EP0241380B1 - Method and device for focusing an antenna array at a test point - Google Patents

Description

The present invention firstly relates to a method of focusing, on at least one point to be examined from a radiation source, the antennas of an array of antennas receiving the radiation from the point with corresponding respective reception phase shifts at the time of travel of said radiation between said point and the respective antennas.

Such a method is used when it is desired to obtain, from the microwave radiation coming from an object to be analyzed, a microwave image of this object. To this end, a set of antennas is organized to form a network, this term being taken in a similar, but broader, sense than that which it possesses in optics, and this network of antennas is successively focused on each of the points to examine of the object, so as to build, point by point, the microwave image of this object. Microwave imaging systems have, in particular, applications in biomedical engineering for the detection and treatment of tumors, for example, as well as in civil engineering, for the detection of buried objects for example, or for control of materials treated by microwave radiation (polymerization, thawing, drying ...). Furthermore, the antennas known by the name of electronically scanned antennas, used for example in radar and in telecommunications, are organized in a fixed array of antennas which has maximum sensitivity in a variable direction electronically controllable, and implements such a process, corresponding to focusing on a point located at an infinite distance, and defined only by its direction.

A focusing method is already known in the microwave radiation field, in which the signal received by each antenna is phase-shifted in a microwave phase shifter, the phase-shifted signals are added and the summation signal subjected to microwave detection. The phase law, which determines the particular phase shift to be applied to each received signal, is established so that the contributions from the point on which the network is focused are in phase at the time of summation, the contributions from other points having any phases with respect to each other. Thus, after summation, the signal obtained mainly represents the contribution of the focal point.

This known method has, on the one hand, the disadvantage of a difficult implementation from a practical point of view, because it requires the use of as many microwave phase shifters as there are antennas in the array. However, a microwave phase shifter is a relatively complex component, with a high cost price and a large size. In cases where the number of antennas reaches several hundred, even several thousand, this solution is therefore very expensive. In addition, it is clear that the size of the phase shifters conditions the distance between an antenna and the neighboring antennas, that is to say the pitch of the network. However, a not-too-large antenna will give a poor quality image. On the other hand, a drawback of this method is that it does not allow simultaneous focusing on several points.

To overcome these drawbacks, it is known to use a synthetic focusing process. In such a method, each received signal is not phase-shifted, which is subjected directly to coherent microwave detection, that is to say a detection making it possible to know the signal detected in module and in phase. Module and phase of each signal received are then acquired by a computer. The computer performs digital processing of all of this data, in order to extract the contribution from any point of the object. Such processing therefore amounts to synthetic focusing as opposed to analog focusing obtained using microwave phase shifters.

• le rapport signal à bruit est très inférieur au rapport signal à bruit du procédé de focalisation analogique;
• les seuls algorithmes de traitement numériques qu'il est possible de mettre en oeuvre ne correspondent qu'à des géométries de réseau particulières et en nombre limité (géométrie plane, cylindrique, sphérique ...);
• la mise en oeuvre pratique du procédé, qui nécessite autant de détections cohérentes microondes successives que d'antennes, impose l'utilisation d'un multiplexeur microonde à très grand nombre de voies, donc d'un prix de revient et d'un encombrement élevé;
• les détections cohérentes effectuées sur chaque signal rendent impossible le traitement des signaux incohérents, c'est-à-dire des signaux dont la phase varie de façon aléatoire, comme les signaux microondes émis par l'objet lui-même utilisés par exemple en thermographie pour obtenir une image de la température des différents points de l'objet ;
• la focalisation simultanée sur plusieurs points nécessite d'utiliser autant de calculateurs que de points de focalisation.

The synthetic focusing method has the following drawbacks, however:

• the signal to noise ratio is much lower than the signal to noise ratio of the analog focusing method;
• the only digital processing algorithms that it is possible to implement correspond only to specific network geometries and in limited number (plane, cylindrical, spherical geometry, etc.);
• the practical implementation of the method, which requires as many coherent successive microwave detections as there are antennas, requires the use of a microwave multiplexer with a very large number of channels, therefore of a cost price and a large size ;
• the coherent detections carried out on each signal make it impossible to process incoherent signals, that is to say signals whose phase varies randomly, such as the microwave signals emitted by the object itself used for example in thermography for obtain an image of the temperature of the various points of the object;
• simultaneous focusing on several points requires the use of as many computers as there are focal points.

Also known, in the field of acoustic radiation, from the article by R.D. GATZKE et al. "Electronic Scanner for a Phased-Array Ultrasound Transducer" in Hewlett-Packard Journal, volume 34, n ° l2, December l983, pages l3-20, a method of focusing, on at least one point to examine from a radiation source , antennas of an antenna array receiving the radiation from the point with respective reception phase shifts, corresponding to the travel time of said radiation between said point and the respective antennas, in which the signals delivered by the antennas are amplitude modulated by at least one and the same low frequency modulation signal, respectively with modulation phase shifts corresponding to the reception phase shifts.

In this known method, the modulated signals are each delayed by a specific delay before undergoing an addition. This therefore implies the use of as many delay circuits as there are antennas in the network and complicates the law followed by the modulation phase shifts.

