FR2686457A1 - Electronic scanning antenna - Google Patents

Abstract

The invention relates to an antenna of the sector-scanning type, including a plurality of transducers (T). It includes at least one lens (L) arranged in such a way that at least some of the said transducers (T) are arranged substantially in the focal plane of the said lens (L). Several lenses (L1... L5) can be arranged along at least one given direction, each having, substantially in its focal plane (S), a different group of transducers (T). Electronic processing is then carried out by multiplexing the output signals from the transducers of each group in a multiplexer (MPX1... MPX5). The outputs of the multiplexers (MPX1... MPX5) drive the inputs of a beam-forming circuit (PA1... PA5, R1... R5). Thus a significant reduction is obtained in the number of electronic processing channels. Application to sonars, radars and to echography.

Description

ELECTRONIC SCAN ANTENNA
The present invention relates to an antenna of the electronic scanning type.

In this type of antenna, a single transmission and reception beam is formed at each transmission recurrence. To do this, it is associated with an electronics whose function is to produce signals to focus the transmission and reception and to ensure the electronic scanning of the image sector. The rejection of the image lobes out of the field is necessary to obtain good images, resulting in a step between sensors which is between 0.5 and one time the value of the wavelength
In such a system, it is necessary to implement a large number of sensors on reception and emission to form a single beam at a time, which has the consequence that the processing electronics are very complicated and very expensive. .

 The subject of the present invention is an antenna of the sector electronic scanning type in which the number of electronic circuits used for transmission and / or reception can be reduced in a significant proportion.

 The invention thus relates to an antenna of the sector electronic scanning type, comprising a plurality of transducers, characterized in that it comprises at least one focusing element arranged in such a way that the output signals of at least one group of said transducers correspond to an arrangement of the transducers of said group substantially in a focal surface of said focusing element.

 The objective of the focusing element is to focus the waves on a transducer, and each transducer corresponds to a given direction e. By thus associating one or more focusing elements with all or part of the transducers, the number of electronic processing channels is reduced, as will be shown in the rest of the description.

 A focusing element may be a lens. By arranging several lenses in at least one given direction, each lens having substantially a different group of transducers in its focal area, an antenna whose signals can be processed in a simple manner by multiplexing, in other words by successively addressing each transducer of the same group and by application of a delay given to the emission and the reception.

 According to a first embodiment, at least one group of transducers is located on the focal surface of the corresponding lens, each transducer having the same width x.

 According to a second embodiment, at least one group of transducers forms a plane array whose width varies between x for the axial sensor x / 2 for the extreme sensor. According to a preferred embodiment, x has the value x = f> / 1, f denoting the focal length of a lens, 1 its aperture and the wavelength of the transducer signals.

 According to a preferred embodiment, a focusing element is constituted for each group of transducers by a group of delay elements having ny delays, each delay element having an input connected to the output of a corresponding transducer of said group, and each element retarder having an output for producing a multiplexed signal of said delays. This gives an "electronic" equivalent of a lens, with a "virtual" focal surface.

 The invention also relates to an antenna as defined above in which the electronic processing means comprise at least one multiplexer having a plurality of first terminals coupled to the transducer of a group and a second terminal coupled to a delay circuit whose delay is function of a control signal, said switch being addressable to electrically connect any one of the first terminals to the second terminal.

 Such an antenna may comprise a plurality of multiplexers, the outputs of the corresponding delay circuits being coupled to inputs of a summing circuit whose output produces the image signal, the control signals of the delay circuits being chosen so as to produce the desired scan on show and on reception.

 An antenna that can be used both for transmission and reception can be obtained by virtue of the fact that at least one multiplexer is arranged to receive a transmission signal.

 According to a particularly advantageous variant, for forming beams in parallel, the antenna comprises a plurality of beam forming circuits each having a plurality of inputs each of which is connected to at least one transducer of each group of transducers.

