RADAR TARGET SIMULATOR WITH CONTINUOUS RANGE EMULATION AND CORRESPONDING SIMULATION METHOD

Description

[0001] The invention relates to a radar target simulator with no lower target distance limitation and continuous distance emulation and a radar target simulation method with no lower distance limitation and continuous distance emulation.

[0002] Generally, in times of an increasing number of applications employing radar systems such as autonomous vehicles, there is a growing need of a radar target simulator with no lower target distance limitation and continuous distance emulation and a radar target simulation method with no lower distance limitation and continuous distance emulation in order to ensure a correct functioning of such applications especially on the basis of highly accurate and efficient measurement and testing.

[0003] US 4,661,818 A relates to a delay-simulator for testing a radio altimeter of the type which emits a frequency-modulated wave, the modulation being according to linear segments of slope p, and which compares the frequencies of the emitted wave and of the wave which is received after reflection to provide a distance measurement. The delay-simulator is connected between the transmitting antenna and the receiving antenna of the altimeter, and comprises two signal-processing chains which are arranged, in parallel, between a receiving section and a single-sideband mixer, the output terminal of which is connected to a transmitting section. The first chain is formed by first devices, which generate a quantity G, which is proportional to p, and second devices, which generate a signal possessing a frequency fd, the ratio of fd and G assuming a predetermined, adjustable ratio, Q. The second chain is formed by a coupler which is connected to a variable attenuator. The single-sideband mixer beats substractively between the two signals which it receives. Furthermore, it is the object of said delay-simulator "to simulate, for a distance-measuring apparatus of the FM-CW type, all the altitudes included within a range of distances covering several km, or even several tens of km, in steps of a value ranging between a few tens of cm and several m" (see column 2, lines 4 to 8).

[0004] Accordingly, there is an object to provide a radar target simulator with no lower target distance limitation and continuous distance emulation and a radar target simulation method with no lower distance limitation and continuous distance emulation, which allow for highly accurate and efficient measurements or testing, respectively.

[0005] This object is solved by the features of claim 1 for a radar target simulator and the features of claim 12 for a radar target simulation method. The dependent claims contain further developments.

[0006] According to the invention, a radar target simulator according to claim 1 is provided. Advantageously, this allows for highly accurate and efficient measurements or testing. Further advantageously, said tracking may preferably be performed continuously and / or in real-time or nearly in real-time.

[0007] According to a first preferred implementation form of the invention, the ramp slope estimating unit is configured to determine a corresponding frequency slope and / or a corresponding direction preferably on the basis of the tracking information. Advantageously, for instance, direction can be positive or negative, which allows for a high flexibility.

[0008] For example, the first delay time may be generated with the aid of digital and / or analog means preferably comprising at least one of an electrical, acoustical, optical delay line, a field programmable gate array (FPGA), or any combination thereof.

[0009] According to a further preferred implementation form of the invention, the radar target simulator comprises a second mixer especially for applying a Doppler frequency. Advantageously, for instance, the Doppler frequency can be applied in a particularly efficient manner. Further advantageously, the second mixer may preferably be implemented in a digital manner. For instance, the second mixer may especially be implemented as a mixing stage of an FPGA. In this context, it is noted that this may analogously apply for further mixers.

[0010] According to a further preferred implementation form of the invention, the ramp slope estimating unit comprises a second delay unit especially for delaying the receive signal according to a second delay time. In addition to this or as an alternative, the ramp slope estimating unit comprises a complex output especially for providing corresponding phase information preferably on the basis of the tracking information. Advantageously, for example, the second delay time may be generated with the aid of digital and / or analog means preferably comprising at least one of an electrical, acoustical, optical delay line, or any combination thereof. Further advantageously, the complex output especially providing corresponding phase information may allow for a determination of the respective positive and negative slope delay time.

[0011] According to a further preferred implementation form of the invention, the ramp slope estimating unit comprises a third mixer especially for mixing the receive signal with a delayed version of the receive signal. Advantageously, for instance, accuracy can be increased in a particularly efficient manner. Further advantageously, said delayed version may be generated on the basis of the second delay time. In other words, the delayed version of the receive signal may be generated with the aid of the second delay unit.

