Ultra-wideband measuring bridge

Abstract

Measuring bridge (1) comprising a first matching element (2), a second matching element (3) and a third matching element (4) , wherein all matching elements (2, 3, 4) consist of at least three resistors (21, 22, 23, 31, 32, 33, 41, 42, 43) arranged in a T-structure, wherein a second resistor (32) of the second matching element (3) is connected to a second resistor (22) of the first matching element (2) and a third resistor (43) of the third matching element (4) is connected to a third resistor (23) of the first matching element (2) and wherein a second resistor (42) of the third matching element (4) can be connected to a measuring object (7), wherein a third resistor (33) of the second matching element (3) can be connected to a calibration standard (5), characterized by that a first resistor (31, 41) of the second and third matching element (3, 4) is connected to a signal input of a component (11) in the form of a balun (111) or a differential amplifier (112) that suppresses a common-mode component on its two signal inputs.

Description

[0001] The invention relates to a measuring bridge, which can be used, for example, within a network analyzer to supply an excitation signal to a device under test and to simultaneously supply reflections of the device under test to measuring electronics, and a balun, which can be used in the measuring bridge, wherein the measuring bridge has a very high bandwidth.

[0002] Publication CZ 2009 0770 A3 discloses a measuring system for measuring extreme impedances of microwave circuit elements. US 7 126 347 B1 discloses a broadband differential reflection bridge with a 50-ohm input and a 50-ohm output or a 100-ohm output. DE 10 2007 021 899 A1 discloses a microwave circuit with a defective ground structure filter. DE 40 17 412 A1 discloses a bidirectional bridge for use as a reflectormeter test device.

[0003] In addition to measuring reflections from a device under test (DUT), a measuring bridge can also be used to investigate the DUT's transmission behavior in more detail. In this case, one terminal of the DUT is connected to the first port of the network analyzer, while the other terminal is connected to the second port. An excitation signal is generated within the network analyzer and applied to the DUT at its first terminal. The signal transmitted by the DUT is output, for example, at the second terminal and fed to the second port of the network analyzer. This second port is also connected to a measuring bridge to feed the transmitted signal to the measurement electronics, which preferably measure its magnitude and phase.

[0004] It is important that the bandwidth of the usable frequency range is as large as possible, i.e., extends over several decades, which in turn requires a measuring bridge that has high directivity at both low and high frequencies.

[0005] A measuring bridge suitable for a wide frequency range is already known from DE 10 2006 005 040 A1. This bridge comprises a resistive bridge for a low frequency range and a hybrid coupler for a high frequency range, with switches providing the ability to toggle between these two units. A disadvantage of DE 10 2006 005 040 A1 is that, due to the different modules used for different frequencies, the bridge's construction is complex and its operation requires extensive calibration routines.

[0006] The object of the present invention is therefore to create a measuring bridge that is easier to manufacture and can be operated over a very wide bandwidth, and a balun that can be used in this context.

[0007] The problem is solved by the features of claim 1. Advantageous further developments of the measuring bridge according to the invention are specified in the dependent claims.

[0008] The measuring bridge according to the invention comprises a first matching element, a second matching element, and a third matching element, each matching element consisting of at least three resistors arranged in a T-structure. A second resistor of the second matching element is connected to a second resistor of the first matching element, and a third resistor of the third matching element is connected to a third resistor of the first matching element. A second resistor of the third matching element is connectable to a device under test. A third resistor of the second matching element is connectable to a calibration standard. A first resistor of the second and third matching elements is each connected to a signal input of a component in the form of a balun or a differential amplifier, which suppresses a common-mode component on its two signal inputs.

[0009] It is particularly advantageous to use three symmetrical matching elements consisting of at least three resistors arranged in a T-structure, because this ensures that only a common-mode signal is present at the output of two matching elements in a balanced measuring bridge.

[0010] A further advantage of the measuring bridge according to the invention is that the third resistor of the second matching network can be connected to the calibration standard, and the first resistor of the second and third matching networks is each connected to a signal input of the component that suppresses a common-mode component on its two signal inputs. It is particularly advantageous here that the signals at the output of the second and third matching networks are fed to such a component. If the measuring bridge is balanced, no voltage can be measured at the output of this component. If the measuring bridge is not balanced, a differential-mode signal is superimposed on the common-mode signal, which is not suppressed by the component and can therefore be measured at the component's output. This occurs when the termination impedance of the device under test differs from the termination impedance of the calibration standard.

[0011] The measuring bridge according to the invention also offers an advantage if the resistors of the first, second, and third matching network are designed as thin-film resistors on a substrate, e.g., a ceramic or quartz substrate, or another support medium. These resistors can be precisely trimmed to their exact value using a laser, while simultaneously minimizing the parasitic capacitances and inductances of these resistors compared to discrete components.

