RF front end with power sensor calibration

Abstract

A radar sensor that features: a monolithic integrated microwave circuit (MMIC); at least one RF signal path integrated into the MMIC for an RF signal; at least one power sensor (32) integrated into the MMIC, which is coupled to the RF signal path and configured to generate a sensor signal (P1) representing the power of the RF signal during normal operation of the MMIC; a chip pad (40) for receiving an RF test signal (S) generated by an external test device (60). TEST ) during a calibration operation of the MMIC, wherein the chip pad (40) is coupled to the at least one power sensor (32) so that during the calibration operation of the MMIC the at least one power sensor (32) receives the RF test signal (S TEST ) receives and uses as a sensor signal (P1, P 1, DIG ) generates a signal that corresponds to the power of the RF test signal (S TEST ) represents.

Description

TECHNICAL FIELD

[0001] Embodiments of the present invention relate to the field of integrated radar sensors, which typically comprise a radar transceiver integrated in a monolithic microwave integrated circuit (MMIC). BACKGROUND

[0002] Radio frequency (RF) transmitters and receivers are found in a wide variety of applications, particularly in wireless communications and radar sensors. In the automotive sector, there is a growing demand for radar sensors used in adaptive cruise control (ACC, or radar cruise control) systems. Such systems can automatically adjust a vehicle's speed to maintain a safe distance from other vehicles ahead (as well as from other objects and pedestrians).

[0003] Modern radar systems use highly integrated RF circuits that can combine all core functions of a radar transceiver's RF front end in a single package (single-chip radar transceiver), often referred to as an MMIC (Monolithic Microwave Integrated Circuit). Such RF front ends typically include, among other components, a voltage-controlled oscillator (VCO), power amplifiers (PA), directional couplers, mixers, and analog-to-digital converters (ADC), as well as associated control circuitry for controlling and monitoring the RF front end. Automotive radar applications are subject to various standards related to road safety, such as the ISO 26262 functional safety standard entitled "Road vehicles - Functional safety."To ensure the functional safety of a radar sensor and / or to comply with legal regulations, the RF frontend should operate with well-defined operating parameters.

[0004] For example, the RF frontend should provide an RF output signal (e.g., the transmission signal to be fed to one or more antennas) with a defined output power to achieve a desired sensor performance. Furthermore, a maximum output power should not be exceeded during operation of the RF frontend to comply with legal regulations. Furthermore, the output power of the transmission signal should not be less than a minimum output power to ensure reliable operation of the radar sensor. To meet functional safety standards, the MMIC should be capable of detecting radar sensor malfunctions, for example, when the RF output power is outside specified limits. Consequently, there is a need to measure the power in an RF signal path in the RF frontend.Publication US 5 204 614 A describes a calibratable broadband microwave power sensor constructed from diodes. Publication US 2007 / 0 266 350 A1 describes testing the RF performance of an MMIC using an external test device. Publication US 6 748 203 B1 also describes an RF circuit with power sensors. However, measurement circuits (power sensors) integrated into radar sensors to date have low accuracy, and consequently, comparatively wide guard bands are currently considered in the design of RF front ends and during production tests to ensure that the desired operating range is not exceeded. Wide guard bands result in a comparatively high number of defective products and corresponding rejects. Consequently, there is a need to improve the accuracy of power measurements in integrated RF front ends. SUMMARY

[0005] An exemplary embodiment of the present invention relates to a radar sensor with a monolithic microwave integrated circuit (MMIC). The radar sensor has at least one RF signal path for an RF signal integrated in the MMIC, as well as at least one power sensor integrated in the MMIC, which is coupled to the signal path and configured to generate a sensor signal representing the power of the RF signal during normal operation of the MMIC. The MMIC further comprises a chip pad for receiving an RF test signal generated by an external test device during calibration operation of the MMIC. The chip pad is coupled to the at least one power sensor, so that during calibration operation of the MMIC, the at least one power sensor receives the RF test signal and generates a signal as a sensor signal representing the power of the RF test signal.

[0006] Another exemplary embodiment relates to a system comprising an MMIC with a radar transmitter and / or a radar receiver and an automatic testing equipment (ATE). The MMIC includes at least one RF signal path for an RF signal, at least one power sensor coupled to the RF signal path, and a chip pad for receiving an RF test signal. The power sensor is configured to receive the RF test signal and provide a sensor signal representing the power of the RF test signal. The ATE is configured to generate the RF test signal and feed it into the chip pad.

