Abstract:
An in-vehicle pulse wave radar device is required to detect an object in a wide range of several tens of centimeters to several tens of meters and therefore the receiving circuit requires an amplification circuit having a wide dynamic range covering the receiving pulse waves of both a large signal level and a small signal level. The pulse wave radar device according to the invention controls the amplification degree of the receiving circuit downward at the time of measuring an object at a short distance immediately after transmission of the transmitting pulse wave and upward with time for measuring an object at a long distance by increasing the amplification degree of the receiving circuit progressively with the elapse of time after transmission of the transmitting pulse wave.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a pulse wave radar device using the millimeter wave or submillimeter wave band, or in particular to a pulse wave radar device having an improved detection ability for the reflected wave from an object at a short distance.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, a radar DEVICE has come to be mounted on an automotive vehicle for the purpose of collision prevention and auto cruising. In this in-vehicle radar DEVICE, the reciprocating distance to an object is determined by multiplying the velocity of light by the time from the transmission of the transmitting pulse wave to the receiving of the reflected wave from the object. Thus, the distance to the object is calculated by measuring the time required from the transmission of the transmitting pulse wave to the receiving of the reflected wave from the object.  
         [0005]     This pulse wave radar device makes the preparation for receiving the receiving pulse wave reflected from the object at the shortest distance after transmitting the transmitting pulse wave. The in-vehicle pulse wave radar device is required to detect an object within a wide range of several tens of cm to several tens of m, and therefore the receiving circuit requires an amplifier circuit having a wide dynamic range from the receiving pulse wave of a large signal to the receiving pulse wave of a small signal.  
         [0006]     In the prior art, as the receiving circuit of the pulse wave radar device requiring a wide dynamic range, an automatic gain control (AGC) circuit is used. The configuration of the conventional pulse wave radar device using the AGC circuit is shown in  FIG. 1  (see Japanese Patent Application Laid-Open No. 6-174826, for example).  
         [0007]     In  FIG. 1 , reference numeral  11  denotes a pulse generating circuit,  12  an oscillator,  21  a mixing circuit,  22  a power amplifier circuit,  23  a transmitting antenna,  31  a receiving antenna,  82  a pre-amplifier circuit,  83  a mixing circuit,  84  an intermediate frequency amplifier circuit,  85  a detection circuit, and  86  an AGC circuit. In the mixing circuit  21 , the pulses from the pulse generating circuit  11  are modulated by the oscillation wave from the oscillator  12 , after being amplified in the power amplifier circuit  22 , emitted as a transmitting pulse wave from the transmitting antenna  23 .  
         [0008]     In the mixing circuit  83 , the receiving pulse wave received by the receiving antenna  31  is demodulated by the oscillation wave from the oscillator  12 , and amplified by the intermediate frequency amplifier circuit  84 . During the amplification, the output of the detection circuit  85  is controlled by negative feedback through the AGC circuit  86  to produce a constant value.  
         [0009]     Also, the variable attenuation circuit using the PIN diode as shown in  FIG. 2  is used for a variable gain amplifier circuit. In  FIG. 2 , reference characters R 71  to R 76  denote resistors, C 71  to C 74  denote capacitors, and D 71  to D 73  denote PIN diodes.  
         [0010]     The PIN diodes D 71  to D 73  are DC blocked by the capacitors C 71  to C 74 , and the resistance value of the PIN diodes D 71  to D 73  is variably controlled by the bias voltage from a DC power supply. Once the resistance value of the PIN diodes is variably controlled, the attenuation amount of the variable attenuation circuit is changed, and thus the amplification degree of the variable gain amplifier circuit is controlled.  
       SUMMARY OF THE INVENTION  
       [0011]     In order to secure the distance resolution of 1 m or less, the pulse width of the transmitting pulse is required to be reduced to about the order of nanosecond. Once the width of the receiving pulse is reduced to the order of nanosecond, the AGC circuit described above is required to be controlled by feedback within a very short time. Thus, the AGC operation as the negative feedback control system becomes unstable and the stable circuit operation is difficult to achieve.  