[0012] Furthermore, it is particularly advantageous if the measuring bridge according to the invention comprises a bias device connected to the second resistor of the third matching element, and if the measuring bridge according to the invention comprises a dummy bias device connected to the third resistor of the second matching element, and if the dummy bias device influences the measuring bridge in the same way as the bias device, thereby making the measuring bridge symmetrical. It is particularly advantageous that such a measuring bridge can also be used to test objects that require a bias voltage for their operation. The line branch to the object being measured has the same electrical properties as the line branch to the calibration standard. The dummy bias device preferably comprises the same components, with the difference that it does not necessarily have to provide a bias voltage.

[0013] Furthermore, the common-mode suppressing component within the measuring bridge according to the invention can be formed by a balun according to the invention, wherein the balun comprises a first section containing a substrate on the upper side of which a first signal line and at least one second signal line are guided, and on the lower side of which a thin-film resistive layer is formed below the signal lines. It is particularly advantageous that such a thin-film resistive layer, formed directly below the signal lines, ensures that the common-mode component is attenuated more strongly than the differential-mode component. A balun constructed in this way is suitable for operation up to very high frequencies.

[0014] A further advantage exists if the balun according to the invention comprises a second section, wherein the second section has a coaxial line whose inner conductor is connected to the first signal line and whose outer conductor is connected to the second signal line at a first end of the coaxial line, and wherein at least one first ferrite surrounds the coaxial line. The use of such a ferrite surrounding the coaxial line enables the balun to reliably suppress the common-mode component even at very low frequencies.

[0015] Furthermore, the balun according to the invention offers an advantage if the thin-film resistive layer in the first section has a DGS structure (Defective Ground Structure), such that the thin-film resistive layer is split into two separate thin-film resistive layers. The width of the DGS structure is to be optimized according to the substrate material used, and it is formed directly between the two signal lines on the underside of the substrate. Such a DGS structure has the advantage that the differential-mode signal is attenuated far less than the common-mode signal, thereby increasing the common-mode rejection ratio of the balun.

[0016] A further advantage of the balun according to the invention is that the outer conductor of the coaxial cable is connected to the reference ground at a second end and / or that the balun has a compensation line, preferably consisting of a wire, the first end of which is connected to the first signal line. Such a compensation line ensures that the inner conductor has the same inductance with respect to the housing ground as the outer conductor, and thus the bridge is balanced even at low frequencies when the termination impedance of the device under test is equal to the termination impedance of the calibration standard.

[0017] Finally, the balun according to the invention offers a further advantage if the end of the coaxial cable furthest from the first section of the balun is connected to a first pot core, wherein the first pot core comprises a coaxial cable wound in a ferrite core, and / or if the compensation cable is connected to a second pot core, wherein the second pot core comprises a cable wound in a ferrite core, in particular a wire, and wherein the second pot core has the same electrical properties as the first pot core. Such a pot core ensures that the common-mode component is reliably suppressed even at lower frequencies. A measuring bridge in which such a balun is used can therefore be operated over a very wide bandwidth.

[0018] Various embodiments of the invention are described below by way of example with reference to the drawing. Identical objects have the same reference numerals. The corresponding figures in the drawing show in detail: Fig. 1A an embodiment of an equivalent circuit diagram of the measuring bridge according to the invention; Fig. 1B a further embodiment of an equivalent circuit diagram of the measuring bridge according to the invention; Fig. 1C another embodiment of an equivalent circuit diagram for the measuring bridge according to the invention; Fig. 2A an embodiment for the realization of three matching elements for the measuring bridge according to the invention; Fig. 2B an embodiment for the realization of a calibration standard of the measuring bridge according to the invention; Fig. 2C is an embodiment of the measuring bridge according to the invention, which is suitable for connection to a separate calibration standard; Fig. 3 an embodiment of a chocked balun; Fig. 4 an embodiment of a resistive balun; Fig. 5 a representation that describes the field distribution for common-mode and differential-mode signals in a resistive balun; Fig. 6 an embodiment of a resistive balun according to the invention with a DGS structure; Fig. 7 an embodiment of an embodiment of a combination according to the invention of a balun with ferrites and a compensation line and a resistive balun; Fig. 8 an embodiment of an embodiment of a combination according to the invention of a balun with ferrites and a compensation line and a resistive balun; Fig. 9 another embodiment of a combination according to the invention of a balun with ferrites and a compensation line and a resistive balun with a DGS structure; Fig. 10 an embodiment of an embodiment of a combination according to the invention of a balun with ferrites and a compensation line and a resistive balun with a DGS structure; Fig. 11 an embodiment of an embodiment of a combination according to the invention of a balun with ferrites and a compensation line and a resistive balun with a DGS structure together with pot cores; and Fig. 12 an embodiment of a separate routing of a coaxial line and a compensation line through spatially separated ferrites.