[0007] Another exemplary embodiment further relates to a method for calibrating one or more power sensors integrated in an MMIC of a radar sensor. The method comprises generating an RF test signal having a defined power. The RF test signal is fed into a chip pad of the MMIC of the radar sensor, which is coupled to at least one power sensor contained in the MMIC. Consequently, the power sensor provides a corresponding sensor signal representing the measured power of the RF test signal. Furthermore, the method comprises calculating at least one calibration parameter based on the defined power of the RF test signal and the respective sensor signal, and storing the calibration parameter(s) in a memory integrated in the MMIC. BRIEF DESCRIPTION OF THE ILLUSTRATIONS

[0008] The invention will be better understood with reference to the following description and figures. The components depicted in the figures are not necessarily to scale, but emphasis is placed on illustrating the principles underlying the invention. Furthermore, like reference numerals designate corresponding parts throughout the figures. Regarding the figures: Fig. Figure 1 shows a block diagram illustrating the basic structure of the receive path of an RF transceiver chip; Fig. Figure 2 is a block diagram illustrating the receive / transmit path in a monostatic radar transceiver chip; Fig. Figure 3 illustrates the use of power sensors in an RF front end of a radar transceiver; Fig. Figure 4 shows a block diagram illustrating the calibration of a power sensor used, for example, in the RF frontend of Fig. 3 is integrated; and Fig. Figure 5 shows a flowchart illustrating an exemplary method for calibrating power sensors used in an RF front end integrated in an MMIC. DETAILED DESCRIPTION

[0009] Embodiments of the present invention are discussed below in the context of a radar transceiver (radar sensor). Such radar sensors can be used in a variety of applications, for example, in automobiles as sensors for measuring distance and speed relative to vehicles ahead (adaptive cruise control).

[0010] A so-called "single-chip radar" can contain circuitry that provides the core functions required for distance and / or speed measurement in a single chip (MMIC, Monolithic Microwave Integrated Circuit). Consequently, the chip can include, among other things, RF oscillators, amplifiers, mixers, filters, analog-to-digital converters, and digital signal processors. Fig. Figure 1 illustrates an RF receiver (or the receiving path of an RF transceiver) as used, for example, in a radar distance measuring device (radar sensor). Accordingly, the RF transceiver / receiver 1 includes a mixer 10 to which an RF input signal S RX and an RF oscillator signal S LO which is used to convert the RF input signal S RX down-converted to the baseband or an intermediate frequency band (IF band). The RF input signal S RX can be from an antenna (in Fig. 1 not shown) and can be pre-amplified (see RF amplifier 12) before being fed to the mixer 10. In the present example, the RF oscillator signal is generated by a local oscillator (LO) 11, which may include a voltage-controlled oscillator (VCO) coupled in a phase-locked loop (PLL). The RF oscillator signal S LO However, depending on the actual application, it can also be provided by other circuit arrangements. In radar applications, the local oscillator signal S LO usually frequency modulated to implement a so-called frequency modulated continuous wave (FMCW) radar sensor. When used in a radar distance measuring device, the RF oscillator signal S LO in the range between approximately 24 GHz and 77 GHz (77 GHz in this example). However, higher or lower frequencies may also be applicable.

[0011] As mentioned, the mixer 10 mixes the RF input signal S RX ' (amplified antenna signal) into the baseband (or IF band). The respective baseband signal (mixer output signal) is converted to S BB The baseband signal S BB is then subjected to analog filtering (filter 13) to suppress unwanted sidebands or image frequencies. Filter 13 can be a low-pass filter or a band-pass filter. The filtered baseband signal (filter output signal) is fed to S BB Receivers that use a mixer to downconvert the RF input signal to the baseband or an IF band are known as heterodyne receivers and will therefore not be discussed in more detail. The filtered baseband signal S BB ' is then sampled and converted into a digital signal S RXDIG(analog / digital converter 14), which is then further processed in the digital domain by a digital signal processor (performed, for example, by digital signal processor 15). In the case of downconversion to an IF band, the IF signal can also be digitized for digital demodulation of the IF signal. Digital signal processing can be performed, for example, by a digital signal processor that executes suitable software instructions.

