Level gauge

Description

[0001] The invention relates to a level measuring device whose functionality can be checked.

[0002] In automation technology, particularly for process automation, field devices are often used to record various measured variables. The measured variable to be determined can be, for example, a level, flow, pressure, temperature, pH value, redox potential, conductivity, or dielectric value of a medium in a process plant. To record the corresponding measured values, the field devices each contain suitable sensors or are based on suitable measuring principles. A wide variety of field device types are manufactured and distributed by the Endress + Hauser Group.

[0003] Radar-based measurement methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. A key advantage of radar-based measurement methods is their ability to measure the fill level virtually continuously. In the context of this patent application, the terms " radar " or " High frequency " to radar signals with frequencies between 0.03 GHz and 300 GHz. Common frequency bands for level measurement are 2 GHz, 26 GHz, 79 GHz, or 120 GHz. The two common measurement principles are the pulse transit time principle (also known as " Pulse radar ") and the FMCW principle ("Frequency Modulated Continuous Wave ") .

[0004] The document DE 100 37 715 A1 shows a radar-based level measuring device from the state of the art.

[0005] In the pulse transit time method, high-frequency signals are emitted in pulses at a defined clock rate towards the filling material using a high-frequency source. Based on this, the transit time until the high-frequency pulses reflected at the filling material's surface arrive is measured by receiving a corresponding received signal. To make it easier to determine the transit time or distance to the filling material using the high-frequency received signal, a time-expanded evaluation signal is created from the received signals. Due to the high pulse frequency, the evaluation signal is created by undersampling the received signal. The undersampling takes place in a receiver by sampling the received high-frequency pulses with correspondingly generated sampling pulses, whose sampling rate deviates by a few thousandths of a percent from the clock rate of the transmitted high-frequency pulses.To ensure the corresponding target deviation between the sampling rate and the clock rate is maintained, the sampling rate is adjusted by a control loop depending on the measured actual deviation. Overall, the evaluation signal thus represents the signal amplitude of the received signal or the reflected high-frequency pulses in a time-expanded manner. The evaluation signal reflects the signal amplitude of the received signal as a function of the distance between the measuring device and the product.

[0006] In contrast to the pulse transit time method, FMCW is based on the high-frequency source emitting the high-frequency signal continuously, but with a modulated frequency. The frequency of the emitted high-frequency signal lies in a defined frequency band around a standardized center frequency. With FMCW, the temporal change in frequency is linear by default and has a sawtooth or triangular shape. However, a sinusoidal change can also be implemented in principle. When implementing the FMCW method, the distance to the filling material or the fill level is determined based on the instantaneous frequency difference between the current received signal and the currently emitted high-frequency signal. The receiver generates the time-expanded evaluation signal by mixing the corresponding electrical RF signals.The distance can be determined based on the frequency of the evaluation signal, since the frequency of the evaluation signal changes proportionally to the distance.

[0007] Based on the pulse transit time method and the FMCW method, level measuring devices can be realized with comparatively low circuit complexity and high level resolution in the sub-millimeter range. Radar-based measuring principles are described in more detail, for example, in "Radar Level Detection," Peter Devine, 2000. ".

[0008] In addition to free-radiating radar measurement, in which the high-frequency signals are transmitted and received via an antenna, there is also the variant of guided radar. Instead of an antenna, an electrically conductive measuring probe (e.g., a coaxial cable or a metal rod) serves as the transmission unit, which is lowered into the container to guide the high-frequency signals. Similar to free-radiating radar, the high-frequency signal is reflected in the measuring probe at the level of the product surface and guided back along the measuring probe to the level measuring device. This variant of radar-based level measurement is also known as "guided radar". TDR " (" Time Domain ReflectometryThe advantage of this variant is that less power is required to operate the level measuring device due to the guided signal radiation. Analogous to freely radiating radar devices based on the pulse transit time or FMCW principle, an evaluation signal is also created to determine the level when implementing the TDR principle.

[0009] Regardless of the measuring principle implemented, safety-relevant level measuring devices are required to be able to monitor the various functional units of the device in such a way that any malfunction of any unit can be detected with sufficient certainty. Corresponding safety specifications are defined, for example, as " Safety Integrity Level x (SILx)"according to the IEC61508 series of standards. If the level measuring device cannot comply with such safety requirements or can only partially comply with them, it is considered unsafe and may only be operated with correspondingly shorter test cycles, if at all. However, such test cycles are complex and therefore undesirable in ongoing production processes. However, verifying the correct function of the high-frequency unit, which comprises the high-frequency source and the receiver, is difficult because failures in the analog range often cannot be clearly identified as defects. This applies, for example, to gradual changes that affect measurement accuracy and are therefore safety-relevant.