Other features and advantages of the invention will appear even better on reading the description which follows, in which
Figure 1 shows a sector scanning antenna of the prior art.

 FIG. 2 represents a sector scanning antenna according to a first embodiment of the invention.

 FIG. 3 represents an antenna element according to a variant of the invention.

 FIG. 4 represents a transducer associated with a lens according to the invention.

 FIG. 5 represents a second embodiment of the invention that is suitable for producing parallel beams.

 FIG. 6 represents a preferred embodiment of the invention, according to which focusing is obtained by electronic delays.

 According to Figure 1 corresponding to the prior art, an electronic scanning antenna comprises a plurality of transducers 1 arranged in at least one dimension to obtain an image line at each recurrence. At each recurrence or firing, all the transducers of the antenna are simultaneously supplied with a given electronic delay law. This electronic delay law is changed at each recurrence, which makes it possible to obtain a complete image sequentially. The law of delay serves both to detach to ensure sectoral scanning and focusing. The same law of recurrence is applied to both transmission and reception. The arrow F indicates the direction of sweeping between the first shot Tl and the last shot TD. Such an antenna has a large number of elementary transducers. Each of these transducers is associated with an electronic circuit ensuring the law of delay both on transmission and on reception. The number of transducers being of the order of several tens, this technique has the disadvantage of requiring the production and processing of a large number of signals, which implies a large number of electronic circuits and a high short .

According to FIG. 2, a sector scanning antenna comprises, as in the previous case, a number of transducers T aligned in at least one direction. A transducer T may consist for example of several ceramics electrically connected to each other. These transducers are divided into different groups, here five in number, called G1, G2, G3, G4 and G5. To each of these groups is associated a lens L1, L2, L3, L4 and L5, so that the transducers T are located substantially in the surface or the focal plane of said lenses. In the case of ultrasound, a cylindrical acoustic lens a few millimeters wide is used. For electromagnetic waves, it is possible to use electromagnetic lenses known per se. Both in the case of a transmission and in the case of a reception, the lenses focus the signals in the direction e on a particular transducer referenced Tg of each of the groups of transducers. These lenses therefore make it possible to produce for each transducer of each group an angular separation, a function for which there is no longer any need for an electronic circuit associated with each of the transducers. The signals of the transducers of each group are addressed by electronic commutation to perform the angular scanning. The signals of the transducers of the group G1 are applied to the inputs of a multiplexer MPX1 whose output is connected to the input of a preamplifier PA1 and the output of which drives a delay circuit R1. It is the same for the transducers of the G2 group with an MPX2 multiplexer, a PA2 preamplifier, and a delay circuit R2, and so on up to the G5 group (MPX5 multiplexer, PA5 preamplifier, and R5 delay circuit). The outputs of the delay circuits are applied to the inputs of a summator
S whose output produces the desired video signal V.

 A control signal CD makes it possible to electrically contact a selected transducer of each group with the input of the corresponding preamplifier. At a given moment a sensor of each group corresponding to a direction e selected, is addressed. The summation of signals to which a delay law is applied in a conventional manner makes it possible to obtain a high resolution image in the direction e.

The above circuit corresponds to the reception of a signal. For a transmission phase, signals E1 to E5 are applied to the inputs of the preamplifiers, respectively PA1 .... PA5, and are sequentially distributed by the respective multiplexers MPX1 to MPX5, as a function of the control signal CD, to one of the transducers T of each group, depending on the scanning direction e. Signals
E1 to E5 have the same law of angular delay as the delay circuits R1 to R5 if we want to obtain the same directivity at transmission and reception. This delay law is formed by delay circuits RE1 RE2, .... RE5 whose output drives transmit preamplifiers PAE1, PAE2 ... PAE5, the outputs of which respectively supply the signals E1 to
E5. As previously described in the case of a reception, the lenses L1 to L5 have the same angular function, which makes it possible to reduce in the same proportion the number of signals and electronic circuits necessary to obtain the angular scanning of the antenna.