To overcome the drawbacks of these known methods, the applicant has sought a focusing method of analog type, which can adapt to any geometry of the network, but does not require the use of a large number of elementary circuits, such as the microwave phase shifters of the known microwave process or the delay circuits of the known acoustic process.

• ledit rayonnement est un rayonnement microonde,
• chaque déphasage de modulation correspond à chaque déphasage de réception pour que leur somme soit la même, quelle que soit l'antenne délivrant le signal,
• les signaux modulés sont additionnés en un signal de sommation,
• la composante microonde du signal de sommation est détectée, et,
• le signal détecté est démodulé par démodulation synchrone.

The invention, which was made at the Signals and Systems Laboratory of the Higher School of Electricity, mixed unit l4 of the National Center for Scientific Research, therefore relates to a focusing process, on at least one point to be examined of a radiation source, antennas of an array of antennas receiving radiation from the point with respective reception phase shifts, corresponding to the travel time of said radiation between said point and the respective antennas, in which the signals delivered by the antennas are amplitude modulated by at least one and the same low frequency modulation signal, respectively with modulation phase shifts corresponding to the reception phase shifts, characterized in that:

• said radiation is microwave radiation,
• each modulation phase shift corresponds to each reception phase shift so that their sum is the same, whatever the antenna delivering the signal,
• the modulated signals are added up into a summation signal,
• the microwave component of the summation signal is detected, and,
• the detected signal is demodulated by synchronous demodulation.

The method of the invention does not use any microwave phase shifter. This is the low modulation signal frequency which is out of phase in low frequency phase shifters, moreover very simple. This result is obtained by modulating in amplitude the signals delivered by antennas. However, as will be discussed below, it is possible to use space-saving and inexpensive microwave modulators.

The method of the invention being an analog type method, it can adapt to unconventional network geometries, it makes it possible to compensate for irregularities in the alignment of the antennas of the network, for example, it has a better signal ratio noise than the synthetic focusing process, and it can adapt to incoherent radiation.

Advantageously, the signals delivered by the antennas are amplitude modulated by a low frequency modulation signal of the square type, with modulation phase shifts.

In this case, the modulation can be carried out using microwave switches, of the PIN diode type for example, therefore with a low cost and size.

• des moyens pour engendrer un signal basse fréquence,
• des moyens pour déphaser le signal basse fréquence de déphasages de modulation correspondant aux déphasages de réception, chaque déphasage de modulation correspond à chaque déphasage de réception pour que leur somme soit la même, quelle que soit l'antenne délivrant le signal
• des moyens pour moduler en amplitude les signaux délivrés par les antennes, par le signal basse fréquence avec les déphasages de modulation respectivement,
• des moyens pour additionner les signaux modulés, délivrant un signal de sommation,
• des moyens pour détecter la composante microonde du signal de sommation, et,
• des moyens pour démoduler le signal détecté par démodulation synchrone.

The invention also relates to a device for implementing the method of the invention of focusing on at least one point to be examined from a radiation source, antennas of an array of antennas receiving the radiation from the point with respective reception phase shifts, corresponding to the travel time of said radiation between said point and the respective antennas delivering signals, characterized in that said radiation is microwave radiation, and provision is made:

• means for generating a low frequency signal,
• means for shifting the low frequency signal of modulation phase shifts corresponding to the reception phase shifts, each modulation phase shift corresponds to each reception phase shift so that their sum is the same, whatever the antenna delivering the signal
• means for amplitude modulation of the signals delivered by the antennas, by the low frequency signal with the modulation phase shifts respectively,
• means for adding the modulated signals, delivering a summation signal,
• means for detecting the microwave component of the summation signal, and,
• means for demodulating the detected signal by synchronous demodulation.

Naturally and as will be seen in the description, the invention can also be implemented for focusing the antennas of an array of transmit antennas.

• la figure l représente un schéma par blocs d'un réseau d'antennes de réception et d'un dispositif de focalisation selon l'invention ;
• la figure 2 représente un schéma détaillé du dispositif de focalisation de la figure l ;
• la figure 3 représente un schéma détaillé du circuit de détection cohérente microonde de la figure 2 ;
• la figure 4 représente un schéma détaillé du circuit de détection cohérente basse-fréquence de la figure 2 ;
• la figure 5 représente un schéma détaillé du circuit de déphasage de la figure 2;
• la figure 6 représente un dispositif de focalisation simultanée sur deux points des antennes d'un réseau d'antennes de réception;
• la figure 7 représente un dispositif de focalisation des antennes d'un réseau d'antennes d'émission;
• la figure 8 représente une première variante du dispositif de focalisation de la figure 2;
• la figure 9 représente une deuxième variante du dispositif de focalisation de la figure 2, et,
• la figure l0 représente un diagramme temporel des signaux du dispositif de focalisation de la figure 2.

