Abstract:
In a method for checking the functionality of a mixer, the mixer is supplied with a high-frequency signal and a high-frequency comparison signal in order to generate a baseband signal. The amplitude of the high-frequency signal is modified as a function of time. A direct-current voltage component of the baseband signal which is output by the mixer is analyzed to determine the functionality of the mixer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for checking the functionality of a mixer, and an electronic circuit system. 
         [0003]    2. Description of Related Art 
         [0004]    Microwave mixers are used in radar systems to mix a high-frequency transmission signal with a received reflection signal, thus obtaining a baseband signal having a lower frequency but which still has the same information content as the reflection signal. It is necessary to monitor the mixer function in safety-relevant systems. However, in the related art either no monitoring, or only simple, insensitive monitoring, of the mixers is used. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    An object of the present invention is to provide a method for checking the functionality of a mixer. Moreover, another object of the present invention is to provide an electronic circuit system for checking the functionality of a mixer. 
         [0006]    In a method according to the present invention for checking the functionality of a mixer, the mixer is supplied with a high-frequency signal in order to generate a baseband signal. The amplitude of the high-frequency signal is modified as a function of time. In addition, a direct-current voltage component of the baseband signal which is output by the mixer is analyzed to determine the functionality of the mixer. The method is advantageously suited for checking the functionality of passive and active mixers. The method is cost-neutral in implementation and is EMC-compliant, and allows simple control and monitoring. 
         [0007]    In one refinement, the mixer is supplied with a high-frequency comparison signal in addition to the high-frequency signal. 
         [0008]    According to one specific embodiment, the mixer is part of a radar system. The high-frequency signal is used as the transmission signal of the radar system, and a reflection signal received by the radar system is used as the comparison signal. This advantageously allows the functionality of the mixer of the radar system to be checked without having to modify the wiring of the mixer. 
         [0009]    A variation of the direct-current voltage component of the baseband signal over time is preferably analyzed. To protect against other influences, the modulation frequency and its amplitude in the spectrum may be verified. 
         [0010]    According to one specific embodiment of the method, the amplitude of the high-frequency signal is modulated using an amplitude modulation frequency. Such an amplitude modulation may advantageously and easily be carried out using an amplifier having an adjustable gain factor, or using another switchable source. 
         [0011]    In one refinement of the method, the magnitude of a signal level of the baseband signal at the amplitude modulation frequency of the high-frequency signal is compared to a fixed limiting value, and the mixer is assessed as functional if the limiting value is exceeded. For such an analysis in the frequency domain, any interfering influences, for example as the result of radar targets, are advantageously eliminated. 
         [0012]    In an additional refinement of the method, during a first time interval the amplitude of the high-frequency signal is modulated using a first amplitude modulation frequency, and during a second time interval is modulated using a second amplitude modulation frequency. A random superimposition of the amplitude modulation frequency by a signal which is generated by a reflection on an object present in the surroundings of the radar system may advantageously be recognized in this way. 
         [0013]    According to another specific embodiment of the method, during a first time interval the high-frequency signal has a first amplitude which is constant over time, and during a second time interval has a second amplitude which is constant over time. The mixer is assessed as functional if the direct-current voltage component of the baseband signal in the second time interval has a different value than in the first time interval. In this specific embodiment, the method may advantageously be carried out even more easily. 
         [0014]    An electronic circuit system according to the present invention includes a mixer for mixing a high-frequency signal and a high-frequency comparison signal, and for outputting a baseband signal. A device is provided for modifying the amplitude of the high-frequency signal as a function of time. An analysis circuit is also provided for assessing the functionality of the mixer based on a comparison of a change in a direct-current voltage component of the baseband signal over time with the change in the amplitude of the high-frequency signal over time. The circuit system is advantageously suited for checking the functionality of passive and active mixers. The circuit system is EMC-compliant and allows simple control and monitoring. 
         [0015]    The mixer is preferably part of a radar system. 
         [0016]    The mixer is advantageously a diode mixer or a Gilbert cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a schematic block diagram of a radar system. 
           [0018]      FIG. 2  shows a schematic illustration of a variation of an amplitude-modulated high-frequency signal and of a baseband signal over time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows a schematic illustration of a radar system  100 . Radar system  100  may be a frequency-modulated continuous-wave radar, for example. Radar system  100  may be used, for example, for adaptive cruise control in a motor vehicle. 
