EP0100274A1 - Matching and isolatung arrangement with ferrite circulator - Google Patents

Abstract

1. A matching and isolating device for coupling a circuit to be matched (1) to a connection point (A), the device comprising a ferrite circulator having three branches (41, 42, 43) made of a metal plate comprising a resonator disk, the three branches being disposed there around at a mutual angular distance of 120 degrees, the first branch being connected to this connection point (A), the second branch to a biassing resistor (R), the third branch to the circuit to be matched, characterized in that said metal plate is disposed inside a structure constituted by two ground planes situated on either side of the plate, that the lengths of the first and of the second branches (41, 42) are substantially equal to a quarter wavelength in the circulator at a mean frequency, and that this length is substantially greater than that of the third branch (43) which is shown only by way of indication for purposes of connection to the circuit to be matched, and that the circulator is selected in such a way that the real part of the impedance of its third branch is equal to the real part of the impedance of the circuit to be matched and that the Q factors of said impedances are the same.

Description

The present invention relates to an adaptation and isolation device, comprising a ferrite circulator, for coupling a circuit to be adapted to a connection point, the circuit to be adapted being assimilable, from the point of view of its impedance, to a series resonant circuit.

Broadband adaptation devices for microwave circuits are based on the use of resonators which make it possible, in the case where the circuit to be adapted is not comparable to a parallel resonant circuit, to achieve the combined impedance of that of the circuit to be adapted in order to obtain a wide band of operating frequencies. In addition, it is known to use a circulator between the microwave circuit with its adaptation device and the access considered, with the aim of achieving insulation or improving the standing wave ratio (R.O.S.). The connection between the circulator and the microwave circuit to be adapted is made, in a conventional manner, by switching to a standardized characteristic impedance (50 ohms for example); this leads to the need for two adaptation devices, one for the circulator and one for the circuit to be adapted, and, consequently, a reduction in bandwidth.

The object of the present invention is to avoid these drawbacks due to the use of the two adaptation devices.

This is obtained by making the circuit adaptation device to be adapted from the ferrite circulator.

According to the invention an adaptation and isolation device for coupling a circuit to be adapted to a connection point, the circuit to be adapted being comparable, from the point of view of its impedance, to a series resonant circuit, the device comprising a adaptation circuit of the circuit to be adapted, of the parallel resonant circuit type, coupled to the circuit to be adapted and a ferrite circulator having a first access coupled to the connection point and a second access coupled to the circuit to be adapted, is characterized in that the circulator comprising a switching circuit whose impedances of the accesses are comparable to those of parallel resonant circuits, the circulator constitutes, by itself, the adaptation circuit.

In a neighboring field, it should be noted that the German patent application DE-A-3 034 034 describes an insulator comprising a circulator. In this isolator, in order to best adapt that of the three inputs of the circulator proper (corresponding to the switching function) on which a dissipative load is coupled, the coupling element chosen is a series oscillating circuit whose impedance added to that of the dissipative load is as close as possible to the combined impedance of the circulator; this series oscillating circuit replaces the impedance transformer, type X / 4, which is usually used to carry out the adaptation and which is usually part of the circulator. This therefore has only a distant relation with the invention which, on the other hand, leads not to replace an X / 4 type impedance transformer (which is part of the circulator) by a series oscillating circuit (added to the circulator ), but to remove the adaptation circuit of the parallel resonant circuit type, usually used to couple a circuit to be adapted to a circulator; the circuit to be adapted is thus connected directly to the circulator without any element replacing the adaptation circuit.

• - les figures 1 et 4 des impédances représentatives de circuits,
• - les figures 2 et 3 des montages relatifs respectivement à l'art antérieur et à l'invention,
• - les figures 5 et 6 des vues montrant un circulateur à ferrites utilisé dans le montage selon la figure 3.

The present invention will be better understood and other characteristics will appear with the aid of the description below and of the figures relating thereto which represent:

• - Figures 1 and 4 of the representative impedances of circuits,
• FIGS. 2 and 3 of the arrangements relating respectively to the prior art and to the invention,
• - Figures 5 and 6 of the views showing a ferrite circulator used in the assembly according to Figure 3.

In the various figures, the corresponding elements bear the same references.

