EP0572293A1 - Pulse compression device, particularly in high frequency transmission - Google Patents

Abstract

The invention relates to a pulse compression device. The pulses to be compressed being frequency-modulated, the device includes at least one transmission line (34) whose cutoff frequency varies along its axis of propagation and means (33) for separating an incident wave and a reflected wave, which means are loaded by the transmission line (34) and receive the pulses to be compressed which reflect along the transmission line, the variation in the cutoff frequency and the frequency modulation of the pulses being tuned. Application: high-power pulse compression in microwave transmission. <IMAGE>

Description

The present invention relates to a pulse compression device, in particular in microwave transmission.

It applies in particular to stealth target detection radars. More generally, it applies to all radars whose function requires the emission of high peak powers during very short duration pulses.

To fight for example against stealth targets, it is necessary to emit pulses of shorter duration than the reaction times, themselves very short, of these stealth targets. It is then necessary, for example, to compress the pulses transmitted in a ratio of approximately 10, at constant average transmitted power. This can lead to compressing, for example, pulses with a duration of 100 ns to 1 MW of peak power into pulses with a duration of around 10 ns at 10 MW of peak power.

Analog compression of frequency modulated pulses is a well known technique in reception. Compression at transmission and at high power level has already been achieved. One embodiment is described in particular in the article "SLED: A METHOD OF DOUBLING SLAC'S ENERGY" by Z.D. Farkas, H.A. Hogg, G.A. Loew and P.B. Wilson, Stanford Linear Accelerator Center, Stanford University, 1974. The device described uses the filling time of a resonant cavity as a delay at the front edge of the pulse relative to the back edge. This method is limited by overvoltages due to resonance, by losses in the cavity and by strong electric fields proportional to the overvoltage in the cavity.

Due to the limited gain of this structure, another device called "Binary Power Compressor" was studied. It is described in the article "Binary Peak Power Multiplier and its application to Linear Accelerator Design" by Z. Farkas - IEEE Trans MTT 34 - 1986, page 1036. He uses two amplification channels in parallel, one of the channels is delayed compared to the other before phase recombination and reduction by two of the pulse duration.

To significantly reduce the duration of the pulses, it is therefore necessary to use a large number of sections of this type, which complicates the device and makes it bulky and expensive.

The invention aims to overcome the aforementioned drawbacks, in particular by making it possible to compress pulses significantly with high possible peak powers.

To this end, the subject of the invention is a device for compressing pulses, the pulses being frequency modulated, characterized in that it comprises at least one transmission line whose cut-off frequency varies along its axis of propagation, a non-transmitted signal being reflected, and means for separating an incident wave and a reflected wave, charged by the transmission line and receiving the pulses to be compressed which are reflected along the transmission line, the variation of the cutoff frequency and frequency modulation of the pulses to be compressed being tuned.

The main advantages of the invention are that it is compact, that it is simple to implement and that it allows significant compression rates.

• la figure 1a, un schéma de base explicitant le fonctionnement d'un dispositif selon l'invention ;
• la figure 1b, un exemple de réalisation de ligne à structure périodique ;
• la figure 1c, un diagramme de fréquence de la ligne précitée ;
• la figure 2a, un exemple théorique d'impulsion à comprimer ;
• la figure 2b, l'impulsion précédente comprimée;
• la figure 3a, le synoptique d'un premier mode de réalisation possible d'un dispositif selon l'invention ;
• la figure 3b, un exemple d'impulsion à comprimer ;
• la figure 3c, l'impulsion précédente comprimée ;
• la figure 4, une structure possible pour une ligne utilisée dans un dispositif selon l'invention ;
• la figure 5, un deuxième mode de réalisation possible d'un dispositif selon l'invention.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the appended drawings which represent:

• Figure 1a, a basic diagram explaining the operation of a device according to the invention;
• FIG. 1b, an exemplary embodiment of a line with periodic structure;
• Figure 1c, a frequency diagram of the above line;
• FIG. 2a, a theoretical example of a pulse to be compressed;
• Figure 2b, the previous compressed pulse;
• FIG. 3a, the block diagram of a first possible embodiment of a device according to the invention;
• FIG. 3b, an example of pulse to be compressed;
• Figure 3c, the previous compressed pulse;
• FIG. 4, a possible structure for a line used in a device according to the invention;
• Figure 5, a second possible embodiment of a device according to the invention.