[0010] The invention is therefore based on the object of providing a level measuring device which can check the functionality of its high-frequency unit.

[0011] The invention is defined by the subject matter of the independent claims.

[0012] The invention is therefore based on designing all essential components of the high-frequency unit redundantly, whereby the functionality of the high-frequency unit is tested by switching (i.e., switching off the previously active component and activating the corresponding, previously inactive component) at least one of the redundant components. The test is performed by examining the evaluation signal for changes in the evaluation signal resulting from the switching. Accordingly, depending on the application, the diagnostic unit of the level gauge can, for example, generate a corresponding error signal if the diagnostic unit classifies the high-frequency unit as non-functional. Alternatively or additionally, it is also conceivable for the diagnostic unit to generate a corresponding "OK" signal as long as the high-frequency unit is classified as functional.

[0013] The term " Unit"In the context of the invention, "electronic circuit" is understood to mean, in principle, any electronic circuit that is suitably designed for the intended purpose. Depending on the requirements, it can therefore be an analog circuit for generating or processing corresponding analog signals. However, it can also be a digital circuit such as an FPGA or a storage medium in conjunction with a program. The program is designed to carry out the corresponding method steps or to apply the necessary computing operations of the respective unit. In this context, different electronic units of the measuring device within the meaning of the invention can potentially also access a common physical memory or be physically operated using the same digital circuit.

[0014] Within the scope of the invention, several properties can in principle be considered as defined properties for which the evaluation signal is checked during switching: In this regard, the diagnostic unit can, for example, be designed to compare an edge steepness, a signal amplitude and / or a corresponding signal propagation time of one of the signal maxima as a defined property of the evaluation signal.

[0015] If the level gauge is based on the TDR principle and the transmission unit is accordingly designed as an electrically conductive measuring probe that extends approximately vertically toward the container bottom when the level gauge is installed, there is also the advantageous option of one of the two high-frequency sources being designed to be inverting, and / or one of the two receivers being designed to be inverting. This changes the polarity of the evaluation signal when the active component is switched, provided the high-frequency unit is functioning. This reduces the radiation emissions of the level gauge, as described, for example, in the publication WO2005 / 062002 A1.In this case, the evaluation unit should preferably be designed in such a way that it inverts the polarity of the evaluation signal again if the polarity changes due to the switching, and that it determines the defined property on the basis of the evaluation signal, which may have been polarity-inverted back.

[0016] Furthermore, the inverting design of one of the receivers or one of the high-frequency sources can be used to test the functionality of the high-frequency unit: For this purpose, the evaluation unit must be designed to determine the polarity of the evaluation signal. Furthermore, the diagnostic unit must be designed to control the evaluation unit in such a way that the polarity of the evaluation signal is determined as a defined property of the evaluation signal before and after switching the active high-frequency source or the active receiver. This allows the diagnostic unit to classify the high-frequency unit as inoperative if the polarity of the evaluation signal does not change upon switching.

[0017] In principle, it is irrelevant for the purposes of the invention whether the level measuring device is based on the FMCW or the pulse transit time principle. In the case of the pulse transit time principle, the receivers can be designed as samplers, for example. To implement the pulse transit time principle, the control unit must also be designed accordingly to control the active high-frequency source in such a way that the high-frequency signal is generated in pulse form according to the pulse transit time method. Furthermore, in the case of the pulse transit time method, the evaluation unit controls the active sampler in such a way that the received signal is subsampled according to the pulse transit time method, so that the evaluation signal is time-discretized.

[0018] The high-frequency source can be designed as a high-frequency oscillator in the case of FMCW as well as in the pulse transit time method, whereby its frequency can be set, for example, by a " Phase Locked Loop (PLL)"can be regulated. In the case of TDR, it is sufficient to design the high-frequency source as a capacitor. In the case of the FMCW method, for example, the receivers can be designed as mixers to mix the incoming received signal with the transmitted high-frequency signal and thus obtain the time-expanded evaluation signal corresponding to the pulse transit time method.