In what follows there will be a question of a circuit to be adapted; it will be a microwave circuit of the amplifier, filter, mixer, etc. type.

In Figure 1 is shown schematically the impedance of a circuit to be adapted in the most general case, that is to say in the case where this impedance is comparable to that of a resonant circuit series RLL-Cl. If the central working frequency, F0, of the circuit to be adapted is different from its resonant frequency, an impedance is added in series with the circuit to be adapted to make these two frequencies equal; in the case of FIG. 1 this impedance is obtained by an inductor L'1. In what follows and in the claims this impedance, to be added optionally, will be considered as part of the circuit to be adapted and the latter will be considered as a series resonant circuit formed of the elements R1-L1-Cl with LI = L + L'l ; and the pulsation ω _o , corresponding to the central working frequency, is such that: L1.C1. ω ² _o = 1.

et son coefficient de surtension

The impedance, at pulsation w, of the circuit to be adapted in FIG. 1 is:

and its overvoltage coefficient

When a circuit to be adapted, of the type discussed above, must be coupled to a connection point through an isolation circuit, it is known to use a ferrite circulator for this purpose. circuit diagram shown in Figure 2.

FIG. 2 represents a circuit to be adapted, 1, which is connected to a connection point, A, by an adaptation circuit, 10, followed by a circulator, 20 to 23. The adaptation circuit, 10, is constituted, conventionally, using one or more parallel resonant circuits; the elements of this circuit 10 are chosen so that it has a purely ohmic input impedance and of value Zc given throughout the working frequency band, when its output is connected to the input of the circuit to be adapted. The circulator, although made up of a single element, has been represented by four blocks so as to distinguish between its different functions: a block, 20, called the switching circuit which relates to the "circulator" function proper and three blocks, 21 to 23, called impedance transformers, connected to each of the three ports of the switching circuit 20 to bring the impedances of the three ports of the circulator to the value Zc, mentioned above. The "circulator" function represented by block 20 makes it possible to isolate the circuit to be adapted 1 from another circuit to be adapted connected downstream of point A, so as to improve the standing wave ratio at point A. The transformers impedance, 21 to 23, have, like the adaptation circuit 10, an impedance adaptation role but, unlike circuit 10, they do not require resonators to play this role since, as it will be indicated later, a circulator presents at its accesses impedances of the type of those of parallel resonant circuits; the impedance transformers 21 to 23 are, most generally, constituted by quarter wave transformers. The impedance transformer 21 is connected to point A, the impedance transformer 22 is connected to the first end of a resistor R, the second end of which is grounded; as for the impedance transformer 23 it is coupled to the circuit to be adapted 1 by the adaptation circuit 10.

FIG. 3 shows how, in the context of the invention, the assembly according to FIG. 2 is modified. The assembly according to FIG. 3 distinguishes from the assembly according to FIG. 2 by the fact that the impedance transformer 23 and the adaptation circuit 10 have been eliminated and replaced, as the case may be, either by a direct link, or by a simple impedance transformer 3 does not having no parallel resonant element; this alternative is shown by a drawing of the periphery of block 3 in broken lines. Such an arrangement, by removing the impedance transformer 23 and the adaptation circuit 10 and therefore its resonator, has a wider bandwidth than the arrangement according to FIG. 2; in addition, due to the reduction in the number of elements, the assembly according to FIG. 3 has less losses than the assembly according to FIG. 2.

In the following description, it will be shown why the assembly according to FIG. 3 is possible.

For a ferrite circulator it is known (see in particular the article: Operation of the ferrite junction circulator, published in IEEE Transactions on microwave theory and techniques, from January 1965, pages 15 to 27), and the measurements allow to verify it , that the impedances of the accesses are comparable to those of parallel resonant circuits.