         [0020]    Radar system  100  has a voltage-controlled oscillator  120 . The voltage-controlled oscillator is used for generating a high-frequency signal  210 . High-frequency signal  210  may have a frequency in the range of 77 GHz, for example. The voltage-controlled oscillator preferably allows setting of the frequency of high-frequency signal  210 . Instead of voltage-controlled oscillator  120 , another component may be used for generating high-frequency signal  210 . 
         [0021]    Radar system  100  also includes an amplifier  130  having an adjustable gain factor. Amplifier  130  has an amplifier input  132 , a modulation input  134 , and an amplifier output  136 . Amplifier input  132  is connected to voltage-controlled oscillator  120 , and receives high-frequency signal  210 . Modulation input  134  receives a modulation signal  220 . The gain factor of amplifier  130  may be adjusted via modulation signal  220  which is present at modulation input  134 . Amplifier  130  amplifies high-frequency signal  210  which is present at amplifier input  132 , and outputs it as an amplified high-frequency signal  230  via amplifier output  136 . If the magnitude of modulation signal  220  present at modulation input  134  changes as a function of time, high-frequency signal  210  present at amplifier input  132  is additionally amplitude-modulated by amplifier  130  and output as an amplitude-modulated amplified high-frequency signal  230 . If a signal which is constant over time is present at modulation input  134 , amplifier  130  does not carry out amplitude modulation. 
         [0022]    Radar system  100  also includes an antenna  150  for transmitting high-frequency signal  230 . Antenna  150  may also be used for receiving a comparison signal  240  which is reflected from objects possibly present in the surroundings of radar system  100 . In this case, a circulator (not illustrated in  FIG. 1 ) separates transmitted high-frequency signal  230  and received comparison signal  240 . Alternatively, separate antennas  150  may be used for the transmission and reception, as illustrated in  FIG. 1 . 
         [0023]    Radar system  100  also includes a mixer  110  having a LO input  112 , an RF input  114 , and a baseband output  116 . Mixer  110  is a microwave mixer for frequency conversion. Mixer  110  may be a passive diode mixer or an active mixer, for example a Gilbert cell. LO input  112  is connected to amplifier output  136 , and receives amplified high-frequency signal  230 . Comparison signal  240  is present at RF input  114 . Signal  230  present at LO input  112  and the signal present at RF input  114  have approximately the same frequency. Mixer  110  may be a homodyne mixer or a monodyne mixer. 
         [0024]    Mixer  110  multiplies amplified high-frequency signal  230  by comparison signal  240 . In other words, amplified high-frequency signal  230  is modulated to comparison signal  240 . Mixer  110  thus generates a baseband signal  250  which is output via baseband output  116 . Baseband signal  250  contains signal components whose frequency corresponds to the difference in the frequencies of amplified high-frequency signal  230  and of comparison signal  240 . 
         [0025]    During normal operation of radar system  100 , the frequency of high-frequency signal  210 , and similarly also of amplified high-frequency signal  230 , is changed in a ramp-shaped manner as a function of time. A modulation signal  220  which is constant over time is present at modulation input  134  of amplifier  130 , so that amplifier  130  does not modulate the amplitude of the amplified high-frequency signal. Amplified high-frequency signal  230  is emitted via antenna  150 . Objects present in the surroundings of radar system  100  reflect amplified high-frequency signal  230  back to antenna  150 , where it is received as comparison signal  240 . Due to the propagation time of amplified high-frequency signal  230  to the reflecting object and back to antenna  150 , the frequency of amplified high-frequency signal  230  has already changed by the time comparison signal  240  is received, so that there is a frequency difference between amplified high-frequency signal  230  and received comparison signal  240  which is a function of the distance of the reflecting object from radar system  100 . Mixer  110  generates baseband signal  250 , whose frequency corresponds to this frequency difference. Based on the frequency of baseband signal  250 , an analysis circuit then deduces the distance of the reflecting object from radar system  100 . In order to also compensate for Doppler shifts caused by relative speeds which exist between radar system  100  and the reflecting object, multiple consecutive measuring cycles may be carried out in which the change in the frequency of high-frequency signal  210  and of amplified high-frequency signal  230  over time occurs with different slopes. 
