DE102020206215A1 - Radar sensor and circuit board for contactless permittivity measurement - Google Patents

Abstract

Circuit board (10) for a radar sensor, comprising a circuit board substrate (20), at least one element (12) arranged on the circuit board substrate (20) for guiding radar signals and at least two different polarization-rotating structures (14, 16, 18) which are at different positions the circuit board (10) are arranged and have different frequency positions (f1, f2, f3) of a maximum polarization rotation of reflected radar waves; as well as radar sensor with such a circuit board (10).

Description

The invention relates to a circuit board for a radar sensor, comprising a circuit board substrate and at least one element arranged on the circuit board substrate for carrying radar signals. The element for guiding radar signals can in particular be a radar antenna or a waveguide transition structure for radar signals.

Disclosure of the invention

A central parameter for the performance of a radar sensor for motor vehicles with antennas in microstrip technology, with a waveguide transition structure for radar signals or with another element for guiding radar signals is the permittivity of the printed circuit board substrate. Deviations of the permittivity from an expected value of the permittivity can lead to systematic errors in the angle estimation of the radar sensor and to a reduced range of the radar sensor. A permittivity of a circuit board that deviates from the expected value can result in the fully assembled circuit board having to be disposed of due to a negative final test.

A test of the permittivity can conventionally be carried out either in the case of the already populated circuit board by using, for example, adaptation monitoring of the MMICs used on the basis of the antenna adaptation. A circuit board that has not yet been populated, on the other hand, can be measured conventionally by making contact with probe heads on ring resonators formed on the circuit board for this purpose, for example. This requires precise positioning of the probes used. The resonator structures used for the test and their supply lines require more space on the circuit board.

Both methods require contact to be made with the circuit board and therefore have disadvantages in terms of production costs. Another disadvantage is that when testing the permittivity of the assembled circuit board with a negative result, at least one MMIC (monolithic microwave integrated circuit) is disposed of, so that cost disadvantages arise.

In addition, due to the complex geometry of the structure to be measured, it is difficult for both methods to clearly trace the measurement result back to the permittivity.

Information about a permittivity of the object can be obtained from microwave radiation reflected on an object. So describes DE 10 2011 078 418 A1 a device for determining material properties by millimeter wave radiation, with a transmitter unit that radiates an arbitrarily polarized millimeter wave signal at a certain angle of incidence onto an object to be examined, a receiver unit that detects a signal reflected from the object to be examined with regard to the intensity and / or phase position, and a processing unit which determines the permittivity and / or layer thickness of the examined object from a part of the reflected signal polarized parallel to the plane of incidence and a part polarized perpendicular to the plane of incidence by means of an ellipsometric evaluation, the transmitting and receiving unit being arranged in front of the object to be examined, that they span an aperture surface and / or that the processing unit creates a spatially resolved image of the examined object from the reflected signals by changing the focusing point.

The object of the invention is to provide a circuit board for a radar sensor that allows a simplified check of the permittivity. It is particularly desirable if the permittivity can already be determined on the unpopulated circuit board.

The object is achieved according to the invention by a circuit board for a radar sensor, comprising a circuit board substrate, at least one element arranged on the circuit board substrate for guiding radar signals and at least two different polarization-rotating structures which are arranged at different positions on the circuit board and different frequency positions of a maximum polarization rotation of reflected radar waves exhibit.

The element for carrying radar signals can also be referred to as an element carrying radar signals or as an element for forwarding radar signals.

A maximum polarization rotation is understood to mean a polarization rotation that is at most close to 90 °. The maximum polarization rotation can also be referred to as the maximum orthogonal polarization rotation. It corresponds to a maximum approximation of a rotation by 90 ° (orthogonal polarization rotation), in particular reaching the polarization rotation by 90 °.