FIG. 4 represents a parallel resonant circuit, formed of a) resistor R2 in parallel on an inductance L2 and a capacitor C2. This figure is the schematic representation of the impedance of an access of a ferrite circulator whose impedance, at the pulsation w is:

où ω_o est la pulsation de résonance du circulateur et est telle que L2.C2. ω² _{= 1} 0The overvoltage coefficient of the resonant circuit according to FIG. 4 is:

where ω _o is the resonance pulsation of the circulator and is such that L2.C2. ω ² _{= 1} 0

When a circulator is used in an assembly, as an isolation device for a microwave circuit, its resonant frequency is chosen, for reasons of operating frequency bandwidth, equal or at least as close as possible to the central working frequency and therefore the resonance frequency of the microwave circuit; this is why in the form L1.C1. w ² _o = relating to the circuit to be adapted from Figure 1 and in the formula L2.C2. ω ² _o = 1, the resonance pulse is, in both cases. represented by ω _o .

en remplaçant, dans cette équation, CI et L2 par leurs valeurs tirées des formulesSo that the parallel resonant circuit according to FIG. 4 can constitute an adaptation circuit for the series resonant circuit according to FIG. 1, that is to say so that the diagram of FIG. 2 can be replaced by the diagram of the FIG. 3 with a connection without impedance transformer between circuits 20 and 1, the impedance Z2 of the parallel resonant circuit must be equal to the conjugate of the impedance Zl, that is to say that

by replacing, in this equation, CI and L2 by their values taken from formulas

• pour ω = ω _o la relation (5) entraîne : R1=R2
• pour ω différent de ω_o la relation (6) entraîne :
  
  ce qui donne en multipliant par
  
  
  ce qui peut s'écrire, compte tenu des formules (2) et (4) Q1 = Q2

L1.C1.ω ² = 1 and _L 2. _C 2. w ² _{= 1} 0 0 it comes, making equality between the real parts on the one hand and the imaginary parts on the other hand

• for ω = ω _o equation (5) results in: R1 = R2
• for different ω ω _o of the equation (6) leads to:
  
  which gives by multiplying by
  
  
  which can be written, given formulas (2) and (4) Q1 = Q2

Knowing the values of R1 and Q1, at the resonance pulsation w _o , of a circuit to be adapted, it is always possible to produce a circulator whose resonance pulsation, the overvoltage coefficient and the resonance impedance are respectively equal, or at least close, to ω _o , Q1 and Rl. Thus it is possible to achieve the optimal broadband adaptation between the circulator and the microwave circuit.

It should be noted that if the resistance R2 of the circulator is clearly different from the resistance RI of the circuit to be adapted, it is possible to carry out an adaptation by an impedance transformer, without parallel resonant element, mounted like the impedance transformer 3 of FIG. 3. The assembly constituted by the impedance transformer 3 associated with the circuit to be adapted, 1, always has the same resonant pulsation ω _o as the circuit to be adapted 1 and its resistance is determined to be equal to R2 ; moreover its pulse overvoltage coefficient w _o is no longer equal to QI but is modified by the impedance transformer, 3, it is therefore necessary that the circulator has been chosen so that its overvoltage coefficient at resonance , Q2, is as close as possible to the overvoltage coefficient of the "circuit 1 and transformer 3" assembly.

• - fréquence de résonance : FO = 3,9 GHz
• - coefficient de surtension : QI = 3,15
• - impédance à la résonance : RI = 15 ohms

An exemplary embodiment of a circulator intended for mounting according to FIG. 3, with direct connection, without the transformer 3, between the circuit to be adapted 1 and the switching circuit 20, will be described below using of Figures 5 and 6. The circuit to be adapted was a circuit using a gallium arsenide field effect transistor, operating in the frequency range 3.6 - 4.2 GHz and having the following characteristics:

• - resonance frequency: FO = 3.9 GHz
• - overvoltage coefficient: QI = 3.15
• - resonance impedance: RI = 15 ohms

The purpose of adapting this circuit was to obtain the minimum noise factor of the field effect transistor.

• - fréquence de résonance : FO = 3,9 GHz
• - coefficient de surtension : Q2 = 2,12
• - impédance à la résonance : R2 = 15 ohms.

The circulator produced (Figures 5 and 6) has the following characteristics:

• - resonance frequency: FO = 3.9 GHz
• - overvoltage coefficient: Q2 = 2.12
• - impedance at resonance: R2 = 15 ohms.

It should be noted that the circulator according to FIGS. 5 and 6 does not perfectly satisfy the condition Q2 = QI; however, the measurements have shown that, in the range of working frequencies (3.6 - 4.2 GHz), the difference between the optimal impedance which allows the minimum noise factor to be obtained and the impedance achieved by the circulator remains low and the noise factor obtained is practically minimum.