         [0026]    If the frequency difference between amplified high-frequency signal  230  and comparison signal  240  is small, the frequency of baseband signal  250  generated by mixer  110  is also small. 
         [0027]    If amplified high-frequency signal  230  and comparison signal  240  have the same frequency, mixer  110  outputs a direct-current voltage at baseband output  116 , or baseband signal  250  has a direct-current voltage component. In the present invention it has been found that for a functional mixer  110 , the magnitude of the direct-current voltage component in baseband signal  250  is a function of the amplitude of amplified high-frequency signal  230 , whereas this is not the case for a defective mixer  110 . In one simplified specific embodiment, it is not necessary to supply mixer  110  with a comparison signal  240 . Even without a comparison signal  240  present, baseband signal  250  which is output by mixer  110  has a direct-current voltage component whose magnitude for a functional mixer  110  is a function of the amplitude of amplified high-frequency signal  230 . 
         [0028]    In both cases, the functionality of mixer  110  may be deduced based on the presence of a dependency of the magnitude of the direct-current voltage component of baseband signal  250  on the amplitude of amplified high-frequency signal  230 . For this purpose, radar system  100  has an amplitude modulation device  160  which is connected to modulation input  134  of amplifier  130 . Amplitude modulation device  160  outputs modulation signal  220  in order to modify the gain factor of amplifier  130  as a function of time, and thus to modulate the amplitude of amplified high-frequency signal  230  which is output by amplifier  130 . Radar system  100  also has an analysis circuit  140  which receives and analyses baseband signal  250  which is output by mixer  110 . Analysis circuit  140  is also connected to amplitude modulation device  160  in order to control the amplitude modulation. Analysis circuit  140  checks whether a direct-current voltage component of baseband signal  250  changes corresponding to the amplitude modulation of amplified high-frequency signal  230  which is carried out by amplitude modulation device  160 . If this is the case, analysis circuit  140  deduces that mixer  110  is functional. 
         [0029]    Such checking of the functionality of mixer  110  preferably takes place during a period of time in which the frequency of high-frequency signal  210  and of amplified high-frequency signal  230  undergoes little or no change over time. A measuring cycle for checking the functionality of mixer  110  may be 1 millisecond, for example. The amplitude modulation of amplified high-frequency signal  230  is then switched off, and radar system  100  is returned to normal operation. 
         [0030]    The amplitude of amplified high-frequency signal  230  may be periodically modulated using an amplitude modulation frequency.  FIG. 2  shows an example of a variation of such an amplitude-modulated amplified high-frequency signal  230  over time.  FIG. 2  also schematically illustrates the expected variation in baseband signal  250  over time when mixer  110  is functional. The magnitude of the direct-current voltage component of baseband signal  250  is likewise modulated using the amplitude modulation frequency. Any phase shift between amplified high-frequency signal  230  and baseband signal  250  has not been taken into account in  FIG. 2 , and plays no role in the further analysis. 
         [0031]    Analysis circuit  140  may analyze amplified-modulated baseband signal  250  in the frequency domain, for example. For this purpose, analysis circuit  140  carries out a Fourier transformation of received baseband signal  250 , and checks whether the spectrum of baseband signal  250  thus obtained has a maximum at the amplitude modulation frequency. Any interfering influences at other frequencies are advantageously eliminated in this way. 
         [0032]    To exclude random superimposition of the amplitude modulation frequency with signal components in baseband signal  250  caused by a reflection on an object in the surroundings of radar system  100 , two or more consecutive cycles may be carried out at different amplitude modulation frequencies. 
         [0033]    In one alternative specific embodiment of the present invention, the analysis of baseband signal  250  may also be carried out by analysis circuit  140  in the time domain. For example, it is not possible for the amplitude of amplified high-frequency signal  230  to be periodically modulated; rather, the amplitude may only be switched between a first and a second value. For the switch between the first value of the amplitude of amplified high-frequency signal  230  and the second value of the amplitude of amplified high-frequency signal  230 , when mixer  110  is functional, the magnitude of the direct-current voltage component of baseband signal  250  should also change. If this is not the case, it may be concluded that mixer  110  is defective.