FR2751415A1 - Unit for measuring the transfer impedance of multi-conductor shielded connectors of large diameter and non-uniform shape for aerospace applications - Google Patents

Abstract

The unit is made up of a tri-coaxial mounting designed to have a chamber (22), called `illumination', in which is placed a connector or similar on test (25). The chamber is placed in one branch of a power divider (21) fed by a electro-magnetic wave source whose frequency is variable over a set range. The unit has an energy absorption load (23,24) at each end of the branches and a measuring means connected to the output of the `illumination' chamber branch. The `illumination' chamber consists of a housing which contains, co- axially to the connector (25), a series (30) of discs made from a good electrically conducting material, whose interior contours are adapted to the profile of the connector under test. The discs are designed so as to adjust the characteristic impedance of the propagation medium to the desired value. The chamber has a cylindrical body (27) fitted at its ends with removable covers (28) with a central passage through which cable connections pass.

Description

TRANSFER IMPEDANCE MEASUREMENT BENCH, PARTICULARLY FOR
CONNECTORS OF IMPORTANT DIAMETER AND / OR PROFILE NOT
UNIFORM
The present invention relates to the measurement of transfer impedances in the frequency domain, more particularly of the connectors and their connections to the connecting cables and more particularly aims at measuring the attenuation provided by these connectors to the transmission of spurious signals. generated by high frequency external electromagnetic disturbances, typically signals in the frequency band 1 KHz to 1GHz.

 In the aeronautical and space fields, protected electrical transmission links are systematically used, consisting, on the one hand, of cables composed of at least one linear conductor and at least one shielding braid around the conductor (s) and, d on the other hand, connectors for connecting cables or between a cable and any appliance, device or electrical circuit.

 To preserve the purity of the signals transmitted by the active conductors, it is necessary to isolate the latter from the surrounding electrical influences, which is the precise role of the shields and possibly over-shielding with which these conductors are provided, both at the level of the cables and of the connectors. .

 The various shielding and over-armor elements are mechanically and electrically connected together in order to preserve the immunity of the conductors, that is to say to minimize the induction of a disturbing potential difference between said shields and over-armor and active conductors, following the circulation in the shields and armor plating of a current generated by the variations of the surrounding electromagnetic fields.

 The ability for example of a connector to attenuate external high frequency disturbances is characterized by the electrical transfer impedance of the connector whose value is inversely proportional to the efficiency of the portection of the connector.

 The objective to be achieved is therefore to produce connectors having, at the level of the shields and armor plating, an excellent electrical connection resulting in a very low transfer impedance, in the required frequency range.

 The principle of measuring such transfer impedances of electrical connectors in the frequency band beyond i KHz and up to 1 OGHz is well known.

An international standard IEC 169-1-3 has been published by the Commission
International electrical engineering, defining the protocol for measuring the impedance of the transfer of electrical connectors in the radio frequency domain.

 To this end, this standard defines the principle of a so-called adapted tri-coaxial measurement assembly in which the interior and exterior coaxial systems are both adapted at their end, that is to say terminated by the characteristic impedance of the line in order to avoid, or at least reduce, the formation of standing waves.

 The external system is supplied through a lateral arm, containing an impedance matching transformer, up to a T-junction playing the role of a power divider.

 The connector under test is placed in one of the branches supplied by the divider.

 A signal generator is connected to said side arm and injects an electromagnetic wave at the desired frequency, in the band 1 KHz to 1 06Hz, and a spectrum analyzer is connected to the measurement gate (connector side).

 This device makes it possible to determine the connector transfer impedance at any frequency and in particular in the band 1KHz to 10GHz, from the measurement of the transmission of the circuit.

 However, this device has drawbacks and limits.

 In particular, the measurement means recommended by standard IEC 169-1-3 includes a test chamber, called the illumination chamber, which has been designed for connectors of uniform or relatively uniform profile and with a maximum diameter of around 40 mm.

This illumination chamber in its current design cannot be increased in volume to accommodate in particular connectors of larger diameter, and cannot moreover allow the testing of connectors with non-uniform profile, since the modification of the dimensions of said chamber is likely generate at certain frequencies resonances or parasitic propagation modes different from TEM mode (Transfer
Electromagnetic) necessary for the proper functioning of the device.