FIG. 5 shows, in dashed lines, the periphery C of the assembly formed by the housing of the circulator and the housing of the circuit to be adapted which are placed side by side. In FIG. 5, only one part 4 is shown, situated in the middle of the circulator, two resonator connection plugs, 31, 32, and the field effect transistor, 11, of the circuit to be adapted 1 of FIG. 3 .

The part 4 constitutes the inner conductive part or ribbon (strip in Anglo-Saxon literature) of a structure of the triplate waveguide type, that is to say of a structure formed by two parallel ground planes and a ribbon arranged parallel to and between the two ground planes. These two ground planes appear in FIG. 6 where they bear the references 61 and 62. The part 4 consists of a metallic plate of silver-plated brass 1.2 mm thick comprising a resonator disc 40, 14 mm in diameter. diameter, drilled with a centering hole, 44; around this disc are arranged three branches, 41, 42, 43, at 120 ° from each other. The branches 41 and 42, of length clearly greater than the branch 43, are soldered to the inner conductor of the coaxial plugs, 31, 32. These plugs, of characteristic impedance equal to 50 ohms, allow connection to point A and to the resistance R (Figure 3); their external conductor connected to the housing of the circulator, has not been shown in FIG. 5. The branch 43, which is in fact there only for identification for connection, is welded to the end of the connection gate, G, of the field effect transistor, 11, of the circuit to be adapted; the length of the gate connection of transistor 11, that is to say the length of the connection between the housing of the transistor and the disc 40, determines the inductance L 1 which was mentioned during the description of Figure 1; for the sake of simplification and because this would bring nothing to the understanding of the invention, the elements connected to the drain, D, of the transistor 11 have not been shown; as for the source of transistor 11, it comprises two output connections, SI and S2, which are both soldered to the housing of the circuit to be adapted.

It should be noted that, in the embodiment described, the gate of the transistor 11 is polarized by means of the circulator; indeed the resistor R which, for the sake of simplification, is connected, in FIG. 3, directly between the circulator and the ground is, in reality, connected by its first end to the circulator (transformer 22 comprising the branch 42) and by its second end to a decoupling circuit; this decoupling circuit behaves as a short circuit at the working frequencies of the circuit to be adapted and as an infinite impedance for the DC bias voltage; this bias voltage is applied to the second end of the resistor R. The decoupling circuit can be produced conventionally by open quarter-wave lines which bring back a short circuit and / or by a capacitor.

In FIG. 3, the impedance transformers 21 and 22 and the switching circuit 20 which constituted the circulator have been discussed. In FIG. 4 the resonator disc 40 and the branches 41 and 42 correspond respectively to the switching circuit 20 and to the impedance transformers 21 and 22; these impedance transformers are of the quarter wave transformers type, that is to say that the length of the branches is substantially equal to a quarter of the length, in the circulator, of the wave at the average frequency of 3, 9 GHz. In fact, the blocks 20, 21 and 22 of FIG. 3 are more precisely to be identified with the association of the elements 40, 41, 42 and the elements between which they are inserted in the circulator and which will be described using in Figure 6.

• - un disque de ferrite 51, 52 dont le diamètre est sensiblement égal au diamètre du disque résonateur 40 (figure 5) et une pièce en silice, 53, 54, qui recouvre presque toute la partie du ruban 4 extérieure au disque 40 (figure 5) ; les pièces 51 à 54 ont une épaisseur de 2,1 mm ; le disque de ferrite 51, 52 est percé d'un trou en son centre,
• - un flasque en aluminium,61, 62, constituant l'un des plans de masse mentionnés plus avant et qui comporte, dans sa face opposée au ruban 4, un creux en forme de cuvette circulaire disposée en regard des disques de ferrite 51, 52
• - dans le creux en forme de cuvette : une lame d'air 71, 72, une rondelle en acier doux de 0,5 mm d'épaisseur, 81, 82, et un aimant permanent, 91, 92, de 1,5 mm d'épaisseur ; le circuit magnétique formé avec les deux aimants 91, 92 fournit un champ de 400 gauss,
• - une barre d'un fer en U, 60 ; ce fer en U recouvre, respectivement par ses deux barres en regard, les deux aimants 91, 92 afin de refermer leurs lignes de champ ; ce fer en U est réalisé avec une bande d'acier doux de 1 mm d'épaisseur et 17 mm de largeur.