A polarization-rotating structure is understood to be a structure which is suitable for irradiating with radar waves of a predetermined polarization and excitation frequency To reflect radar waves at least partially with a rotated polarization. Such a polarization-rotating structure can be formed, for example, by a metallic shape on the printed circuit board substrate. Rectangular polarization rotating structures are off DE 10 2015 225 578 A1 known. This document describes a device for receiving microwave radiation, with a printed circuit board and a receiving antenna attached to it, and with polarization-rotating structures. In one example, diagonally oriented rectangles are arranged in an array so that they form a regularly recurring pattern in rows and columns. The polarization-rotating structures are used to suppress unwanted multipath reflections of received signals between the circuit board and a radome by rotating the polarization.

The at least one element arranged on the printed circuit board substrate for guiding radar signals preferably comprises at least one radar antenna or at least one waveguide transition structure for radar signals. The element for guiding radar signals can in particular be a radar antenna or a waveguide transition structure for radar signals.

The radar antenna can in particular be designed using microstrip technology, that is to say a patch antenna. In particular, it can comprise one or more antenna elements (antenna patches) on the circuit board substrate, in particular antenna elements in the form of conductive surfaces, in particular metal surfaces. The radar antenna is, for example, a receiving antenna and / or transmitting antenna for radar wave radiation, in particular millimeter wave radiation.

The waveguide transition structure for radar signals can, for example, be a conductor structure on the printed circuit board substrate, which is designed for coupling radar waves between the printed circuit board and a waveguide (e.g. a waveguide antenna), for example to excite a waveguide, the wave merging into the waveguide. The waveguide transition structure for radar signals can in particular be a conductor structure in the form of a coplanar waveguide or a microstrip structure or microstrip line. The waveguide transition structure for radar signals is, for example, a coupling-in and / or decoupling structure for radar waves for coupling radar waves between the waveguide transition structure and a waveguide, for example via a coupling piece (housing with waveguide port) applied to the circuit board the waveguide.

The essence of the invention is thus a circuit board which is characterized by different polarization-rotating structures on the circuit board, which allow a contactless determination of the circuit board substrate permittivity with the aid of the polarization suppression property of common radar antennas. If, for example, the polarization-rotating structure is irradiated by means of a horn antenna, with a suitable alignment of the horn antenna, the polarization rotation of the structure can be used in combination with the polarization suppression of the horn antenna in order to achieve a reduction in the received power at the frequency position of the maximum polarization rotation of the polarization-rotating structure. In addition, the geometry of the polarization-rotating structure can be recorded with a camera. The described drop in performance can be evaluated in order to infer the permittivity of the printed circuit board substrate (possibly together with the recorded geometry). Due to the different frequency positions of the maximum polarization rotation of the different polarization-rotating structures, the permittivity can be determined spatially resolved due to the different positions of the structures, and / or redundancy can be achieved in the evaluation for determining the permittivity. Thus, the accuracy of the determination of the permittivity can be improved.

In order to acquire the signals of all different polarization-rotating structures, a frequency scan can be carried out, for example, or broadband excitation can be carried out, for example with a noise signal. On the basis of the different frequency positions of the power drops, which correspond to the respective frequency positions of the maximum polarization rotation of the polarization-rotating structures, conclusions can then be drawn about the permittivity at the position of the respective polarization-rotating structures. In addition, the exact shape of the different polarization-rotating structures can be recorded, for example with a camera, in order to recognize any deviations of the shapes from a given respective shape and, if necessary, to take them into account.

Such a circuit board makes it possible to measure the permittivity of the circuit board at the different positions of the polarization-rotating structures early on, especially before the circuit board is equipped with a high-frequency component, for example an MMIC, and thus to allow any deviations from an expected permittivity recognize. In particular, a measured frequency position of a maximum polarization rotation can be directly assigned to the reflected radar waves via the different polarization-rotating structures with different frequency positions of their maximum polarization rotation of the reflected radar waves relevant location on the circuit board. This can take place, for example, in that, in the case of locally measured polarization-rotating structures, the respectively measured structure is recorded directly with a camera.