[0019] Analogous to the level measuring device according to the invention, the object underlying the invention is also achieved by a corresponding measuring method for checking the functionality of the level measuring device. The method comprises the following steps: Generation of the high-frequency signal by means of the respectively activated high-frequency source, transmission of the high-frequency signal towards the filling material and reception of the corresponding reception signal after reflection at the filling material surface, reception of the reception signal by means of the respectively activated receiver so that a time-extended evaluation signal is generated, determination of the defined property of the evaluation signal, switching the respectively active high-frequency source to passive, and vice versa, and / or switching the respectively active receiver to passive, and vice versa.

[0020] The previous steps are then repeated, except for switching the active high-frequency source or the active receiver. By repeating the steps, the defined property is compared before and after switching, and the high-frequency unit is classified as inoperative if the defined property of the evaluation signal has changed by a defined minimum value due to the switching.

[0021] The process can be automated, for example, by cyclically switching the active high-frequency source or receiver, comparing the defined properties before and after switching, and classifying the high-frequency unit with regard to its inoperability during measurement operation.

[0022] The invention is explained in more detail with reference to the following figures. They show: Fig. 1 : A state-of-the-art TDR-based level measuring device, Fig. 2 : a schematic representation of an evaluation signal, Fig. 3: a block diagram of a level measuring device according to the invention, and Fig. 4 : possible effects of a faulty high-frequency unit on the evaluation curve.

[0023] For a basic understanding of the invention, Fig. 1 A block diagram of a state-of-the-art level measuring device 1' is shown, which is used to measure the fill level L of a filling material 2 located in a container 3. The level measuring device 1' shown is based on the pulse transit time principle, wherein it comprises a measuring probe as a transmission unit 13 according to the TDR method. To determine the fill level L, the measuring probe 13 extends inside the container from the top to just above the container bottom. The installation height h of the measuring probe 13 above the container bottom is known and stored in an evaluation unit 14 of the level measuring device 1'.

[0024] According to the pulse transit time method, a high-frequency signal S HF is transmitted in a pulsed manner via the measuring probe 13 toward the filling material 2. Due to the dielectric value jump there, the high-frequency signal S HF is reflected in the measuring probe 13 at the level of the filling material surface 2 and, after a corresponding signal transit time t, is received in the level measuring device 1' as a received signal E HF. The signal transit time of the signal S HF , E HF depends on the distance d = h - L between the container top and the filling material surface.

[0025] To generate the high-frequency signal S HF , the level measuring device 1' comprises a first high-frequency source 121 as part of a high-frequency unit 12. The first high-frequency source 121 can be designed according to the TDR method, for example, as a capacitor that is discharged accordingly to generate the 100 ps to approximately 1 ns pulse. In the case of freely radiating radar according to the pulse transit time or FMCW method, the first high-frequency source 121 can be designed, for example, as a frequency-controlled high-frequency resonant circuit or as a quartz crystal. In order for the first high-frequency source 121 to generate the high-frequency signal S HF in pulse form according to the TDR method at the required clock rate, the first high-frequency source 121 is controlled in a correspondingly clocked manner by a control unit 11 outside the first high-frequency unit 12.The first high-frequency source 121 feeds the high-frequency signal S HF to be transmitted to the measuring probe 13 via a transmit / receive switch 122. The design of the transmit / receive switch 122 is not predetermined in principle. In the case of TDR, as in the case of the device shown in . Fig. 1 As is the case with the embodiment shown, the transmit / receive switch 122 can, for example, be designed purely as an electrical node. Particularly in the case of free-radiating radar, the transmit / receive switch 122 can, for example, be implemented as a duplexer.

[0026] The received signal E RF arriving from the measuring probe 13 in the high-frequency unit 12 is also fed to a first receiver 123 via the transmit / receive switch 122. According to the pulse transit time principle, the received signal E RF is subsampled in the first receiver 123, generating an evaluation signal A(t) that time-stretches the received signal E RF by a defined factor. The time-stretch factor depends on the sampling rate. To achieve sufficient time-stretching, the corresponding sampling rate must be selected so that it differs from the clock rate of the outgoing signal pulses S RF by only a few thousandths of a second. Accordingly, the sampling rate at which the first receiver 123 samples the received signal E HF is again set at the first receiver 123 by the control unit 11, which also specifies the clock rate of the outgoing signal pulses S HF.Time dilation simplifies the determination of the fill level L based on the received signal E HF from a circuit-technical perspective. In contrast to the method described in . Fig. 2 In the embodiment shown, in the case of freely radiating radar, in addition to the time expansion, the received signal E HF is equalized in the first receiver 123, so that the evaluation signal A(t) has only one polarity - plus or minus - with respect to a fixed reference potential.