The present invention will be better understood with the aid of the following description of several implementations of the method of the invention and of several embodiments of the device of the invention, made with reference to the appended drawings, in which:

• FIG. 1 represents a block diagram of an array of reception antennas and of a focusing device according to the invention;
• 2 shows a detailed diagram of the focusing device of Figure l;
• FIG. 3 represents a detailed diagram of the coherent microwave detection circuit of FIG. 2;
• FIG. 4 represents a detailed diagram of the low-frequency coherent detection circuit of FIG. 2;
• FIG. 5 represents a detailed diagram of the phase shift circuit of FIG. 2;
• FIG. 6 represents a device for simultaneous focusing on two points of the antennas of an array of reception antennas;
• FIG. 7 represents a device for focusing the antennas of an array of transmitting antennas;
• Figure 8 shows a first variant of the focusing device of Figure 2;
• FIG. 9 represents a second variant of the focusing device of FIG. 2, and,
• FIG. 10 represents a time diagram of the signals of the focusing device of FIG. 2.

With reference to FIG. 1, in order to obtain a microwave image of an object 20, there is a series of J antennas l on a surface, here a plane P, exposed to microwave electromagnetic radiation coming from the object 20 The antenna l of row j receives a microwave signal s _j . A focusing device 30 is provided with J inputs receiving the J signals s _l , ..., s _j , ..., and s _J. It is also provided with two outputs delivering two useful signals U _X and V _X , as well as a control input receiving, here by means of a parallel bus, a control signal Δ _X.

The object 20 may itself be the source of radiation received by the antennas 1, or it may act as a secondary source, that is to say as a reflector of radiation emitted by a primary microwave source, intended to illuminate object 20, and not shown in FIG. 1 for the sake of simplicity. In the following, it will be assumed, unless otherwise indicated, that the microwave radiation received by the antennas l is monochromatic of frequency f , either because the radiation source is itself monochromatic of frequency f , or because, the source emitting a radiation in a certain frequency band, a selective focusing device 30 is used, centered on the frequency f .

The series of J antennas 1 arranged regularly in the plane P is called, by analogy with the networks encountered in optics, antenna array. The microwave image is obtained by successively focusing the array of antennas l on each of the points to be examined X of the object 20, during a sequential scanning, point by point, of this object.

By focusing the antennas l of the antenna array on a point X, is meant control of the focusing device 30 using the control signal Δ _X so that the useful signals U _X and V _X at the output of the device are only representative of microwave radiation from point X.

A sequential scanning device, not shown, generates the successive control signals Δ _X and a display device, not shown, synchronized by the sequential scanning device, collects the signals U _X and V _X. The sequential scanning device and the viewing device are of the conventional type used in known imaging systems.

Before describing the focusing device 30 according to the invention, it is useful to clarify the principle of prior art methods.

In these methods, each signal s _j is subjected to a microwave phase shift of a determined angle Δ _X (j), and the J signals are phase-shifted. The law Δ _X (j) which determines the phase shift angle for each signal s _j , called the phase law for point X, is established so that the contributions coming from point X, finding themselves in phase at the time of the summation, interfere constructively, while the contributions coming from the other points, having any phases with respect to each other, interfere destructively. Thus, the result of the summation mainly represents the contribution of point X. We therefore focus the array of antennas l on point X by imposing the phase law Δ _X (j) using the signal Δ _X. The phase law Δ _X (j) can be deduced from knowing the lengths of the paths connecting point X to each of the antennas l.

The focusing device 30 according to the invention will now be described, with reference to FIG. 2. In this figure, each reference 2 designates a microwave switch. There are as many switches 2 as there are inputs, that is to say here J switches.

The switch 2 of row j is provided with a microwave input receiving the signal s _j and a microwave output delivering the signal s ′ _j , as well as a control input receiving a signal C _j .

A microwave summing device 3 is provided with J inputs receiving the J signals s ′ _l , ..., s ′ _j , ... and s ′ _J , and an output delivering a signal s .

A coherent microwave detection circuit 6 is provided a signal input receiving the signal s , a control input and two outputs delivering DA and DB signals.

A microwave oscillator 4 is provided with an output delivering a signal r of coherent microwave detection, connected to the control input of circuit 6. The signal r is a sinusoidal signal of frequency f .

A low frequency coherent detection circuit 8 is provided with two signal inputs receiving the signals DA and DB, a control input and two outputs delivering the signals U _X and V _X.

A low frequency oscillator ll is provided with an output delivering a modulation signal D, connected to the control input of circuit 8. The signal D is a sinusoidal signal of frequency F, of value between substantially a few kilohertz and substantially a few megahertz.

A phase shift circuit l2 is provided with an input receiving the signal D, J commando inputs connected to the parallel duo receiving the control signal Δ _X , and J outputs delivering the J signals C _l , ..., C _j , ... and C _J.

With reference to FIG. 3, the microwave coherent detection circuit 6 comprises two mixers 6l and 62 and a phase shifter 63. The mixers 6l and 62 are of the type comprising two inputs, and one output delivering a signal equal, at all times, to the produces signals received on both inputs. These are devices known to those skilled in the art as ring modulators or balanced mixers.

The mixer 6l receives on an input the signal s and on the other input signals r , and outputs DA signal.

The mixer 62 receives on one input the signal s and on the other input the signal r phase shifted by an angle equal to π / 2, in the phase shifter 63. The mixer 62 outputs the signal DB.