[0012] Fig. Figure 1 illustrates the receive path of an RF receiver or transceiver. In so-called bistatic or pseudo-monostatic radar systems, the receiver can be separate from the transmitter because the transmitter and receiver use separate antennas. Fig. Figure 2 shows an RF transceiver (combined receiver and transmitter) that can be used in a monostatic radar system in which the same antenna is used to transmit and receive RF signals. The transceiver shown in Fig. 2 comprises a directional coupler 22 which is coupled between the mixer 10 and an antenna 20 and is designed to receive the RF signal S from the antenna 20 RX to the mixer 10 (receive path). Furthermore, the directional coupler 22 is designed to direct the RF oscillator signal S LO (amplified by the power amplifier 23) to the antenna 20, which transmits a corresponding electromagnetic radar signal. Apart from the directional coupler 22, the reception path (amplifier 12, mixer 10, filter 13, analog / digital converter 14, signal processor 15) is the same as in Fig. 1 and the description in question is therefore not repeated here.

[0013] The directional coupler 22 can be implemented as a ring coupler (rat-race coupler) formed by striplines. However, other types of directional couplers (e.g., circulators) can also be used. In particular, when using a rat-race coupler, one port of the coupler is terminated by a terminating impedance 21. The directional coupler 22 can be implemented in the same chip package as the other circuit components of the transceiver to provide a single-chip solution. It should be noted that the block diagrams from Fig. 1 and Fig. 2 only represent the basic structure of an RF heterodyne transceiver. Actual implementations are, of course, much more complex. However, the illustrations from Fig. 1 and Fig. 2 is sufficient for the present discussion and for understanding the functions of the embodiments described here.

[0014] In many applications, it may be necessary or desirable to measure RF power in various parts of the RF front-end of an RF transceiver (e.g., in the receive path, in the transmit path, at the reference input of the mixer, etc.). As mentioned above, information about RF power may be required during regular operation of the device, as well as during production testing of the device to ensure compliance with product specifications and regulatory requirements (quality control). To enable testability, RF power sensors are provided in various parts of the RF transceiver chip. That is, the power sensors are included in the RF circuit design as "Design for Testability" (DFT) features. As mentioned above, power sensors can be used not only for production testing but also enable power monitoring during RF front-end operation.For this purpose, the information obtained by one or more power sensors can be provided as a digital or analog signal at a dedicated pin (e.g. output pin) of the MMIC.

[0015] For example, diodes can be used as RF power sensors. Diodes are useful for this purpose because they can be easily included in the MMIC containing the RF front end. Diodes as power sensors (also called power detectors) are well known and therefore will not be discussed in detail here. Generally, diodes are used to rectify an incoming RF signal, operating the diode in its square-law region to generate a signal approximately proportional to the power of the RF signal. To extend the dynamic range of an RF power sensor diode beyond the square-law region, linearity correction circuits or multiple diode stacks can be used. A power sensor can be connected directly to the RF signal path where the power is to be measured.Alternatively, a directional coupler or power divider can be used to couple the power sensor to the RF signal path. Suitable directional couplers can be easily implemented using striplines formed in a metallization layer of the MMIC.

[0016] Simple sensor diodes, which can be integrated together with the circuitry forming the RF front end of an RF transceiver, have limited absolute accuracy. To improve the accuracy of power measurements, the power sensors are calibrated using a test signal with a defined power. The test signal can be generated on-chip using a signal generator circuit that is also integrated in the same chip as the RF front end, but external test signal generators can also be used. In this case, the test signal can be applied to a test pin or a test pad (e.g., needle contact point) of the chip, for example, using a needle probe. The test signal can have the same or a similar frequency as the RF signal (e.g., 77 GHz or 24 GHz) whose power is to be monitored during normal operation. However, this is not necessarily the case.The test signal may have a frequency that is lower than the frequency of the RF signal during operation.

[0017] Fig. 3 is a block diagram showing the RF front end of the radar transceiver from Fig. 2, where the part about digital postprocessing has been omitted. In addition to the example from Fig. 2, the RF frontend comprises power sensors 32 and 36, which are coupled to the RF signal paths by means of directional couplers 31 and 35, respectively. Apart from the additional power sensors and the associated couplers, the circuit is Fig. 3 identical to the circuit from Fig. 2, and reference is made to the corresponding description above. Depending on the actual implementation of the power sensors 32 and 36, the couplers 31 and 35 may be omitted and are therefore optional. In this case, the power transistors are connected directly to the respective RF signal paths without intermediate couplers.

