the same directions on either side of the blade 16. In other words the component of the input wave corresponding to the vector

is divided into two waves with rectilinear polarization in phase.

est de transformer cette dernière en deux ondes à polarisation rectiligne horizontale en sens inverses (figures 2_e à 2_h). Ainsi, comme le montrent les figures 2_h et 2_d, à gauche de la lame 16 on obtient une onde à polarisation rectiligne horizontale

en phase avec l'onde

Par contre à droite de la lame 16 l'onde à polarisation rectiligne

est en opposition de phase avec l'onde à polarisation rectiligne

Dans ces conditions l'onde à polarisation circulaire de composantes

est transformée en une onde à polarisation rectiligne

d'un seul côté de la lame 16 (figure 2_i), les composantes

s'annulant de l'autre côté.On the other hand, the effect of the blade 16 on the wave with vertical rectilinear polarization

is to transform the latter into two waves with horizontal rectilinear polarization in opposite directions (Figures 2 _e to 2 _h ). Thus, as shown in FIGS. 2 _h and 2 _d , to the left of the blade 16 there is obtained a wave with horizontal rectilinear polarization

in tune with the wave

By cons to the right of the blade 16 the wave with rectilinear polarization

is in phase opposition with the wave with rectilinear polarization

Under these conditions the circularly polarized wave of components

is transformed into a wave with rectilinear polarization

on one side of the blade 16 (Figure 2 _i ), the components

canceling out on the other side.

est transformée en une onde à polarisation rectiligne du côté droit de la lame 16.It will be readily understood that a wave with circular polarization in the opposite direction, that is to say of components

is transformed into a wave with rectilinear polarization on the right side of the blade 16.

de chaque côté de la lame métallique 16, à l'arrière du bord en biseau 19. Ces sondes 21 et 22 sont alignées, et donc en positions symétriques l'une de l'autre par rapport à la lame 16.Rectilinear probes 21 and 22 pass through the walls 17 and 18 perpendicularly to the latter, and therefore parallel to the electric field vector

on each side of the metal blade 16, behind the bevel edge 19. These probes 21 and 22 are aligned, and therefore in positions symmetrical to one another with respect to the blade 16.

The probe 21 is connected with its end _21a to the outside of the waveguide 15, to a circuit on a printed circuit board 23 (Figures 3 and 5) parallel to the wall 17 and at a short distance the latter. Likewise, the probe 22 is associated with a printed circuit board 24 identical to the board 23 and of position symmetrical with respect to the plane of the blade 16.

In Figure 4 there is shown, on the one hand, the circuit 23₁ superheterodyne located on the wafer 23, on the other hand, the circuit 24₁ on the wafer 24 and, on the other hand finally, the common local oscillator 25 to these two circuits which is on a printed circuit board 26 (FIGS. 3 and 5) perpendicular to the boards 23 and 24, that is to say parallel to another wall 27 of the waveguide 15 of rectangular section, and at a short distance from this wall 27. As a variant, the plate 26 is applied against the external face of the wall 27.

For the connection (FIG. 5) of the local oscillator 25 common to the superheterodyne circuits 23₁ and 24₁, bridges are provided for connection, respectively between the plate 26 and the plate 23, and 31 between the plate 26 and the plate 24. These bridges overlap edges in contact with the plates 23 and 26 and 24 and 26.

The circuit 23₁ comprises a low noise amplifier 32 receiving the signal from the probe 21 and which is connected to the first input 34₁ of a mixer 34 via a bandpass filter 33. The second input 34₂ of the mixer 34 is connected to a first output of the power divider associated with the local oscillator 25. The output 34₃ of the mixer is connected to the output of the circuit via an intermediate frequency amplifier FI 35.

Likewise, the circuit 24₁ includes an amplifier 36 with low noise, a bandpass filter 37, a mixer 38, one input of which is connected to a second output of the power divider associated with the local oscillator 25 and an intermediate frequency amplifier FI 39.

In the example, and as can be seen in FIG. 3, the printed circuit board 23 rests on a wall 40 parallel to the wall 17 and at a short distance from the latter; a cover 41 is associated with this wall 40. In this way the printed circuit board 23 is sealed in a box formed by the wall 40 and the cover 41. Similarly, the boards 24 and 26 are enclosed in respective boxes with base wall and cover. The set of waveguides 13, 14, 15 and printed circuit boards with their boxes is placed in another protective box 45 (FIG. 2) forming a single piece with the horn 12.

The arrangement of the invention which consists in providing a local oscillator common to circuits 23₁ and 24₁ avoids interference between these two circuits.

In addition, the arrangement which consists in providing probes 21, 22 perpendicular to the walls 17, 18 and connected to circuits on parallel plates 23, 24 and at a short distance from said walls 17, 18, or applied against the latter, allows an embodiment compact receiving device, i.e. to minimize the volume occupied by this device 11. The resulting advantage is a reduction in the wind resistance and a reduction in the risk of obscuring the wave coming from the satellite before it is received by the device.

