Abstract:
Disclosed is a method of bias adjustment for a millimeter wave radar apparatus that can efficiently and highly accurately adjust the bias of an MMIC used in a radio frequency circuit in the millimeter wave radar apparatus. The method comprises: providing a DA converter in a bias circuit in the millimeter wave radar apparatus comprising an antenna, a radio frequency unit, and a processing unit for performing transmission and reception processing of the radio frequency unit; connecting a signal generator in place of the antenna; and connecting a test processing unit and a control apparatus to the radio frequency unit, wherein the control apparatus applies an initial bias value in the form of a digital value to the MMIC, calculates the target value for the digital bias value based on the result of the measurement of the received signal, and takes the target value as the digital bias value for the production processing unit when the radio frequency characteristic of the received signal obtained by applying the target value to the MMIC lies within specified limits.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from, and incorporates by reference the entire disclosure of Japanese Patent Application No. 2007-241043, filed on Sep. 18, 2007. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the bias adjustment of a radio frequency unit in a radar apparatus, and more specifically to a method for adjusting, at low cost and with good accuracy, the bias of a monolithic microwave integrated circuit (MMIC) built into a radio frequency circuit in a millimeter wave radar apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    It is known in the art to provide for use in a vehicle control system such as adaptive cruise control (ACC), a pre-crash system (PCS), or the like, a millimeter wave radar apparatus that uses millimeter waves in the frequency band of 30 GHz to 300 GHz as an apparatus for detecting obstacles (such as other vehicles, roadside constructions, pedestrians, etc.) around the vehicle. The currently used millimeter wave radar is of the type that employs a frequency-modulated continuous wave (FM-CW) system, and can measure the distance and relative velocity of a target by using a simple system. 
         [0006]    A conventional millimeter wave radar apparatus comprises a transmitting/receiving antenna, a radio frequency unit connected to the transmitting/receiving antenna to transmit and receive radio waves (millimeter waves), an analog circuit incorporating a signal processing circuit for processing signals output from the radio frequency unit, a digital signal processor for digitally processing signals output from the analog circuit, and a communication interface for transmitting the signals processed by the digital signal processor to a vehicle system (ACC, PCS, etc.). 
         [0007]    The antenna and the radio frequency unit are scanned together left and right by a motor contained in a scanner unit. The motor is driven by a motor driving circuit, which operates under the control of a signal supplied from the digital signal processor. The digital signal processor converts the beat signal produced by interference between the received wave and the transmitted wave into a digital signal, analyzes the beat frequency in its processing circuit, and detects the target&#39;s position by computing distance, relative velocity, and angle information. The radio frequency unit forming one component element of such a millimeter wave radar apparatus generally comprises a radio frequency circuit constructed from a plurality of MMICs (Monolithic Microwave Integrated Circuits) and a bias circuit for operating the MMICs. 
         [0008]    Each MMIC contains a mixer, multiplier, amplifier, switch, etc., but the optimum point of the gate bias value for the mixer, multiplier, amplifier, etc varies in each MMIC. Further, in the radio frequency unit, since the transmit power and receive gain of the transmitter/receiver need to be adjusted to given target values, the bias value must be adjusted for each individual MMIC. Furthermore, since there exist variations in the gold ribbon connecting the MMICs, in the waveguide converter, and in the IF (Intermediate Frequency) circuit connected to the radio frequency circuit, beside variations among MMICS, bias adjustment has therefore been required. 
         [0009]    The radio frequency unit in the millimeter wave radar apparatus includes a radio frequency circuit incorporating a plurality of MMICs, a bias circuit, and an IF circuit. Power is supplied to the radio frequency circuit and the bias circuit from a power supply circuit through respective regulators. 
         [0010]    The MMICs built into the radio frequency circuit function as an oscillator, mixer, multiplier, amplifier, switch, etc., as described above, and are interconnected by a gold ribbon. The transmit signal output from the MMIC at the final stage is fed through the waveguide converter and the waveguide and transmitted out from the antenna. The signal reflected by the target and received by the antenna is input via the waveguide and the waveguide converter into the MMICs for processing, and the received signal processed by the MMICs is supplied to the IF circuit and sent to the analog circuit. 
         [0011]    The analog circuit includes, in addition to the signal processing circuit, a microcomputer comprising a control processor, a memory, an I/O (Input/Output), and an external I/O. The signal from the I/F circuit is supplied via the signal processing circuit and the I/O to the control processor for processing, and the processed signal is output via the external I/O to the vehicle system. 