         [0012]     To cope with this problem, the object of this invention is to provide a pulse wave radar device having a high distance resolution which can measure the distance by accurately receiving the receiving pulse having a large receiving level difference between a receiving pulse wave of a large signal from a near object and a receiving pulse wave of a small signal from a remote object.  
         [0013]     Also, in the conventional variable attenuation circuit used for the amplifier circuit, each PIN diode is required to be controlled by a separate DC power supply, and further, the existence of three current paths disadvantageously leads to a large power consumption.  
         [0014]     To solve this problem, another object of the invention is to provide a pulse wave radar device including a receiving circuit capable of variable gain amplification and detection with a simple circuit configuration and a small in power consumption.  
         [0015]     In order to achieve the objects described above, according to this invention, there is provided a pulse wave radar device wherein a distance to an object at a short distance is measured immediately after transmitting the transmitting pulse wave by controlling the amplification degree of the receiving circuit downward, while an object at a great distance is measured upon elapse of a predetermined time after transmitting the transmitting pulse wave with the amplification degree of the receiving circuit controlled progressively upward.  
         [0016]     In the case where the amplification degree of the receiving circuit is controlled in this manner, a receiving pulse wave having a large signal level is reflected from the object at a short distance, and therefore can be amplified or detected at an appropriate signal level by receiving it with a small amplification degree. From an object at a large distance, on the other hand, a receiving pulse wave small in signal level is reflected, and therefore can be amplified or detected at an appropriate signal level by receiving it with a large amplification degree. Also, the receiving circuit having no feedback path for the amplification can amplify stably.  
         [0017]     Specifically, according to this invention, there is provided a pulse wave radar device including: a transmitting circuit for periodically transmitting the transmitting pulse wave modulated from a transmitting pulse; a transmitting antenna for radiating the transmitting pulse wave from the transmitting circuit; a receiving antenna for receiving the receiving pulse wave reflected from an object; a receiving circuit for amplifying with a variably gain and detecting the receiving pulse wave from the receiving antenna; and a gain control circuit for controlling the amplification degree of the receiving circuit; wherein the gain control circuit periodically controls the amplification degree of the receiving circuit in accordance with the period in which the transmitting circuit transmits the transmitting pulse wave, and increases the amplification degree of the receiving circuit after the transmitting circuit transmits the transmitting pulse wave.  
         [0018]     According to this invention, a pulse wave radar device is provided in which the receiving pulse waves having a large receiving level difference including the receiving pulse wave of a large signal from an object at a short distance and a receiving pulse wave of a small signal from an object at a great distance can be accurately received and the distance can be measured.  
         [0019]     In the pulse wave radar device according to this invention, the gain control circuit can reduce the amplification degree of the receiving circuit while the transmitting circuit transmits the transmitting pulse wave.  
         [0020]     Even in the case where an excessively large receiving pulse wave is input by the leakage of the transmitting pulse wave in the pulse wave radar device or the leakage of the transmitting pulse wave to the receiving antenna from the transmitting antenna, the receiving circuit can be protected from saturation.  
         [0021]     In the pulse wave radar device according to this invention, the gain control circuit may reduce the amplification degree of the receiving circuit after the elapse of a predetermined time following the transmission of the transmitting pulse wave during the period when the transmitting circuit transmits the transmitting pulse wave.  
         [0022]     After the pulse wave radar device transmits the transmitting pulse wave, after the elapse of the reciprocating propagation time corresponding to the maximum detection distance of the pulse wave radar device, the echo or the multiplex reflection of the transmitting pulse wave is received, and therefore the operation error due to the unrequired receiving pulse wave can be prevented.  
         [0023]     In the pulse wave radar device according to the invention, the gain control circuit may control the amplification degree of the receiving circuit upward in proportion to the fourth power of the time elapsed from the transmission of the transmitting pulse wave from the transmitting circuit.  