[0018] The level of the output signals provided by power sensors 32 and 36 represents the current RF power level in the respective RF signal path. In the present example, output signal P1 represents the RF power of the oscillator signal at the reference input of mixer 10. Similarly, output signal P2 represents the RF power of the amplified antenna signal arriving at the RF input of mixer 10. Additional power sensors can be included in other parts of the RF frontend. In most applications, the output signals P1, P2 of power sensors 32, 36 will be voltage signals, which can be digitized for further processing in the digital domain.

[0019] Fig. 4 is a block diagram illustrating the calibration of a power sensor (e.g., power sensor 32 as shown in Fig. 3) using an RF test signal S TEST, which has a defined power level. In Fig. 4, a directional coupler 31 is shown which couples the power sensor 32 to an RF signal path in which RF power is to be measured. As described above with reference to Fig. As mentioned in Figure 3, the coupler 31 may be optional depending on the actual implementation. For the purpose of sensor calibration, the power sensor 32 is coupled to a chip pad 40, which is configured to be contacted with a needle probe 41 during a production test. In general, the chip pad 40 is a circuit node that carries the test signal S TEST The needle probe 41 is used to receive an RF test signal S TEST to be fed into the power sensor 32 via the chip pad 42, whereby the test signal S TESThas an RF signal power that can be set to one or more defined levels. The test signal can be generated using the external test device 60. Alternatively, an on-chip signal generator can be used (in this case, the needle probe 41 is not required). During calibration, the power sensor 32 measures the power of the RF test signal S TEST and provides a corresponding power signal P1, which represents the instantaneous power level of the signal S TEST The sensor output signal P1 can be digitized by an analog-to-digital converter (ADC) 45 and the resulting digital signal P 1,DIG can be a control unit 50 (which also includes the digital signal processor 15, see Fig. 1, may contain).

[0020] The control unit 50 can be designed to receive the digital sensor signal P 1,DIGcommunicate back to the test device so that depending on the known power levels of the test signal S TEST and the respective sensor output signals P 1,DIG Calibration parameters can be calculated. The calibration parameters can be communicated back to the control unit 50 and stored in a memory 51. During normal operation of the radar sensor, the stored calibration parameters are used by the control unit 50 to correct the respective sensor output signals P1, P2, thus improving the accuracy of the power measurement.

[0021] If the information about the power level of the test signal S TESTis available in the control unit 50, the calibration parameters can be calculated using the control unit 50 instead of the test device 60. Furthermore, if the test signal is generated on-chip, the external test device is not required, and calibration can be repeated regularly or at specific time intervals during the life cycle of the RF transceiver. Thus, a drift in the sensor characteristics of the power sensors can be compensated for by regularly updating the calibration parameters.

[0022] Fig. Figure 5 shows a flow chart illustrating an exemplary calibration procedure performed using the system of Fig. 4 can be accomplished. Accordingly, power sensors contained in an MMIC (e.g. power sensors 32 from Fig. 4) by means of an automatic test device (e.g. test device 60 from Fig. 4) is calibrated. In the present example, an RF test signal having a desired power is generated (step 71). The RF test signal is supplied to a circuit node coupled to at least one power sensor contained in the MMIC (step 72). As a result, the power sensor provides a corresponding sensor signal (see Fig. 4, sensor signal P1), which represents the power of the RF test signal, and at least one calibration parameter can be calculated based on the desired power and the respective sensor signal (step 73). The calibration parameter(s) can then be stored in a memory (step 74, see Fig. 4, Memory 51).

[0023] While various embodiments of the invention have been described, it will be apparent to those skilled in the art that many other embodiments are possible within the scope of the invention. Accordingly, the invention is to be limited except in light of the appended claims and their equivalents. With respect to the various functions performed by the above-described components or structures (assemblies, devices, circuits, systems, etc.), unless otherwise indicated, the terms (including reference to a "means") used to describe such components shall also apply to any other component or structure that performs the specified function of the described component (i.e.which is functionally equivalent), even if it is not structurally equivalent to the disclosed structure which performs the function in the exemplary embodiments of the invention presented herein.

