WO2010007096A2

WO2010007096A2 - System for electrical capacitance tomography

Info

Publication number: WO2010007096A2
Application number: PCT/EP2009/059070
Authority: WO
Inventors: Thomas Scheiwiller; Dario Pierri; Daniel Erhardt
Original assignee: Bühler AG
Priority date: 2008-07-16
Filing date: 2009-07-15
Publication date: 2010-01-21
Also published as: WO2010007096A3

Abstract

The present invention provides an improved system for electrical capacitance tomography imaging of material located within a space delimited by walls. The system comprises multiple electrodes arranged on said walls and defining a capacitance between each pair of electrodes within said multiple electrodes; and a charge/discharge measuring circuit connected or connectable to each of said pairs of electrodes. The charge/discharge measuring circuit comprises switching means for alternately connecting an electrode as an excitation electrode and a measurement electrode. Signal lines connecting electrode surfaces of said electrodes to said charge/discharge measuring circuit are provided with shield lines and/or shield layers.

Description

System for electrical capacitance tomography

The present invention relates to a system for electrical capacitance tomography (ECT) imaging of material, typically a flowable bulk material, located within a space delimited by walls. Such systems comprise multiple electrodes arranged on these walls and defining a capacitance between each pair of electrodes within the multiple electrodes. A charge/discharge measuring circuit is connected or connectable to each of these pairs of electrodes. The charge/discharge measuring circuit may be operated in capacitance measuring mode or in resistance measuring mode using voltage pulses applied to each of the electrode pairs. The charging and discharging currents at each of these capacitances are measured at different times in the charging and discharging cycles. These measured current values are the result of a capacitive component (capacitance, non- conductive) and a resistive component (resistance, conductive). In capacitance tomography, the measured capacitance and resistance values are then used to determine the material distribution within the walls for a given material with known permittivity and resistivity.

Such systems and methods are e.g. known from WO 2009/027782 or WO 2009/027795 the content of which is incorporated herein by reference. Such a system typically can be used to determine the moisture content or the density of a material which is supplied through a conduit delimited by these walls. Based on measurement of moisture and density and determination of a pattern at two axially spaced areas of the conduit, the flow of the material moving through the conduit can be determined.

However, if the material is supplied at a high speed, the system needs to be operated with a relatively high frequency in order to have a good temporal or spatial resolution. If high operating frequencies are used, there is a current risk of interferences between measuring electrodes and excitation electrodes and their respective supply or signal detection lines.

It is therefore an object of the present invention to overcome the drawbacks of the prior art and in particular to improve the accuracy of such an electrical capacitance tomogra- phy (ECT) imaging system. In particular, it is an object of the present invention to provide improved temporal resolution and/or improved spatial resolution of an ECT imaging system.

The present invention provides improvements to such electrical capacitance tomography (ECT) imaging systems according to the independent claims.

According to the invention, the charge/discharge measuring circuit and the driver is connected or connectable to each of said pairs of electrodes by means of signal lines. The signal lines are arranged on a printed circuit board. Each signal line is provided with neighboring shield lines or shield layers which are arranged on the printed circuit board and which are connected or connectable to ground. Such an arrangement allows for efficient shielding of the signal lines with respect to external interferences and at the same time allows easy manufacturing of the system according to the invention. Such a coaxial cable approximation within the printed board has a similar effect as a coaxial cable with, however, a much easier way of manufacturing.

According to a preferred embodiment of the invention, each of the signal lines is provided with a shield line on each lateral side and with a shield layer in a plane above and in a plane below the signal line. The shield lines and shield layers are arranged typically in different planes and preferably may be connected with through contacts between the planes. By such through contacts an even better shielding can be achieved.

Preferably, the through contacts are irregularly spaced such as to avoid constructive interference caused by reflections.

According to a further preferred embodiment each of the electrodes may be provided with two neighboring guard electrodes arranged on said walls and attached to guard lines. Such guard electrodes are known per se in order to make the electric field applied by the electrodes more homogeneous. Preferably, shield lines are provided between signal lines and guard lines. According to still a further preferred embodiment, shield lines connected or connectable to ground may be arranged around the electrodes. A better shielding of neighboring of electrodes thereby can be achieved.