Figure la represents a basic diagram explaining the operation of a device according to the invention. A pulse signal 1, for example microwave, enters a first propagation line 2 of length l₁. At the output of this first propagation line is placed a second propagation line 3 of length I₂, with periodic structure, of period p. An exemplary embodiment of this type of line with periodic structure called a folded guide line is illustrated in FIG. 1b. In a propagation line, a microwave waveguide for example, of rectangular section, are placed metal plates 4 parallel to the section of the line, that is to say perpendicular to its axis of propagation. These metal plates 4 are fixed alternately on one side 311 and on the other opposite side 312 of the line 3, leaving a space between their end and the side of the line to which they are not fixed. Similarly, these metal plates 4 are fixed to one and the other sides 314, 315 adjacent to the two preceding sides 311, 312. The period p of the structure of the line 3 is for example defined by the distance between two successive metal plates 4 fixed in the same way on line 3.

The device according to the invention uses the property of reflection of the waves which a periodic structure has, for certain frequencies, of the type for example of that of FIG. 1b. This reflection, defined in particular by the laws of Bragg or Brillouin, occurs when the reflection, on each cell, the metal plates 4 for example, of the structure, becomes cumulative in a certain frequency band.

par rapport à son instant d'entrée dans cette première ligne, où l₁ est la longueur de la première ligne de transmission 2, à structure non périodique par exemple, et v₁ la vitesse de propagation de l'énergie du signal 1 dans cette première ligne. Lorsque la fréquence du signal f est supérieure à la fréquence de coupure fc, et inférieure à la fréquence de coupure maximale dans le cas d'un diagramme du type de la figure 1c par exemple, la ligne à structure périodique 3 propage le signal 1 comme cela est représenté par des pointillés 7 sur la figure 1a. La ligne à structure périodique 3 étant par exemple ouverte à l'extrémité opposée à celle reliée à la première ligne 2, le signal 1 se réfléchit à cette extrémité. A la sortie de la première ligne 2, le signal 1 se trouve alors retardé de $$\text{τ=2}\frac{\mathit{\text{l}}\text{₁}}{\mathit{\text{v}}\text{₁}}\text{+ 2}\frac{\mathit{\text{l}}\text{₂}}{\mathit{\text{v}}\text{₂}}$$

où l₁ et v₁ représentent les mêmes grandeurs que précédemment et l₂ et v₂ représentent respectivement la longueur de la ligne à structure périodique 3 et la vitesse de propagation de l'énergie du signal 1 dans cette ligne.FIG. 1c illustrates by a diagram the relationship between the frequency transmitted f through such a line with periodic structure and the wave number β, equal to 2π / λ, of propagation in the structure, λ being the guided wavelength in the structure. The diagram in Figure 1c defines the bandwidth of the line. In particular, this passband is bounded to the low frequencies by a frequency fc constituting a line cutoff frequency, when β is a multiple of 2π / L where L is equal to the aforementioned period p. When the frequency of the signal 1 is lower than the frequency fc at the input of the device of FIG. 1a, the pulse is reflected at the input 5 of the line 3 with periodic structure. At the exit of the first line 2, the reflected pulse 6 is delayed by $$\text{τ = 2}\frac{\mathit{\text{l}}\text{₁}}{\mathit{\text{<figure-callout id="1" label="v" filenames="imgf0001.png" state="{{state}}">v</figure-callout>}}\text{₁}}$$