Referring to Figure 4, the low frequency coherent detection circuit 8 includes four mixers 8l, 82, 83 and 84, two phase shifters 85 and 85 ′, a subtractor 86 and an adder 88, and two low-pass filters 87 and 89 .

The mixers 8l to 84 are of a type comparable to that of the mixers 6l and 62. The mixer 8l receives on one input the signal DA and on the other input the signal D, and it outputs a signal SA. The mixer 82 receives on one input the signal DB and on the other input the signal D phase shifted by an angle equal to π / 2 in the phase shifter 85, and it outputs the signal SB. The mixer 83 receives on one input the signal DA and on the other between the signal D phase shifted by an angle equal to π / 2 in the phase shifter 85 ′, and it outputs the signal SC. The mixer 84 receives the signal DB on one input and the signal D on the other input, and outputs the signal SD.

The subtractor 86 receives on its two inputs the signals SA and SB; its output is connected to filter 87, which outputs the signal U _X.

The adder 88 receives on its two inputs the signals SC and SD; its output is connected to filter 89 which outputs the signal V _X.

Referring to Figure 5, the phase shift circuit l2 includes J controllable phase shifters l2l. The phase shifter l2l of rank j is provided with a signal input receiving the signal D, with a control input for the angle Δ _X (j) of phase shift, receiving a control signal also called Δ _X (j) in for the sake of simplicity, and of a signal output delivering the signal C _j , phase shifted by the angle Δ _X (j) relative to the modulation signal D.

The J control inputs of the J phase shifters l2l constitute the parallel bus to which is applied the signal Δ _X composed of the J signals Δ _X (j).

The switches 2 are here PIN diode switches, well known to those skilled in the art.

The adder 3 and the mixers 6l and 62 are circuits of the type known by a person skilled in the art for microwave use, while the mixers 8l to 84, the subtractor 86 and the adder 88, the low-pass filters 87 and 89 and phase shifters 12l are circuits of the type known to those skilled in the art for use at low frequency.

The operation of the focusing device 30 which has just been described is now explained, with reference to FIG. 10.

et que le signal D, de fréquence F, délivré par l'oscillateur ll, est de la forme $$\text{D = cos (2 πFt)}$$

It is assumed that the signal r , not shown, of frequency f , delivered by the microwave oscillator 4, is of the form $$\text{r = cos (2 π ft)}$$

and that the signal D, of frequency F, delivered by the oscillator ll, is of the form $$\text{D = cos (2 πFt)}$$

The signal C _j at the output of the phase shifter l2l of rank j is then worth: $${\text{VS}}_{\text{j}}{\text{= cos (2 πFt + Δ}}_{\text{X}}\text{(j))}$$

expression dans laquelle A_j est l'amplitude de signal s_j et δ_j son angle de phase par rapport au signal r.Suppose then that the microwave signal received by the antenna of rank j is of the form: $${\text{s}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{cos (2 πft + δ}}_{\text{j}}\text{)}$$

expression in which A _j is the signal amplitude s _j and δ _j its phase angle with respect to the signal r .

dans cette expression, la fonction échelon Ech[C_j] est égale à l si C_j est positif et égal à 0 si C_j est négatif.Then the signal s ′ _j at the output of the switch 2 of row j is written: $${\text{s ′}}_{\text{j}}{\text{= s}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]}$$

in this expression, the echelon function Ech [C _j ] is equal to l if C _j is positive and equal to 0 if C _j is negative.

Thus, we can say that the signal s ′ _j is amplitude modulated by a square type modulation signal, as shown in Figure l0.

c'est-à-dire : $${\text{r.s′}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.[cos (4 πft + δ}}_{\text{j}}{\text{) + cos (δ}}_{\text{j}}{\text{)]. Ech [C}}_{\text{j}}\text{]}$$

The product of the signal r by the component s ′ _j of the signal s , carried out, in the microwave coherent detection circuit 6, by the mixer 61, gives a signal: $${\text{rs ′}}_{\text{j}}{\text{= cos (2 πft). AT}}_{\text{j}}{\text{cos (2 πft + δ}}_{\text{j}}{\text{). Ech [C}}_{\text{j}}\text{]}$$

that is to say : $${\text{rs ′}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.[cos (4 πft + δ}}_{\text{j}}{\text{) + cos (δ}}_{\text{j}}{\text{)]. Ech [C}}_{\text{j}}\text{]}$$

The first component of this signal, at frequency 2f, is here filtered by the mixer 6l. If this were not the case, a low pass filter would eliminate this component. Thus, the contribution DA _j of the signal s _j of the signal DA at the output of the microwave coherent detection circuit 6 is worth: $${\text{DA}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/ 2. cos δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]}$$

For the same reasons, and taking into account the phase shift of π / 2 introduced by the phase shifter 63, the contribution DB _j of the signal s _j to the signal DB at the output of the coherent microwave detection circuit 6 is worth: $${\text{DB}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/ 2. sin δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]}$$

$${\text{SB}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.sin δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{] . cos (2 πFt +}\frac{\text{π}}{\text{2}}\text{)}$$

$${\text{SC}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.cos δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{] . cos (2 πFt +}\frac{\text{π}}{\text{2}}\text{)}$$

$${\text{SD}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.sin δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{] . cos (2 πFt)}$$