[0027] To determine the fill level L, the first receiver 123 transmits the evaluation signal A(t) to an evaluation unit 14. If the first receiver 123 is a digital sampler, this is already done in digital form. The determination of the fill level L by means of the evaluation signal A(t) by the evaluation unit 14 is carried out using Fig. 2 illustrated in more detail: Fig. 2illustrates the temporal amplitude curve of the received signal E HF or the time-extended evaluation signal A(t). The distance d between the container top and the filling material surface is proportional to the time axis of the evaluation signal A(t) of the received signal E HF . In the ideal case, i.e. without any external interference, the received signal E HF comprises three signal maxima M a . The first signal maximum M a can be assigned to the internal reflection of the high-frequency signal S HF at the transmit / receive switch 122. The second signal maximum M a in the received signal E HF is caused by the surface of the filling material 2, while the third signal maximum M a is caused by the probe end 131 of the measuring probe 13.

[0028] With the aid of any filtering methods, the evaluation unit 14 is able to determine the signal propagation time t M of the signal maximum M a caused by the product surface. Based on appropriate calibration data, the evaluation unit 14 calculates the corresponding distance d from this signal propagation time t M . Using the relationship L = h - d or the known installation height h, the fill level L can in turn be determined from the distance d.

[0029] A key prerequisite for the evaluation unit 14 to be able to correctly determine the fill level L is the faultless functioning of the high-frequency unit 12, since this does not lead to an obvious failure of the high-frequency unit 12, depending on the impairment. As in Fig. 4aAs shown, a malfunctioning high-frequency unit 12 can, for example, lead to a gradual offset of the received signal E HF or an offset of the evaluation signal A(t) with increasing operating time. This may unnoticed distort the signal propagation time t M of the fill level maximum MA and thus the determined distance value d. The result of a gradual degradation of a high-frequency amplifier of the high-frequency unit 12 is again shown in Fig. 4billustrated: Due to such an error mechanism, the evaluation signal S HF or the underlying reception signal E HF can be attenuated in its amplitude, so that the evaluation unit 14, in case of doubt, does not detect the signal maximum MA that is based on the filling material surface, but incorrectly uses a different signal maximum MA to determine the distance d or the filling level L. Even such a malfunction of the high-frequency unit 12 is not immediately recognizable from the outside. However, a sudden failure of one of the units is also conceivable and may not be recognized depending on the situation. Thus, the level measuring device 1' can be used in applications that have corresponding safety requirements such as " SIL " must be observed, may not be used.

[0030] A possible embodiment of the level measuring device 1 according to the invention, with which a possible malfunction of the high-frequency unit 12 can be diagnosed, is therefore shown in Fig. 3 described in more detail: In principle, the structure and functionality of the Fig. 3 level measuring device 1 shown in Fig. 1 shown embodiment variant. In addition, however, the high-frequency unit 12 of the level measuring device 1 according to the invention comprises a second high-frequency source 121' and a second receiver 123'. It is advantageous if the second high-frequency source 121' is designed identically to the first high-frequency source 121, for example, again as a capacitor. The same applies to the second receiver 123' with respect to the first receiver 123.

[0031] In the case of free-radiating radar, i.e. contrary to the Fig. 3In the embodiment shown, it is advantageous with regard to the high-frequency sources 121, 121' if they generate the high-frequency signal S HF at the same frequency in order to ensure the same signal behavior.

[0032] The control unit 11 can selectively activate one of the two high-frequency sources 121, 121' using a first switch 120. The control unit 11 can, in turn, selectively activate one of the two receivers 123, 123' using a second switch 124. In this context, the switches 120, 124 can be designed, for example, as transistors whose gate / base are controlled by the control unit 11.

[0033] In relation to the high frequency sources 121, 121', the term " Activate" to the switching on of the high-frequency source 121, 121' to be activated as well as the connection of the high-frequency source 121, 121' to be activated with the measuring probe 13 or with the transmit / receive switch 122. With reference to the term " Switch " means that the high-frequency source 121, 121' to be deactivated is separated from the measuring probe 13 and / or that it is switched off. With regard to the two receivers 123, 123', the term " Activate " to the sampling of the received signal E HF and the transmission of the corresponding evaluation signal A(t) to the evaluation unit 14. This means in connection with the term " Switch " in turn, that the receiver 123, 123' to be switched inactive no longer samples the received signal E HF as of the activation of the other receiver 123, 123' and / or no longer transmits the evaluation signal A(t) to the evaluation unit 14.