with respect to its instant of entry into this first line, where l₁ is the length of the first transmission line 2, with non-periodic structure for example, and v₁ the speed of propagation of the energy of signal 1 in this first line . When the frequency of the signal f is greater than the cutoff frequency fc, and less than the maximum cutoff frequency in the case of a diagram of the type of FIG. 1c for example, the line with periodic structure 3 propagates the signal 1 as this is represented by dotted lines 7 in FIG. 1a. The periodically structured line 3 being for example open at the end opposite to that connected to the first line 2, the signal 1 is reflected at this end. At the exit of the first line 2, the signal 1 is then delayed by $$\text{τ = 2}\frac{\mathit{\text{l}}\text{₁}}{\mathit{\text{<figure-callout id="1" label="v" filenames="imgf0001.png" state="{{state}}">v</figure-callout>}}\text{₁}}\text{+ 2}\frac{\mathit{\text{l}}\text{₂}}{\mathit{\text{<figure-callout id="2" label="v" filenames="imgf0001.png" state="{{state}}">v</figure-callout>}}\text{₂}}$$

where l₁ and v₁ represent the same quantities as previously and l₂ and v₂ represent respectively the length of the line with periodic structure 3 and the speed of propagation of the energy of signal 1 in this line.

It is therefore possible to thus create a different delay depending on the frequency of the applied signal.

The basic diagram of a device according to the invention presented in FIG. 1a in fact corresponds to a single transmission line whose cutoff frequency varies in the direction of its axis of propagation. In the case of FIG. 1a, this cutoff frequency takes only two values, it is for example very low over the length l₁, so as to allow all the signals involved to pass, and it takes the value fc at the level of the entrance to the periodic structure. Thus, in a theoretical case illustrated by FIG. 2a, where a pulse is modulated so as to contain two successive signals S₁ and S₂ represented as a function of time t, the signal S₁ having a frequency higher than that of the signal S₂, it is possible define a period of the line with periodic structure 3 so as to obtain a cutoff frequency f₁ lower than the frequency of the signal S₁ but greater than that of the signal S₂. Then, taking into account the propagation speeds v₁ and v₂ above, it is possible to define the lengths l₁ and l₂, also mentioned above, to delay the signal S₁ with respect to the signal S₂ so that it is superimposed on the signal S₂ at the output of the global line consisting of lines 2, 3 with different cutoff frequencies as shown in FIG. 2b where the mixture of the two signals has not been shown but only their temporal arrangements. Provided that the line has at its output means, not shown, for separating an incident wave and a reflected wave, a microwave circulator for example, it is possible to recover at the output of this line a pulse whose width T₂ is reduced compared to the length T₁ of the incoming pulse.

FIG. 3a represents the block diagram of a possible embodiment of a device according to the invention. The pulse to be compressed arrives via a line 31 at the input 32 of means 33 for separating an incident wave and a reflected wave, a microwave circulator for example. The separation means 33 are loaded by a transmission line 34 of structure similar to the line with periodic structure 3 of FIG. 1a. The separation means 33 can be loaded by line 34 via a transmission line 35 or have an opening directly closed by line 34. The pulses to be compressed can for example have a radar recurrence frequency between a few kilohertz and a few hundred kilohertz. These pulses are modulated, for example, by a microwave signal. For the production of the device according to the invention, the structure of the line 34 and the modulation of the pulses are for example adapted to compress the latter. Line 34 connected to the separation means 33 has a structure similar to line 3 in Figure 1a but without this structure being necessarily periodic. According to the invention, the line 34 has a structure such that its cut-off frequency, low for example, varies along its axis of propagation. In this sense, it is analogous to the overall line of FIG. 1a consisting of the two lines 2, 3 where the variation in cutoff frequency is made at the transition of the two lines. In the case of the device of FIG. 3a, this variation is adapted to the modulation of the pulses to be compressed. In particular, if this modulation is linear, the variation in the cutoff frequency along the axis of propagation of line 34 is also linear.