[0019] Fig. Figure 1A shows an exemplary embodiment of an equivalent circuit diagram of the measuring bridge 1 according to the invention. The measuring bridge 1 according to the invention comprises a first matching element 2, a second matching element 3, and a third matching element 4. The first matching element 2 consists of at least three resistors 21, 22, 23 arranged in a T-structure. The second matching element 3 also consists of at least three resistors 31, 32, 33, also arranged in a T-structure. The same applies to the third matching element 4, which also consists of at least three resistors 41, 42, 43, arranged in a T-structure. The second resistor 22 of the first matching element 2 is connected to the second resistor 32 of the second matching element 3. The third resistor 23 of the first matching element 2 is connected to the third resistor 43 of the third matching element 4.

[0020] The third resistor 33 of the second matching network 3 is connected to a calibration standard 5. This calibration standard 5 is either formed directly within the measuring bridge 1 or it can be connected to the measuring bridge 1. In this case, the measuring bridge 1 has a first connection element 6, which can be, for example, a terminal socket, to which the separate calibration standard 5 is connected, in particular screwed.

[0021] On the other side, the second resistor 42 of the third matching element 4 is connected to the object 7 to be measured. The object 7 is preferably screwed to a second connecting element 8, the second connecting element 8 preferably being designed as a terminal socket and connected to the second resistor 42 of the third matching element 4.

[0022] The first resistor 21 of the first matching network 2 is connected to a signal generator 9. This signal generator 9 can be located within a network analyzer and connected to the first resistor 21 of the first matching network 2 via a third connection element 10. The third connection element 10 is preferably also a connector. However, it is also possible that the measuring bridge 1 is integrated directly within a network analyzer as a plug-in card, in which case the third connection element 10 is simply a plug connector or intersubstrate connector.

[0023] The resistors of the first matching network 2, the second matching network 3, and the third matching network 4 must be selected such that the signal generator 9 sees a matched load. Furthermore, the second matching network 3 must have an impedance in the direction of the calibration standard 5 (match) that corresponds to that of the calibration standard 5, so that no further reflections occur. The same applies to the third matching network 4, whose resistors are selected such that no additional reflections occur when a device under test 7 is connected, which has the same impedance as the calibration standard 5. For a 50-ohm system, for example, all resistors within the first matching network 2, the second matching network 3, and the third matching network 4 have a resistance value of approximately 16.78 ohms.

[0024] Furthermore, the first resistor 31 of the second matching network 3 and the first resistor 41 of the third matching network 4 are connected to a component 11 which suppresses a common-mode component on its two signal inputs. In the exemplary embodiment, the common-mode suppressing component 11 is a Fig. Figure 1A shows a balun 111. It is clearly visible that a first signal line 121 connects the inner conductor of the balun 111 to the first resistor 31 of the second matching network 3. Conversely, a second signal line 122 connects the first resistor 41 of the third matching network 4 to an outer conductor of the balun 111. The balun 111 can be constructed very simply, for example, using a coaxial cable surrounded by ferrites. The end of the balun 111 facing away from the measuring bridge 1 is connected to a fourth terminal 13, to which the network analyzer's measurement electronics, in particular various mixer stages and / or analog-to-digital converters, are connected. The outer conductor of this side of the balun 111 is connected to the reference ground. The balun 111 converts a differential signal into a ground-referenced signal.

[0025] If the device under test 7 terminates the connection between the second resistor 42 and the device under test 7 in the same way as the calibration standard 5, the signals on the first signal line 121 and the second signal line 122 are equal in amplitude and phase. The balun 111 suppresses this common-mode component, so that no voltage with respect to ground can be measured at the fourth terminal 13. The situation is different if the second terminal 8 is not terminated by the device under test 7 in the same way as by the calibration standard 5. In this case, the signals on the first signal line 121 and the second signal line 122 are not exactly equal, so that the differential-mode component with respect to ground can be measured at the fourth terminal 13.

[0026] This setup allows the reflection behavior of the object 7 to be measured without difficulty. As already explained, the calibration standard 5 can also be arranged within the measuring bridge 1, thus eliminating the need for the first connection element 6. In this case, however, the measuring bridge 1 exhibits a slight asymmetry, which must be determined by means of a calibration procedure, because the object 7 still needs to be connected to the measuring bridge 1 via the second connection element 8. To achieve the highest possible bandwidth, the third resistor 33 of the second matching network 3 should therefore preferably be connected to a first connection element 6. The first connection element 6 should have the same electrical properties as the second connection element 8.The achievable isolation between the generator path (signal generator 9) and the measurement path (connection element 13) therefore depends on how well the connection for the object under test 7 is replicated by the calibration standard 5 in the case “Adapted”.