         [0012]    The input side of each MMIC in the radio frequency unit is connected to a voltage dividing circuit comprising a trimmable resistor and a conventional resistor in the bias circuit. The bias circuit divides the output voltage of the regulator through the trimmable resistor and the conventional resistor, and applies the resulting bias value to the MMIC. In the prior art, the resistance set value of each trimmable resistor has been determined by examining the IF signal output from the IF circuit, and then, trimming of the trimmable resistor has been performed using the thus set value. 
         [0013]    However, once the trimming is done, the value of the trimmable resistor can no longer be changed and as a result, there has been a problem that even if a temperature monitor is added, temperature compensation cannot be performed, and since determining whether the value is set correctly or not can only be checked in the inspection step, and therefore it is not possible to detect faults once the radar apparatus is shipped to the market. 
         [0014]    In view of this, JP-A-2005-227031 discloses a bias adjustment method that uses a current monitor circuit for monitoring the sum of the drain currents flowing in the respective MMICs in accordance with the variations between the MMICs, and that converts the detection signal of the current monitor circuit into a digital signal, computes a gate voltage set value for producing a predetermined drain current, and applies the thus computed value to each MMIC through a D/A converter. According to the technique disclosed in JP-A-2005-227031, since expensive trimmer resistors are not used, not only the cost, but also the number of adjusting steps can be reduced compared with the conventional gate bias circuit that performs the adjustment using the bias control circuit constructed with trimmer resistors. 
         [0015]    The technique disclosed in JP-A-2005-227031 obtains the gate voltage set value by monitoring the sum of the drain currents flowing in the respective MMICs and by determining the gate voltage set value so that the drain current value becomes equal to the predetermined current value; however, the purpose of the gate bias adjustment of the radio frequency unit is not only to adjust the optimum bias point of each MMIC, but also to adjust the radio frequency characteristics such as the transmitter power and receiver gain, and with the technique disclosed in JP-A-2005-227031 there has been the problem that highly accurate adjustment of the radio frequency characteristics cannot be accomplished because of large variations in the correlation between the drain current of each MMIC and the radio frequency characteristics. 
       SUMMARY OF THE INVENTION 
       [0016]    Accordingly, it is an object of the present invention to provide a method of bias adjustment for a radio frequency unit in a millimeter wave radar apparatus that can efficiently and highly accurately adjust the bias of an MMIC used in the radio frequency unit in the radar apparatus. It is also an object of the invention to provide a radio frequency unit and a radar apparatus that can implement such a bias adjustment method. 
         [0017]    To achieve the above object, there is provided according to the present invention a method of bias adjustment for a radio frequency unit in a radar apparatus, and more particularly a method for use in a radar apparatus comprising a transmitting/receiving antenna, a radio frequency unit connected to the transmitting/receiving antenna and containing at least one MMIC and a bias circuit for the MMIC, and a processing unit, connected to the radio frequency unit, for processing transmission and reception of radio waves, wherein the method is used for adjusting an analog bias value to be applied to the bias circuit. 
         [0018]    In a first mode of the present invention, the bias circuit to which the method of the invention is applied is provided with a D/A converter which is connected to the gate terminal of the MMIC. In a second mode of the present invention, the bias circuit to which the method of the method is applied is provided with a D/A converter which is connected to the gate terminal of the MMIC and an A/D converter for converting a received signal output from the radio frequency unit into a digital value for input to the processing unit. 
         [0019]    The adjustment method comprising the following steps can be applied to the bias circuit of the first mode, i.e., the adjustment method comprises the steps of: connecting a radio wave signal generator to an antenna connecting end of the radio frequency unit so as to cause the received signal to be output from the radio frequency unit; in this condition, connecting a test processing unit in place of the processing unit; connecting to the test processing unit a control apparatus to which the received signal is to be input; and causing the control apparatus to execute the following steps. The steps that the control apparatus is caused to execute include: outputting an initial value of a digital bias value to the D/A converter through the test processing unit; subsequently measuring a radio frequency characteristic of the received signal output from the radio frequency unit in response to the initial value, calculating a target value for the digital bias value based on the measured radio frequency characteristic, and supplying the target value to the D/A converter through the test processing unit; measuring the radio frequency characteristic of the received signal output from the radio frequency unit in response to the target value and determining whether the measured radio frequency characteristic lies within specified limits; if the measured radio frequency characteristic lies within the specified limits, then determining the target value as a bias adjusting value to be applied to the MMIC; and if the measured radio frequency characteristic lies outside the specified limits, then recalculating the target value until the measured radio frequency characteristic comes within the specified limits. 