 According to another drawback of the known device, it comprises in the power line of the power divider a multi-stage transformer for impedance matching between the T-junction and the power line in the frequency band 1KHz to 10GHz . This transformer is adapted to the characteristic impedance, generally 50 Q, of the generator of the supply line, and aims to match the impedances of the two branches of the measuring device to this same value of 50 Q.

 However, the geometry of this multi-stage transformer is very precise and therefore requires very careful and expensive machining.

 Furthermore, the adoption of this type of transformer leads to the consequence that in low frequency conditions (from 0 to 1 MHz) the transfer function is not the same as at high frequencies (from 1 GHz to 10 GHz), which must be taken into account when calculating the transfer impedance according to the test frequency.

 The present invention aims primarily to eliminate the drawbacks of the limitations of the testing capacities of known devices by designing an illumination chamber suitable for testing in particular connectors which may have a diameter greater than 40 mm and / or a profile. nonuniform while allowing the characteristic impedance of the propagation medium to be adjusted to the correct value, in particular in the frequency range from 1KHz to 1OGHz.

 To this end, the subject of the invention is a transfer impedance measurement bench, in particular for connectors or the like of large diameter and / or of non-uniform profile, consisting of a measurement assembly of the suitable tricoaxial type comprising a chamber. , called illumination, in which the connector or the like under test is placed, placed in one of the branches of a power divider supplied by an electromagnetic wave source of variable frequency within a determined range, a load of energy absorption at each end of said branches and of the measurement means connected to the outlet of the branch of the illumination chamber, characterized in that said illumination chamber consists of a housing in which are stacked, coaxially at connector or the like under test, a series of contiguous washers made of a material which is good conductor of electricity and the internal contour of each of which is adapted to the part opposite the profile of said connector or the like, so as to adjust the characteristic impedance of the propagation medium to the desired value.

 The term “connector” or the like is generally understood to mean any type of single-pin or multi-pin electrical connector, with shielding and possibly shielding of the conductive elements, as well as all types of mono or multi-conductor cables, each conductor being or not individually surrounded by a shielding braid, all the conductors, unshielded or shielded, being itself possibly surrounded by a shielding braid.

 By connector is also meant a single connector, male or female, or a couple of connectors, male and female, assembled, or even a connector-connection base assembly between cable and equipment box.

 The principle of adapting the internal contour of each washer consists in delimiting, from one end to the other of the line placed in the chamber and constituted by the connector or the like under test and its end connections, successive zones in profile, that is to say of constant diameter or slight variation in profile, and to place opposite each of said zones a washer whose internal edge, that is to say the internal diameter varies, by facing the other of the puck, according to a determined law. In general, this law is either a constant, the internal diameter then being constant and adapted to the considered portion of the profile of the connector, or a linear variation also adapted to said considered portion of the profile.

 The washer material is a good conductor of electricity, in particular a metal such as aluminum.

 Such an illumination chamber allows the measurement of transfer impedance of connectors or cables with a diameter greater than 40 mm, in particular diameters up to 60 mm, as well as connectors having a non-uniform axial profile, that is to say that is to say with a local diameter varying very substantially from one end to the other of the connector or of the pair of connectors assembled.

 According to another characteristic of the device of the invention, each energy absorption charge is made of a single material, consisting of a machinable ferromagnetic resin and shaped into two removable cylindroconical half-shells, which can be assembled directly around the connecting cable from the connector under test to the bench measurement doors, the shapes and dimensions of the half-shells being determined so as to obtain, in combination with a terminal resistive load removably mounted at the end of each branch, a characteristic impedance of said branches the desired value.

 Such absorbent fillers are simpler to produce, more reliable and easier to assemble and dismantle, in particular for installing and removing the connector under test, than the absorbent fillers of conventional measurement benches.

 The invention also aims to adapt a measurement bench incorporating an illumination chamber and absorbent loads of the above type, to an operation with a characteristic impedance of 100 Q instead of the traditional characteristic impedance of 50 Q.

 To this end, the invention relates to a bench of the type defined above and characterized in that it comprises a power divider in T, the two opposite branches of which are determined, as regards the diameter of the internal conductor and the internal diameter of the external conductor, so as to have a characteristic impedance of 100 Q, the lateral arm being devoid of impedance transformer.