According to a further aspect of the invention, the signal lines connecting the electrodes to the charge/discharge measuring circuit and to the driver are arranged in an Y- configuration with a joint line for connection to the driver and to the measuring circuit. Two antiparallel diodes are arranged in parallel between said joint line and said driver.

Such an arrangement separates the measuring circuit electrically from the driver during measurement and therefore reduces interference effects from the driver during measurement. While it is understood that such a diode arrangement can be advantageously used in context with shield lines as described above, it is apparent to a skilled person that advantages also can be achieved with other arrangements without shield lines.

According to still another aspect of the present invention the signal lines are arranged on a printed circuit board whereby neighboring signal lines have a lateral spacing of at least 3 mm. While it is known to skilled persons that a certain distance may be required between neighboring lines in order to avoid cross talking between neighboring lines, the inventors have surprisingly found that a much bigger distance is required in the present context than ordinarily would be expected. In particular, signal lines may not be attached to neighboring pins of standard connectors used to connect the signal lines to a computer. While it will appreciated that such a distant arrangement may be particularly advantageous in context with shield lines as mentioned above, it will be apparent to the skilled person that such an arrangement can also be used without the shield lines described herein above.

The printed circuit board typically may be provided with a plug connector to which a connector cable attached to a calculating unit such as a personal computer or a digital processor may be connected. According to a preferred embodiment of the invention, a plug connector is used which has a number of poles which is at least ten times higher than the number of electrodes of the system. The signal lines are then attached to pins of the plug connector which are separated by at least five pins which preferably are connected or connectable to ground. It has been found that even with shielded plug connectors interference effects between neighboring signal lines are too strong for the purpose of the present application if signal lines are connected to neighboring or to too close pins.

According to still another aspect of the present invention, the signal lines arranged on the printed circuit board may be provided with delay lines which are arranged in a manner such that all signal lines have the same lengths. Especially when the measurement of the current pulse during a cycle is separated in a capacitive part and in a resistive part an exact timing of the analysis of the signal is necessary and delays between the different signal lines connecting different electrodes can be avoided with such delay lines. While such delay lines are particularly preferred in context with shield lines, anti- parallel diodes or signal lines arranged at a large lateral distance, delay lines also may be advantageously used for electrical capacitance tomography imaging systems without such arrangements.

The invention and its specific embodiments will now be described with reference to the accompanying drawings in which:

Fig. 1 a shows wave form diagrams of various excitation and measurement pulses;

Fig. 1 b shows a simplified circuit diagram for switches connecting the electrodes to a measuring circuit;

Fig. 2 shows the shape of a typical current pulse;

Fig. 3 schematically shows a coaxial cable approximation within a printed circuit board as compared to a normal coaxial cable in a cross section view;

Fig. 4 shows a top view of signal lines and neighboring shield lines in a schematic representation;

Fig. 5 shows a top view of signal lines, shield lines and guard lines on a printed circuit board; Fig. 6 to 7 show a schematic representation of signal lines during excitation and during measurement of a first embodiment;

Fig. 8 and 9 show a schematic representation of signal line 5 between a sensor and a driver during excitation and during measurement of a preferred embodiment of the invention;

Fig. 10 shows a top view of a printed circuit board with highlighted signal lines;

Fig. 11 shows a top view of a printed circuit board with highlighted guard lines;

Fig. 12 shows a top view of a printed circuit board with highlighted shield lines;

Fig. 13 shows an enlarged view of an electrode of an embodiment according to the present invention;

Fig. 14 shows an enlarged view of the connections to an electrode of an embodiment according to the present invention

Fig. 15 shows delay lines arranged on a printed circuit board of an embodiment according to the present invention, and

Fig. 16 shows a schematic view of electrodes arranged on a wall of a conduit in top view (left hand side) and in front view (right side).

Excitation and measurement pulses

Fig. 1 a shows waveform diagrams of excitation and measurement pulses used with a charge/discharge measuring circuit. The FPGA code is very flexible. Its S3 / S4 pulses can be adjusted with 10 ns resolution (at 10 MHz excitation frequency) and shifted by 0 or 5 ns with respect to the excitation pulse in order to adjust the time sequence of the excitation and measuring signals to each other. Such a precise adjustment may be necessary in view of the length of the signal lines or the difference in sensor size. In this context, also the time for operating the analog switches need to be taken into consideration. During the measurement, the FPMG ensures an appropriate timing on the basis of preset delays and patterns for the switches.