         [0020]    The adjustment method comprising the following steps can be applied to the bias circuit of the second mode, i.e., the adjustment method comprises the steps of: connecting a radio wave signal generator to an antenna connecting end of the radio frequency unit so as to cause the received signal to be output from the radio frequency unit; in this condition, connecting a test processing unit in place of the processing unit and providing a control apparatus to be connected to the test processing unit; and causing the control apparatus to execute the following steps. The steps that the control apparatus is caused to execute include: outputting an initial value of a digital bias value to the D/A converter through the test processing unit; subsequently acquiring from the test processing unit a measurement result of a radio frequency characteristic of the received signal output from the radio frequency unit in response to the initial value, calculating a target value for the digital bias value based on the measurement result, and supplying the target value to the D/A converter through the test processing unit; determining whether the measurement result of the radio frequency characteristic of the received signal acquired from the test processing unit in response to the target value lies within specified limits; if the measurement result lies within the specified limits, then determining the target value as a bias adjusting value to be applied to the MMIC; and if the measurement result lies outside the specified limits, then recalculating the target value until the measurement result comes within the specified limits. 
         [0021]    A radio frequency unit according to the present invention that achieves the above object comprises: at least one MMIC for outputting a signal to be transmitted from an antenna, and for processing a signal received by the antenna; an IF circuit to which the received signal processed by the MMIC is input; a control processor for outputting a bias value adjusting signal based on a bias value determined from a receive gain of the received signal output from the IF circuit and on a predetermined bias value; a bias circuit to which the bias value adjusting signal is input; and a radio frequency circuit containing at least one MMIC whose bias value is adjusted in accordance with the bias value adjusting signal output from the bias circuit. 
         [0022]    Further, a radar apparatus according to the present invention that achieves the above object comprises the above radio frequency unit and the antenna connected to the radio frequency unit. 
         [0023]    According to the present invention, since the IF signal output from the radio frequency unit and the transmit power of the antenna represent the characteristics containing the variations in radio frequency characteristics, other than the variations among the MMICs, caused by variations in connection losses between the MMICs (variations due to gold ribbon connection losses), variations in radio frequency characteristics dependent on the handling of the MMICs, and variations in radio frequency of waveguide conversion, adjusting the gate bias based on the IF output and the transmit power automatically achieves the two purposes of the gate bias adjustment, i.e., the adjustment of the optimum bias point of each MMIC and the adjustment of the radio frequency characteristics such as transmit power, receive gain, etc., and thus the adjustment can be accomplished with extremely high accuracy. 
         [0024]    Usually, after the gate bias adjustment, the radio frequency unit is inspected for radio frequency characteristics in an inspection process before shipment from the factory; accordingly, by utilizing the equipment used for the inspection of the radio frequency characteristics, the present invention can be readily implemented without requiring any special equipment. Furthermore, while a drain current monitor circuit has been required in the prior art, the present invention eliminates the need for a drain current monitor circuit, and can thus reduce the cost. Further, since the gate bias adjusting value can be adjusted with high accuracy, and the adjustment can be made by software, the adjustment can be easily automated, achieving a reduction in time required for the adjustment and thus achieving an efficient and highly accurate bias adjustment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings, wherein: 
           [0026]      FIG. 1  is a block diagram schematically showing the configuration of a conventional millimeter wave radar apparatus; 
           [0027]      FIG. 2  is a block circuit diagram showing the configuration of a bias circuit incorporated in a radio frequency unit in the conventional millimeter wave radar apparatus; 
           [0028]      FIG. 3  is a block circuit diagram explaining the configuration of a first embodiment of a bias circuit incorporated in a radio frequency unit in a millimeter wave radar apparatus according to the present invention, and a method of bias adjustment for the same. 
           [0029]      FIG. 4  is a flowchart explaining the bias adjustment method for the radio frequency unit shown in  FIG. 3 . 
           [0030]      FIG. 5  is a block circuit diagram showing the configuration of the millimeter wave radar apparatus incorporating the radio frequency unit of the present invention adjusted by the adjustment method shown in  FIGS. 3 and 4 . 