[0027] As will be explained later, the resistors of the first, second, and third matching network 2, 3, 4 are preferably thin-film resistors formed on a substrate 25, e.g., a ceramic or quartz substrate, or another suitable support material. The calibration standard 5, if integrated directly into the measuring bridge 1, can also be implemented as one or more thin-film resistors.

[0028] Fig. Figure 1B shows a further embodiment of an equivalent circuit diagram of the measuring bridge 1 according to the invention, wherein a bias voltage can be applied to the object 7 under test. This is done via a so-called bias device 20. The bias device 20 provides an adjustable direct current, decoupled from the high frequency, at the second terminal element 8, to which the object 7 under test is connected. The insertion of such a bias device 20 generally reduces the directivity achievable with the measuring bridge 1. To compensate for the effects of the bias device 20 on the directivity, the measuring bridge according to the invention is supplemented by a dummy bias device 21 on the side of the calibration standard 5. The circuit design of this dummy bias device 21 is the same as that of the bias device 20. The difference is that the dummy bias device 21 does not supply a direct current to the calibration standard 5.The measuring bridge 1 according to the invention is thereby brought into a symmetrical state because changes in the phase position due to the bias device 20 are also caused by the dummy bias device 21 on the calibration standard 5.

[0029] Another difference between the embodiment from Fig. 1A and the embodiment from Fig. 1B consists in the fact that the second resistor 22 of the first matching element 2 and the second resistor 32 of the second matching element 3 have been combined to form a first equivalent resistance 221. This equivalent resistance has a value of approximately 33.56 ohms for the 50-ohm system. Furthermore, the third resistor 23 of the first matching element 2 and the third resistor 43 of the third matching element 4 have also been combined to form a second equivalent resistance 222. This second equivalent resistance 222 also has a value of approximately 33.56 ohms for the 50-ohm system. The remaining operation of the measuring bridge 1 according to the invention is described below. Fig. 1B corresponds to the functioning of the measuring bridge 1 according to the invention. Fig. 1A, therefore, regarding the other components, refer to the explanations on Fig. 1A is referred to.

[0030] Fig. Figure 1C shows a further embodiment of an equivalent circuit diagram of the measuring bridge 1 according to the invention, wherein a differential amplifier 112 has been used for the common-mode suppressing component 111. This differential amplifier 112 should, however, have a sufficiently high common-mode rejection ratio. This allows the spatial arrangement of the measuring bridge 1 according to the invention to be reduced in size. However, the upper cutoff frequency is significantly lower when using a differential amplifier 112 compared to using a balun 111. Thus, differential amplifiers 112 that have a sufficiently high common-mode rejection ratio are currently only suitable up to an upper frequency range of approximately 5 GHz. The remaining circuit structure consists of Fig. 1C corresponds to the circuit structure from Fig. 1B and Fig. 1A, which is why reference is made to the previous sections of the description.

[0031] Fig. Figure 2A shows an embodiment for the realization of the three matching elements 2, 3, 4 of the measuring bridge 1 according to the invention. The measuring bridge 1 according to the invention is realized using thin-film technology based on microstrip lines. The first resistor 31 and the third resistor 33 of the second matching element 3, the first resistor 41 and the second resistor 42 of the third matching element 4, and the first resistor 21 of the first matching element 2 are clearly visible.

[0032] As already mentioned Fig. As explained in Figure 1B, the second resistor 32 of the second matching network 3 and the second resistor 22 of the first matching network 2 are combined to form a first equivalent resistor 221. The same applies to the third resistor 23 of the first matching network 3 and the third resistor 43 of the third matching network 4, which are combined to form the second equivalent resistor 222. These resistors are thin-film resistors that can be trimmed to their nominal value using a laser.

[0033] The measuring bridge 1 according to the invention is formed on a substrate 25, e.g. on a ceramic 25 or a quartz substrate 25 or another suitable support medium 25. The parasitic losses of a ceramic support are significantly lower at high frequencies than those of the known plastics used in printed circuit boards.