[0012] According to a further preferred implementation form of the invention, the ramp slope estimating unit comprises a frequency estimating unit, preferably a Fourier transformation unit, especially for applying a Fourier transform, preferably a discrete Fourier transform, to a signal resulting from mixing the receive signal with a delayed version of the receive signal. Advantageously, for example, inaccuracies can be reduced further. Further advantageously, additionally or alternatively, the frequency estimating unit may be configured to estimate the respective frequency on the basis of corresponding phase information of the respective beat signal especially after mixing with the aid of the third mixer. In addition to this or as an alternative, the frequency estimating unit may be configured to apply a Fourier transform with an argumentum maximi estimation or function.

[0013] According to a further preferred implementation form of the invention, the signal generation unit comprises or is a controlled oscillator, preferably a numerically controlled oscillator or a voltage-controlled oscillator. In addition to this or as an alternative, the modulation frequency is set on the basis of a relation between the ramp slope of the radar under test and a respective processing time of the radar target simulator. Advantageously, for example, the signal generation unit can be implemented in a particularly cost-efficient manner. Further advantageously, additionally or alternatively, the signal generation unit may comprise multiple controlled oscillators, preferably multiple numerically controlled oscillators or multiple voltage-controlled oscillators, especially for multiple targets or the simulation thereof, respectively.

[0014] According to a further preferred implementation form of the invention, the signal generation unit is configured in a manner that it behaves like a frequency-shifting unit especially for creating Doppler effects. Advantageously, for instance, simulation of moving radar targets may especially not be limited to stationary radar targets.

[0015] Advantageously, for example, due to the fact that the respective beat signal is relatively small in frequency, there is no need for the usage of high capability components, which additionally reduces costs.

[0016] According to a further preferred implementation form of the invention, the radar target simulator comprises an amplifying unit, preferably an adjustable amplifying unit, being especially configured to set a gain with respect to a response signal to be transmitted to the radar under test. Advantageously, for instance, with the aid of said gain, a radar cross section of the respective radar target can be adjusted in a particularly accurate and efficient manner. Further advantageously, preferably as an alternative, the radar target simulator may comprises an attenuation unit being especially configured to set an attenuation with respect to a response signal to be transmitted to the radar under test.

[0017] Advantageously, for example, at least a part of the transmitting unit and the receiving unit can be based on the same components, which additionally reduces costs and therefore leads to an increased efficiency.

[0018] According to a further preferred implementation form of the invention, the receiving unit comprises at least one of an antenna, a fourth mixer, an analog-to-digital converter, or any combination thereof. In addition to this or as an alternative, the transmitting unit comprises at least one of an antenna, a fifth mixer, a digital-to-analog converter, or any combination thereof. Advantageously, for instance, the receiving unit and / or the transmitting unit can be implemented in a simple manner, thereby reducing inefficiencies.

[0019] According to the invention, a radar target simulation method according to claim 12 is provided.

[0020] Exemplary embodiments of the invention are now further explained with respect to the drawings by way of example only. In the drawings: Fig. 1shows an exemplary embodiment of an inventive radar target simulator or essential parts thereof, respectively; Fig. 2shows an exemplary embodiment of the invention as a whole; and Fig. 3shows a flow chart of an embodiment of the invention.

[0021] With respect to Fig. 1, a block diagram of an exemplary embodiment of an inventive radar target simulator 10 or some exemplary parts thereof, respectively, with no lower target distance limitation and continuous distance emulation is shown. According to Fig. 1, the radar target simulator 10 comprises a receiving unit 11 configured to receive a radar signal from a radar under test 12 and to provide a corresponding receive signal.

[0022] In addition to this, the radar target simulator 10 comprises a ramp slope estimating unit 13 configured to track the ramp slope of the radar under test 12 on the basis of the receive signal especially as tracking information. As it can further be seen from Fig. 1, the radar target simulator 10 comprises a digital target generation unit 14, wherein the ramp slope estimating unit 13 is exemplarily comprised by said digital target generation unit 14.

[0023] Generally, it is noted that it might be particularly advantageous if the complete target generation unit or back-end, respectively, is a digital system or unit as the above-mentioned digital target generation unit 14. However, with respect to said target generation unit or target generation back-end, it is further noted that other solutions could be completely analog, as well as a mixed analog / digital implementation.