 The preamplifier sets PA1 .... PA5 and delay circuits R1 ... R5 and the summing circuit S constitute a conventional beam-forming circuit.

 According to FIG. 3, the sensors Tm associated with a lens Lm are arranged on the focal surface of the lens Lm and directed towards it. For example, the transducers may be formed according to a plurality of concave surfaces by a known method as described, for example, in Japanese Patent 57-181299. It is also possible to have the transducers on a plane array, as shown in FIG. 2, the width of each transducer decreasing from x to x / 2 from the center to the edges so as to increase the interception. This decrease can be linear (in first approximation). In addition, it is advantageous to proportionally reduce the gap between the sensors to ensure the continuity of the scan, which leads to increase the number of transducers, which however remains lower than the case of the prior art.

 It will also be noted that the depth of field is determined by the horizontal pupil 1 of the lens and by the position of the retina carrying the transducers relative to the plane of the lenses. These two parameters will be adjusted according to the operational problem to be solved. It will be understood that, with respect to the position of the transducers vis-à-vis the focal surface of the lens, the term "substantially" in the focal surface of the lens means in the sense of the application that one must take into account the depth of field as mentioned above. On the other hand, and independently of any consideration of depth of field, the second solution consisting of a plane array comprising transducers whose width is variable as a function of the position relative to the axis of the lens, implies that it It is necessary that only a transducer located in the vicinity of the axis of the lens is located on the focal surface thereof.

Referring now to FIG. 4, we will indicate how the width of an elementary transducer T can be determined. An elementary sensor T must intercept the opening 1 of the lens L. Let ec be the angle under which the transducer T sees the lens L. We can write in first approximation ec ^ 'of l / f =
with f = focal length of the lens L # = wavelength of the signals of the transducer T,
c = width of the transducer T.

 As a result, lc <f / 1.

 In order to optimize the width of the elementary sensor T, each transducer must be on the focal surface of the lens and mechanically oriented in order to intercept the pupil of the lens. As indicated above, an alternative solution consists of a plane array whose sensor width varies from f # / 1 for the axial sensor Tmo to f> / 21 for the extreme sensors, the variation law being linear (first approximation).

 We will now be able to quantify the advantages of the system compared to the prior art.

 By way of example, f = 1 is chosen, this corresponds to a numerical aperture of the lens equal to 1. For an antenna of length L, the number of processing channels Ncl is equal to 2L />. In the case of a conventional antenna, the transducers must be spaced by> / 2 in order to obtain a +90 image lobe rejection.

 In the case of an antenna comprising at least one lens according to the invention and of length L ', the pitch between the sensors may be equal to #, due to the fact that it is no longer necessary to obtain a reduction image lobes, this being obtained thanks to the directivity of the lens. The total number of sensors NC2 is equal to L '. This already allows a gain in ratio 2 compared to the prior art. In addition, and since there are n transducers T per group G, the number of electronic processing channels is also decreased by a nc ratio.

 Thus, 2 x nc times less electronic processing channels and 2 times less T transducers will be needed than in the case of a prior art electronic scanning antenna.

DIGITAL APPLICATION
An = l / A for a frequency f of 3 MHz, A = 0.5 mm and for lenses of width 1 = 3 mm, an improvement factor equal to 12 is obtained with respect to an antenna of the art. prior, twelve times less electronic processing channels that in a scanning antenna of the prior art.

 To increase this factor, it would be necessary to increase the pupil of the lens, this being however obtained at the expense of the angular field of the lens and the depth of field. The optimization of this factor will therefore be a compromise depending on the operational choices (sector, range, frequency of the signal).

 A further improvement relating to the rejection of the image lobes at + 90 ° is achieved by placing screens between the groups of transducers, for example by over-molding a sufficient thickness of an acoustically transparent material (for example polyurethane) that can be machined to obtain convex or concave surfaces according to the index of the material that forms the lenses A saw cut between each lens constitutes the screen (air gap).