In the low frequency coherent detection circuit 8, the contributions SA _j , SB _j , SC _j and SD _j (the latter two not shown for the sake of simplicity) of the signal s _j to the signals SA, SB, SC and SD are equal to: $${\text{HER}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.cos δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]. cos (2 πFt)}$$

$${\text{SB}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.sin δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]. cos (2 πFt +}\frac{\text{π}}{\text{2}}\text{)}$$

$${\text{SC}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.cos δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]. cos (2 πFt +}\frac{\text{π}}{\text{2}}\text{)}$$

$${\text{SD}}_{\text{j}}{\text{= A}}_{\text{j}}{\text{/2.sin δ}}_{\text{j}}{\text{. Ech [C}}_{\text{j}}\text{]. cos (2 πFt)}$$

et des harmoniques.However, the function Ech [C _j ] can be broken down into a fundamental: $$\frac{\text{l}}{\text{π}}{\text{cos (2 πFt + Δ}}_{\text{X}}\text{(j))}$$

and harmonics.

ou non, donnera naissance à une composante continue ou lentement variable.Only the fundamental, multiplied by the signal D, out of phase with $$\frac{\text{<figure-callout id="2" label="π" filenames="imgf0006.png" state="{{state}}">π</figure-callout>}}{\text{2}}$$

or not, will give rise to a continuous or slowly variable component.

$${\text{V}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/4π [cos δ}}_{\text{j}}{\text{. sin(Δ}}_{\text{X}}{\text{(j)) + sin δ}}_{\text{j}}{\text{. cos (Δ}}_{\text{X}}\text{(j)]}$$

soit $${\text{U}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/4π. cos [ δ}}_{\text{j}}{\text{+ Δ}}_{\text{X}}\text{(j)]}$$

$${\text{V}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/4π. sin [δ}}_{\text{j}}{\text{+ Δ}}_{\text{X}}\text{(j)]}$$

Thus, after passing through the subtractor 86 and into the adder 88, and low-pass filtering in the filters 88 and 89, the contributions U _Xj and V _Xj of the signal s _j to the signals U _X and V _X are written: $${\text{U}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/ 4π [cos δ}}_{\text{j}}{\text{. cos (Δ}}_{\text{X}}{\text{(j)) - sin δ}}_{\text{j}}{\text{. sin (Δ}}_{\text{X}}\text{(j))]}$$

$${\text{V}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/ 4π [cos δ}}_{\text{j}}{\text{. sin (Δ}}_{\text{X}}{\text{(j)) + sin δ}}_{\text{j}}{\text{. cos (Δ}}_{\text{X}}\text{(j)]}$$

is $${\text{U}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/ 4π. cos [δ}}_{\text{j}}{\text{+ Δ}}_{\text{X}}\text{(j)]}$$

$${\text{V}}_{\text{Xj}}{\text{= A}}_{\text{j}}{\text{/ 4π. sin [δ}}_{\text{j}}{\text{+ Δ}}_{\text{X}}\text{(j)]}$$

It appears that the phase shift Δ _X (j) of the modulation signal is added to the phase shift δ _j of the microwave signal. So everything happens as if the signal s _j was phase shifted by an angle Δ _X (j) in a microwave phase shifter. Thus, if we choose a phase law so that the angle Δ _X (j) corresponds to the angle δ _j so that their sum remains constant whatever j , the signals U _X and V _X , equal to the sum of all U _Xj and V _Xj respectively, are well representative of the radiation emitted by the focusing point X.

The focusing device 70 shown in Figure 6 allows the simultaneous focusing on two points X and Y of the object 20, to continuously observe what is happening at these two particular points without having to form a complete image, for example to follow the evolution of their temperature in the case of certain biomedical applications.

For this purpose, the focusing device 70 is provided with two buses receiving the signals Δ _X and Δ _Y representative phase laws Δ _X (j) and Δ _Y (j) corresponding to the points X and Y to be observed. The focusing device 70 is also provided with two groups of two outputs continuously supplying signals representative of the points X and Y, here for example the previously defined signals U _X , V _X , U _Y and V _Y.

The focusing device 70 differs from the device 30 of FIG. 2 essentially in that it comprises two oscillators 7ll and 7ll ′ delivering two signals D _l and D₂ respectively, of frequency F _l and F₂ respectively. The two signals D _l and D₂ are of the same type as the signal D already encountered.

The output signal from the oscillator 7ll is applied to a phase shift circuit 7l2 analogous to the circuit l2 in FIG. 2. The circuit 7l2 is controlled by the signal Δ _X.

Similarly, the output signal from the oscillator 7ll ′ is applied to a phase shift circuit 7l2 ′ analogous to the circuit l2 in FIG. 2, controlled by the signal Δ _Y.

Each circuit 7l2 and 7l2 ′ delivers a set of J modulation signals analogous to the signals C _l , ..., C _j ..., and C _J of FIG. 2. The two modulation signals of rank j control two switches 72 mounted in parallel downstream of an antenna 7l, of row j . The switches 72, 2J in number, and the antennas 71, the number of J, are similar to the switches 2 and the antennas 1 in FIG. 2.