FIG. 3b illustrates a pulse of duration T₁ whose modulation is linear. At the start of the pulse the frequency of the signal contained in the pulse is equal to a value f₁, the frequency of the following signals varying linearly up to the frequency of the end of pulse signal having a value f₂, f₂ being less than f₁ . The low cut-off frequency along line 34 can therefore vary for example linearly from the value f₂ at its input end to the value f₁ at its other end. The length of line 34 is for example calculated so that all successive signals constituting the pulse to be compressed are superimposed on the last signal of frequency f₂ at the output of line 34. The law of variation of the cut-off frequency, low for example, along the axis of propagation of line 34 and the frequency modulation law of the pulse to be compressed are therefore tuned so that all the successive signals constituting the pulse are found, after reflection, in synchronism on the side of the input port of line 34. The compressed pulses are then obtained at output 36 of the separation means 33. FIG. 3c illustrates a pulse compressed of duration T₂ obtained for example by the compression of the pulse of duration T₁ of FIG. 3b. This compressed pulse consists of the mixture of signals of different frequencies delayed with respect to each other.

In the exemplary embodiment described by FIGS. 3a, 3b and 3c, the cut-off frequencies used are low cut-off frequencies of the passband evolving along the propagation axis of line 34. It is possible to use the high cut-off frequencies of this passband, in this case, the signal to be compressed is modulated in such a way that the frequency of the signals it contains increases from the start of the pulse instead of decreasing as in the exemplary embodiment described.

The frequency modulation of the pulse to be compressed need not be linear. However, such modulation facilitates the production of a device according to the invention.

The variation of the cutoff frequency along the axis of line 34 can be obtained in several ways. In a line with periodic structure such as that of FIG. 1b, the cutoff frequency depending on the period or the pitch p between two groups of consecutive and identical metal plates, one solution to vary this cutoff frequency consists in varying this not p. The line then no longer has a properly periodic structure but nevertheless retains its reflection properties, the bandwidth evolving with the step p. Another solution consists, for example, in varying the height or the width of the transmission line 34. Still in the case of a line of the type of FIG. 1b, it is possible to vary the geometry of the metal plates 4 while retaining their periodic arrangement. It is also possible to vary both this arrangement and the geometry of the metal plates 4.

FIG. 4 shows another embodiment of the transmission line 34 with variable cutoff frequency. Metal plates or valves 41 are arranged along line 34, a guide of rectangular section for example, perpendicular to its axis of propagation. The valves 41 are all fixed on the same side 341 of the line and centered for example on the middle of this side. The variation of the cutoff frequency along this line can be achieved by varying the pitch between the valves 41 or by varying their height. Other embodiments are possible for this line 34, this line can in particular be made of dielectric material whose geometry varies as a function of the axis of propagation of the line.