 Another advantage of an antenna according to the invention is that it is possible to integrate the multiplexers in the probe, which makes it possible to reduce in a factor nc the number of channels to be transmitted by the connecting cable of the probe to the processing electronics and therefore the diameter of the cable. On the other hand, the level of the signal flowing in each wire of the transducer is greater than in the conventional case because of the gain of the lens relative to that of the transducer, or about 8 dB (10 log6 in the digital application ci above), which makes it easier to overcome electrical noise.

 It thus appears, thanks to the significant reduction in the number of processing channels that it is conceivable to produce digital processing machines at a reasonable cost.

According to FIG. 5, beams are formed in parallel in order to increase the frame rate in order to carry out either a Doppler processing or an image-to-image integration process in order to reduce the noise in particular. For this purpose, a plurality of beam forming circuits FF1, FF2 and FF3, etc. are implemented. Each input of such a beamforming circuit is connected to at least one transducer of each group of transducers. For example, the beam forming circuit FF2 has its inputs connected to the axial transducers To of each of the transducer groups G1 to G4 corresponding to the lenses L1 to L4. In the same way, the beamforming circuits FF1 and FF3 each have their inputs connected to transducers T occupying similar positions in the different groups G1 to
G4, and so on for other beam forming circuits. We can thus have up to nc beam forming circuits per antenna. Alternatively, the inputs of the beam forming circuits may also receive the signals of several (p) transducers of the same group, arranged in the same way for each of the groups. In this case, it is necessary to add to each beam forming circuit FF q MPX electronic switches with p inputs (q being the number of groups of transducers). To connect all transducers
T, then q.nc/p MPX electronic switches and nc / p FF beamformer circuits are required. Of course, for p = nc, we find the configuration of Figure 2.

 The outputs of the beam forming circuits FF1, FF2, FF3... Produce parallel video signals V1, V2, V3.

 FIG. 6 represents a preferred embodiment of the invention in which an electronic equivalent of a lens is produced.

 Each transducer group forming a sub-antenna has nc transducers numbered from 1 to nc. There are K sub-antennas (K = N / nc). Each transducer is connected to a bidirectional delay circuit RS1, RS2 ... RSC, each delay circuit producing nv delays on nv corresponding outputs. These delays are chosen so as to compensate for the differences in operation between each transducer and a predetermined virtual focal point.

A focus is considered satisfactory in the area where the difference in operation does not exceed
These outputs are multiplexed by MPX1 ... MPXc multiplexers whose output is connected to an input of a duplexer. The first group of transducers has referenced duplexers (D11, D21 ... DC1) whose other input receives an emission delay signal ER1. The second group of transducers has referenced duplexers (D21, D22 ... DC2) whose other input receives an emission delay signal ER2; the last (Kth) group of transducers has referenced duplexers (D1K, D2K, ... DCK) whose other input receives a transmission delay signal ERK. The outputs of the duplexers (D11, D21 ... DC1) of the first group are connected to the corresponding inputs of an adder S1. The outputs of the duplexers (D12, D22 ...

DC2) of the second group are connected to the corresponding inputs of an adder S2. The outputs of the duplexers (D1K, D2K ... DCK) of the last group of transducers are connected to the corresponding inputs of a summator SK. The output of the summator S1 is connected to the input of the programmable delay circuit R1. The output of summator S2 is connected to the input of the programmable delay circuit
R2. The output of the summator SK is connected to the input of the programmable delay circuit RK. As already mentioned, the programmable delays R1 ... RK provide the formation of the track in the pointing direction and also vary in time to ensure dynamic focusing.

 At the show, we do the same treatment as at the reception. The antenna operates sequentially with recurrence transmission and reception. There is only one beam per recurrence. At each recurrence, there is a pointing direction and a track is formed. The sequencing of the multiplexers is such that one changes position, that is to say of delay, with each recurrence.