[0034] Fig. Figure 2B shows an embodiment for the realization of a calibration standard 5 which is formed directly on the measuring bridge 1 according to the invention. It is clearly visible that a first dashed area 26 within Fig. 2B the excerpt from Fig. 2A contains. Regarding the first dashed area 26, reference is made to the descriptive sections. Fig. 2A referred. The calibration standard 5 can be replicated by a simple 50-ohm resistor, especially at low frequencies when the phase shift of the supply lines and the influences of the transitions for coupling the device under test 7 are negligible. Within Fig. In Figure 2B, this calibration standard 5 is represented by two 100-ohm thin-film resistors 271 and 272, which are connected in parallel to the reference ground on the back side of the substrate 25 (i.e., on the back side of the support material 25) via vias 281 and 282. These two 100-ohm thin-film resistors 271 and 272 can also be trimmed to their exact nominal value using a laser. A conductor track 29 is also shown, which connects the third resistor 33 of the second matching network 3 to the two 100-ohm thin-film resistors 271 and 272. This conductor track 29 has a V-shaped recess 30 at its end facing the calibration standard 5. This prevents the current flow from reaching the end, thereby reducing the capacitive stray field.

[0035] Fig. Figure 2C shows an embodiment of the measuring bridge 1 according to the invention, which is suitable for connection to a separate calibration standard 5. It is clearly visible in Fig. 2C in the middle of area 26 from Fig. 2B, which contains the three matching elements 2, 3, 4, as is the case with Fig. 2A has been described. Furthermore, the measuring bridge 1 according to the invention comprises Fig. 2C comprises a first connection element 6, a second connection element 8, and a third connection element 10. The connection elements 6, 8, and 10 can be terminal sockets. A separate calibration standard 5 can be screwed onto the first connection element 6, while the second connection element 8 is connected to the device under test 7. The third connection element 10 can be connected to the signal generator 9, as already explained. It is important that the first connection element 6 has the same electrical properties as the second connection element 8. This means that the phase shifts caused by the first connection element 6 and the second connection element 8, as well as the signal attenuation at the two connection elements 6 and 8, must be approximately the same. Preferably, the third connection element 10 also has the same electrical properties.It is clearly visible that the measuring bridge 1 according to the invention has a housing 31 in addition to the substrate 25. This housing 31 is preferably made of a conductive metal, such as aluminum, and prevents interference radiation from coupling into the measuring bridge 1.

[0036] Fig. Figure 3 shows an embodiment of a chocked balun 111. As already described in Fig. As explained in section 1A, the first signal line 121 is connected to the inner conductor 35 of the balun 111, whereas the second signal line 122 is connected to the outer conductor 36 of the balun 111. The balun 111 from Fig. 3 has a coaxial structure. The inner conductor 35 is radially surrounded by an outer conductor 36. In the embodiment shown Fig. 3. Two ferrites, 371 and 372, are still present, radially surrounding the outer conductor 36. The interfering common-mode currents I MThe currents on the outer conductor 36 are suppressed by means of the ferrites 371 and 372. This interfering common-mode current I M which would lead to an unbalanced signal component at the output of the balun 111, is, as already explained, suppressed by the ferrites 371, 372 on the sheath of the coaxial cable, so that the same current I is present in both output lines 35, 36 B flows in opposite directions.

[0037] The ferrites 371, 372 are preferably designed as ferrite rings, which can have different lengths and sizes and can be made of different ferrite materials. These ferrites 371, 372 can be arranged at different intervals on the coaxial conductor. The sequence and arrangement of the ferrites 371, 372 depends on the frequency range to be covered and must be adapted in each case. The adaptation can be carried out by connecting both the first terminal element 6 to the calibration standard 5 and the second terminal element 8 to a known calibration standard 5. A signal is fed into the measuring bridge 1 according to the invention via the third terminal element 10 above the signal generator 9. The ferrites are then moved along the outer conductor 36 until the smallest possible voltage minimum is measured at the fourth terminal element 13. With ideal components, this voltage would be zero.

[0038] However, problems arise with increasing frequencies, caused by resonance phenomena on the outer conductor 36 of the coaxial cable 38. Furthermore, the behavior of the ferrite materials at higher frequencies is not ideal and is partly undefined. For this reason, a different balun concept is preferred for balancing frequencies above, for example, 4 GHz.

[0039] Fig. Figure 4 shows an embodiment of a resistive balun 111, which can be used for higher frequencies. The first signal line 121 and the second signal line 122 are clearly visible. These two signal lines 121 and 122 are balanced lines arranged on the top side of the substrate 25. A ground plane is arranged on the back side of the substrate 25, with a resistive layer 32 embedded within the ground plane, located below the first signal line 121 and the second signal line 122. A housing 31 also ensures that this resistive balun 111 is shielded.