         [0024]     Assuming that the reflectivity of the object is constant, the intensity of the receiving pulse wave from the object is inversely proportional to the fourth power of the distance to the object. By increasing the amplification degree of the receiving circuit by the gain control circuit in proportion to the fourth power of the time elapsed from the transmission of the transmitting pulse wave, therefore, the amplification and detection at a substantially constant signal level are made possible.  
         [0025]     The receiving circuit of the pulse wave radar device according to the invention includes a variable attenuation circuit including a first PIN diode and a second PIN diode connected in cascade between an input terminal and an output terminal with the input terminal at the N polarity, a third PIN diode connected between the junction point of the first and second PIN diodes and a common terminal with the common terminal at the P polarity, an input resistor connected between the input terminal and the common terminal and an output resistor connected between the output terminal and a control power terminal, wherein the amplification degree of the receiving circuit can be controlled by controlling the voltage applied to the control power terminal.  
         [0026]     According to the invention, there is provided a pulse wave radar device having a receiving circuit capable of variable gain amplification and detection with a simple circuit configuration and small in power consumption.  
         [0027]     The pulse wave radar device according to the invention may further includes a reciprocating propagation time calculation circuit for calculating the reciprocating propagation time with respect to an object based on the time difference between the timing at which the transmitting pulse wave is transmitted from the transmitting circuit and the timing at which the receiving pulse wave is received by the receiving circuit.  
         [0028]     The provision of the reciprocating propagation time calculation circuit makes it possible to calculate the reciprocating propagation time. Once the reciprocating propagation time can be calculated, the distance to the object can be calculated based on the thus calculated reciprocating propagation time.  
         [0029]     According to this invention, a pulse wave radar device having a high distance resolution is provided in which the receiving pulse waves having a large receiving level difference including a receiving pulse wave of a large signal from an object at a short distance and a receiving pulse wave of a small signal from an object at a great distance can be accurately received and the distance can be measured.  
         [0030]     Also, according to the invention, there is provided a pulse wave radar device having a receiving circuit capable of variable gain amplification and detection with a simple circuit configuration and small in power consumption. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a block diagram for explaining the configuration of a pulse wave radar device using the conventional AGC circuit.  
         [0032]      FIG. 2  is a block diagram for explaining the configuration of a variable attenuation circuit using the conventional PIN diode.  
         [0033]      FIG. 3  is a block diagram for explaining an example of the pulse wave radar device according to an embodiment of the invention.  
         [0034]      FIG. 4  is a block diagram for explaining an example of the pulse wave radar device according to another embodiment of the invention.  
         [0035]      FIG. 5  is a diagram for explaining an example of configuration of the reciprocating propagation time calculation circuit.  
         [0036]      FIG. 6  is a diagram for explaining the operation of a pulse wave radar device according to the invention.  
         [0037]      FIG. 7  is a diagram for explaining the operation of a pulse wave radar device according to the invention.  
         [0038]      FIG. 8  is a diagram for explaining the operation of the variable attenuation circuit used in the receiving circuit of the pulse wave radar device according to the invention.  
         [0039]      FIG. 9  is a diagram for explaining the operation of the variable attenuation circuit used in the receiving circuit of the pulse wave radar device according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]     Embodiments of the invention are explained below with reference to the drawings. This invention is not limited to the embodiments described below.  