Fig. 1 a shows 50% amplitude reduction which is achieved by omitting one out of two S3 and S4 cycles. An additional switch S5 connecting the measurement signal to ground was introduced to cause gaps between S3 and S4. The measuring signal must not be floating at any time. This is also required while the amplitude reduction is active.

Typically, the electrodes, e.g. eight electrodes are arranged evenly spaced on the circumference of a tubular wall. A signal produced by the excitation electrode and received by a diametrically opposed measurement electrode is relatively weak due to the distance between such electrodes. Contrary to this, a relatively strong signal is measured by an electrode which is just neighboring the excitation electrode. In order to compensate for such difference in signals, the amplitude can be corrected by omitting some of the measurement cycles determined by closing and opening of the switches S3 and S4 (see figure 1 b). As shown in figure 1a, about 50% of the amplitude is removed or corrected. However, it is also possible to have a larger part of the amplitude corrected e.g. by omitting not only one out of two cycles but e.g. by omitting four out of five cycles (reduction to 20%).

Contrary to the disclosure in WO 2009/027795, there is no overlap between closing of switches S3 and S4. Furthermore, it has been found by the inventors that whenever switch S3 or S4 is open, the electrode 10b (see figure 1 b) should be ground through closing of switch S5 (see figure 1 b). However, contrary to what has been suggested in WO 2009/027795 there should be no overlap between closing of switch S5 and closing of switches S3 and S4. With the exception of the above-mentioned differences, the op- eration of the excitation and discharge/charging measurement circuit corresponds to what has been disclosed in WO 2009/027795 which is incorporated herein by reference.

Fig. 1 b shows a simplified circuit diagram for S3, S4 and S5. In figure 1 b there is schematically shown a driver circuit 13 providing an excitation voltage. The excitation electrode 10a (by way of example in figure 1 b) can be alternatingly connected to the excitation voltage V^exι by closing a switch S1 and to ground by closing a switch S2. The measurement electrode 10b is connected to a charge/discharge measuring circuit 11 which will not be described in more detail but which has been disclosed in WO 2009/027795. By closing the switches S3, S4 and S5 as shown in the wave diagram in figure 1 a, the current pulse generated between the electrodes 10a, 10b during a charge/discharge cycle can be determined as explained in WO 2009/027795.

It should be noted that the electrode 10a has been shown as an excitation electrode and the electrode 10b as a measuring electrode in figure 1 b by way of reference. The electrodes will be alternatingly connected as an excitation electrode and as measurement electrode in a manner known to a skilled person in order to get a distribution pattern of material located within a space delimited by a wall on which the electrodes are arranged (not shown in detail herein).

Fig. 2 shows the shape of a typical current pulse during one measuring cycle with 10 possible measuring pulse width adjustments (10 steps of 10 ns each). Thus, the pulse width can be adjusted in 10 ns steps, thus allowing to precisely select the desired portion of the overall current pulse of interest, for instance the capacitive or the resistive portion of the pulse. Measurement windows of 10 ns can be selectively used for determining the current pulse during a specific cycle. In particular, a capacitive part during around the first 15 to 20 ns of the positive pulse can be used to measure a capacitive part C of the signal whereas a second part between 20 and 50 ns can be used to measure the resistive part R of the current pulse. A similar differentiation will be made for the negative current pulse between 50 and 100 ns.

This separation into a capacitive part and into a resistive part is especially used if the moisture content and the quantity of material shall be determined with one and the same sensor. The analog switches connecting the measuring signal are controlled by a FPGA operating at a frequency of 100 MHz. The pulse thus can be adjusted in steps of 10 ns. Any pattern of 10 ns steps can be programmed for the control of the analog switches.

Circuit hardware

The lines ("striplines") from the driver circuit to the electrode surfaces and back to the analog switch are extremely delicate. In particular, interferences between lines used for excitation and lines used for signal detection have to be taken care of. Preferably, as shown in Fig. 3 signal lines 20 are provided as "coaxial cable approximations" on a printed circuit board on which said signal lines 20 are arranged. Such coaxial cable approximation comprises a band-shaped top layer 22a following the path of the signal line 20 as a band-shaped conductor, a band-shaped bottom layer 22b following the path of the signal line 20 as a band-shaped conductor and a middle layer comprising two parallel outer shield lines 21 and a central signal line 20 adjacent to each other halfway in between the top layer 22a and the bottom layer 22b. The band-shaped top and bottom layers 22a, 22b and the left-hand side and right-hand shield lines 21 are all grounded, while the signal line 20 is not grounded, i.e. insulated with respect to the grounded bands and layers. As a result, the signal line 20 can be used as a signal carrier and is shielded by the grounded layers 22a, 22b as well as by the grounded shield lines 21 to the left and to the right of it. Figure 3 shows by way of a comparison a coaxial cable on the right hand side.