However, it is particularly advantageous when measuring the circuit board to assign the responses of the respective polarization-rotating structures based on their different frequency positions to the respective different polarization-rotating structures, and thus to the corresponding positions on the circuit board. For example, the different frequency positions of a maximum polarization rotation of reflected radar waves of the different polarization-rotating structures can be assigned in an ascending order of the frequency positions to the different positions of the circuit board in a predetermined order. Deviations in the permittivity of the printed circuit board substrate can lead, for example, to a shift in the frequency position in which the order relationship between the frequency positions of the structures is maintained.

It is therefore particularly advantageous if the different polarization-rotating structures have different design frequencies at which, in the case of a predetermined uniform permittivity of the printed circuit board substrate, the respective frequency positions of a maximum polarization rotation of reflected radar waves would occur, and the different design frequencies each have a spacing that is greater than a shift in the frequency position of the maximum polarization rotation of reflected radar waves compared to the respective design frequency of this frequency position, expected due to a variation range of the permittivity of the type of printed circuit board used. In this way it can be ensured that the frequency positions of the maximum polarization rotation of reflected radar waves in the printed circuit board produced are assigned in ascending order to predetermined positions on the circuit board. The corresponding design frequency can therefore first be determined from the measured frequency positions and assigned to them, and the permittivity at the relevant position of the circuit board can be deduced from the deviation of the measured frequency position from the design frequency. This can be done, for example, by comparing the measured frequency position with fields simulated for different permittivities.

Depending on the shape of the polarization-rotating structure and depending on the permittivity of the printed circuit board substrate at the position of the polarization-rotating structure, a maximum polarization-rotating effect will occur at a certain frequency of the reflected radar waves or excitation frequency. The polarization-rotating structure can thus be designed for a given position of the frequency of its maximum polarization-rotating effect. For example, a rectangular polarization-rotating structure can be designed specifically for an expected permittivity of the printed circuit board substrate at the location of the structure in order to obtain a specific frequency position of a maximum polarization rotation of reflected radar waves. For this purpose, for example, different edge length ratios of the rectangular shape can be specified in a field simulation and the respective reflection properties can be simulated until a suitable edge length ratio is found. If necessary, this can be followed by a step of scaling the edge lengths and the excitation frequency, followed by a further field simulation to check the properties achieved by the scaling. If necessary, the edge length ratios can then be varied further with renewed field simulations.

The different polarization-rotating structures preferably have different reference frequency positions of a maximum polarization rotation of reflected radar waves, which reference frequency positions would result in the case of a uniform permittivity of the circuit board substrate corresponding to an average value of the permittivities of the circuit board substrate at the locations of the polarization-rotating structures, with a frequency spacing between the Frequency position of a maximum polarization rotation of reflected radar waves of a respective polarization-rotating structure and the associated reference frequency position is smaller than a smallest distance of the relevant reference frequency position from each adjacent reference frequency position. This means that any local deviations in the permittivity of the printed circuit board substrate at the locations of the polarization-rotating structures from a mean value of the permittivities does not change anything in the assignment of the polarization-rotating structures to the sequence of the frequency positions of the maximum polarization rotation of the different polarization-rotating structures, in ascending order of the frequency positions .

The different polarization-rotating structures preferably have different shapes, which, assuming a uniform permittivity of the printed circuit board substrate, correspond to different reference frequency positions of a maximum polarization rotation, a distance between the frequency position of a maximum polarization rotation of reflected radar waves of a respective polarization-rotating structure and the relevant reference frequency position being smaller is a smallest distance between the reference frequency position and any other reference frequency position. In other words, the relevant reference frequency position of a polarization-rotating structure is closer to the frequency position of the maximum polarization rotation of the relevant polarization-rotating structure than to any other reference frequency position.

The at least one element arranged on the printed circuit board substrate for guiding radar signals is preferably at least one planar element.