[0040] Fig. Figure 5 shows an embodiment which determines the field distribution for common-mode and differential-mode signals in the resistive balun 111. Fig. 4 describes in more detail. The left figure in the drawing of Fig. Figure 5 describes the field distribution for a common-mode signal in a resistive balun 111. It is clearly visible that the current flows in the same direction in both signal lines 121 and 122. As already mentioned in Figure 5, the signal lines 121 and 122 are... Fig. As discussed in section 4, the substrate 25 separates the signal from an underlying resistive layer 32. The substrate 25 is preferably a ceramic. The field lines of the electric field are clearly visible. As already explained, with the common-mode signal, the current in both signal lines 121 and 122 flows in the same direction. The circuit is closed by the current in the ground plane to the back of the substrate. This current can be attenuated by means of the resistive layer 32 on the back of the substrate.

[0041] The situation is different, however, with a push-pull signal, as shown in the right-hand diagram. Fig. Figure 5 shows this. In this case, the currents in the first signal line 121 and in the second signal line 122 flow in opposite directions. Such a push-pull signal is only slightly affected by the resistive layer, as the field distribution of the electric field shows.

[0042] Fig. Figure 6 shows an embodiment of a resistive balun 111 according to the invention with a DGS structure 39 (Defective Ground Structure). To further reduce the influence of the resistive layer 32 on the push-pull signal, the portions of the resistive layer 32 located directly beneath the first signal line 121 and the second signal line 122 are removed. The DGS structure 39 splits the resistive layer 32, which is a thin-film resistive layer 32, into two separate thin-film resistive layers 32. The width of the DGS structure 39 is adapted to the substrate material used and the frequency range to be covered. It is formed directly between the two signal lines 121 and 122 on the underside of the substrate 25. In the example from Fig. 6 the width of the DGS structure 39 is significantly larger than the distance between the first signal line 121 and the second signal line 122.

[0043] The attenuation of the push-pull signal can be further reduced by decreasing the slot width between the first signal line 121 and the second signal line 122. This concentrates the electromagnetic field in the area of ​​the slots and reduces the field components in the area of ​​the resistive layer 32, thus bringing the first signal line 121 and the second signal line 122 closer together. The slot width can be selected to cover a range from, for example, a few micrometers to, for example, a few millimeters.

[0044] The use of such a DGS structure 39 results in the field concentration on the resistive layer 32 decreasing in the case of a differential-mode signal, whereas in the case of a common-mode signal the electromagnetic fields are concentrated on the edges of the resistive layer 32, which significantly dampens the common-mode signal more than the differential-mode signal.

[0045] However, balancing frequencies below 4 GHz would require a very long resistive layer 32, which would lead to disproportionately large losses of the push-pull signal at high frequencies.

[0046] For this reason, the balun 111 according to the invention preferably combines the concepts from Fig. 3, Fig. 4 and Fig. 6. Therefore, it shows Fig. Figure 7 shows an embodiment of a balun 111 according to the invention, comprising both ferrites 371, 372, 373, 374, 375, and a resistive component. The balun 111 according to the invention consists of a first section 50 and a second section 51. The first section 50 shows the structure of the resistive part, as it is related to the Fig. 4, Fig. 5 and Fig. Section 6 was explained. The second section, 51, shows the structure as it leads to the Fig. 3 was explained. It is clearly visible that the first section 50 comprises a substrate 25, on the upper side of which the first signal line 121 and at least one second signal line 122 are routed. On the underside of the substrate 25, in the exemplary embodiment, Fig. 7 below the signal lines 121, 122 a thin-film resistive layer 32 is formed.

[0047] As already explained, this thin-film resistive layer 32 attenuates the common-mode signal on the first and second signal lines 121, 122 significantly more than the differential-mode signal on the two lines. The first section 50 of the balun 111 according to the invention ensures that the common-mode signal is effectively suppressed for frequencies above approximately 4 GHz. As already explained, this thin-film resistive layer 32 would have to be very long in order to also attenuate a low-frequency common-mode signal effectively. For this reason, the second section 51 of the balun 111 according to the invention consists of a coaxially constructed balun 111, wherein the coaxial conductor 38 is surrounded by several ferrites 371 to 375. The inner conductor 35 of the coaxial line 38 of the second section 51 of the balun 111 according to the invention is connected to the first signal line 121, wherein the outer conductor 36 is connected at a first end of the coaxial line 38 to the second signal line 122.

[0048] Furthermore, the second section 51 of the balun 111 according to the invention optionally includes a compensation line 52, which is preferably a wire. This compensation line 52 is also connected at its first end to the first signal line 121 and thus to the inner conductor 35 of the coaxial line 38. Due to the ferrites 371 to 375, the inductance of the outer conductor 36 of the coaxial line 38 increases, whereas, due to the coaxial design, the inductance of the inner conductor 35 does not increase. This would lead to an asymmetrical design of the balun 111 according to the invention, depending on the respective operating frequency. For this reason, the inner conductor 35 is connected to a separate compensation line 52, which is also preferably routed through the same ferrites 371 to 375 as the coaxial line 38.In this case, the inductance of the inner conductor 35 with respect to the housing ground increases to the same extent as the inductance of the outer conductor 36 with respect to the housing ground. The balun 111 according to the invention exhibits symmetrical behavior regardless of the operating frequency.