If guard electrodes are additionally used, guard lines 30 may be arranged adjacent the signal lines 20. In a similar manner as with respect to the signal lines 20, also the guard lines 30 can be shielded by neighboring shield lines 21 as shown in figure 4 and with bottom layer and top layer shields (not shown in figure 4).

Using the grounded strips as a shield reduces the tendency of the striplines to oscillate and allows higher operating frequencies to be used. Thus, operating frequencies up to 10 MHz have been achieved.

Preferably, as shown in Fig. 5, relatively thin signal lines 20 are laterally provided with the shield lines 21 (which are somewhat broader). Furthermore, there are guard lines 30 alternating with the signal lines 30. The shield lines 21 are provided with spaced through-contacts 25. This helps avoid unwanted oscillations in the three-layer shield structure. Preferably, these through- contacts 25 are irregularly spaced in order to prevent constructive interference among reflections and reduce them to noise.

Sthpline Y-constellations

It is rather difficult or even impossible to optimally adjust these strip lines, since the physical location of the driver and of the analog switches connecting the electrode 10b to the measuring circuit 11 can not be the same. Therefore, the sensor surface 10a, the driver circuit 13 and the analog switch S3 or S4 are connected by a Y-constellation, as shown in Figs. 6 and 7.

Fig. 6 shows the Y-constellation line in an excitation mode where an excitation pulse from the driver circuit 13 is applied to the sensor surface 10a and the analog switch S3 or S4.

Fig. 7 shows the Y-constellation stripline in a measuring mode where a measuring pulse from the sensor surface 10a is received by the driver circuit 13 and the analog switch S3 or S4.

The measuring signal is thus distributed on two separate lines 20a, 20b, the first one 20a between the sensor surface 10a and the driver circuit 13 and the second one 20b between the sensor surface 10a and the analog switch S3 or S4. Due to this Y- constellation, optimum termination of this measuring line arrangement is practically impossible.

Preferably, as shown in Figs. 8 and 9, the circuitry may be provided with two antiparallel diodes 40 arranged in parallel between the driver 13 on the one hand and the sensor and the analog switch S3, S4 on the other hand.

Fig. 8 shows the improved Y-constellation stripline in excitation mode where an excitation pulse from the driver circuit is applied to the sensor surface and the analog switch. A relatively strong excitation pulse from the driver circuit passes the diode arrangement. Fig. 9 shows the Y-constellation stripline in measuring mode where a measuring pulse from the sensor surface is received by the driver circuit and the analog switch. A relatively weak measurement pulse from the sensor surface does not significantly pass the diode arrangement, thus virtually severing the driver circuit from the measurement signal.

Due to the antiparallel diodes, the Y-constellation is shortened to the point where it does no longer have any negative effects on the measurement signal.

As will be shown with reference to figure 10 to 12 and 15, the shield lines 21 , the guard lines 30 and the signal lines 20 are arranged on a printed circuit board 15.

Excitation and measurement lines provided with inserted plug type connector A plug type connector 41 may be inserted into the excitation and measurement paths of the excitation and measurement lines 20 and the guard lines 30 as well as the shield lines 21. However, in order to avoid parasitic capacitances in the excitation and measurement paths of the lines due to the plug connector interfaces and due to the closeness of adjacent lines, the signal lines 20 are arranged laterally at a distance L of about 5 mm. Typically, a connector 41 with 104 poles is used in order to attach 8 measuring signal lines and 16 guard signal lines. The measuring signal lines 20 are shown in bold in figure 10 whereas the guard signal lines 30 are shown in bold in figure 11. The use of a connector 41 having much more pins than necessary for connecting the measuring signal lines 20 and the guard signal lines 30 (typically for eight electrodes and thus for eight measuring signals 104 pins) allows to connect the signal lines 20 to connecting pins 42a, 42b which are separated by further connecting pins 42c which may be used for connection of guard lines 30 (see figure 11 ) or which may be attached to ground. By such an arrangement, the distance between signal carrying parts within the connector can be kept sufficiently high. Typically, a connector of the type Samtec QFSS-052-01 - L-D-A can be used.