           [0031]      FIG. 6  is a block circuit diagram explaining the configuration of a second embodiment of a bias circuit incorporated in a radio frequency unit in a millimeter wave radar apparatus according to the present invention, and a method of bias adjustment for the same. 
           [0032]      FIG. 7  is a flowchart explaining the bias adjustment method for the radio frequency unit shown in  FIG. 6 . 
           [0033]      FIG. 8  is a block circuit diagram showing the configuration of the millimeter wave radar apparatus incorporating the radio frequency unit of the present invention adjusted by the adjustment method shown in  FIGS. 6 and 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Before describing the preferred embodiments, an explanation will be given regarding the bias adjustment of a radio frequency unit in the conventional radar apparatus shown in  FIGS. 1 to 2 . 
         [0035]      FIG. 1  shows the configuration of the conventional millimeter wave radar apparatus  10 . The millimeter wave radar apparatus  10  comprises a transmitting/receiving antenna  1 , a radio frequency unit  2  connected to the transmitting/receiving antenna  1  to transmit and receive radio waves (millimeter waves), an analog circuit  3  incorporating a signal processing circuit for processing signals output from the radio frequency unit  2 , a digital signal processor  4  for digitally processing signals output from the analog circuit  3 , and a communication interface  5  for transmitting the signals processed by the digital signal processor  4  to a vehicle system  6  (ACC, PCS, etc.). 
         [0036]    The antenna  1  and the radio frequency unit  2  are together scanned left and right by a motor  8  contained in a scanner unit  7 . The motor  8  is driven by a motor driving circuit  9  which operates under the control of a signal supplied from the digital signal processor  4 . The digital signal processor  4  converts the beat signal produced by interference between the received wave and the transmitted wave into a digital signal, analyzes the beat frequency in its processing circuit, and detects the target&#39;s position by computing distance, relative velocity, and angle information. The radio frequency unit  2  forming one component element of such a millimeter wave radar apparatus  10  generally comprises a radio frequency circuit constructed from a plurality of MMICs (Monolithic Microwave Integrated Circuits) and a bias circuit for operating the MMICs. 
         [0037]    Each MMIC contains a mixer, multiplier, amplifier, switch, etc., but the optimum point of the gate bias value for the mixer, multiplier, amplifier, etc. varies in each MMIC. Further, in the radio frequency unit, since the transmit power and receive gain of the transmitter/receiver need to be adjusted to given target values, the bias value must be adjusted for each individual MMIC. Furthermore, since there exist variations in the gold ribbon connecting the MMICs, in the waveguide converter, and in the IF (Intermediate Frequency) circuit connected to the radio frequency circuit, besides variations among the MMICs, the bias adjustment has therefore been required. 
         [0038]      FIG. 2  shows the configuration of the radio frequency unit  2  and analog circuit  3  in the conventional millimeter wave radar apparatus  10  shown in  FIG. 1 . The radio frequency unit  2  includes a radio frequency circuit  20  incorporating a plurality of MMICs  21 , a bias circuit  22 , and an IF circuit  23 . Power is supplied to the radio frequency circuit  20  and the bias circuit  22  from a power supply circuit (not shown) through respective regulators  24  and  25 . 
         [0039]    The MMICs  21  built into the radio frequency circuit  20  have the functions of an oscillator, mixer, multiplier, amplifier, switch, etc., as described above, and are interconnected by a gold ribbon (labeled Au in the figure). The transmit signal output from the MMIC  21  at the final stage is fed through the waveguide converter  26  and the waveguide  27  and transmitted out from the antenna  1 . The signal reflected by the target and received by the antenna  1  is input via the waveguide  27  and the waveguide converter  26  into the MMICs  21  for processing, and the received signal processed by the MMICs  21  is supplied to the IF circuit  23  and sent to the analog circuit  3 . 
         [0040]    The analog circuit  3  includes, in addition to the signal processing circuit  35 , a microcomputer  30  comprising a control processor  31 , a memory  32 , an I/O  33 , and an external I/O  34 . The signal from the I/F circuit  23  is supplied via the signal processing circuit  35  and the I/O  33  to the control processor  31  for processing, and the processed signal is output via the external I/O  34  to the vehicle system  60 . 
