Abstract:
An automotive radar test system includes circuitry for multiple down and up conversions of a signal from the automotive radar. Conditioning circuitry delays an intermediate frequency signal (IF 2 ) obtained after a second down conversion to simulate the delay of a return signal from an object located a particular distance from the automotive radar, and to attenuate the IF 2  signal to simulate variable target sizes, and to generate a Doppler shift in the IF 2  signal to simulate target speed. The conditioned signal is up-converted and transmitted back to the automotive radar system to determine if the automotive radar provides accurate readings for distance, size and speed. The radar test system further couples the second IF signal to a spectrum analyzer to determine if the automotive radar is operating in the desired bandwidth and to a power meter to determine if the automotive radar is transmitting at a desired power level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for testing the performance accuracy of an automotive radar system. More particularly, the present invention relates to a downconversion of the automotive radar system signals in a radar target simulator in a manner to enable improved performance and lower cost radar components and assemblies. 
     2. Background 
     Automobile manufacturers have begun producing automotive radar systems. An automotive radar transmits a signal from an antenna typically located in the grill area of an automobile. The automotive radar then determines, from the delay of a return signal received by the antenna, the distance an object causing there turn signal. The automotive radar can also determine, from the Doppler frequency shift of the return signal, the speed an object causing the return signal is traveling. The automotive radar system can also determine the size of an object causing the return signal through the radar cross section (RCS). Automotive Radar Systems in the United States are configured to operate nominally within a 76-77 GHz frequency band allocated by the Federal Communications Commission (FCC) for automotive radar systems. 
     To assure proper performance of an automotive radar system, the device must be regularly tested. Testing is performed to assure the automotive radar system is operating within the required frequency range specified by the FCC. Testing is also performed to assure that the system is radiating adequate power. Test measurement is further made to assure that the automotive radar system is making proper calculations of distance to an object creating a return signal. 
     Test systems have been developed that utilize a single frequency conversion scheme to simulate moving targets, with a know radar cross section, at a prescribed distance. One such system is disclosed in U.S. Pat. No. 5,920,281 entitled “Radar Test System For Collision Avoidance Automotive Radar.” The system receives a 76-77 GHz signal from the automotive radar system and down-converts the signal to a fixed intermediate frequency (IF) at which the signal is analyzed to determine operating power and frequency and conditioned to simulate a target. The conditioned signal is then up-converted and re-radiated, either from a separate antenna or from the same antenna. For signal analysis, the system provides at least two IF signal outputs, one for monitoring frequency using a spectrum analyzer and another for monitoring power. 
     A test system can be configured to provide a signal return from a target with a known RCS, at a prescribed distance and moving at a certain velocity as described in U.S. Pat. No. 6,087,995 entitled “Universal Auto radar Antenna Alignment System.” For each target distance, the RCS level can be varied to simulate different target sizes. The delay is simulated using delay lines (coaxial or Bulk Acoustic) of various lengths, in parallel. 
     Simulated velocity is achieved by introducing false Doppler into the returned signal as further described in U.S. Pat. No. 6,087,995. False Doppler is achieved by offsetting the LO of the down-converter from that of the up-converter. One of the LO sources is a synthesizer referenced to a crystal resonator, while the other is a tunable synthesizer referenced to the crystal resonator. The two LO sources use the same crystal reference to ensure Doppler stability. 
     SUMMARY 
     In accordance with the present invention, a radar test system is provided for an automotive radar system, which provides multiple up-conversions and down-conversions. The multiple frequency conversion schemes provide for lower cost components and improved performance over previous radar test systems. 
     The radar test system in accordance with the present invention provides for conditioning an IF signal, lower in frequency than previous systems, to assure that an automotive radar system is simulating targets by providing variable parameters in the conditioned signal, including distance (using a SAW or coaxial device), and size (or RCS). Further variable speed (or Doppler shift) is provided at the first upconversion from the lower IF frequency rather than upconversion to the original RF frequency, enabling lower cost components to be used and more accuracy to be achieved. Testing to assure the automotive radar is operating within a desired frequency band can be performed by providing an external connection from the radar test system to a spectrum analyzer after the first downconversion. Similarly, power measurements can be provided using an external connection to a power meter after the first downconversion. A logarithmic detector is used after the first downconversion to detect the lower IF frequency, enabling the LO to be adjusted to assure the IF frequencies remain fixed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with respect to particular embodiments thereof, and references will be made to the drawings in which: 
     FIG. 1 shows the components of a radar test system in accordance with the present invention; 
     FIG. 2 illustrates delay modules connected in series, which maybe used in place of the parallel delay devices of FIG. 1; 
     FIG. 3 illustrates how delay modules can also be made of a series of parallel-connected devices; 
     FIG. 4 shows the components of a millimeter-wave frequency-conversion module which may be used to provide components for the circuit of FIG. 1; and 
     FIG. 5 shows the components of a gain equalizer, which maybe used in the circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows components of the radar test system  100  in accordance with the present invention included which uses a double up-conversion as well as a double down-conversion. The radar test system receives a signal from the automotive radar system (RF in )  102 , which should be in the 76-77 GHz bandwidth, utilizing an antenna  104 . The antenna  104  used can be a standard gain pyramidal horn. Absorbing material is preferably provided around the antenna to minimize the reflections from the edges of the antenna and housing of the radar test system. 