 At each recurrence, the programmable delays of transmission (signals ER1, ER2 ... ERK) and those of reception (circuits R1, R2 ... RK) are identical.

The delays thus comprise two components identical to the transmission and the reception: a delay to carry out the misalignment, (R1 ... RK, ER1 ... ERK), and a delay to realize a focusing (circuits
RS1 ... RSC). The circuits RS1 ... RSC each comprise nv bidirectional delay circuits, for example LC type delay lines.

It is possible to integrate in the probe the preformation of the channels corresponding to an "electronic" focusing, namely the fixed delay circuits.
RS1 ... RSC which are advantageously analog, bidirectional multiplexers MpXl ... MPXC, for each group, or in all N multiplexers of this type, the duplexes transmission-reception (D11, D21 ... DCl ...

DCK) also in the name of N and the K summators (S1, S2 ... SK). Thus, the number of connections in the probe / machine connection cable (K coaxial cables for transmission, and K for reception) is reduced to 2K (instead of 2N).

 Another substantial advantage over the lens implementation is that the transducers no longer have to respect the 1c <f # / 1.

Claims (14)

 1) An antenna of the sector scanning type comprising a plurality of transducers, characterized in that it comprises at least one focusing element (L) arranged in such a way that the output signals of at least one group of said transducers (T) correspond to an arrangement of the transducers of said group substantially in a focal surface of said foci element (L).  2) Antenna according to claim 1, characterized in that it comprises a plurality of focusing elements (L1 ... L5) arranged for different groups of transducers.  3) Antenna according to one of claims 1 or 2, characterized in that a focusing element is a lens (L), the transducers of said group being disposed substantially in the focal surface of the lens (L).  4) Antenna according to claim 3, characterized in that the lenses (L1 ... L5) are separated by screens (10).  5) Antenna according to claim 3, characterized in that at least one group of transducers (T) is located on the focal surface (S) of the corresponding lens (L), each transducer having the same width x.  6) Antenna according to one of claims 3 to 5, characterized in that at least one group of transducers (T) forms a plane array whose width varies between x for the axial sensor x / 2 for the extreme sensor.  7) Antenna according to one of claims 5 or 6, characterized in that x = f, \ / 1, f denoting the focal length of a lens, 1 its aperture, and the wavelength of the transducer signals (T ).  8) Antenna according to claim 2, characterized in that a focusing element is constituted for each group of transducers (1 ... nc) by a group of delay elements (RS1 ... RSC) having nv delays, each delay element having an input connected to the output of a corresponding transducer of said group, and each delay element having an output for producing a multiplexed signal said nv delays.  9) Antenna according to claim 8, characterized in that it comprises for each group of transducers, a multiplexing element (S1 ... SC) for multiplexing the outputs of the delay elements (Rs1 ... RSC) of said group.  10) Antenna according to claim 9, characterized in that it comprises between the output of each delay element and the corresponding input of the multiplexer element, a duplexer (D11 ... DCK) transmission / reception.  11) Antenna according to one of the preceding claims, characterized in that it comprises electronic processing means comprising at least one multiplexer (MPX) having a plurality of first terminals coupled to the transducers (T) of a group and a second terminal coupled a delay circuit (R) whose delay is a function of a control signal, said multiplexer (MPX) being addressable for electrically connecting any one of the first terminals to the second terminal.  12) Antenna according to claim 11, characterized in that it comprises a plurality of multiplexers (MPX1 ... MPX5), in that the outputs of the corresponding delay circuits are coupled to inputs of a summing circuit (S) of which the output produces a video signal, and in that the control signals of the delay circuits (R) are selected to produce the desired scan on reception.  13) Antenna according to one of claims 11 or 12, characterized in that at least one multiplexer (MPX1 ... MPX5) is arranged to receive a transmission signal.  14) Antenna according to one of claims 2 to 13, characterized in that it comprises a plurality of beam forming circuits (FF) each having a plurality of inputs each of which is connected to at least one transducer (T) of each group transducers.