[0049] Fig. Figure 8 shows an embodiment of a combination according to the invention of a balun 111 with ferrites 371 to 375 and a resistive balun 111. In contrast to Fig. Figure 7, which shows a top view of an embodiment of the balun 111 according to the invention, illustrates Fig. Figure 8 shows a spatial view of an embodiment of the balun 111 according to the invention. The first section 50 and the second section 51 are also clearly visible. The first section 50 forms the resistive part of the balun 111 according to the invention, whereas the second section 51 shows the so-called “choked balun”. Fig. Figure 8 also shows the compensation line 52, which is guided through the same ferrites 371 to 375 as the coaxial line 38, thus making the balun 111 according to the invention symmetrically constructed. A housing 31 shields the balun 111 according to the invention from the environment.

[0050] Fig. Figure 9 shows a further embodiment of a combination according to the invention of a balun 111 with ferrites 371 to 375 and a resistive balun 111 with a DGS structure 39. The DGS structure 39, which ensures that the common-mode component is attenuated significantly more than the differential-mode component, is clearly visible. The remaining structure consists of Fig. 9 corresponds to the one from Fig. 7, which is referred to in this context.

[0051] Fig. Figure 10 shows an embodiment of a combination according to the invention of a balun 111 with ferrites 371 to 375 and a compensation line 52 and a resistive balun 111 with a DGS structure 39. Fig. Figure 10 shows a spatial view of the balun 111 according to the invention, whereas Fig. Figure 9 shows a top view of the same. The housing 31, which shields the balun 111 according to the invention, is also clearly visible.

[0052] Fig. Figure 11 shows an embodiment of a combination according to the invention of a balun 111 with ferrites 371 to 375 and a compensation line 52 together with pot cores 601, 602 and a resistive balun 111 with a DGS structure 39. The compensation line 52 is insulated from the coaxial line 38. This can be achieved by providing either the compensation line 52 and / or the coaxial line 38 with heat-shrink tubing.

[0053] Two pot cores 601 and 602 are clearly visible, serving to extend the operating frequency range up to frequencies in the kHz range. The end of the coaxial line 38 located away from the first section 50 of the balun 111 is connected to a first pot core 601, which comprises a coaxial line 38 wound in a ferrite core. Furthermore, the end of the compensation line 52, located away from the first section 50 of the balun 111, is connected to a second pot core 602, which comprises a line 52 wound in a ferrite core and has approximately the same electrical properties as the first pot core 601.Preferably, the pot cores 601, 602 are not connected to the coaxial cable 38 and the compensation cable 52, but the coaxial cable 38 is wound in the first pot core 601, whereas the compensation cable 52 is wound in the second pot core 602.

[0054] The compensation line 52, which preferably has electrical insulation, is connected to the reference ground at the output of the second pot core 602. The outer conductor 38 is also connected to the reference ground at the output of the first pot core 601. Measuring electronics (not shown) determine the magnitude and phase of the voltage of the inner conductor 35 relative to the reference ground.

[0055] Fig. Figure 12 shows an embodiment of a separate routing of a coaxial line 38 and a compensation line 52 through various ferrites 701 to 70. 10 or 711 to 71 10 In this case, the coaxial cable 38 is connected through the ferrites 701 to 70.10 guided, whereas the compensation line 52 through the ferrites 711 to 71 10 is guided. The height through which the ferrites 701 to 70 10 The increase in inductance of the outer conductor 38 should be equal to the height through which the ferrites 711 to 71 pass. 10 increase the inductance of the compensation line 52.

[0056] Within the scope of the invention, all described and / or drawn features can be combined with one another as desired. The invention is not limited to the described embodiments. Naturally, for example, more than 10 ferrites or fewer than 10 ferrites can also be used.