[0024] Moreover, with respect to the receiving unit 11, it is noted that said receiving unit 11 exemplarily comprises an antenna or an antenna array, a mixer 16c, and an analog-to-digital converter 17. In this context, a down-conversion is performed with the aid of the mixer 16c after the antenna or antenna array, respectively, especially for a simpler handling of the corresponding signal in the baseband region. In addition to this, the above-mentioned analog-to-digital converter 17 performs the respective conversion from analog to digital signal domain.

[0025] The radar target simulator 10, especially the digital target generation unit 14 thereof, comprises a decision unit 15. Said decision unit 15 is configured to decide if the receive signal should be passed to a first delay unit 18a or to the ramp slope estimating unit 13.

[0026] With respect to said first delay unit 18a, it is noted that the first delay unit 18a is configured to delay the receive signal according to a first delay time Δτ 1 .With respect to the first delay time Δτ 1 , it is noted that said first delay time Δτ 1 especially sets the respective target distance.

[0027] Again, with respect to the decision unit 15, it is noted that said decision 15 is configured to pass the corresponding receive signal to the first delay unit 18a in the case that the corresponding target distance R is larger than a minimum target distance R min . Otherwise, if the corresponding target distance R is lower or equal to said minimum target distance R min , the decision unit 15 is configured to pass the corresponding receive signal to the ramp slope estimating unit 13, especially to a second delay unit 18b thereof. Furthermore, if the corresponding target distance R is lower or equal to the minimum target distance R min , the respective frequency modulation of the radar under test may preferably be tracked.

[0028] Advantageously, a priori information about the respective modulation scheme is not necessary due to on-the-fly tracing of the frequency modulation. With respect to the minimum target distance R min , it is noted that R min preferably is the shortest distance which may be set by the radar target simulator 10 or the digital target generation unit 14. This value may especially be driven by the respective internal minimum delay time. Advantageously, R min does not comprise a lower bound. Additionally, R min does not comprise an upper bound. Accordingly, the radar target simulator 10 allows for a full control over all generated radar target signal properties at the same time.

[0029] Further advantageously, moving radar targets generated by the radar target simulator 10 may approximate as close as zero meter. In this context, even for the physical air gap between the radar under test 12 and the radar target simulator 10 there is no limitation. For instance, this helps indicating the respective radar behavior directly before an accident.

[0030] Again, with respect to the ramp slope estimating unit 13, it is noted that it might be particularly advantageous if the ramp slope estimating unit 13 is configured to determine the corresponding frequency slope and / or the corresponding direction preferably on the basis of the tracking information. As it can further be seen from Fig. 1, the ramp slope estimating unit 13 exemplarily comprises the above-mentioned second delay unit 18b for delaying the receive signal according to a second delay time Δτ 2 . Furthermore, the ramp slope estimating unit 13 exemplarily comprises a mixer 16b especially for mixing the receive signal with a delayed version of the receive signal, preferably the Δτ 2 -delayed version of the receive signal.

[0031] Moreover, the ramp slope estimating unit 13 exemplarily comprises a discrete Fourier transformation unit 21 especially for applying a Fourier transform, preferably a discrete Fourier transform, to a signal resulting from mixing the receive signal with the delayed version of the receive signal. It is noted that the ramp slope estimating unit 13 may advantageously comprise a complex output especially for providing corresponding phase information preferably on the basis of the tracking information.

[0032] It is further noted that it might be particularly advantageous if with the above-mentioned second delay unit 18b, the additional mixing stage 16b, the discrete Fourier transformation unit 21, the respective ramp slope of the radar under test is tracked. This information or the tracking information is exemplarily fed to a signal generation unit in the form of a numerically controlled oscillator 22. In this context, it is noted that with respect to the signal generation unit or the numerically controlled oscillator 22, respectively, an argumentum maximi function unit 23 may be connected upstream as exemplarily shown.

[0033] Accordingly, the radar target simulator 10 or the ramp slope estimating unit 13 may preferably comprise an argumentum maximi function unit. In this exemplary case, the ramp slope estimating unit 13 comprises the argumentum maximi function unit 23.

[0034] The above-mentioned signal generation unit or the above-mentioned numerically controlled oscillator 22, respectively, is exemplarily configured to generate an offset frequency especially in order to shift the corresponding radar target towards the desired beat frequency and / or towards the desired radar target range. In this context, it is generally noted that the radar target simulator 10 may preferably be configured to mix said offset frequency generated by the signal generation unit or the numerically controlled oscillator 22 with the receive signal.