By such an arrangement, all measuring signals can be arranged in one row separated from the guard signals. In addition, a ground plate 43 is arranged between the measuring signals (left hand side in figure 10) and the guard signals (right hand side, see figure 11 ). As shown in figure 10, each signal line 20 is provided laterally on both sides with a shield line 21. The shield lines 21 are further connected to the upper and lower shield layers (see figure 3, not visible in figure 10) by means of through-contacts 25.

Figure 11 shows in bold the guard lines 30 arranged neighbouring the signal lines 20 and separated therefrom by shield lines 21 which are connected to ground. The guard lines 30 are attached to pins 43d of the connector 41 arranged on the right hand side and separated from the pins to which the signal lines 20 are connected by the grounded plate 43.

Figure 12 shows in bold the shield lines 21 which are attached to grounded pins of the plug connector 41.

In order to avoid parasitic capacitances in the excitation and measurement paths of the lines due to closeness of adjacent lines ground lines are also used between the signal lines 20 and the guard lines 30.

As shown in Fig. 10 to 12, the signal lines 20, guard lines 30 and shield lines 21 are arranged adjacent to each other and substantially in a plane defined by the printed circuit board 15.

For clarity, the top and bottom layers are not shown in Figs. 10, 11 and 12.

However, these layers are also attached to the shield lines and to ground through a plurality of contacts 25.

Screening of the sensor surfaces

Fig. 13 shows electrode surfaces 10a, 10b and 10n and guard surfaces with additional ground shield tracks 23 and ground area surrounding the electrode and the guard surfaces 12a, 12b and 12n. This further improves the shielding. In addition, the tracks of the top and bottom layers are connected via the through-contacts 25, thus further improving the ground connection. In Fig. 13, the additional ground lines 23 are shown. For clarity, the bottom layers are again not shown in Fig. 13. Figure 13 further shows a central measuring and excitation electrode 10a and a part of the neighbouring measuring and excitation electrode 10b and 10n. Axially adjacent the measuring and excitation electrodes 10a, 10b, 10n are respective guard electrodes 12a, 12b, 12n on one side and 12a-, 12b-, 12n- on the other axial side.

Line arrangement between electrode surfaces

As shown in Fig. 14, the signal lines 20 on the electrodes are shielded by shield tracks 23 attached to the lines 21 (not shown) on both sides along the entire length of each line all the way to the corresponding electrode surface. The size or length of the physical connection between the end of the line and the electrode surface has been reduced to a minimum and is much shorter than the length of the line. As a result, cross-talk between adjacent lines in the Fig. 14 arrangement is eliminated. In Fig. 14, while the shield lines 23 are shown, again no bottom layers are shown.

As shown in Fig. 15, all signal lines 20 have the same or nearly the same length by the provision of serpentine like delay lines 26. This reduces skew between the leading edges of simultaneous signals. Also, each signal line 20 is provided with shield lines 21 on both of its sides. Guard lines 30 having a similar length adjustment are provided on each side of these signal lines 20 and separated therefrom by the shield lines 21.

The present invention is primarily directed to an improved design of the circuit and the circuit board in order to improve the quality of the detected signals and in order to improve spatial and/or temporal resolution. The operation of the system and the design of the measuring circuit as well as the general use of the system for the determination of mass distribution in a conduit is made basically in a manner known to those skilled in the art and e.g. particularly as disclosed in WO 2009/027795 and WO 2009/027782.

Figure 16 schematically shows a top view (left hand side) and a front view (right hand side) of a system for measuring the moisture and density of material M contained in a conduit delimited by a tubular wall W. On the tubular wall W there are arranged 8 electrodes (10a, 10b, ... 10n) evenly spaced on the circumference of the wall W. These electrodes 10a, 10b form respective pairs of electrodes. They are alternatingly used as measuring and excitation electrodes so that 8 x 7 (e.g. 56) different pairs of measuring and excitation electrodes can be used. Each of these electrodes is formed as a planar electrode connected to or connectable to the driver and the measuring circuit by signal lines as shown in the previous figures. The left hand side of figure 16 schematically shows a front view of the tubular conduit delimited by the wall W. Measuring and excitation electrodes 10a, 10b, 10n arranged on the wall. They are axially delimited by guard electrodes 12a, 12b, 12n and 12a', 12b', 12n' connected to a driver in a manner as shown with reference to the figures described above. In order to determine the flow of material moving in the conduit delimited by the wall W, a same arrangement of measuring and excitation electrodes and guard electrodes is arranged spaced in the axial direction A on the wall W (not shown).