         [0041]    The input side of each MMIC  21  in the radio frequency unit  2  is connected to a voltage dividing circuit comprising a trimmable resistor  28  and a conventional resistor  29  in the bias circuit  22 . The bias circuit  22  divides the output voltage of the regulator  25  through the trimmable resistor  28  and the conventional resistor  29 , and applies the resulting bias value to the MMIC  21 . In the prior art, the resistance set value of each trimmable resistor  28  has been determined by examining the IF signal output from the IF circuit  23 , and then, trimming of the trimmable resistor  28  has been performed using the thus set value. 
         [0042]    However, once the trimming is done, the value of the trimmable resistor can no longer be changed; as a result, there has been the problem that even if a temperature monitor is added, temperature compensation cannot be performed, and besides, since determining whether the value is set correctly or not can only be checked in the inspection step, and therefore it is not possible to detect faults once the radar apparatus is shipped to the market. 
         [0043]    The present invention is intended to solve the above problem, and the specific embodiments of the invention will be described in detail below. For simplicity of explanation, the same components as those used in the prior art millimeter wave radar apparatus  10  shown in  FIGS. 1 and 2  will be referred to by the same reference numerals. 
         [0044]      FIG. 3  is a diagram for explaining the configuration of a first embodiment of a bias circuit  40  incorporated in a radio frequency unit  2 A in a millimeter wave radar apparatus according to the present invention, along with its connection to an analog circuit  3 A provided for testing purposes, and a method of bias adjustment for the same. The radio frequency unit  2 A includes, in addition to the bias circuit  40 , a radio frequency circuit  20  and an IF circuit  23  which are identical to those shown in  FIG. 1 . Power from a power supply circuit not shown is supplied to the radio frequency circuit  20  through a regulator  24 . The analog circuit  3 A shown in  FIG. 3  is for testing purposes (for bias adjustment) and is not actually used in the millimeter wave radar apparatus in which the radio frequency unit  2 A is incorporated. 
         [0045]    The MMICs  21  built into the radio frequency circuit  20  have the functions of an oscillator, mixer, multiplier, amplifier, switch, etc., as earlier described, and are interconnected by a gold ribbon. The transmit signal output from the MMIC  21  at the final stage is fed through the waveguide converter  26  and the waveguide  27  to the antenna side. In this embodiment, a signal generator  11  is connected in place of the antenna  1  shown in  FIG. 2 . The signal generator  11  is a measuring instrument that can generate the same signal as that received at the antenna  1 . The signal generator  11  may be replaced by a power meter. The signal generated by the signal generator  11  is fed via the waveguide  27  and the waveguide converter  26  into the MMICs  21  for processing, and the signal processed by the MMICs  21  is supplied to the IF circuit  23 . The IF signal output from the IF circuit  23  is supplied via a spectrum analyzer (designated SA in the figure)  12  and a personal computer (hereinafter abbreviated PC)  13  to the analog circuit  3 A. 
         [0046]    The bias circuit  40  of this embodiment comprises a plurality of D/A converters (D/AC- 1  to D/AC-n)  41  connected to the inputs of the respective MMICs  21  in the radio frequency circuit  20 , and a plurality of A/D converters (A/DC- 1  to A/DC-n)  42  connected to the outputs of the respective D/A converters  41 . On the other hand, the analog circuit  3 A includes a microcomputer  30 A comprising a control processor  31 , a memory  32 , an I/O  33 , an external I/O  34 , and a gate voltage output setting circuit  36 . The output of the gate voltage output setting circuit  36  is connected to the input terminals of the respective D/A converters  41  in the bias circuit  40 , and the outputs of the respective A/D converters  42  in the bias circuit  40  are coupled to the control processor  31 . 
         [0047]    The configuration shown in  FIG. 3  is used when performing the bias adjustment by the bias circuit  40 ; therefore, as described above, the signal generator  11  as a measuring instrument is connected to the waveguide  27 , and the output of the IF circuit  23  is coupled via the spectrum analyzer  12  as a measuring instrument and via the PC  13  to the external I/O  34  of the microcomputer  30 A in the analog circuit  3  and fed to the control processor  31 . 
         [0048]      FIG. 4  is a flowchart for explaining the bias adjustment method implemented in the radio frequency unit  2 A of  FIG. 3  using the measuring instruments (signal generator  12  and spectrum analyzer  12 ). The processing steps shown in the flowchart are carried out by the PC  13  connected to the analog circuit  3 A shown in  FIG. 3 . 