Figure 6 is a sectional view along the axis XX (Figure 5) of the circulator. This view shows that, on either side of the ribbon 4, are arranged two symmetrical structures with respect to the ribbon and which comprise, from the ribbon:

• - A ferrite disc 51, 52 whose diameter is substantially equal to the diameter of the resonator disc 40 (Figure 5) and a piece of silica, 53, 54, which covers almost the entire part of the tape 4 external to the disc 40 (Figure 5 ); parts 51 to 54 have a thickness of 2.1 mm; the ferrite disc 51, 52 is pierced with a hole in its center,
• - An aluminum flange, 61, 62, constituting one of the ground planes mentioned above and which comprises, in its face opposite to the strip 4, a hollow in the form of a circular bowl arranged opposite the ferrite discs 51, 52
• - in the bowl-shaped hollow: an air knife 71, 72, a mild steel washer 0.5 mm thick, 81, 82, and a magnet permanent, 91, 92, 1.5 mm thick; the magnetic circuit formed with the two magnets 91, 92 provides a field of 400 gauss,
• - a bar of a U-shaped iron, 60; this U-shaped iron covers, respectively by its two facing bars, the two magnets 91, 92 in order to close their field lines; this U-shaped iron is made with a strip of mild steel 1 mm thick and 17 mm wide.

An insulating cylindrical rod, 45, made of nylon (registered trademark), passes through the holes drilled in the tape 4 (hole 44) and in the ferrite discs, 51, 52, in order to ensure the positioning of these parts one by one. compared to others.

A very thin silver foil, not shown in FIG. 6, is placed between the disc 51 and the silica piece 53 on the one hand and the flange 61 on the other hand, to ensure good electrical contact between these elements. Another sheet, not shown, ensures good contact between the disc 52 and the silica piece 54 on the one hand and the flange 62 on the other.

The housing of the circulator comprises, between the flanges 61 and 62 and the U-shaped iron, 60, a socket 31, fixed to the flanges 61 and 62, by means of screws not shown.

This ferrite circulator was manufactured, according to the characteristics to be obtained in order to carry out the adaptation of the circuit to be adapted which was mentioned above; for this, the choice fell first on an existing circulator having characteristics fairly close to those to be obtained, then, after successive tests, the circulator described with the aid of FIGS. 5 and 6 was produced.

It should be noted that the invention is not limited to the example described; in this way, in particular, it is possible, in the assembly according to FIG. 3, to remove the impedance transformer 22, that is to say, practically the branch 42 of the ribbon 4 (see FIG. 5) , then it suffices to give R the value presented, at resonance, by the corresponding output of the resonator.

It should also be noted that the circulator can be used as an adaptation element of a circuit to be adapted, not only on the input of the circuit, as in the example described, but also on the circuit output; thus the same circulator can serve as an adaptation element for two circuits to be adapted: a first circuit to be adapted having its output coupled to one of the ports of the circulator and a second circuit to be adapted having its input coupled to another access point of the circulator.

Claims (3)

1. Adaptation and isolation device for coupling a circuit to be adapted (1) to a connection point (A), the circuit to be adapted being comparable, from the point of view of its impedance, to a series resonant circuit, the device comprising an adaptation circuit (R2-L2-C2) of the circuit to be adapted, of the parallel resonant circuit type, coupled to the circuit to be adapted and a ferrite circulator (20-21-22) having a first access coupled to the point of connection and a second access coupled to the circuit to be adapted, characterized in that the circulator comprising a switching circuit whose impedances of the accesses are comparable to those of parallel resonant circuits, the circulator constitutes, by itself, the circuit adaptation. 2. Device according to claim 1, characterized in that the circuit to be adapted is directly connected to an access to the switching circuit. 3. Device according to claim 2, characterized in that, the circuit to be adapted being comparable to a series resonant circuit (R1-L1-Cl) having a resonant pulse ω _o , an impedance RI at resonance and an overvoltage coefficient Ql, the circulator is chosen so that, from the point of view of the access to which the circuit to be connected is connected, it can be assimilated to a parallel resonant circuit of pulsation ω _o , of impedance R2 at resonance and of overvoltage coefficient Q2 with R2 and Q2 substantially equal to RI and Ql respectively.

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