[0035] Again, with respect to the radar target simulator 10, the radar target simulator 10, especially the transmit path 19 thereof, exemplarily comprises a mixer 16a especially for applying a Doppler frequency. Furthermore, the radar target simulator 10, especially the transmit path 19 thereof, exemplarily comprises an amplifying unit, preferably an adjustable amplifying unit 20, being especially configured to set a gain with respect to a response signal to be transmitted to the radar under test.

[0036] With respect to said gain, it is noted that with the aid of setting this gain, the respective radar cross section of the corresponding radar target may be adjusted. Moreover, the radar target simulator 10, especially the transmit path 19 thereof, comprises a transmitting unit 24 being especially configured to transmit a response signal to be transmitted to the radar under test 12.

[0037] Exemplarily, said transmitting unit 24 comprises an antenna or an antenna array, a mixer 16d, a digital-to-analog converter 25. Said digital-to-analog converter 25 performs the respective conversion to an analog signal which is up-converted to the radio frequency region by the mixer 16d.

[0038] With respect to the above-mentioned transmit path 19, it is noted that an output of the signal generation unit or the numerically controlled oscillator 22, respectively, is connected to a first input of a mixer 16e, whereas a second input of said mixer 16e is provided with the same input signal as the second delay unit 18b. Furthermore, an output of the mixer 16e is connected to a first input of an adder 26, whereas a second input of said adder 26 is connected to an output of the first delay unit 18a. Moreover, an output of the adder 26 is connected to an input of the above-mentioned mixer 16a, whereas an output of said mixer 16a may connected to an input of the above-mentioned amplifier 20.

[0039] In the context of the invention, it is generally noted that other implementations could comprise different forms to create the delay, especially the first delay time and / or the second delay time. Therefore, signal samples of different time steps may be created. In this context, cost efficient analog components may comprise optical delay lines especially with directly modulated lasers, surface acoustic wave-delay elements, bulk acoustic wave-delay elements, simple wires especially with a defined length, or any combination thereof.

[0040] Now, with respect to Fig. 2, an exemplary embodiment of the inventive radar target simulator 30 is depicted especially as a whole and in an abstracted manner in comparison to the exemplary embodiment according to Fig. 1.

[0041] Advantageously, in this context, the demands for the digital backend 33 can be reduced as less bandwidth is required for processing especially due to the incorporation of a backscatter transponder and / or the usage of an (external) analog mixer.

[0042] In the context of embodiment according to Fig. 2, it is further noted that especially in the case of the implementation of multiple transponders, the respective radar target simulator 30 may comprise multiple digital-to-analog converters 31N - instead of the single digital-to-analog converter 25 of the first exemplary embodiment of the radar target simulator 10 according to Fig. 1 - and multiple corresponding elements as illustrated by the multiple blocks 32N.

[0043] This option would advantageously allow to create targets with different angle of arrival. Furthermore, for each angle of arrival, a respective digital-to-analog converter and / or transponder may preferably be employed.

[0044] In addition to this, with respect to the digital backend 33, it is noted that said digital backend 33 may provide several interfaces. In this context, at least one of an universal serial bus (USB) interface, an inter-integrated circuit (I2C) interface, an Ethernet (Eth) interface, a parallel (PAR) interface, a serial peripheral interface (SPI), or any combination thereof may preferably be provided.

[0045] With respect to both embodiments according to Fig. 1 and Fig. 2, it is noted that it might be particularly advantageous if the basis for said embodiments is to track the respective frequency modulation and to apply this information in a modulation in order to shift the frequency towards the set radar target distance.

[0046] In this context, on the basis of the ramp slope of, for instance, a frequency modulated continuous wave radar and the delay time of, for example, the respective digital back-end, an offset frequency or a modulation frequency may preferably be derived. Furthermore, said offset frequency may especially be applied to the current receiving frequency. It is further noted that the respective complex output is especially configured to provide the corresponding phase information, wherein said phase information may preferably give the corresponding direction.

[0047] Advantageously, said offset frequency or said modulation frequency may preferably be set by the ramp slope and / or the corresponding direction and / or the corresponding radar target distance. Further, the offset frequency or the modulation frequency are applied to the respective real-time frequency especially in order to get the desired response. Furthermore, the respective Doppler shifter may especially be configured to give the corresponding relative speed of the respective radar targets.