         [0049]    In step  401 , the initial bias value is set. The PC  13  connected to the analog circuit  3 A sets a generally suitable value as the initial bias value by using a digital value, and sends this value to the control processor  31  via the external I/O  34 . Then, the control processor  31  supplies this initial bias value (digital value) to the D/A converters  41  via the gate voltage output setting circuit  36 , and the bias value converted to analog form is applied to the respective MMICs  21 . With the thus set bias value, the MMICs  21  in the radio frequency circuit  20  receive the signal from the signal generator  11  and process the received signal, and the received signal thus processed is output from the IF circuit  23 . 
         [0050]    In step  402 , the received signal output from the IF circuit  23  is frequency analyzed by the spectrum analyzer  12 , and the result is fed to the PC  13 . The PC  13  automatically measures the radio frequency characteristics based on the output of the spectrum analyzer  12 , and detects the receive gain. In the next step  403 , based on the receive gain measured in response to the initially set bias value, the PC  13  calculates the target bias value as the target value for the bias value to be applied to the MMICs  21 . Then, in step  404 , the target bias value is sent as a bias adjusting value to the microcomputer  30 A in the analog circuit  3 A, and the bias adjusting value is written to the memory  32  via the external I/O  34  and the control processor  31 , while at the same time, the bias adjusting value is supplied to the D/A converters  41  via the gate voltage output setting circuit  36 . 
         [0051]    Then, in step  405 , the MMICs  21  to which the bias adjusting value converted to analog form has been applied are again made to receive the signal from the signal generator  11 , and the received signal output from the IF circuit  23  is frequency analyzed by the spectrum analyzer  12 , and the result is fed to the PC  13 . The PC  13  automatically measures the radio frequency characteristics of the received signal, and detects the receive gain of the radio frequency circuit  20  to which the bias adjusting value has been applied. In the next step  406 , it is determined whether the detected receive gain lies within specified limits; if it is not within the specified limits, the process returns to step  403  to recalculate the target bias value for the MMICs  21 , and the process from step  404  to step  406  is repeated. The process from step  403  to step  406  is repeated until it is determined in step  406  that the detected receive gain of the radio frequency circuit  20  lies within the specified limits. 
         [0052]    If it is determined in step  406  that the detected receive gain of the radio frequency circuit  20  lies within the specified limits, the process proceeds to step  407  where the bias adjusting value retrieved from the memory  32  of the test analog circuit  3 A is transferred to a memory of a production analog circuit (described hereinafter) which is combined with the radio frequency unit  2 A thus measured. 
         [0053]      FIG. 5  is a diagram showing the configuration of the millimeter wave radar apparatus  10 A incorporating the radio frequency unit  2 A of the present invention adjusted by the adjustment method shown in  FIGS. 3 and 4 . The radio frequency circuit  20  in the radio frequency unit  2 A is identical to the radio frequency circuit  20  shown in  FIG. 3 . The analog circuit  3 B combined with the radio frequency unit  2 A comprises a microcomputer  30 B, which is identical in configuration to the microcomputer  30 A of the test analog circuit  3 A described with reference to  FIG. 3 , and a signal processing circuit  35  connected to the I/O  33 . The bias adjusting value transferred under the control of the PC  13  at the time of testing is held in the memory  32  of the microcomputer  30 B. The vehicle system  60  is connected to the external I/O  34 . 
         [0054]    In the millimeter wave radar apparatus  10 A incorporating the radio frequency unit  2 A according to the first embodiment of the present invention, the control processor  31  reads the bias adjusting value stored in the memory  32  of the microcomputer  30 B in the analog circuit  3 B, and supplies it, via the gate voltage output setting circuit  36  and the D/A converters  41 , as a bias value to the MMICs  21  in the radio frequency circuit  20 . The bias value applied to the MMICs  21  is constantly monitored by the control processor  31  via the A/D converters  42 , and if the bias value applied to the MMICs  21  in the radio frequency circuit  20  deviates from the bias adjusting value stored in the memory  32 , the control processor  31  corrects the bias value. In this way, the correct bias value is always applied to the MMICs  21  of the present invention. 
         [0055]    The D/A converters  41  may be configured to be able to change the gate voltage output setting as needed if the bias value applied to the MMICs  21  in the radio frequency circuit  20  deviates from the bias adjusting value stored in the memory  32 . In this case, the D/A converters  41  can digitally control the bias value for output to the MMICs  21 . 