     The received signal is coupled through a high pass filter  105  to a first down-converter  106 , along with a local oscillator (LO) signal LO 1 . The downconverter  106  downconverts the received signal to a first intermediate frequency signal (IF 1 ). Based on the detected power levels of the radar signal, as discussed in more detail subsequently, the frequency of the LO 1  signal is adjusted such that the signal IF 1  is at a fixed frequency. 
     The IF 1  signal is provided from a buffer amplifier and low pass filter to remove higher harmonics. The IF 1  signal may be coupled off from node  108  through a power divider  109  to a spectrum analyzer  110  and a power meter  112 . The spectrum analyzer  110  may be utilized to determine if the automotive radar system is operating within prescribed bandwidth limits. The power meter  112  enables determination of the radiated power from the automotive radar system. 
     The signal IF 1  is next down-converted in a second down-converter  114  to a signal IF 2  using a second LO signal LO 2   a . The signal IF 2  is then passed through a buffer and low pass filter to remove higher frequency harmonics. The IF 2  signal is then delayed in time using delay device  116  to impose a desired group delay to simulate a particular distance. In the embodiment shown, the delay device  116  is a parallel combination of a Surface-Acoustic Wave (SAW) delay device and a coaxial cable. Switches connect one of either the SAW device or coaxial cable to provide the desired delay. The delay device  116  may alternatively comprise series connected delay modules as shown in FIG.  2 . 
     The delay modules of FIG. 2 can contain either SAW devices, or desired lengths of coaxial cable as desired. The delay modules can also contain a series of parallel connected devices as shown in FIG.  3 . The switches in the delay modules can connect to a short through line to effectively provide minimal delay, or through either a SAW device or coaxial cable length to provide the delay shown. With the 10-meter, 20 meter and 30 meter delays shown, a stepped delay can be provided in steps of 10 m from 0-60 meters. Different delay lengths can be used to provide different steps, and a different number of modules can be used to provide a greater range or steps as desired. 
     The signal from the delay device  116  can be coupled at node  118  to a log detector  120 . The log detector  120  provides a calibrated measure of the integrated power and enables the LO to be adjusted to assure IF 1  and IF 2  remain at their required fixed frequency. The IF 2  signal is then further attenuated using variable attenuator  122  to simulate a particular RCS level. From the attenuator, the IF 2  signal is provided through a gain equalizer  124 . The gain equalizer  124  can include coaxial cavity-tuned equalizers to reduce amplitude variations over the frequency range of the signal received, as discussed in more detail subsequently. 
     The IF 2  signal is then buffered and provided to an up-converter  126  along with a LO signal LO 2   b , the upconverter  126  converting the IF 2  signal to an IF 1  signal. The IF 1  signal is then provided through a band pass filter and buffer amplifier to remove harmonics and intermnodulation products created by the first upconverstion, and provided to a second upconverter  128  along with an LO signal LO 2   b . The signal LO 2   b  is equal to the signal LO 2   a  plus a Doppler shift. The second upconverter  128  converts the IF 1  signal back to the original 76-77 GHz band plus any Doppler shift to provide the signal RFout. The signal from the second upconverter  128  is re-radiated through a second antenna  130  to the automotive radar system. 
     The LO signals, LO 1 , LO 2   a  and LO 2   b  are provided from synthesizers  132 ,  133  and  137 , all being referenced to a crystal reference oscillator  140 . The crystal oscillator  140  drives the synthesizer  132  which includes a phase locked loop  134  in combination with a YIG Tuned Oscillator (YTO)  162 . The output of the synthesizer  132  is provided through a power divider  164  which provides signals down two paths. One path is through a x4 multiplier  205 , bandpass filter  210  and x2 multiplier  210  to mixer  106  to provide the LO 1  signal for downconversion. A second path is through the x4 multiplier  305 , bandpass filter  307  and x2 multiplier  315  to provide the LO 1  signal to mixer  128  for upconversion. 
     The crystal reference oscillator  140  also drives synthesizer  133 . The synthesizer  133  includes a phase locked loop  166  in combination with a dielectric resonator oscillator (DRO)  135 . The output of synthesizer  137  provides the LO 2   a  signal for downconversion to mixer  114 . 