[0048] Finally, Fig. 3 shows a flow chart of an embodiment of the inventive radar target simulation method with no lower target distance limitation and continuous distance emulation. In a first step 100, a radar signal is received from a radar under test. Then, in a second step 101, a corresponding receive signal is provided. Furthermore, in a third step 102, the ramp slope of the radar under test is tracked on the basis of the receive signal especially as tracking information.

[0049] It might be particularly advantageous if the radar target simulation method further comprises the step of determining the corresponding frequency slope and / or the corresponding direction preferably on the basis of the tracking information. In addition to this or as an alternative, the radar target simulation method may comprise the step of delaying the receive signal according to a first delay time.

[0050] In further addition to this or as a further alternative, the radar target simulation method may comprise the step of applying a Doppler frequency. Further additionally or further alternatively, the radar target simulation method may comprise the step of delaying the receive signal according to a second delay time.

[0051] It is noted that it might be particularly advantageous if the radar target simulation method comprises the step of providing corresponding phase information preferably on the basis of the tracking information especially with the aid of a complex output. In addition to this or as an alternative, the radar target simulation method may further comprise the step of mixing the receive signal with a delayed version of the receive signal.

[0052] Additionally or alternatively, the radar target simulation method may comprise the step of applying a Fourier transform, preferably a discrete Fourier transform, to a signal resulting from mixing the receive signal with a delayed version of the receive signal.

[0053] In further addition to this or as a further alternative, the radar target simulation method may comprise the step of setting a modulation frequency preferably on the basis of the tracking information and / or the corresponding direction and / or the corresponding radar target distance especially with the aid of a signal generation unit.

[0054] In this context, it might be particularly advantageous if the signal generation unit comprises or is a controlled oscillator, preferably a numerically controlled oscillator or a voltage-controlled oscillator. In addition to this or as an alternative, it might be particularly advantageous if the modulation frequency is set on the basis of a relation between the ramp slope of the radar under test and a respective processing time of the radar target simulator.

[0055] Furthermore, the radar target simulation method may comprise the step of configuring the signal generation unit in a manner that it behaves like a frequency-shifting unit especially for creating Doppler effects. In addition to this or as an alternative, the radar target simulation method may comprise the step of configuring the signal generation unit to shift the respective radar target towards the corresponding beat frequency and / or towards the corresponding radar target range especially by setting the modulation frequency.

[0056] It is additionally or alternatively noted that it might be particularly advantageous if the modulation frequency is based on or equal to a multiplication of the ramp slope of the radar under test with the difference of the respective processing time of the radar target simulator and the respective real radar target time.

[0057] Additionally or alternatively, the radar target simulation method may comprise the step of shifting the respective radar target towards the corresponding beat frequency and / or towards the corresponding radar target range especially by setting the modulation frequency.

[0058] In further addition to this or as a further alternative, the radar target simulation method may comprise the step of setting a gain with respect to a response signal to be transmitted to the radar under test especially with the aid of an amplifying unit, preferably an adjustable amplifying unit. It is further noted that it might be particularly advantageous if the radar target simulation method comprises the step of transmitting a response signal to be transmitted to the radar under test.

Claims

1. A radar target simulator (10) comprising: a receiving unit (11) configured to receive a radar signal from a frequency modulated continuous wave radar under test (12) and to provide a corresponding receive signal, a transmitting unit (24) configured to transmit a response signal to be transmitted to the radar under test (12), a ramp slope estimating unit (13), and a signal generation unit (22), wherein the ramp slope estimating unit (13) is configured to track the ramp slope of the radar under test (12) on the basis of the receive signal as tracking information, wherein the signal generation unit (22) is configured to set a modulation frequency on the basis of the tracking information and a target distance of a simulated radar target to the radar under test (12), wherein the modulation frequency is applied to the receive signal, and wherein, by setting the modulation frequency, the signal generation unit (22) is configured to shift the radar target towards a beat frequency corresponding to said target distance with no lower target distance limitation and continuous distance emulation, and wherein the radar target simulator (10) further comprises: a first delay unit (18a) configured for delaying the receive signal according to a first delay time (Δτ1), and a decision unit configured to decide if the receive signal should be passed to the first delay unit (18a) or to the ramp slope estimating unit (13), wherein the decision unit (15) is configured to pass the receive signal to the first delay unit (18a) in case the target distance is larger than a minimum target distance, and to pass the receive signal to the ramp slope estimating unit (13) in case the target distance is lower or equal to said minimum target distance, wherein, in order to apply the modulation frequency to the receive signal, an output of the signal generation unit (22) is connected to a first input of a mixer (16e) and a second input of said mixer (16e) is provided with the receive signal, and wherein, in order to transmit said response signal to the radar under test (12), an output of said mixer (16e) is connected to a first input of an adder (26), whereas a second input of said adder (26) is connected to an output of said first delay unit (18a), and an output of the adder (26) is connected to said transmitting unit (24).