 The invention thus makes it possible to produce two types of measurement bench using the same types of illumination chamber with adaptation washers and absorbent charges and adapted to operation, one with a characteristic impedance of 50 Q and the other with a characteristic impedance of 100 Q.

 The bench at 50 Q includes a power divider of the conventional type with lateral arm with multi-stage impedance transformer, while the bench at 100 Q comprises a divider without transformer, the washers of the illumination chamber and the absorbent charges being dimensioned accordingly as a function of the characteristic impedance of the bench.

 The 50 Q bench is suitable for impedance transfer measurements in the frequency range 1KHz to 10 MHz, while the 100 Q bench is better suited for measurements in the range 1KHz to 10GHz because it allows to obtain a constant input input impedance as a function of the frequency, that is to say a uniform transfer function, which is not the case with conventional benches in which it is necessary to perform a phase inversion when we go from a low frequency power supply (1 KHz to 10 MHz) to a high frequency power supply (10 MHz to 10 GHz).

Other characteristics and advantages will emerge from the description which follows of an embodiment of the device of the invention, description given by way of example only and with reference to the appended drawings in which
- Figure 1 schematically illustrates a tri-coaxial measurement arrangement
adapted according to standard IEC-169-1-3;
- Figure 2 shows schematically a tri-coaxial measurement bench
according to the invention;
- Figure 3 shows schematically a test specimen
- Figures 4a to 4c illustrate the disassembly / assembly phases of the
Bench illumination chamber and installation of a test tube
test;
- Figure 5 is an axial sectional view of the illumination chamber of
Figure 2 provided with another set of washers adapted to a
connector with a profile different from that of Figure 2
- Figure 6 is an axial sectional view of one of the end loads
of the bench
- Figure 7 is a left view of the device of Figure 6, and
- Figure 8 is a sectional view of a power divider whose
two branches have an impedance of 100 Q.

 In FIG. 1, there is schematically shown a connector transfer impedance measurement arrangement of the adapted tri-coaxial type recommended by standard IEC 169-1-3, for tests in the frequency domain covering the frequency bands 1 KHz. at 10 MHz and 10 MHz at 10 GHz.

 This assembly comprises a so-called illumination chamber 1 in which a pair of connectors 2 under test is placed, a power divider 3 in one of the branches of which the illumination chamber 1 is arranged, an absorption load of energy, respectively 4 and 5, arranged at the end, on the one hand (4), of the chamber i and, on the other hand (5), on the other branch of the divider 3.

 The lateral arm 6 of the divider 3 comprises a multi-stage impedance transformer 7 capable of being connected by a door A to a supply line 8 in the desired frequency band, for example 1 GHz to 1 OGHz.

 A door B, called measurement, at the end of the branch comprising the connectors under test 2, is connected a measurement device (not shown) comprising in particular a spectrum analyzer, this device, as well as the wave generator (not shown) supplying door A, being arranged in an armored room symbolized at 9.

 Gate C, at the end of the other branch of the bench, is used for calibration.

 The doors B and C are each connected to a semi-rigid cable 10 passing through one of the absorbent charges 4,5 and connected to the pair of connectors in test 2, as illustrated in FIG. 3.

 In FIG. 3 are diagrammatically represented two plug connectors 2 each of which plug body 11 is connected by a copper connection sleeve 12 to a semi-rigid cable 10 the end of which receives an end plug 13 defining the door B or vs.

 In such a tri-coaxial assembly, the interior coaxial systems (active conductors of the connectors 2) and exterior (external shielding of the cables 10, body of the plug 11 and metallic envelopes of the chamber 1 of the divider 3 and of the loads 4.5) are all two adapted to their ends, that is to say terminated by the characteristic impedance (generally 50 Q) of the supply line, in order to avoid, or at least to decrease, the formation of standing waves.

 The impedance transformer 7 of the lateral arm 6 has an input (gate A) of 50 Q adapted to the generator impedance and an output impedance (25 Q) on each branch of the divider 3. The adaptation to 50 Q at each end of the branches (doors B and C) is produced by resistive charges 14 associated with absorbent charges 4,5.