[0034] Also in the case of Fig. 3In the embodiment of the level measuring device 1 according to the invention shown, the control unit 11 basically specifies the clock rate of both high-frequency sources 121, 121'. The same applies to the two receivers 123, 123', whose sampling rates are specified by the control unit 11.

[0035] By means of the redundant design of the high-frequency unit 12 with two high-frequency sources 121, 121' and two receivers 123, 123', it is possible for the control unit 14 to switch the first switch 120 or the second switch 124, for example, cyclically during measuring operation or during a test operation of the level measuring device 1. The switching of the two switches 120, 124 can take place either simultaneously or - if necessary cyclically - offset from one another. At least one evaluation curve A(t) is recorded before and after each switching, with the evaluation unit 14 determining a previously defined property from the two evaluation curves A(t), such as their polarity, the signal amplitude and / or the corresponding signal propagation time t M of one of the signal maxima MA.

[0036] The switching, the recording of the corresponding evaluation curves A(t), and the respective determination of the defined property before and after the switching are coordinated by an appropriately designed diagnostic unit 15 outside the high-frequency unit 12. Furthermore, the diagnostic unit 15 compares the defined property before the switching with the corresponding value after the switching. Should the property change beyond a limit value, the diagnostic unit 15 classifies the high-frequency unit 12 as inoperative and, if necessary, outputs a corresponding error signal. If the defined property is the signal amplitude of one of the signal maxima MA, any decrease in amplitude due to the switching beyond the limit value can be interpreted, for example, as a gradual degradation of a high-frequency amplifier of the high-frequency unit 12, as described in Fig. 4b is shown.

[0037] In the Fig. 3 In the embodiment of the level measuring device 1 according to the invention shown, the second high-frequency source 121' and the second receiver 123' are designed to be inverting. Since the level measuring device 1 is based on the TDR method, this offers the possibility of inverting the polarity of the received signal E HF or the evaluation signal A(t) when switching the active receiver 123, 123' without switching the active high-frequency source 121, 121'—or vice versa. This not only reduces the emissions of the level measuring device 1, but it can in turn be used to check the functionality of the high-frequency unit 12: If the diagnostic unit 15 does not detect a polarity change despite switching either the active high-frequency source 121, 121' or the active receiver 123, 123', the high-frequency unit 12 is to be classified as non-functional.

[0038] It goes without saying that the evaluation signal A(t) can in principle be tested not only for one but also for several properties by the diagnostic unit 15 before and after switching, whereby the diagnostic unit 15 already classifies the high-frequency unit 12 as inoperative in this case if one of the defined properties of the evaluation signal A(t) has changed by at least a defined value due to the switching.

[0039] The Fig. 3The embodiment of the level measuring device 1 according to the invention shown is based on the pulse transit time principle and comprises a measuring probe 13 as a transmission unit according to the TDR method. In this regard, it should be noted that the inventive redundant design of the high-frequency unit 12 with two high-frequency sources and two receivers, as well as the corresponding testing of the functionality, can in principle also be implemented in the case of freely radiating radar or when implementing the FMCW principle. List of reference symbols

[0040] 1 , 1'Level measuring device 2Filling material 3Container 11Control unit 12High-frequency unit 13Transmission unit 14Evaluation unit 120First switch 121 ,121'High-frequency source 122Transmitter / receiver switch 123 ,123'Receiver 124Second switch A(t)Evaluation signal dDistance E HF Received signal hInstallation height LLevel M a Signal maximum S HF High-frequency signal tSignal propagation time