2. The radar target simulator (10) according to claim 1, wherein the ramp slope estimating unit (13) is configured to determine a corresponding frequency slope and / or a corresponding direction preferably on the basis of the respective tracking information.

3. The radar target simulator (10) according to any of the claims 1 to 2, wherein the radar target simulator (10) comprises a second mixer (16a) especially for applying a Doppler frequency.

4. The radar target simulator (10) according to any of the claims 1 to 3, wherein the ramp slope estimating unit (13) comprises a second delay unit (18b) especially for delaying the receive signal according to a second delay time (Δτ2), and / or wherein the ramp slope estimating unit (13) comprises a complex output especially for providing corresponding phase information preferably on the basis of the respective tracking information.

5. The radar target simulator (10) according to any of the claims 1 to 4, wherein the ramp slope estimating unit (13) comprises a third mixer (16b) especially for mixing the receive signal with a delayed version of the receive signal.

6. The radar target simulator (10) according to any of the claims 1 to 5, wherein the ramp slope estimating unit (13) comprises a frequency estimating unit (21) especially for applying a Fourier transform, preferably a discrete Fourier transform, to a signal resulting from mixing the receive signal with a delayed version of the receive signal.

7. The radar target simulator (10) according to any of the claims 1 to 6, wherein the signal generation unit (22) comprises or is a controlled oscillator, preferably a numerically controlled oscillator or a voltage-controlled oscillator.

8. The radar target simulator (10) according to any of the claims 1 to 7, wherein the modulation frequency is set on the basis of a relation between the ramp slope of the radar under test (12) and a respective processing time of the radar target simulator (10).

9. The radar target simulator (10) according to any of the claims 1 to 8, wherein the signal generation unit (22) is configured in a manner that it behaves like a frequency-shifting unit especially for creating Doppler effects.

10. The radar target simulator (10) according to any of the claims 1 to 9, wherein the radar target simulator (10) comprises an amplifying unit (20), preferably an adjustable amplifying unit, being especially configured to set a gain with respect to a response signal to be transmitted to the radar under test (12).

11. The radar target simulator (10) according to any of the claims 1 to 10, wherein the receiving unit (11) comprises at least one of an antenna, a fourth mixer (16c), an analog-to-digital converter (17), or any combination thereof, and / or wherein the transmitting unit (24) comprises at least one of an antenna, a fifth mixer (16d), a digital-to-analog converter (25), or any combination thereof.

12. A radar target simulation method, the radar target simulation method comprising the steps of: receiving a radar signal from a frequency modulated continuous wave radar under test, providing a corresponding receive signal, tracking a ramp slope of the radar under test on the basis of the receive signal as tracking information by means of a ramp slope estimation unit (13), setting a modulation frequency on the basis of the tracking information and a target distance of a simulated radar target to the radar under test (12) by means of a signal generation unit (22), wherein the modulation frequency is applied to the receive signal, and wherein, by setting the modulation frequency, the radar target is shifted towards a beat frequency corresponding to said target distance with no lower target distance limitation and continuous distance emulation, and deciding if the receive signal should be passed to a first delay unit (18a) for delaying the receive signal according to a first delay time (Δτ1) or to the ramp slope estimating unit (13), wherein the receive signal is passed to the first delay unit (18a) in case the target distance is larger than a minimum target distance, and the receive signal is passed to the ramp slope estimating unit (13) in case the target distance is lower or equal to said minimum target distance, wherein said applying of the modulation frequency to the receive signal involves providing an output of the signal generation unit (22) to a first input of a mixer (16e) and providing the receive signal to a second input of said mixer (16e), and providing an output of said mixer (16e) to a first input of an adder (26), providing an output of said first delay unit to a second input of said adder (26), and providing an output of said adder (26) to a transmitting unit (24) for transmitting a response signal to the radar under test (12).