Claims

[1] A radar sensor that has: a monolithic microwave integrated circuit (MMIC); at least one RF signal path for an RF signal integrated in the MMIC; at least one power sensor (32) integrated in the MMIC, coupled to the RF signal path and configured to generate a sensor signal (P1) representing the power of the RF signal during normal operation of the MMIC; a chip pad (40) for receiving an RF test signal (S TEST ) during calibration of the MMIC, wherein the chip pad (40) is coupled to the at least one power sensor (32) such that during the calibration operation of the MMIC, the at least one power sensor (32) detects the RF test signal (S TEST ) and outputs it as a sensor signal (P1, P 1, DIG ) generates a signal that represents the power of the RF test signal (S TEST ) represents. [2] The radar sensor according to claim 1, further comprising: a memory (51) integrated in the MMIC for storing one or more calibration parameters which are based on the sensor signal (P1, P 1, DIG ) displayed power and the respective power of the RF test signal (S TEST ) can be calculated. [3] The radar sensor according to claim 2, further comprising: a control unit (50) integrated in the MMIC, which is coupled to the memory (51) and is designed to process the sensor signal (P1, P 1,DIG ) and, during normal operation of the MMIC, the sensor signal (P1, P 1,DIG ) based on one or more stored calibration parameters. [4] The radar sensor according to claim 1 or 2, further comprising: a control unit (50) integrated in the MMIC, which is designed to receive the sensor signal (P1, P 1,DIG) and to communicate with an external test device (60). [5] The radar sensor according to claim 1 or 2, further comprising: at least one directional coupler (31) or power splitter integrated in the MMIC to connect the at least one power sensor (32) to the respective signal path(s). [6] A system that has: an MMIC with a radar transmitter and / or a radar receiver, wherein the MMIC has at least one RF signal path for an RF signal, at least one power sensor (32) coupled to the RF signal path and a chip pad (40) for receiving an RF test signal (S TEST ), wherein the power sensor (32) is designed to detect the RF test signal (S TEST ) and a sensor signal (P1, P 1,DIG ) representing the power of the RF test signal; an automatic test device (ATE) (60) which is designed to test the RF test signal (S TEST ) and feed this into the chip pad (40). [7] The system of claim 6, wherein the MMIC further comprises a control unit (50) configured to communicate with the ATE (60). [8] The system according to claim 6 or 7, wherein the ATE (60) is designed to receive the sensor signal (P1, P 1,DIG ) and based on a known power level of the RF test signal (S TEST ) and the sensor signal (P1, P 1,DIG ) to calculate at least one calibration parameter; and wherein the ATE (60) is configured to communicate the calculated calibration parameter(s) to the MMIC. [9] The system according to claim 6 or 7, wherein the ATE (60) is configured to receive from the MMIC the power level of the RF test signal (S TEST) and based on the power level of the RF test signal (S TEST ) and the sensor signal (P1, P 1,DIG ) to calculate at least one calibration parameter. [10] The system according to claim 8 or 9, further comprising: a memory (51) for storing the calibration parameter(s). [11] A method for calibrating one or more power sensors included in an MMIC of a radar sensor, the method comprising: Generating an RF test signal (S TEST ) with a defined performance; Feeding the RF test signal (S TEST ) into a chip pad (40) of the MMIC of the radar sensor, wherein the chip pad (40) is coupled to the at least one power sensor (32) contained in the MMIC, wherein the power sensor (32) generates a corresponding sensor signal (P1, P 1,DIG ) that provides the measured power of the RF test signal (S TEST ) represents; Calculating at least one calibration parameter based on the defined power of the RF test signal (S TEST ) and the respective sensor signal (P1, P 1,DIG ); Saving the calibration parameter(s) in a memory integrated in the MMIC. [12] The method according to claim 11, wherein the RF test signal (S TEST ) is generated by an automatic test device (ATE) (60) which is a separate device from the MMIC. [13] The method according to claim 12, wherein the feeding of the RF test signal (S TEST ) in the circuit nodes (40): Connecting a probe (41) to a chip pad of the MMIC forming the circuit node, and Feeding the RF test signal (S TEST ) into the circuit node (40) coupled to the at least one power sensor (32) via the probe (41) and the chip pad. [14] The method according to claim 11 or 12, wherein the RF test signal (STEST ) is generated by an on-chip signal generator contained in the MMIC. [15] The method according to claim 11, wherein the calculated calibration parameter(s) are stored in a memory (51) included in the MMIC. [16] The method according to one of claims 12 or 13, wherein the at least one calibration parameter is calculated by the ATE (60). [17] The method according to any one of claims 11 to 15, wherein the at least one calibration parameter is calculated by the control unit (50) included in the MMIC.

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