The J output signals of the J groups of the two switches 72 in parallel are added in a microwave summator 73, which delivers a signal s .

A coherent microwave detection circuit 76, analogous to circuit 6 in FIG. 3, receives the signal s on its signal input and deliberates two signals DA and DB on its two outputs.

A microwave oscillator 74, analogous to oscillator 4 of FIG. 2, delivers a signal r to the control input of circuit 76.

Two low frequency coherent detection circuits 78 and 78 ′ are provided, analogous to circuit 8 in FIG. 4. Each circuit 78 and 78 ′ receives the signals DA and DB on its two signal inputs, and, on its control input the signal D _l and signal D₂ respectively. Circuit 78 outputs the signals U _X and V _X , and circuit 78 ′ signals U _Y and V _Y.

The operation of the focusing device 70 is as follows. Because the frequencies F _l and F₂ of the signals D _l and D₂ are different, the low frequency coherent detection circuit 78 demodulates only the components of the signals DA and DB modulated at the frequency F _l in the switches 72, that is ie those which correspond to the phase shift law Δ _X , determined by the phase shift circuit 7l2, focusing the network on the point X. For the same reason, the circuit 78 ′ demodulates only the components of the signals DA and DB modulated at the frequency F₂, that is to say those which correspond to the phase shift law Δ _Y , focusing the grating on the point Y.

Naturally, it is possible to extend the focusing device which has just been described to the simultaneous focusing on a number of points greater than two, by choosing the different modulation frequencies in order to avoid any risk of intermodulation.

Naturally, the focusing method of the invention can be extended to imaging systems in which a focused antenna array is used to illuminate a point of the object to be observed and an unfocused antenna array, or even a single omnidirectional antenna, to receive radiation from the illuminated point.

FIG. 7 represents a device implementing such a method. Block 50 represents the focusing device of a network of K transmitting antennas 5l to focus on the point X ′ of the object 20 ′.

The focusing device 50 comprises a microwave oscillator 54, delivering a microwave emission signal e , of frequency f .

Each antenna 5l is connected to the output of the oscillator 54 by means of a microwave switch 52, of the same type as the switches 2 in FIG. 2. The switch 52 of rank k is provided with an input of command receiving a signal C _k .

A phase shift circuit 5l2, analogous to the phase shift circuit l2 in FIG. 5, is provided with a signal input and with K outputs delivering the signals C _l , ..., C _k , ..., and C _K each signal C _k being out of phase with respect to the signal received on the input of circuit 5l2 by an angle Δ _X (k) controlled by the signal Δ _X applied to the control bus of block 5l2.

An oscillator 511, analogous to the oscillator ll of FIG. 2, delivers a signal D ′ of frequency modulation F, at the input of the block 5l2.

On reception, a reception antenna 40, here unique, receives the signals coming from the object 20 ′. It is followed by a coherent microwave detection circuit 56, analogous to circuit 6 in FIG. 3. Circuit 56 receives, on its control input, the output signal from oscillator 54. The two outputs of circuit 56 are connected to a coherent detection circuit 58, analogous to circuit 8 in FIG. 4. The control input of circuit 58 receives the modulation signal D′. Circuit 58 outputs the signals U _X and V _X.

The operation of the focusing device 50 is similar to that of the device 30. The phase shifts Δ _X (k) introduced on the modulation signals of the microwave signals emitted produce the same effect, on the signal received and subjected to coherent microwave microwave detection. f , in circuit 56, and at coherent low frequency detection at frequency F in circuit 58, that phase shifts Δ _X (k) introduced on the microwave signals transmitted. If these phase shifts Δ _X (k) are chosen to correspond to the phasing, at point X, of the signals coming from the K antennas 5l, the transmission network has therefore been focused on point X.

Obviously, it is possible to use simultaneously a focused transmission network using switches controlled at the frequency F _E and a focused reception network using switches controlled at the frequency F _R. Under these conditions, two successive low frequency coherent detections should be carried out, one at the frequency F _E , the other at the frequency F _R. As these coherent detections are followed by a low pass filtering to keep only the low part of the frequency spectrum of the output signal, it is equivalent to carry out a single coherent low frequency detection at the beat frequency F _B between F _E and F _R , that is: $${\text{F}}_{\text{B}}{\text{= | F}}_{\text{E}}{\text{- F}}_{\text{R}}\text{|}$$

Such a situation is encountered, for example, in imaging systems using crossed linear networks. In such systems, linear networks are used instead of planar networks, the direction of the transmission network being perpendicular, for example, to the direction of the reception network. These crossed linear arrays are well known to those skilled in the art for their better longitudinal spatial resolution and the reduced number of antennas used.

Obviously, it is possible, with the method of the invention, to focus the antennas of a transmission network simultaneously on several points, by transposing the device 70 of FIG. 6 to transmission.

In the foregoing description, it has been implicitly considered, in the case for example of a reception antenna array, that the output of each antenna was a microwave signal guided by a guide structure of the waveguide, cable type. coaxial or ribbon line, for example. In this case, the summation is carried out by a microwave summing device, like the summing device 3 of FIG. 2, provided with J inlet access and an outlet access, each access being connectable to a guide structure of the type defined here. -above.