         [0056]    In the first embodiment of the present invention, the adjustment can be made with an accuracy substantially equal to the measurement accuracy. More specifically, according to the present invention, when an adjustment is made so that the radio frequency circuit  20  will have a gain of 20 dB, for example, the adjustment can be made with an accuracy of ±1 dB or less, i.e., the gain of the radio frequency circuit  20  can be adjusted with an error not greater than 5%. 
         [0057]      FIG. 6  is a diagram for explaining the configuration of a bias circuit  40 A incorporated in a radio frequency unit  2 B in a millimeter wave radar apparatus according to a second embodiment of the present invention, along with its connection to an analog circuit  3 C, and a method of bias adjustment for the same. The radio frequency unit  2 B includes, in addition to the bias circuit  40 A, a radio frequency circuit  20  and an IF circuit  23  which are identical to those shown in the first embodiment. The radio frequency circuit  20  is the same as that shown in the first embodiment, and power is supplied to it from a power supply circuit (not shown) through the regulator  24 . The analog circuit  3 C shown in  FIG. 6  is for testing purposes (for bias adjustment) and is not actually used in the millimeter wave radar apparatus in which the radio frequency circuit  20  is incorporated. 
         [0058]    As in the first embodiment, the signal generator  11  as a measuring instrument capable of generating the same signal as that received at the antenna is connected in place of the antenna to the radio frequency circuit  20 . The signal generated by the signal generator  11  is fed via the waveguide  27  and the waveguide converter  26  into the MMICs  21  for processing, and the signal processed by the MMICs  21  is supplied to the IF circuit  23 . In the first embodiment, the IF signal output from the IF circuit  23  was supplied to the analog circuit  3 B via the spectrum analyzer  12  and the PC  13 , but in the second embodiment, the IF signal output from the IF circuit  23  is supplied, via an IF A/D converter (A/DC-n+1)  43 , directly to the control processor  31 A. 
         [0059]    The bias circuit  40 A of the second embodiment is identical in configuration to the bias circuit  40  of the first embodiment, except for the IF A/D converter  43 , and comprises a plurality of D/A converters  41  connected to the respective MMICs  21  in the radio frequency circuit  20  and A/D converters  42  connected to the outputs of the respective D/A converters  41 . On the other hand, the configuration of the analog circuit  3 C of the second embodiment is substantially the same as that of the analog circuit  3 A of the first embodiment, the only difference being that the I/O  33  is omitted. The output of the gate voltage output setting circuit  36  is connected to the input terminals of the respective D/A converters  41  in the bias circuit  40 A, and the outputs of the respective A/D converters  42  in the bias circuit  40 A are coupled to the control processor  31 A. 
         [0060]    The configuration shown in  FIG. 6  is used when performing the bias adjustment by the bias circuit  40 A, but differs from the first embodiment in that the IF signal output from the IF circuit  23  is supplied via the IF A/D converter  43  to the control processor  31 A of the microcomputer  30 C in the analog circuit  3 C, and in that the spectrum analyzer placed in front of the PC  13  is omitted. In this embodiment also, the signal from the signal generator  11  is input to the PC  13 . 
         [0061]      FIG. 7  is a flowchart explaining the bias adjustment method in the radio frequency unit  2  shown in  FIG. 6 , and the flowchart is carried out by the PC  13  shown in  FIG. 6 . 
         [0062]    In step  701 , the initial bias value is set. The PC  13  connected to the analog circuit  3 C sets a generally suitable value as the initial bias value by using a digital value, and sends this value to the control processor  31 A via the external I/O  34 . Then, the control processor  31 A supplies this initial bias value to the D/A converters  41  via the gate voltage output setting circuit  36 , and the bias value converted to analog form is applied to the respective MMICs  21 . With the thus set bias value, the MMICs  21  in the radio frequency circuit  20  receive the signal from the signal generator  11 , and supplies the received signal to the IF circuit  23  which outputs the IF signal. 
         [0063]    The IF signal output from the IF circuit  23  is converted by the IF A/D converter  43  into a digital value which is supplied to the control processor  31 A of the microcomputer  30 C in the analog circuit  3 C. The control processor  31 A of the second embodiment is provided with a signal processing function, and processes the IF signal supplied via the IF A/D converter  43  and supplies the processed received signal to the PC  13  via the external I/O  34 . 