The at least one element arranged on the printed circuit board substrate for guiding radar signals is preferably formed by at least one metallic mold on the printed circuit board substrate.

Preferred embodiments emerge from the subclaims.

The polarization-rotating structures are preferably planar structures. In particular, the polarization-rotating structures preferably each extend only in one plane. The polarization-rotating structures are preferably formed by flat, two-dimensional structures on the printed circuit board substrate.

The polarization-rotating structures are preferably each formed by a metallic shape on the printed circuit board substrate. In this way, a reflective structure can be created particularly well.

The different polarization-rotating structures preferably have different rectangular shapes. The shapes can differ, for example, by their different sizes and / or different edge length ratios of the edges of the rectangular shapes.

The different polarization-rotating structures are preferably rectangular metal surfaces.

The different polarization-rotating structures are preferably each formed by a conductive island structure on the printed circuit board substrate, that is to say by an unconnected, conductive structure. As a result, the reflective properties of the structures can be improved and are independent of other structures or circuit elements on the circuit board. The unconnected structures can also be referred to as conductor islands or conductive metallic islands.

The circuit board preferably has at least one first conductive layer, the different polarization-rotating structures being formed by conductive surfaces on the circuit board substrate, and the conductive surfaces and the at least one element for guiding radar signals being formed in the at least one first conductive layer of the circuit board . In this way, the conductive layers can be produced inexpensively and without additional effort in the same layer in which the at least one element for guiding radar signals is produced. The conductive layer is preferably a metallic layer and the conductive surfaces are preferably metal surfaces.

The printed circuit board preferably comprises a rear-side conductive layer and a front-side conductive layer which is structured in order to form the different polarization-rotating structures and the at least one element for guiding radar signals. The circuit board preferably further comprises the circuit board substrate arranged between the rear-side conductive layer and the front-side structured conductive layer. The conductive layer on the back can shield interference and the polarization-rotating structures can be produced in a defined manner. The rear-side conductive layer and the front-side conductive layer can be or comprise, for example, a rear-side metallization layer and a front-side metallization layer.

The object is further achieved by a radar sensor with a circuit board of the type described, in particular with a monolithic integrated microwave circuit, also referred to as MMIC (Monolithic Microwave Integrated Circuit), applied to the circuit board. The features of the circuit board are particularly advantageous here because they allow the permittivity of the circuit board to be checked, in particular a spatially resolved permittivity check, at an early stage in the manufacturing process, for example before the monolithic integrated microwave circuit is fitted to the circuit board.

• 1 eine schematische Darstellung einer Leiterplatte für einen Radarsensor;
• 2 eine schematische Querschnittsansicht der Leiterplatte;
• 3 eine schematische Darstellung eines Prüfverfahrens, das bei der Leiterplatte angewendet werden kann;
• 4 eine schematische Darstellung zur Erläuterung der polarisationsdrehenden Wirkung einer polarisationsdrehenden Struktur;
• 5 eine schematische Darstellung von Auslegungsfrequenzen und tatsächlichen Frequenzlagen maximaler Polarisationsdrehungen von polarisationsdrehenden Strukturen einer Leiterplatte;
• 6 eine schematische Darstellung von Referenz-Frequenzlagen der unterschiedlichen polarisationsdrehenden Strukturen der Leiterplatte; und
• 7 eine schematische Darstellung eines Radarsensors mit der Leiterplatte.

Embodiments of the invention are explained in more detail below with reference to the drawing. Show it:

• 1 a schematic representation of a circuit board for a radar sensor;
• 2 a schematic cross-sectional view of the circuit board;
• 3 a schematic representation of a test method that can be applied to the circuit board;
• 4th a schematic illustration to explain the polarization-rotating effect of a polarization-rotating structure;
• 5 a schematic representation of design frequencies and actual frequency positions of maximum polarization rotations of polarization-rotating structures of a circuit board;
• 6th a schematic representation of reference frequency positions of the different polarization-rotating structures of the circuit board; and
• 7th a schematic representation of a radar sensor with the circuit board.