 More specifically, the absorbent fillers 4,5 are conical in shape and consist of four segments of a foam coated with an absorbent layer, the four segments being in the form of wedges inserted between an external frustoconical metallic envelope and a conical metallic spacer 4a , 5a interposed between the absorbent load 4,5 and the connecting cable 10. The resistive load 14 is connected by welding to said external frustoconical envelope and to said internal spacer 4a, 5a.

Such an arrangement allows the measurement of the transfer impedance (Zt) of the pair of connectors 2 according to the formula (at high frequencies, that is to say beyond 1 MHz)
Zt = S21 (f) .100.62
where S21 is the transmission of the assembly at the frequency f.

 The principle of measurement consists in injecting on the door A an electromagnetic wave whose energy is distributed in an equal way in the two opposite branches (B, C). The wave which propagates in branch B induces a current on the skin of the connectors, which in turn induces a voltage inside the connectors, the measurement of which is carried out using the coaxial cable 10 on door B.

 This principle is well known and will not be detailed further.

 The known measuring device illustrated in FIG. 1 has drawbacks and limits which the device of the invention, shown diagrammatically in FIG. 2 according to one embodiment, proposes to eliminate or reduce.

 The measurement bench according to the invention comprises the same basic elements, namely a power divider 21, with its entry or supply door A, an illumination chamber 22 placed in one of the branches of the divider 21 and a pair of absorbent fillers placed, one (23) at the end of the illumination chamber 22 and the other (24), at the end of the other branch of the divider 21.

 The pair of connectors to be tested 25, for example of the type illustrated in FIG. 3, is arranged coaxially in the chamber 22, with its cables 26 for connection to the doors B and C arranged as in the known device, that is to say coaxially with the divider 21 and charges 23.24.

 The elements 21 to 24 are removable modular elements, easily removable and reassemblable.

 The illumination chamber 22 consists of a cylindrical metal housing 27 closed at its two ends by a circular flange 28, removable thanks to a screw closure system 29.

 According to an essential characteristic of the device of the invention, the chamber 22 is lined with a set 30 of washers made of a material that conducts electricity well, in particular a metal, for example aluminum, stacked coaxially with the chamber and therefore to the connectors under test 25.

 The washers have a variable thickness and an internal axial passage of variable profile, as a function of the local section of the profile along the connector assembly 25 which faces each washer.

 Some washers, such as washers 31 and 32, have a cylindrical central passage 33, while others, such as washers 34 to 37, have a frustoconical central passage 38. The objective is to constitute around the connectors 25 a sort of vault of revolution whose configuration and distance from the connectors make it possible to adjust the impedance characteristic of the propagation medium (inside the illumination) to the desired value so as to avoid, or at least reduce, at certain frequencies, the phenomena of resonance or propagation of waves in parasitic mode different from the propagation mode, of the TEM type, necessary for the proper functioning of this type of measurement bench.

The profile of the washers 30 is determined by experiments as a function of each type of connector to be tested.

 The walls (33 and 38) of the central passages of the washers, or internal sections of said washers, are located opposite sections of changing profiles of the connector assembly 25. In other words and schematically, each internal section (33, 38) a washer corresponds to an average profile of the section opposite the connector assembly.

 The installation in the illumination chamber 22 of the connector assembly to be tested is, as illustrated by FIGS. 4a to 4c, very easy.

 It suffices to remove the housing 39 containing the load 23, which is fixed to the chamber by a flange 40 and screws 41, then to remove the adjoining flange 28, as well as the first washer 34 (FIG. 4c).

 Then, the connector assembly 25, with its cables 26 but without the end sockets 42, is introduced into the bench as illustrated in FIG. 4b.

 Finally, the washer 34-flange 28-housing 39 assembly is replaced (FIG. 4c).

 The divider 21 also comprises (FIG. 4a), at the end of its two opposite branches, a flange 43 for its fixing by screws 44, on the one hand, to the chamber 22 and, on the other hand, to the housing 45 of the load 24, identical to the housing 39.

 It should be noted the simplification brought by the invention to the production of absorbent fillers 23, 24 which consist of the same single material and which are in direct contact with the connecting cables 26 thanks to a rectilinear internal passage formed in the load axis.