         [0064]    In step  702 , the PC  13  automatically measures the radio frequency characteristics based on the output of the external I/O  34 , and detects the receive gain. In the next step  703 , based on the receive gain measured in response to the initially set bias value, the PC  13  calculates the target bias value for the MMICs  21 . The target bias value is written to the memory  32  via the external I/O  34  and the control processor  31 , while at the same time, it is supplied to the D/A converters  41  via the gate voltage output setting circuit  36 , and the target bias value converted to analog form is applied to each MMIC  21 . The MMICs  21  in the radio frequency circuit  20  receives the signal from the signal generator  11  with the thus set target bias value, and the processed received signal is output as the IF signal from the IF circuit  23 . 
         [0065]    The IF signal output from the IF circuit  23  is converted by the IF A/D converter  43  into a digital value which is supplied to the control processor  31 A of the microcomputer  30 C in the analog circuit  3 C. The control processor  31 A processes the received signal supplied via the IF A/D converter  43 , and supplies the processed received signal to the PC  13  via the external I/O  34 . In step  704 , the radio frequency characteristics of the received signal are automatically measured to detect the receive gain. 
         [0066]    In the next step  705 , it is determined whether the detected receive gain lies within specified limits; if it is not within the specified limits, the process returns to step  703  to recalculate the target bias value for the MMICs  21 , and the process from step  704  to step  706  is repeated. The process from step  703  to step  705  is repeated until it is determined in step  705  that the detected receive gain lies within the specified limits. 
         [0067]    If it is determined in step  705  that the detected receive gain lies within the specified limits, the process proceeds to step  706  where the bias adjusting value retrieved from the memory  32  of the test analog circuit  3 C is transferred to a memory of a production analog circuit (described hereinafter) which is combined with the radio frequency unit  2 B thus measured. In the second embodiment, since the need for the measurement by the spectrum analyzer is eliminated, and the signal used for processing the received signal of the radar apparatus is directly processed in the microcomputer  30 C, the adjustment can be made with higher accuracy. 
         [0068]      FIG. 8  is a diagram showing the configuration of the millimeter wave radar apparatus  10 B incorporating the radio frequency unit  2 B of the second embodiment of the present invention adjusted by the adjustment method shown in  FIGS. 6 and 7 . The radio frequency unit  2 B is identical to the radio frequency unit  2 B shown in  FIG. 6 . The analog circuit  3 D combined with the radio frequency unit  2 B comprises a microcomputer  30 D which is identical in configuration to the microcomputer  30 C of the test analog circuit  3 C described with reference to  FIG. 6 . The bias adjusting value transferred under the control of the PC  13  at the time of testing is held in the memory  32  of the microcomputer  30 D. The vehicle systems  60  is connected to the external I/O  34 . 
         [0069]    In the millimeter wave radar apparatus  10 B incorporating the radio frequency unit  2 B according to the second embodiment of the present invention, the control processor  31 A reads the bias adjusting value stored in the memory  32  of the microcomputer  30 D in the analog circuit  3 D, and supplies it, via the gate voltage output setting circuit  36  and the D/A converters  41 , as a bias value to the MMICs  21  in the radio frequency circuit  20 . The bias value applied to the MMICs  21  is constantly monitored by the control processor  31 A via the A/D converters  42 , and if the bias value applied to the MMICs  21  in the radio frequency circuit  20  deviates from the bias adjusting value stored in the memory  32 , the control processor  31 A or the D/A converters  41  correct the bias value. In this way, the correct bias value is always applied to the MMICs  21  of the present invention. 
         [0070]    The antenna  1  is connected to the waveguide  27  of the radio frequency circuit  20 , and the signal received by the antenna  1 , processed by the radio frequency circuit  20 , and output from the IF circuit  23  is converted by the IF AD converter  43  into a digital signal which is supplied to the control processor  31 A in the analog circuit  3 D and processed by the signal processing function of the control processor  31 A. 
         [0071]    The detection and setting of the bias adjusting value of the D/A converters  41  in the radio frequency unit  2 B may be performed in the adjusting circuit shown in  FIG. 6 , and the analog circuit  3 B shown in  FIG. 5 , not the analog circuit  3 D shown in  FIG. 8  incorporating the control processor  31 A having a signal processing function, may be employed as the analog circuit to be combined with the radio frequency unit  2 B. 
         [0072]    Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will understand that many modifications are possible in the exemplary embodiments without departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.