Claims

1. A radar-based fill level measuring device for measuring a fill level (L) of a filling material (2) in a container (3), comprising: - An electrically conductive measurement probe (13), which extends roughly perpendicular toward the base of the container when the fill level measuring device (1) is installed and which can be used to transmit high-frequency signals (SHF) toward the filling material (2) and can then, following reflection on the surface of the filling material, be used to receive these as receive signals (EHF), characterized by - a high-frequency unit (12), with o two activatable high-frequency sources (121, 121'), each of which is configured to generate the high-frequency signal (SHF), ∘ two activatable receivers (123, 123'), each of which can be used to time-dilate the receive signal (EHF), ∘ wherein one of the two high-frequency sources (121, 121') is configured to be inverting, and / or wherein one of the two receivers (123, 123') is configured to be inverting, - a control unit (11), which is configured ∘ to activate one of the high-frequency sources (121, 121') in each case in such a way that the activated high-frequency source (121, 121') generates the high-frequency signal (SHF), and ∘ to activate one of the receivers (123, 123') in each case in such a way that the activated receiver (123, 123') converts the receive signal (EHF) into a time-dilated evaluation signal (A(t)), - an evaluation unit (14) which is configured o to determine a defined property and a signal duration (tM) of at least one signal maximum (MA) based on the evaluation signal (A(t)), and ∘ to determine the fill level (L) based on the signal duration (tM) of the signal maximum (Ma), and - a diagnostics unit (15) which is configured o to control the control unit (11) in such a way that the high-frequency source (121, 121') which is active in each case and / or the receiver (123, 123') which is active in each case is / are deactivated in order to invert the polarity of the receive signal (EHF) and / or the evaluation signal, ∘ to control the evaluation unit (14) in such a way that the defined property of the evaluation signal (A(t)) is at least determined both before and after switching ∘ in order to classify the high-frequency unit (12) as non-functional if the defined property of the evaluation signal (A(t)) changes at least by a defined value due to switching.

2. The fill level measuring device as claimed in one of the preceding claims, wherein the diagnostics unit (15) is configured to compare an edge steepness, a signal amplitude, and / or a corresponding signal duration (tM) of the at least one signal maximum (MA) as a defined property of the evaluation signal (A(t)).

3. The fill level measuring device as claimed in claim 1 or 2, wherein the evaluation unit (14) is configured ∘ to invert the polarity of the evaluation signal (A(t)) if the polarity changes due to switching, and ∘ to determine the defined property based on the evaluation signal (A(t)) with the inverted polarity.

4. The fill level measuring device as claimed in claims 1 to 3, wherein the evaluation unit (14) is configured to determine the polarity of the evaluation signal (A(t)), wherein the diagnostics unit (15) is configured ∘ to control the evaluation unit (14) in such a way that the polarity of the evaluation signal (A(t)) is determined as its defined property both before and after switching the active high-frequency source (121, 121') and / or the active receiver (123, 123'), and ∘ to classify the high-frequency unit (12) as non-functional if the polarity of the evaluation signal (A(t)) does not change due to switching, or if the polarity of the evaluation signal (A(t)) changes due to switching.

5. The fill level measuring device as claimed in one of the preceding claims, wherein the receivers (123, 123') are configured as samplers, and wherein the control unit (13) is configured - to actuate the high-frequency source (121, 121') which is active in each case in such a way that the high-frequency signal (SHF) is generated in the form of pulses, and - to actuate the sampler (123, 123') which is active in each case in such a way that the receive signal (EHF) is subsampled according to the time-of-flight method in such a way that time discretization is carried out on the evaluation signal (A(t)).

6. The fill level measuring device as claimed in at least one of the preceding claims, wherein the diagnostics unit (15) is configured to generate a fault signal if it classifies the high-frequency unit (12) as non-functional.

7. A method for checking the functionality of the fill level measuring device (1) as claimed in one of the preceding claims, comprising the following process steps: - Generating the high-frequency signal (SHF) using the high-frequency source (121, 121') which has been activated in each case, - transmitting the high-frequency signal (SHF) toward the filling material (2) and receiving the corresponding received signal (EHF) following reflection on the surface of the filling material, - receiving the receive signal (EHF) using the receiver (123, 123') which has been activated in each case in such a way that a time-dilated evaluation signal (A(t)) is generated, - determining at least one signal maximum (MA) of the evaluation signal (A(t)), - determining at least one defined property of the at least one signal maximum (MA, MA'), - switching the high-frequency source (121, 121') which is active in each case to passive, and vice versa, and / or switching the receiver (123, 123') which is active in each case to passive, and vice versa, and then repeating the previous process steps, - comparing the defined property before and after switching, and - classifying the high-frequency unit (12) as non-functional if the defined property of the evaluation signal (A(t)) has changed at least by a defined value due to switching.

8. The method as claimed in claim 7, wherein - the switching of the high-frequency source (121, 121') which is active in each case and / or the receiver (123, 123') which is active in each case, - the comparison of the defined property before and after switching, and - the classification of the high-frequency unit (12) regarding its functionality are carried out cyclically.