Claims

1. A system for electrical capacitance tomography for imaging of material located in a space delimited by walls (w),

said system comprising multiple electrodes (10a, 10b, ... 10n) arranged on said walls and defining a capacitance between each pair of electrodes (10a, 10b) within said multiple electrodes;

a charge/discharge measuring circuit (11 ) and a driver circuit (13) connected or connectable to each of said pairs of electrodes (10a, 10b) by means of signal lines (20, 20a, 20b) arranged on a printed circuit board (15)

wherein said signal lines (20, 20a, 20b) are provided with neighboring shield lines (21 ) and/or shield layers (22) arranged on said printed circuit board (15) and connected or connectable to ground.

2. A system according to claim 1 , wherein each of said signal lines (20) is provided with a shield line (21 ) on each lateral side and with a shield layer (22a, 22b) in a plane above and in a plane below said signal line (21 ).

3. A system according to one of the claims 1 or 2, wherein shield lines (21 ) and shield layers (22a, 22b) arranged in different planes are connected to each other with through contacts (25).

4. A system according to claim 3, wherein said through contacts (25) are irregularly spaced.

5. A system according to one of the claims 1 to 4, wherein each of said electrodes (10a, 10b, .... 10n) is provided with two axially arranged neighbouring guard electrodes (12a, 12b ... 12n) arranged on said walls (W) and connected or connectable to a driver by means of guard lines (30) and wherein shield lines (21 ) are provided between said signal lines (20) and said guard lines (30).

6. A system according to one of the claims 1 to 5, wherein additional shield lines (23) connected or connectable to ground arranged along said electrodes (10a, 10b ... 10n).

7. A system for electrical capacitance tomography imaging of material located within space delimited by walls (W), in particular a system according to one of the claims 1 to 6, said system comprising multiple electrodes (10a, 10b ... 10n) arranged on said walls and defining a capacitance between each pair of electrodes (10a, 10b) within said multiple electrodes;

a charge/discharge measuring circuit (11 ) and a driver (13) connected or connectable to each of said pairs of electrodes (10a, 10b) by means of signal lines (20) in an Y-configuration with a joint line,

wherein two antiparallel diodes (40) are arranged in parallel between said joint line and said driver (13).

8. A system for electrical capacitance tomography imaging of material located within space delimited by walls (W), in particular according to one of the claims 1 to 7,

a charge/discharge measuring circuit (11 ) and a driver (13) connected to or connectable to each of said pairs of electrodes by means of signal lines (20) arranged on a printed circuit board (15),

wherein neighbouring signal lines (20) have a lateral spacing (L) of at least 3 mm, preferably of at least 5 mm.

9. A system according to claim 8, wherein said signal lines (20) are connected to a connector plug (41 ) having a number poles at least ten times higher than the number of electrodes (10a, 10b ... 10n),

wherein neighbouring signal lines (20) are connected to pins (42a, 42b) of said plug (41 ) separated by at least 5 pins preferably connected to ground.

10. A system according to claim 9, the system further comprising guard electrodes (12a, 12b, ... 12n) provided with guard lines (30), wherein said guard lines are attached to pins (42d) of said plug connector (41 ) which are separated from pins (42a, 42b) to which signal lines (20) are attached by a grounded plate (43).

11. A system for electrical capacitance tomography imaging of material located within a space delimited by walls (W), in particular according to one of the claims 1 to 10,

said system comprising multiple electrodes (10a, 10b, .... 10n) arranged on said walls and defining a capacitance between each pair of electrodes within said multiple electrodes

a charge/discharge measuring circuit (11 ) and a driver (13) connected or connect- able to each of said pairs of electrodes (10a, 10b) by means of signal lines (20) arranged on a printed circuit board (15),

wherein the signal lines (20) are provided with delay lines (26) arranged in such a manner that all signal lines (20) have the same length.