This is not compulsory, and FIG. 8 shows for example a device in which the receiving antennas are dipole antennas l ′, regularly arranged on a panel l00 made of insulating material. The panel l00 is placed in front of a single antenna 4l, which plays the role of summing of the microwave signals picked up and radiated again by the antennas l ′, these signals no longer being, as previously, supported by a guide structure. In this case, the switches may be simple diodes 2 ′, the switching signals C _l , ..., C _j , ... and C _J being for example applied by means of connections 2l which are not very disturbing for the electromagnetic field. In a known manner, such connections are made, for example, of carbon wires so as to be sufficiently resistive to have only a slight influence on the electromagnetic field. One can also consider, to remove the connections 2l, to use diodes 2 'photoconductive switched using light signals applied for example using a laser beam.

The antenna 4l, playing the role of summing device, is directly connected to a coherent microwave detection circuit, identical to the circuit 6 in FIG. 2, in the case of a network of reception antennas. The rest of the device is unchanged.

In order to reduce the number of mixers used in the device 30 of FIG. 2, it is possible, instead of having each mixer perform the same task continuously, having them perform different tasks over time. Thus, the focusing device 90 of FIG. 9 represents a variant of the device of the invention, using only two mixers instead of six.

With reference therefore to FIG. 9, a mixer 96l, identical to the mixer 6l in FIG. 3, is provided with a first input receiving the signal s at the output of the summator 3, with a second input and with an output.

A mixer 98l, identical to the mixer 8l in Figure 4, is provided with a first input connected to the output of the mixer 96l, a second input and an output connected to the input of a low-pass filter 987, identical to the low-pass filter 87 of FIG. 4 .

The output of an oscillator 94, identical to the oscillator 4 of FIG. 2, is connected to the second input of the mixer 96l via a controllable phase shifter 963. The output of an oscillator 9ll, identical to the the oscillator ll of FIG. 2 is connected to the second input of the mixer 981 via a controllable phase shifter 985.

The controllable phase shifters 963 and 985 are likely to phase shift by an angle equal to 0, or equal to π / 2, as a function of a signal applied to the control input with which each of them is provided.

A processing and control circuit 9l, for example with a microprocessor, and provided with two outputs connected to the control inputs of the phase shifters 963 and 985, an input connected to the output of the low-pass filter 987, and two outputs delivering the signals U _X and V _X.

The operation of the focusing device 90 is as follows: the processing and control circuit 9l controls the phase shifters 963 and 985 sequentially, so that the previously defined signals SA, SB, SC and SD appear the year after the other at the input of the filter 987. The circuit 9l stores the various filtered signals and processes the corresponding data to add them and deliver the signals U _X and V _X previously defined.

In the above description, we have always considered the case where the signals picked up are coherent signals, that is to say periodic signals of defined phase, to which coherent microwave detection can be applied, as in circuits 6, 76, 6 ′ and 96l-63, as the case may be. The invention is not limited to such coherent radiation and can also be applied to thermography, for example.

In this case, an image representative of the temperatures of the various points of the object is constructed, from microwave signals of which the object is itself the source. These signals being inconsistent, that is to say of random phase, they must be detected with particular detection devices of known type, for example quadratic detection devices with or without prior frequency changes. The devices of FIGS. 2, 6, 8 and 9 must therefore be modified, in this case, so that the coherent microwave detection circuits 6, 76, 6 ′ and 96l-963 are replaced by suitable devices.

Finally, in all of the foregoing, it has been considered that the microwave signals emitted or received were subjected to amplitude modulation by all or nothing using switches placed on the microwave channels. It is clear that this solution is materially the simplest to implement. Such all-or-nothing modulation amounts to a product by a modulation signal of the square signal type. The spectrum of such a signal consists of a fundamental component and harmonics. However, as has been seen, given the low-pass filtering carried out at the end of the chain, the influence of the harmonics is zero. The same result would therefore be obtained, except for a level factor, by replacing the microwave switches 2 by produced modulators, for example ring modulators, controlled by the signals C _l , ... C _J , ... and C _J.

The device 70 for simultaneous focusing on several points of FIG. 6 could then be modified by placing only one modulator downstream of each antenna, and by controlling this modulator with the sum of the two corresponding signals coming from the phase shift circuits 7l2 and 7l2 ′.

In the above description, the antennas are generally organized on a surface to form an array. Of course, this is not compulsory, and, as has been pointed out for crossed networks, the antennas can be organized in a line, straight or curved, to form a linear network.

In the case of simultaneous focusing on several points, it is not compulsory to use as many switches in parallel on each channel as there are focal points. A single switch can be used controlled by a suitable signal, for example the signal resulting from the product of the step functions relating to each of the phase shifted modulation signals.

Claims (11)

1. A method for focusing, on at least one point (X) to be examined of a radiating source (20), antennae (1;71;1′) of an antenna array receiving the radiation from the point with respective reception phase shifts (δ_j) corresponding to the time of path of said radiation between said point (X) and the respective antennae, in which signals (s_j) delivered by the antennae are amplitude modulated by at least a same low frequency modulation signal (D), respectively with modulation phase shifts (Δx(j)) corresponding to the reception phase shifts, characterized in that :

- said radiation is a microwave radiation,

- each modulation phase shift (Δ_X(j)) corresponds to each reception phase shift (δ_j) so that their sum (δ_j + Δ_X(j)) is the same whatever the antenna delivering the signal (s_j),

- the modulated signals (s′_j) are added in a summation signal (s),

- the microwave component of the summation signal (s) is detected, and

- the detected signal (DA,DB) is demodulated by synchroneous demodulation.