     The local oscillator signals L 02   a  and L 02   b  are from separate oscillator sources that track each other to ensure frequency stability. To enable tracking, the output of the synthesizer  133  provides the signal LO 2   a  as well as the input of an I-Q mixer  148 . The I-Q mixer  148  includes components to compare the frequency from each source and a phase locking scheme to offset the LO 2   a  oscillator  135  from the DRO oscillator  136  used to generate the LO 2   b  signal. The offset frequency constitutes a Doppler shift (or simulated target speed) for the simulated radar target. This technique of generating the false Doppler, described in more detail subsequently, allows suppression of the carrier and sidebands to levels better than 50 dBc. 
     More details of the radar test system in accordance with the present invention are described in sections to follow. 
     I. Millimeter Wave Up/Down Converter Modules 
     The radar test system, in one embodiment, uses a millimeter-wave frequency-conversion module as shown in FIG.  4 . The components of the module of FIG. 4 can be used for the components  205 ,  210 ,  215  and  106  of FIG. 1, and are similarly labeled. The components of FIG. 4 can, likewise, be used for the components  305 ,  310 ,  315  and  128  of FIG.  1 . The millimeter-wave module can be configured for use as a frequency up-converter or down-converter by switching between two IF amplifiers  220  and  225  with opposite gain direction, or by physically rotating the IF amplifier during manufacture. 
     MMICs operating outside of their manufacturer&#39;s frequency ranges can still be used for frequency multiplication and mixing. This provides a low-cost alternative at the expense of deterioration in the mixer-conversion loss. Mixer-conversion loss is improved by making use of one of the IF amplifiers  220  or  225 . The dependence of the mixer conversion-loss on the LO frequency can be eliminated by using a waveguide diplexer at the input port of the module, which provides LO leakage signals with a path to a matched resistive load. 
     I. Double Conversion Scheme Effects 
     The multiple conversion scheme used in the system of FIG. 1 enables a lower system cost and improved performance. The following sections describe the benefit of the multiple conversion scheme. 
     A. Doppler Shift 
     The radar test system performs Doppler simulation using a scheme of phase locking the two oscillators  135  and  136  of FIG. 1 with an offset as disclosed in U.S. Pat. No. 6,384,772, which is incorporated herein by reference. The components of the phase locking scheme include a power splitter  406  for distributing the signal from oscillator  136  to the first input of mixers  408  and  410 . A power splitter  412  distributes the signal from oscillator  135  to the second input of mixer  410  and to the second input of mixer  408  with a phase shift φ 1  in phase shifter  414  to generate first I and Q signals from mixers  408  and  410 . Higher frequency components of the first I and Q signals are filtered out by low pass filters  418  and  420  and the signals are applied to first inputs of multipliers  422  and  424 . 
     A numerically controlled oscillator  137  receives a numerical control signal from Doppler control unit  404  to provide a Doppler offset which may be from 0 to 50 KHz, or another frequency range depending on design requirements. The output of the oscillator  137  is provided directly to multiplier  422  to provide a second I signal for multiplying by the first Q signal. The output of oscillator  137  is further provided to multiplier  424  through a phase shifter  426  to multiply a second Q signal by the first I signal. The phase shifter  426  provides a phase shift φ 1  either matching the phase shift of phase detector  414 , or with an additional 180 degrees from the phase shift φ 1 . The output of multiplier  422  is subtracted from the output of multiplier  424  in summer  428 . The output of summer  428  then provides a voltage control signal to DRO  136 . 
     As configured, the phase locking circuitry creates a phase detector, so that the output of the summer  428  provides a DC signal sin(φ 2 ), where φ 2  is a phase difference between the signals combined from oscillators  135  and  136 . The circuitry enables stable tracking of a minimal frequency offset such as from 0-50 KHz with the oscillators operating in the range of 10 GHz. 
     The double-conversion system utilized in the radar test system improves on prior Doppler schemes by allowing the two oscillators  135  and  136  to be at a lower frequency than a single conversion system would allow. This reduces the cost of the oscillators  136  and  137  and the I-Q mixer  148 . The double-conversion also allows the oscillators  135  and  136 , which provide the LO 2  signals, to act as fundamental local oscillators to mixers  114  and  126 , thus eliminating the need for additional frequency multipliers. This allows the oscillators  135  and  136  to have lower phase noise requirements and a lower cost. The double-conversion scheme also improves system performance because it reduces the effect on the system of the spurious sidebands generated by the I-Q mixer  148 . These sidebands would have a bigger effect on the system if the local oscillator had to be multiplied up in frequency. This technique of generating the Doppler shift may allow suppression of the spurious sidebands to levels better than 50 dBc. 
     Although the oscillators  135  and  136  are shown phase-locked to a common reference  140 , they may alternatively be free-running. Although accuracy of the free running components will not be as high, component costs are still reduced from a single conversion scheme because of the reduced cost of low frequency components. 