FIG. 5 shows another possible embodiment of a device according to the invention. This consists of a 3dB coupler 51 having an input 52 through which the pulses to be compressed enter, an output 53 delivering the compressed pulses and two branches 54, 55 through which the signals entering by input 52 pass, the signals passing in each of the branches having the same amplitude, equal to 1 / √2 times half that of the input signals, hence the name of 3dB coupler, and being in quadrature. The branches 54, 55 of the coupler 51 are each loaded by a line 56, 57 whose cutoff frequency varies along the axis of propagation. These lines 56, 57 are for example similar to that of FIG. 1b, but with a variable pitch between the metal plates 58 or with a variable dimension of these plates, these variations being adapted to the frequency modulation of the pulses to be compressed. These lines could also be similar to that presented in FIG. 4. The branches 54, 55 of the coupler 51 have terminations 59, 60 which make it possible to adapt them to the input of lines 56, 57, because the input section of the latter is reduced by the presence of the metal plates 58. The operation of the device in FIG. 5 uses the properties of the coupler 51 and of the lines 56, 57 which are associated with it, the coupler notably playing the role of the means for separating a incident wave and a reflected wave. A pulse to be compressed enters via input 52 of coupler 51 and then divides into two quadrature pulses of equal amplitude, equal to 1 / √2 times half the amplitude of the incoming pulse, one of the pulses thus created is propagating in one branch 54 and the other impulse in the other branch 55. Then they enter the lines 56, 57 via the terminations 59, 60. These impulses propagate and are reflected inside the lines 56 , 57 as described above in line 34 of the device in FIG. 3a. In particular, if the pulse to be compressed is linearly frequency modulated, the cut-off frequency varies linearly along the lines 56, 57 according to methods previously described. In the case of the device in FIG. 5, the structures of the lines 56, 57 are preferably identical. After reflection in these lines 56, 57, their structure having been adapted to the frequency modulation of the pulse entering the coupler 51 in order to compress it in a similar manner for example to line 34 of the device of Figure 3a, the two pulses present at the outlet of the branches are compressed. The pulse from a branch 54 is divided into two quadrature pulses of equal amplitude, one returning to the input 52 of the coupler and the other propagating to the output 53 of the coupler. Similarly the impulse from the other branch 55 is divided in the same way. The two pulses being previously of equal amplitude and in quadrature, the pulses propagating towards the input 52 are found in phase opposition and of the same amplitude therefore cancel out while the pulses propagating towards the output 53 of the coupler, in phase, recombine to form the initial compressed pulse.

The device of FIG. 5 makes it possible, in particular thanks to the coupler, to compress high power pulses and can therefore be advantageously used for the compression of pulses in microwave transmission, for radar applications for example.

The examples of devices presented apply to microwave electromagnetic waves but their principle can apply to other types of waves.

Claims (10)

Pulse compression device, the pulses being frequency modulated, comprising at least one transmission line (34, 56, 57) having at least one cutoff frequency (fc), a non-transmitted signal being reflected, and means for separation (33, 51) of an incident wave and a reflected wave, charged by the transmission line (34, 56, 57) directing the pulses to be compressed towards the transmission line, and receiving the pulses reflected by said line , characterized in that the transmission line (34) consists of a waveguide of rectangular section where metal plates (4, 41, 58) are arranged on one side (341) parallel to the section of the guide, the variation of the cutoff frequency (fc) and the frequency modulation of the pulses to be compressed being tuned. Device according to claim 1, characterized in that the metal plates (4, 58) are fixed alternately on one side (311) and on the other opposite side (312) of the guide and alternately to one (314) and to the other (315) on the two sides adjacent to the previous sides (311, 312), leaving a space between an unfixed end of a plate (4, 58) and a side. Device according to claim 1, characterized in that the metal plates (41) are fixed on the same side (341) of the guide. Device according to any one of the preceding claims, characterized in that the frequency modulation is linear. Device according to any one of the preceding claims, characterized in that the variation in the cut-off frequency (fc) along the axis of propagation of the line (34, 56, 57) is obtained by varying the no distance between the metal plates (4, 41, 58). Device according to any one of the preceding claims, characterized in that the variation in the cut-off frequency (fc) along the axis of propagation of the line is obtained by varying the geometry of the metal plates (4, 41, 58). Device according to any one of the preceding claims, characterized in that a variation in the line cut-off frequency (fc) along the line propagation axis is obtained by variation of the section of the guide. Device according to any one of the preceding claims, characterized in that the separation means (33) consist of a microwave circulator. Device according to one of claims 1 to 7, characterized in that the separation means consist of a coupler (51), the coupler (51) having two branches (54, 55) each charged by a transmission line ( 56, 57) whose cutoff frequency (fc) varies along the axis of propagation. Device according to any one of Claims 4 to 9, characterized in that the variation in the cut-off frequency (fc) of the transmission line is linear.

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