2. The method as claimed in claim 1, wherein said signals (s_j) delivered by the antennae are amplitude modulated by a low frequency modulation signal of square type, with modulation phase shifts (Δ_X(j)).

3. The method as claimed in claim 1 or 2, wherein the detected signal (DA,DB) is demodulated by multiplying, on the one hand, by the modulation signal (D), on the other by the modulation signal (D) phase shifted by π/2, then by low pass filtering.

4. The method as claimed in any of claims 1 to 3, wherein the microwave component of the summation signal (s) is detected by multiplying, on the one hand, by a microwave detection signal (r), on the other, by this microwave detection signal (r) phase shifted by π/2, then by low pass filtering.

5. A focusing device for implementing the method of claim 1, for focusing, on at least one point (X) to be examined of a source (20) of radiation, antennae (1;71,1′) of an antenna array receiving the radiation from the point with respective reception phase shifts (δ_j), corresponding to the time of path between said point (X) and the respective antennae delivering signals (s_j), characterized in that said radiation is a microwave radiation, and these is provided :

- means (11;711;711′;911) for generating a low frequency signal (D;D₁,D₂)

- means (12;712;712′) for phase shifting the low frequency signal (D;D₁,D₂) by modulation phase shifts ( Δ_X(j)) corresponding to the reception phase shifts (δ_j), each modulation phase shift ( Δ_X(j)) corresponds to each reception phase shift (δ_j) so that their sum (δ_j + Δ_X(j)) is the same whatever the antenna delivering the signal (s_j),

- means (2;72;2′) for modulating in amplitude the signals (s_j) delivered by the antenna, by the low frequency signal (D;D₁,D₂) with modulation phase shifts ( Δ_x(j)), respectively,

- means (3;73;41) for adding the modulated signals (s′_j), delivering a summation signal (s),

- means (4,6;74,76;94,963,961,6′) for detecting the microwave component of the summation signal (s), and,

- means (8;78,78′,985,981) for demodulating the detected signal by synchroneous demodulation .

6. The device as claimed in claim 5, wherein said means for amplitude modulating the signals (s_j) delivered by the antennae comprise microwave switches (2;72;2′).

7. The device as claimed in claim 5 or 6, wherein said means (8;78,78′,985,981) for demodulating the detected signal include :

- means (85,85,985) for phase shifting said low frequency signal (D;D₁,D₂) by an angle equal to π/2,

- means (81-84;981) for multiplying the detected signal, on the one hand by the modulation signal (D;D₁,D₂) and on the other by the phase shifted modulation signal, and

- means (87,89;987) for filtering the low frequency components of the multiplied detected signal.

8. The device as claimed in any of claim 5 to 7, wherein said means (4,6;74,76;94,963,961,6′) for detecting the microwave component of the summation signal (s) include :

- means (4,74,94) for generating a microwave detection signal (r),

- means (63;963) for phase shifting said microwave detection signal (r) by an angle equal to π/2,

- means (61,62;961) for multiplying the summation signal (s), on the one hand, by the microwave detection signal (r) and, on the other, by the phase shifted microwave detection signal and for filtering the low frequency components from said multiplied summation signal.

9. A method of focusing, on at least one point (X′) to be examined of an object (20′) illuminated by microwave radiation, antennae (51) of a first antenna array transmitting the radiation towards the point with respective transmission phase shifts corresponding to the time of path of said radiation between the respective antennae and said point (X′), at least one antenna (40) receiving the radiation from said point (X′), characterized in that :

- the signals delivered to the antennae are obtained from the same microwave transmission signal (e) modulated in amplitude by at least a first low frequency modulation signal (D′), respectively with modulation phase shifts (Δ_X(k)) corresponding to transmission phase shifts so that their sum is the same whatever the antenna,

- the microwave component of the received signal is detected, and

- the detected signal is demodulated by synchroneous demodulation.

10. The method as claimed in claim 9, wherein several antennae receiving radiation from said point (X′) are provided, arranged so as to form a second array, each antenna receiving the radiation from the point with respective reception phase shifts, wherein :

- the signals delivered by the reception antennae are modulated in amplitude by at least one same second low frequency modulation signal, respectively with modulation phase shifts corresponding to the reception phase shifts, each modulation phase shift (Δ_X(j)) corresponding to each reception phase shift (δ_j) so that their sum (δ_j + Δ_X(j)) is the same whatever the antenna delivering the signal (s_j),

- the modulated received signals are added into a summation signal before detection of the microwave component of the received signal.

11. The method as claimed in claim 10, wherein the detected signal is demodulated by multiplying by at least one signal at the beat frequency (|F_E - F_R|), between the frequency (F_E) of the first low frequency modulation signal and the frequency (F_R) of the second low frequency modulation signal.

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