         [0041]      FIG. 3  is a block diagram for explaining an example of a pulse wave radar device according to an embodiment of the invention. The configuration of the pulse wave radar device is explained with reference to this block diagram. In  FIG. 3 , numeral  11  denotes a pulse generating circuit for generating transmitting pulses having a predetermined period, numeral  12  denotes an oscillator adapted to oscillate at a modulation frequency, numeral  21  denotes a mixing circuit for modulating the transmitting pulse at a modulation frequency, numeral  22  denotes a power amplifier circuit for amplifying the power of the transmitting pulse wave, numeral  23  denotes a transmitting antenna for radiating the transmitting pulse wave, numeral  31  denotes a receiving antenna for receiving the receiving pulse wave, numeral  32  denotes a pre-amplifier circuit for amplifying the receiving pulse wave, numeral  33  denotes a mixing circuit for demodulating the receiving pulse wave, numeral  34  denotes an intermediate frequency amplifier circuit for amplifying the demodulated receiving pulse, numeral  35  denotes a detection circuit for detecting the demodulated receiving pulse, numeral  36  denotes a reciprocating propagation time calculation circuit for calculating the reciprocating propagation time with respect to an object, and numeral  41  denotes a gain control circuit for periodically controlling the amplification degree of the receiving circuit.  
         [0042]     The transmitting circuit includes the pulse generating circuit  11 , the oscillator  12 , the mixing circuit  21  and the power amplifier circuit  22 . The receiving circuit includes the oscillator  12 , the pre-amplifier circuit  32 , the mixing circuit  33 , the intermediate frequency amplifier circuit  34  and the detection circuit  35 .  
         [0043]     First, with reference to  FIG. 3 , the configuration of the transmission system of the pulse wave radar device is explained. The pulse generating circuit  11  generates transmitting pulses having a predetermined period. The predetermined period is preferably longer than the reciprocating propagation time of the radio wave corresponding to the maximum detection distance of the pulse wave radar device. The mixing circuit  21  mixes the transmitting pulse from the pulse generating circuit  11  with the modulating wave from the oscillator  12  and outputs a transmitting pulse wave. The power amplifier circuit  22  amplifies the power of the transmitting pulse wave from the mixing circuit  21 , and the transmitting antenna  23  emits the transmitting pulse wave from the power amplifier circuit  22 . The transmitting antenna  23  may be configured of a plurality of antennas.  
         [0044]     Next, with reference to  FIG. 3 , the configuration of the receiving system of the pulse wave radar device is explained. The receiving antenna  31  receives the receiving pulse wave reflected from the object. The receiving antenna  31  may also include a plurality of antennas. The transmitting antenna may be shared with the receiving antenna. The pre-amplifier circuit  32  amplifies a small receiving pulse wave. The mixing circuit  33  demodulates the receiving pulse from the receiving pulse wave by detection with the oscillation wave in a frequency band used by the pulse wave radar device. In the intermediate frequency amplifier circuit  34 , the amplification degree controlled by the gain control circuit  41  is amplified to a signal level suitable for detecting the demodulated receiving pulse from the mixing unit  33 . The detection circuit  35  regenerates the receiving pulse by detecting the demodulated receiving pulse from the intermediate frequency amplifier circuit  34 .  
         [0045]     The reciprocating propagation time calculation circuit  36  calculates the reciprocating propagation time with respect to the object based on the time difference between the timing at which the transmitting pulse wave is transmitted by the transmitting circuit and the timing at which the receiving circuit receives the receiving pulse wave. In  FIG. 3 , the time difference is between the timing at which the pulse generating circuit  11  outputs the transmitting pulse and the timing at which the detection circuit  35  outputs the receiving pulse. The delay time in the transmitting circuit, the transmitting antenna, the receiving antenna and the receiving circuit is measured in advance. The reciprocating propagation time calculation circuit  36  preferably corrects the reciprocating propagation time with respect to the object by subtracting the delay time measured in advance.  
         [0046]     Though not shown, the last stage of the reciprocating propagation time calculation circuit  36  includes an arithmetic circuit to calculate the distance to the object based on the reciprocating propagation time and the propagation speed.  