1 shows schematically a circuit board for a radar sensor in a plan view of the front side of the circuit board. The circuit board 10 comprises at least one element for guiding radar signals, exemplified as a radar antenna 12th in the form of a patch antenna, as well as several, in the example shown three, polarization-rotating structures 14th , 16 , 18th different shape. The polarization rotating structures 14th , 16 , 18 are each represented by a rectangular metal surface on the circuit board substrate 20th the circuit board 10 educated. The polarization rotating structures 14th , 16 , 18th have an orientation of the rectangles that is 45 ° compared to an orientation of the radar antenna 12th is rotated.

2 shows schematically a cross-sectional view of the circuit board 10 . The circuit board 10 comprises the electrically insulating circuit board substrate 20th , a first conductive layer 22nd in the form of a metallization layer in which the radar antenna 12th and the polarization rotating structures 14th , 16 , 18th are formed, and a backside conductive layer 24 in the form of a metallization layer. In addition to the high-frequency positions shown, the circuit board 10 comprise further layers, not shown.

3 shows schematically in a side view a test method for determining the permittivity of the printed circuit board substrate 20th the circuit board 10 . By means of an antenna 30th , for example a horn antenna, is the circuit board 10 irradiated with a radar wave signal. The antenna 30th is for example according to the orientation of the radar antenna 12th aligned with the circuit board. At the respective different frequency positions of a maximum polarization rotation of the polarization-rotating structures 14th , 16 , 18th the reflected, polarization-rotated component of the radar signal by the horn antenna receiving the reflected signal 30th suppressed due to the polarization, which is now orthogonal to the reflected signal, so that at these frequency positions there are power drops in the intensity when the signal is received. The geometry of the polarization rotating structures 14th , 16 , 18th can also be equipped with a camera 32 are recorded.

4th shows a schematic example of a polarization-rotating structure 14th in the form of a rectangular metallic shape on the circuit board substrate 20th with different edge lengths a, b. A field vector E of the incident radar waves can be in vector components parallel to the short edge a and to the long edge b of the polarization-rotating structure 14th be disassembled. The shape of the polarization rotating structure 14th is roughly matched to the radar frequency used, so that in the frequency range of the measurement and depending on the actually existing permittivity of the circuit board substrate 20th at the location of the polarization rotating structure 14th a certain frequency position f _{1 of} the maximum polarization rotation of the reflected radar waves results, in which the field vector R of the reflected radar waves is rotated in its polarization by 90 ° with respect to the field vector E of the incident radar waves.

This drop in power is shown in the measured reflected power Ref of the portion of the reflected radar waves received with the same polarization as a function of the frequency f. This is shown schematically in FIG 1 on the polarization rotating structure 14th shown as a graph. Correspondingly, further polarization-rotating structures designed for a different design frequency of the frequency position of the maximum polarization rotation 16 and 18th a power drop can be seen at a frequency position f ₂ or f ₃ , as shown schematically in FIG 1 is represented by graphs.

5 shows a diagram in which the different design frequencies F ₁ , F ₂ , F _{3 of} the polarization-rotating structures 14th , 16 , 18th are shown schematically. The design _{frequencies F 1} , F ₂ , F ₃ are the frequency positions of the maximum polarization rotation of the different polarization-rotating structures expected for an expected permittivity ε _{r of the printed circuit board substrate} 14th , 16 , 18th . For permittivities of the printed circuit board substrate that deviate from this expected value 20th For example, the actual frequency positions f ₁ , f ₂ , f _{3 of} the maximum polarization rotation of reflected radar waves of the different polarization-rotating structures can differ 14th , 16 , 18th result in frequency ranges which are indicated schematically by the ranges ΔF ₁ , ΔF ₂ , ΔF3. The deviating permittivity thus leads to shifts in the frequency positions f ₁ , f ₂ , f _{3 of} maximum polarization rotation compared to the design _{frequencies F 1} , F ₂ , F ₃ , but the deviations are within the range of variation ΔF ₁ expected due to the possible range of variation of the permittivity, ΔF ₂ , ΔF ₃ remain and can therefore be clearly assigned during the measurement. From the actual, measured frequency position f ₁ , f ₂ , f _{3 of} the maximum polarization rotation of a polarization-rotating structure 14th , 16 , 18th it is therefore possible to infer the permittivity at the location of this polarization-rotating structure.