 In comparison, the conventional absorbent device (Figure 1) is more complex (absorbent material in four elements inserted between an outer casing and an auxiliary internal spacer 4a), more difficult to carry out and impractical to use because it cannot be dismantled, the resistive loads 14 being welded to the terminal metal parts.

 The absorbent loads 23, 24 can be changed as a result by mounting the terminal resistive loads 14 ′ on a part 39a or 45a forming a flange, removably attached to the end of the housing 39 or 45, so as to put in place or remove very easily said charges 23.24.

 When another type of connector is to be tested, the washers 31, 32, 34, 35, 36, 37 will be replaced by other washers of thickness and profile of the central passage suitable for this other type.

 FIG. 5 shows another pair 25 ′ of connectors having a profile different from that of pair 25. It can be observed that the washers 46 disposed inside the housing 27, 28 of the illumination chamber 22 are all different from those of FIG. 2, with regard to one and / or the other of the following parameters: thickness, axial profile of the central passage, average distance of the internal edge of the washer from the axis of the connectors.

 It will be noted that, for example, the second washer 46 starting from the right has a central passage 47 with a large variation in diameter, from one face to the other of the washer, in order to take account of a significant variation in diameter. the section of the connector assembly 25 'between the parts 48 and 49 of the latter, over a relatively small axial distance. This will result in a relatively thin washer. This will be done by approximation and averaging by decomposing the axial profile of the connectors into successive sections, from one end to the other of the line section disposed inside the illumination chamber.

 The coaxial divider 21 shown in FIGS. 4a to 4c is of a conventional type with conventional 7 ′ impedance transformer of the type 50 Q - 25 Q, like that (7) of FIG. 1.

 In accordance with the invention, it is possible to produce a measurement bench with an illumination chamber and absorbent charges of the type of FIG. 2, operating with a characteristic impedance of 100 Q instead of 50, by judicious dimensioning of the divider 21, associated with a replacement of the washers of the illumination chamber and of the absorbing and resistive end charges 123, 24, 14 ') by others adapted to 100 Q.

 There is shown in Figure 8 such a divider 21 'without impedance transformer, consisting of a supply conduit 50 connected to a conduit 51 of the horizontal branch of the T of the divider.

 In the conduit 50 is disposed a central conductor 52 of constant diameter, while in the conduit 51 is arranged one of the semi-rigid cables 26 of the connector assembly under test, the contact between the core 52 and the cable 26 being ensured by a conventional contact finger 52a.

According to the invention, the ratio between the diameter of the conductor 52 and the internal diameter of the conduit 50, as well as the ratio between the diameter of the cable 26 and the internal diameter of the conduit 51 are determined so as to have on the arm of supply 50 a characteristic impedance of 50 Q and on the output of each branch of the divider 21 'an impedance of 100 Q, from the formula

Where: Q is the relative permittivity of air;
D is the internal diameter of the external conduit
d is the diameter of the internal conductor.

 The use of such a divider 21 'makes it possible to obtain, over the entire frequency range from 1 kHz to 1 GHz, a constant input impedance of the circuit as a function of the frequency, that is to say a function uniform transfer, which is not the case with a divider (21) of the type with impedance transformer 50 2 - 25 Q.

 A characteristic impedance of 100 Q on each branch of the divider, on the other hand, requires adapting the impedance of the washers of the illumination chamber as well as that of the absorbent and resistive end charges 23, 24, 14 '.

 This poses no problem due to the modular design of the washers and loads and their ease of installation.

 Figures 6 and 7 illustrate an embodiment of the charges 23,24 of the invention.

 These consist of two half-shells 24a, 24b assembled and integral, made of an appropriate material, good absorbing electromagnetic waves and having mechanical characteristics suitable for precise machining, as well as for successive assembly and disassembly without damage. .

A particularly suitable material for this purpose is the material sold under the trademark IMOSORB by the company
MICROWAVE INSTRUMENTATION and made of a ferromagnetic resin.

 The two assembled half-shells 24a, 24b form a load 24 of cylindro-conical shape, in direct contact with the connecting cable 26, the conical part being directed towards the power divider. A passage is formed coaxially with the load 24 in correspondence with the diameter of the connecting cable 26.

 The charges 23 and 24 are identical, as well as their casing (39,45) which is cylindro-conical.