         [0047]     In the pulse wave radar device shown in  FIG. 3 , the gain control circuit  41  periodically controls the amplification degree of the intermediate frequency amplifier circuit  34  in accordance with the period in which the pulse generating circuit  11  outputs the transmitting pulses. Specifically, the receiving pulse wave from an object at a short distance is received at a large signal within a short time after emitting of the transmitting pulse wave. The receiving pulse wave from an object at a large distance, on the other hand, is received at a small signal level after the elapse of a long time from the emitting of the transmitting pulse wave. In view of this, the gain control circuit  41  controls the amplification degree of the intermediate frequency amplifier circuit  34  to increase progressively with time after the transmitting pulse is output by the pulse generating circuit  11 .  
         [0048]     This control operation makes it possible for the intermediate frequency amplifier circuit  34  to output the receiving pulse of a substantially constant signal level for both the receiving pulse waves from the objects at both short and long distances. Thus, even the detection circuit  35  having a small dynamic range can operate accurately. Also, the pulse wave radar device according to the invention, which is not controlled by feedback, can operate in stable fashion with a high distance resolution.  
         [0049]     Another example of the pulse wave radar device is shown in  FIG. 4 . In  FIG. 4 , the same reference numerals as those in  FIG. 3  denote the same component elements. The pulse wave radar device shown in  FIG. 4  is different from the pulse wave radar device shown in  FIG. 3  in that in the former, the amplification degree of the pre-amplifier circuit  32  is periodically controlled by the gain control circuit  41  in accordance with the period in which the transmitting pulse is output from the pulse generating circuit  11 . The gain control circuit  41  controls the amplification degree of the pre-amplifier circuit  32  to increase after the transmitting pulse is output from the pulse generating circuit  11 .  
         [0050]     This control operation makes it possible for the pre-amplifier circuit  32  to output the receiving pulses of a substantially constant signal level for both the receiving pulse waves from the objects at both short and long distances. Thus, even the mixing circuit  33 , the intermediate frequency amplifier circuit  34  and the detection circuit  35  having a small dynamic range can operate normally. Thus, this pulse wave radar device is more advantageous than the pulse wave radar device described in  FIG. 3  in that in the former, the magnitude of the dynamic range of the mixing circuit  33  and the intermediate frequency amplifier circuit  34  can be relaxed. Also, for lack of feedback control, the pulse wave radar device according to this embodiment can operate in stable fashion with a high distance resolution.  
         [0051]     The reciprocating propagation time calculation circuit shown in  FIG. 3  or  4  is so configured that a S-R flip-flop circuit, in which the timing at which the transmitting pulse is output from the pulse generating circuit  11  provides a set input and the timing at which the detection circuit  35  outputs the receiving pulse provides a reset input, is combined with a low-pass filter for extracting the low-frequency component of the output of the S-R flip-flop circuit.  FIG. 5  shows an example of the configuration of the reciprocating propagation time calculation circuit. In  FIG. 5 , numeral  36  denotes the reciprocating propagation time calculation circuit, numeral  37  denotes the S-R flip-flop circuit, and numeral  38  denotes the low-pass  
         [0052]     In  FIG. 5 , the S-R flip-flop circuit  37  is such that the timing at which the transmitting pulse is output from the pulse generating circuit provides a set input (Set in  FIG. 5 ), and the timing at which the detection circuit outputs the receiving pulse provides a reset input (Reset in  FIG. 5 ). The S-R flip-flop circuit  37  outputs a signal in such a manner that the on time is long in the case where the period from the timing at which the pulse generating circuit outputs the transmitting pulse to the timing at which the detection circuit outputs the receiving pulse is long, and the on time is short in the case where the period is short from the timing at which the pulse generating circuit outputs the transmitting pulse to the timing at which the detection circuit outputs the receiving pulse.  
         [0053]     The low-pass filter  38  extracts the low-frequency component from the output of the S-R flip-flop circuit  37 . Specifically, in the case where the period from the timing at which the pulse generating circuit outputs the transmitting pulse to the timing at which the detection circuit outputs the receiving pulse is long, the output of the low-pass filter  38  has a large low-frequency component, while in the case where the period from the timing at which the pulse generating circuit outputs the transmitting pulse to the timing at which the detection circuit outputs the receiving pulse is short, the output of the low-pass filter  38  has a small low-frequency component. By detecting the output of the low-pass filter  38 , therefore, the reciprocating propagation time with respect to the object can be calculated. The distance to the object can be digitally indicated by converting the output of the low-pass filter  38  into a digital signal.  