Conversely, on the basis of the measured frequency positions f ₁ , f ₂ , f _{3 of} the polarization-rotating structures 14th , 16 , 18th a determination of corresponding comparison frequencies (reference frequencies) ref ₁ , ref ₂ , ref ₃ take place, which are based on the assumption of a uniform permittivity of the printed circuit board substrate 20th and otherwise unchanged polarization-rotating structures 14th , 16 , 18th would result. So much for the permittivity of the printed circuit board substrate 20th at the locations of the polarization rotating structures 14th , 16 , 18th assumes different values, as in 6th shown, the measured frequency positions (such as f ₁ ) then in areas around the respective comparison _{frequency positions (e.g. ref 1} ) that are smaller than the distances between the adjacent comparison _{frequency positions (here Δ (ref 1} , ref ₂ )).

7th shows schematically a radar sensor with the circuit board 10 including the element for carrying radar signals in the form of the radar antenna 12th , one of the polarization rotating structures 14th , 16 , 18th and a monolithic microwave integrated circuit MMIC 40 on the circuit board 10 . The radar sensor is a radar sensor for motor vehicles.

In the examples above, instead of the antenna 12th the element for guiding radar signals also has a waveguide transition structure 12th for radar signals, at which a coupling piece 41 (Housing with waveguide port) for a waveguide 42 is arranged as schematically dashed in FIG 7th is shown.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102011078418 A1 [0006]
• DE 102015225578 A1 [0011]

Claims (10)

Circuit board (10) for a radar sensor, comprising a circuit board substrate (20), at least one element (12) arranged on the circuit board substrate (20) for guiding radar signals and at least two different polarization-rotating structures (14, 16, 18) which are at different positions the circuit board (10) are arranged and have different frequency positions (f ₁ , f ₂ , f ₃ ) of a maximum polarization rotation of reflected radar waves. PCB according to Claim 1 wherein the at least one element (12) arranged on the printed circuit board substrate (20) for guiding radar signals comprises at least one radar antenna (12) or at least one waveguide transition structure (12) for radar signals. PCB according to Claim 1 or 2 , in which the polarization rotating structures (14, 16, 18) are planar structures. Circuit board according to one of the preceding claims, in which the polarization-rotating structures (14, 16, 18) are each formed by a metallic mold on the circuit board substrate (20). Printed circuit board according to one of the preceding claims, in which the different polarization-rotating structures (14, 16, 18) have different rectangular shapes. Circuit board according to one of the preceding claims, in which the different polarization-rotating structures (14, 16, 18) are each formed by a conductive island structure on the circuit board substrate (20). Circuit board according to one of the preceding claims, wherein the circuit board has at least one first conductive layer (22), wherein the different polarization-rotating structures (14, 16, 18) are formed by conductive surfaces on the circuit board substrate (20), and wherein the conductive surfaces and the at least one element (12) for guiding radar signals are formed in the at least one first conductive layer (22) of the circuit board. Circuit board according to one of the preceding claims, wherein the circuit board has a rear-side conductive layer (24) and a front-side conductive layer (22) which is structured around the different polarization-rotating structures (14, 16, 18) and the at least one element (12) for guiding radar signals comprises. Radar sensor with a printed circuit board (10) according to one of the preceding claims. Radar sensor after Claim 9 , with a monolithic integrated microwave circuit (40) applied to the printed circuit board (10).

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