In the variant shown in FIGS. 7 and 8, to separate the waves with circular polarization in opposite directions, a waveguide of rectangular or square section is not used, but a waveguide 50 of circular section which is separated , in longitudinal direction, in two parts by a wall 51 having an elongated slot 52 in a determined direction. Upstream of the slot 52 a probe 53 passes through the wall of the waveguide, in a radial direction, and is connected to a printed circuit board 54 which is applied against a flat outer face 55 of the device 15 ʹ for separating the waves at circular polarization in opposite directions.

The wall 51 with its slot 52 elongated parallel to the direction of the probe 53, constitutes a short-circuit for waves with rectilinear polarization such as the electric field E is parallel to the probe. On the other hand, this wall 51 leaves the passage to waves with perpendicular polarization. Thus a probe 56 crosses radially the wall of the waveguide 50 in a direction perpendicular to that of the probe 53, but downstream of the wall 51. The part of the waveguide 50 which contains the end of the probe 56 is closed by an end wall 57 constituting a short circuit for all the waves, whatever their polarization.

The probe 56 is, like the probe 53, connected to a circuit located on a printed circuit board 58 applied against a flat external face 59 perpendicular to the face 55.

The circuits on the pads 54 and 58 are analogous to the circuits 23₁ and 24₁ described in relation to FIG. 4. As in this embodiment, a common local oscillator 25ʹ is provided. This is placed on another printed circuit board 60 connected to the circuits on the boards 54 and 58 by bridges 61 and 62 (FIG. 8). The trigger guard 61 overlaps the edges in contact 54 _a and 60 _has plates 54 and 60; similarly the trigger guard 62 overlaps edges in contact 58 _b and 60 _b of the plates 58 and 60. To receive the plate 60 the device 15ʹ has an oblique face 70 of short length relative to the faces 55 and 59 (FIG. 8) forming an interruption of the joint edge 71 to said faces 55 and 59. In the example, this face 70 is inclined by about 45 ° relative to the adjacent faces 55 and 59. This face 70 is inside the rectangular parallelepiped that constitutes the part 15ʹ; the depth of the withdrawal is sufficient for the wafer 60 and the oscillator 25 non not to exceed this rectangular parallelepiped either, which maintains the compact nature of the device.

The waveguide 50 is formed of three pieces of cast aluminum, the first 63 ending in a nose 64 closed by the wall 51 and the second, 65, having a countersink 66 receiving by force adjustment the nose 64. The third piece is the end plug 57.

In the embodiments described, the local oscillator 25 common to the circuits of the two channels is on a printed circuit board while the other circuit elements are on two separate boards. Other distributions of the circuit elements are however possible. Thus in one example the common local oscillator 25 and the IF amplifiers 35, 39 are on the same printed circuit board.

Claims (11)

1. Device for simultaneous reception of separate television broadcasts with the aid of waves with circular polarisation in opposite directions comprising a waveguide (15, 15') containing means (16) converting the waves with circular polarisation into waves with plane polarisation or of different directions, and, for each wave with plane polarisation, a superheterodyne-type circuit (23₁, 24₁), the two circuits including a common local oscillator (25, 25'); characterised in that the waves with plane polarisation are of opposite directions; the waveguide (15, 15') has a parallelepipedal general external shape; and two probes (21, 22; 53, 56) pass through the walls of the waveguide perpendicularly to external faces and are connected to the circuits on printed circuit boards applied against the said external faces, or parallel to and at a slight distance from these faces.
2. Device according to Claim 1, characterised in that the common local oscillator (25, 25') is arranged on a printed circuit board on a third external face of the waveguide (15, 15').
3. Device according to Claim 1 or 2, characterised in that the waveguide (15, 15') has a parallelepipedal general internal shape with a depolarising blade (16) for carrying out the conversion and the separation of the waveguides with circular polarisation in opposite directions, this plate being parallel to two internal faces of the waveguide and preferably at an equal distance from the latter, the probes (21, 22) being perpendicular to these internal faces.
4. Device according to Claim 3, characterised in that the probes (21, 22) are in alignment with one another.
5. Device according to Claims 2 and 3, characterised in that the printed circuit board for the local oscillator (25) which is common to the two circuits is on one face, or parallel to and at a slight distance from one face, perpendicular to the two faces having the boards connected to the probes.
6. Device according to Claim 5, characterised in that the printed circuit board on which the local oscillator (25, 25') is arranged has edges in contact with corresponding edges of the boards of the circuits to which the probes are connected, the link between a power divider associated with the common local oscillator and the circuit boards connected to the probes being made by link bridges (30, 31) straddling the contacting edges.
7. Device according to Claim 1, characterised in that each circuit includes a low-noise amplifier (32, 36).
8. Device according to Claim 2, characterised in that the two probes (53, 56) have perpendicular directions, the printed circuit boards to which these two probes are connected being therefore in perpendicular planes, and in that the common local oscillator (25') is on a printed circuit board oriented at about 45° with respect to the first two boards.
9. Device according to Claim 8, characterised in that the parallelepiped constituted by the waveguide (15') has, along one of its edges, a recess of niche in the bottom of which the board (60) is arranged including the common local oscillator (25, 25'), this board and this oscillator not projecting from the rectangular parallelepiped.
10. Device according to Claim 2, characterised in that the common local oscillator (25, 25') is at an equal distance from the probes (21, 22; 53, 56).
11. Application of the device according to any one of the preceding claims to the reception of television broadcasts transmitted by geostationary satellite.

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