         [0054]     In calculating the distance to the object, the delay time in the transmitting circuit or the receiving circuit of the pulse wave radar device is corrected by shifting the bias of the output level of the reciprocating propagation time calculation circuit by an amount corresponding to the delay time, or the delay time is corrected when calculating the distance to the object based on the output of the reciprocating propagation time calculation circuit.  
         [0055]     The reciprocating propagation time calculation circuit may alternatively be a pulse count circuit operated in such a manner that the timing at which the pulse generating circuit outputs the transmitting pulse provides a set input, while the timing at which the detection circuit outputs the receiving pulse provides a reset input. During the time from the set input to the reset input, pulses of a predetermined period are generated, and the pulse count circuit counts the number of the pulses during the particular time, thereby making it possible to calculate the reciprocating propagation time with respect to the object.  
         [0056]     Both of the reciprocating propagation time calculation circuits described above can calculate the distance to the object by dividing the reciprocating propagation time by twice the speed of light.  
         [0057]     With reference to  FIGS. 6, 7 , the operation of the pulse wave radar device is explained. In  FIGS. 6, 7 ,  FIGS. 6A, 6B ,  6 C,  7 D,  7 E,  7 G,  7 F show the signal waveforms at the points A, B, C, D, E, G, F, respectively, shown in  FIGS. 3, 4 ,  5 . An explanation is given below by appropriately referring to the reference characters shown in  FIGS. 3, 4 , and  5 .  
         [0058]      FIG. 6A  shows the transmitting pulses output toward the mixing circuit  21  by the pulse generating circuit  11 . The period of the transmitting pulses is set longer than the reciprocating propagation time corresponding to the maximum detection distance of the pulse wave radar device. The transmitting pulse width is preferably shorter than the time corresponding to the distance resolution of the object.  
         [0059]      FIG. 6B  shows the signal output by the pulse generating circuit  11  toward the gain control circuit  41 . The amplification degree of the receiving circuit is preferably reduced for a predetermined length of time before transmission of the transmitting pulse wave so that the gain control circuit  41  reduces the amplification degree of the receiving circuit upon the elapse of the reciprocating propagation time corresponding to the maximum detection distance of the pulse wave radar device after transmitting the transmitting pulse wave during the period of transmission of the transmitting pulse wave. The amplification degree during this predetermined length of time is preferably minimum. The operation error can be prevented by reducing the sensitivity against the unnecessary receiving pulses caused by following a detour or by multiple reflection after receiving the receiving pulse wave from the object at the maximum detection distance.  
         [0060]     Also, during the period when the pulse generating circuit  11  outputs the transmitting pulses, the gain control circuit  41  preferably reduces the amplification degree of the receiving circuit. During this period, the amplification degree is preferably minimized. The saturation of the receiving circuit can be prevented which otherwise might be caused by the leakage of the transmitting pulse wave into the pulse wave radar device or the leakage from the transmitting antenna  23  to the receiving antenna  31 . Once the receiving circuit is saturated, the receiving pulse wave from the object could not be normally received until the saturation is restored.  
         [0061]      FIG. 6C  shows the signal with which the gain control circuit  41  controls the amplification degree of the receiving circuit The amplification degree of the intermediate frequency amplifier circuit  34  or the pre-amplifier circuit  32  is controlled as shown in  FIG. 6C . Specifically, the amplification degree of the intermediate frequency amplifier circuit  34  or the pre-amplifier circuit  32  is controlled upward gradually with time after transmission of the transmitting pulse wave. A long distance to the object lengthens the reciprocating propagation time of the radio wave, and the signal level of the receiving pulse wave is reduced along with distance. By gradually increasing the amplification degree, therefore, the signal level variation of the receiving pulse wave can be reduced after amplification. As a result, the dynamic range of the receiving circuit can be reduced.  