     B. Delay Lines 
     The radar test system may simulate target distance by providing a group delay to the signal. The double-conversion scheme allows the radar test system to utilize a Surface Acoustic Wave (SAW)  116  filter as a delay line. The radar test system may be designed to use a single SAW device  116  or multiple SAW devices, in series or in parallel, as described previously, to achieve either a single delay or multiple delays, respectively. As compared to a Bulk Acoustic Wave or a coaxial delay line, a low frequency SAW delay line reduces system costs and allows the delay line to be mounted directly onto a surface mount board with other surface mount components. 
     In one embodiment, the radar test system also suppresses the leakage around the SAW delayline to at least 50 dB below the delayed signal. This maybe accomplished by mounting the SAW device on a surface-mount board with a ground plane printed on the board surrounding the SAW, with the exception of the input and output traces. The board with the SAW device is placed in a metallic housing, or metal can. Thin metallic walls of the can are mounted such that contact is made with the top metallic lid of the SAW device, to the ground plane on the board around the SAW device, to the walls to the side of the SAW device, and to the lid of the radar test system module housing. In an alternative embodiment, the SAW can be mounted upside down in a metal cavity in the radar test system housing. 
     C. RCS Control 
     The radar test system uses a low-frequency attenuator  122  to attenuate the radar signal to simulate different target sizes or RCS. Use of the double-conversion scheme enables the use of a low-frequency attenuator, which reduces cost. The attenuator may be surface mounted and integrated into the same module as the SAW delay device. 
     D. Gain Equalizer 
     The radar test system uses a low-frequency gain equalizer  124 , which is enabled by using the double-conversion scheme. FIG. 5 shows a diagram of one embodiment of the gain equalizer  124 . The gain equalizer includes a cascade of tuned resonant circuits  510  that are coupled to the main transmission line  520 . The coupling factor and resonant frequency of each resonator is tuned by varying the surface mount capacitors  530 . The equalizer includes one or more of the tuning sections  510 , where each section varies the amplitude over a specific frequency band. Resistors, such as  540 , are used to control the width of each resonant section. 
     E. Signal Power Measurement 
     A low-frequency RF detector  120  is placed after the SAW delay line  116 . The SAW delay  116  acts as a bandwidth-limiting RF filter. Using the SAW device as such a filter allows the RF detector  120  to accurately measure the integrated power of the radar signal without the use of an external power meter, thus reducing the overall system cost. 
     F. LO Tuning 
     The signal from the automotive radar system can occupy a frequency bandwidth referred to as BWn, such as 300 MHz, within the allocated bandwidth of 76-77 GHz, referred to as BWw. To detect the signal from the automotive radar system, the radar test system must operate over the wider bandwidth of BWw, since the radar signal frequency from the automotive radar will be unknown and can be located anywhere in the band BWw. Therefore, the radar test system intermediate frequencies IF 1  and IF 2  must also occupy the wider bandwidth of BWw. 
     In order to use a low-cost SAW delay line with the narrow frequency bandwidth of BWn, the radar test system of the present invention uses a special LO tuning scheme. A single tunable synthesizer  132  provides the LO and LO 1  for both of the IF mixers  106  and  128 . The output of oscillator  162  can be multiplied up if necessary, as it is in FIG. 1 using multipliers such as  205 ,  215 ,  305  and  315 . In order to detect the presence of a radar signal at an RF frequency in the 76-77 GHz band (BWw), the oscillator  162  is swept over a bandwidth of at least BWw. The radar signal from the automotive radar, RF, is down-converted using mixer  106  to a frequency of IF 1(tune) =RF−LO 1(tune) , which is then down-converted using mixer  114  to IF 2(tune) . As the oscillator is tuned in frequency using controller  113 , the RF detector  120  is used to monitor the power of the signal passing through the SAW device  116 . The controller  113  uses measurements from the RF detector  120  to tune the LO so that IF 2(tune) =IF 2 , IF 2  being the center frequency of the SAW device where the RF detector will have a maximum power reading. The radar test system can use this maximum power reading to establish the actual frequency of the radar signal, RF, in the band BWw. Once the frequency RF is established, the oscillator frequency LO 1  may be set to the tuned frequency to enable the radar test system to perform the target simulation function. Continued monitoring of the IF 1  signal by controller  113  and the phased locked loops  160  and  166  adjusts the LO 1  frequency to assure IF 1  remains fixed. 
     The tunable oscillator  162  may be free running or phase-locked. If the oscillator  162  is phase-locked to an internal or external reference, such as crystal reference  140 , then the frequency of the tuned oscillator  162  is known accurately and can be used to determine the frequency of the radar signal accurately. This feature allows the radar test system to determine and display to the user the frequency of the radar without the use of an external spectrum analyzer, thus reducing the overall system cost. 
     Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many other modifications will fall within the scope of the invention, as that scope is defined by the claims provided to follow.