Claims

[1] Measuring bridge (1) comprising a first matching element (2), a second matching element (3) and a third matching element (4) , wherein all matching elements (2, 3, 4) consist of at least three resistors (21, 22, 23, 31, 32, 33, 41, 42, 43) arranged in a T-structure, wherein a second resistor (32) of the second matching element (3) is connected to a second resistor (22) of the first matching element (2) and a third resistor (43) of the third matching element (4) is connected to a third resistor (23) of the first matching element (2) and wherein a second resistor (42) of the third matching element (4) can be connected to a measuring object (7), wherein a third resistor (33) of the second matching element (3) can be connected to a calibration standard (5), characterized by , that a first resistor (31, 41) of the second and third matching element (3, 4) is connected to a signal input of a component (11) in the form of a balun (111) or a differential amplifier (112) that suppresses a common-mode component on its two signal inputs. [2] Measuring bridge according to claim 1, characterized by , that a first resistor (21) of a first matching element (2) can be connected to a signal generator (9) and / or that a second resistor (22) of the first matching element (2) is combined with a second resistor (32) of the second matching element (3) to form a first equivalent resistor (221) and / or that a third resistance (23) of the first matching element (2) is combined with a third resistance (43) of the third matching element (4) to form a second equivalent resistance (222). [3] Measuring bridge according to one of the preceding claims, characterized by , that the resistors (21, 22, 23, 31, 32, 33, 41, 42, 43) of the first, second and third matching network (2, 3, 4) are designed as thin-film resistors on a substrate (25) and / or that the substrate (25) is a ceramic or quartz substrate. [4] Measuring bridge according to claim 2, characterized by , that the calibration standard (5) is formed by at least one thin-film resistor (271, 272) on the substrate (25) and / or that a conductor track (29) connecting the at least one calibration standard (5) designed as a thin-film resistor (271, 272) to the second matching element (3) has a V-shaped recess (30) at its end facing the calibration standard (5), which reduces a capacitive stray field, and / or that the measuring bridge (1) has a first connection element (6) via which the second matching element (3) can be connected to a separate calibration standard (5), wherein the first connection element (6) has the same electrical properties as a second connection element (8) which connects the third matching element (4) to the object being measured (7). [5] Measuring bridge according to claim 1, characterized by , that the measuring bridge (1) includes a bias device (20) which is connected to the second resistor (42) of the third matching element (4), that the measuring bridge (1) includes a dummy bias device (21) which is connected to the third resistor (33) of the second matching element (3) and that the dummy bias device (21) is constructed in the same way as the bias device (20), making the measuring bridge (1) symmetrical. [6] Measuring bridge according to any one of claims 1 to 5, characterized by , that a balun (111) is inserted into the measuring bridge (1) as a common-mode suppressing component (11), wherein the balun (111) has a first section (50) wherein the first section (50) comprises a substrate (25) on the top of which a first signal line (121) and at least a second signal line (122) are guided and on the bottom of which a thin-film resistive layer (32) is formed below the signal lines (121, 122). [7] Measuring bridge according to claim 6, characterized by , that the balun (111) includes a second section (51), wherein the second section (51) has a coaxial line (38) whose inner conductor (35) is connected to the first signal line (121) and whose outer conductor (36) is connected at a first end of the coaxial line (38) to the second signal line (122), and where at least one first ferrite (371, 372, 373, 374, 375; 701, 70 10) surrounds the coaxial cable (38). [8] Measuring bridge according to claim 6 or 7, characterized by , that the first signal line (121) and the second signal line (122) are arranged close to each other and / or that the substrate (25) is a ceramic or quartz substrate. [9] Measuring bridge according to one of claims 6 to 8, characterized by , that the thin-film resistive layer (32) has a DGS structure (39) which splits the thin-film resistive layer (32) into two separate thin-film resistive layers (32), wherein the width of the DGS structure (39) is adapted to the substrate material used and the frequency range to be covered, and wherein it is formed directly between the two signal lines (121, 122) on the underside of the substrate (25). [10] Measuring bridge according to claim 7, characterized by , that the outer conductor (36) of the coaxial line (38) is connected at a second end to the reference ground and / or that the balun (111) has a compensation line (52) whose first end is connected to the first signal line (121) and / or that the compensation line (52) is a wire. [11] Measuring bridge according to claim 10, characterized by , that the compensation line (52) together with the coaxial line (38) is surrounded by at least one first ferrite (371, 372, 373, 374, 375) or that the compensation line (52) of at least one further ferrite (711, 71) 10 ) is surrounded, with at least one further ferrite (711, 71) 10 ) has the same properties as at least one first ferrite (371, 372, 373, 374, 375; 701, 7010) and / or that a second end of the compensation line (52) is connected to the reference mass. [12] Measuring bridge according to claim 10 or 11, characterized by , that the end of the coaxial line (38), which is away from the first section (50) of the balun (111), is connected to a first pot core (601), wherein the first pot core (601) comprises a coaxial conductor wound in a ferrite or wherein the coaxial conductor (38) is wound in the first pot core (601) and / or that the compensation line (52) is connected to a second pot core (602), wherein the second pot core (602) comprises a conductor wound in a ferrite, or wherein the compensation line (52) is wound up in the second pot core (602) and wherein the second pot core (602) has the same electrical properties as the first pot core (601).

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