 The half-shells 24a, 24b are themselves fixed by their planar external end face on an insulating flange washer 53, which can itself be secured to a metallic annular ring 54 which can itself be attached to an external flange 55 of housing 39.

 In FIGS. 6 and 7, resistive loads of impedance adaptation are shown at 14 ′, analogous to the resistive loads 14 of the conventional assembly (FIG. 1), and connected between the external washer 54 and an internal metal washer 56 in connection electric with socket 42.

 As a variant, the set of resistive loads 1 4 ′ can be replaced by a washer shown diagrammatically at 14 "in FIG. 6 and made of a resistive material ensuring a homogeneous distribution over the entire circumference, said washer being attached to the flange (53, 54,56) and ensuring the electrical connection between parts 54 and 56.

 The loads 23, 24 have a different morphology and dimensioning depending on the impedance adaptation that one wants to achieve.

 The housings 27, 28, 39, 45 as well as the divider 21 are made of an electrically conductive material, for example brass.

 The measuring bench of the invention can be used for measuring the transfer impedance of connectors, generally a pair of male-female connectors, the diameter of which can reach 60 mm for example and the length 150 mm, whatever the complexity of the profile, as well as of cables, in particular large diameter multiconductor cables, regardless of the shielding system of the active conductors, without the size of these connectors or cables causing impedance mismatching, in particular in the range 1 KHz - 1OGHz, detrimental to the accuracy of the measurement.

 Tests have shown that, with the bench of the invention, mismatch losses of less than 0.5 dB were noted for frequencies of the order of 1 GHz.

Claims (8)

 1. Transfer impedance measurement bench in particular for connectors or the like of large diameter and / or non-uniform profile, consisting of a measurement arrangement of the suitable tri-coaxial type comprising a chamber (22), called illumination, in which the connector or the like under test (25) is arranged, placed in one of the branches of a power divider (21) supplied by an electromagnetic wave source of variable frequency within a determined range, a load of energy absorption (23,24) at each end of said branches and of the measurement means connected to the outlet of the branch of the illumination chamber, characterized in that said illumination chamber (22) consists of '' a housing in which are stacked, coaxially with the connector or the like under test (25), a series (30) of washers placed side by side in material which is good conductor of electricity and the internal contour of each of which is adapted to the opposite part of the profit l of said connector or the like, so as to adjust the characteristic impedance of the propagation medium to the desired value  2. Measuring bench according to claim 1, characterized in that said housing consists of a cylindrical body (27) provided at its ends with flanges (28) removably mounted on said cylindrical body and provided in their center with a passage of the cable (26) connecting the connector under test (25) to the output doors (B, C) of the test bench.  3. Measuring bench according to claim 1 or 2, characterized in that the washers (30) have a central coaxial passage whose wall is cylindrical (33) or frustoconical (38).  4. Measuring bench according to one of claims 1 to 3, characterized in that the washers are metallic.  5. Measuring bench according to one of claims1 to 4, characterized in that each energy absorption load (23,24) is made of a single material, consisting of a machinable ferromagnetic resin and shaped into two half cylindrical-conical shells (24a, 24b), removable, assembled directly around the connecting cable (26) from the connector under test to the measurement doors (B, C) of the bench, the shapes and dimensions of the half-shells (24a, 24b ) being determined so as to obtain, in combination with a terminal resistive load (14 ') removably mounted at the end of each branch, an impedance characteristic of said branches of the desired value.  6. Measuring bench according to one of claims 1 to 5, characterized in that it comprises a power divider (21 ') in T whose two opposite branches are determined, as for the diameter of the internal conductor (26) and to the internal diameter of the external conductor (51), so as to have a characteristic impedance of 100 Q, the lateral arm (50,52) being devoid of impedance transformer.  7. Measuring bench according to claim 5, characterized in that said resistive loads (14 ') are removably mounted on the housings (39,45) containing the absorbent charges (23,24), said housings themselves being removably mounted on the illumination chamber (22) or the power divider (21,21 ').  8. Measuring bench according to claim 5, characterized in that the resistive loads (14 ') consist of a washer made of a resistive material (14 ") attached to an end flange (53,54,56) mounted to it - even removably on the housing (39,45) containing the absorbent load (23,24), said housing itself being removably mounted on the illumination chamber (22) or the power divider (21, 21 ').