         [0062]     Also, the gain control circuit  41  preferably controls the amplification degree of the receiving circuit to increase in proportion to the fourth power of the time elapsed after transmission of the transmitting pulse wave. The signal level of the transmitting pulse wave emitted from the transmitting antenna  23  decreases in inverse proportion to the square of the propagation distance. Similarly, the signal level of the receiving pulse wave from the object decreases in inverse proportion to the square of the propagation distance. This indicates that the signal level variation in the receiving circuit can be substantially minimized by controlling the amplification degree of the receiving circuit to increase in proportion to the fourth power of the time elapsed from the transmission of the transmitting pulse wave.  
         [0063]     In  FIG. 6 , the pulse generating circuit outputs the signal shown in  FIG. 6B . However as an alternative, the signal as shown in  FIG. 6A  may be output by the pulse generating circuit  6 A so that the gain control circuit  41  may output the signal as shown in  FIG. 6C .  
         [0064]      FIG. 7D  shows a pulse waveform output by the pulse generating circuit  11  toward the reciprocating propagation time calculation circuit in timing with the output of the transmitting pulse. The timing of the pulses shown in  FIG. 7D  may coincide with that of the pulses shown in  FIG. 6A , or taking the delay in the transmitting circuit or the receiving circuit of the pulse wave radar device into consideration, may be delayed behind the timing of the pulses shown in  FIG. 6A . In the case where the timing of the pulses shown in  FIG. 7D  is rendered to coincide with the timing of the pulses shown in  FIG. 6A , the delay is corrected in the reciprocating propagation time calculation circuit or the last stage thereof.  
         [0065]      FIG. 7E  shows the receiving pulses from the detection circuit  35 . The time difference in timing between the pulse waveform shown in  FIG. 7D  and the pulse waveform shown in  FIG. 7E  corresponds to the reciprocating propagation time of the radar wave. From this reciprocating propagation time, the distance to the object can be calculated.  
         [0066]      FIGS. 7G, 7F  show the operation waveforms in the reciprocating propagation time calculation circuit  36  including a combination of the S-R flip-flop circuit  37  and the low-pass filter  38  shown in  FIG. 5 . The time length from the timing at which the pulse ( FIG. 7D ) is input from the pulse generating circuit  11  to the timing at which the receiving pulse ( FIG. 7E ) is input from the detection circuit  35  corresponds to the pulse width of the pulse signal ( FIG. 7G ) output from the S-R flip-flop circuit  37 . The pulse width of the pulse signal ( FIG. 7G ) is narrow when the distance to the object is short and wide when the distance to the object is long.  
         [0067]     The pulse signal ( FIG. 7G ) output from the S-R flip-flop circuit  37  is such that the signal voltage is constant and only the pulse width changes with the reciprocating propagation time with respect to the object. The low-pass filter  38 , as shown in  FIG. 7F , extracts the DC component of the pulse signals having a constant voltage and different pulse widths output from the S-R flip-flop circuit  37 . Specifically, the pulse signal is converted into the DC signal ( FIG. 7F ) having a signal level corresponding to the pulse width of the pulse signal ( FIG. 7G ) output from the S-R flip-flop circuit  37 . The signal voltage of this DC signal assumes a signal level corresponding to the reciprocating propagation time with respect to the object.  
         [0068]     As described above, even a receiving circuit having a small dynamic range can accurately receive the receiving pulse waves having a large difference in receiving level, including the receiving pulse wave of a large signal level from an object at a short distance and the receiving pulse wave of a small signal level from an object at a long distance, and therefore, a pulse wave radar device can be provided which can measure the distance over a wide range of distance from short to long distances.  
         [0069]     Next, the amplifier circuit for controlling the amplification degree is explained. The amplification degree can be controlled either by changing the amplification degree of the amplifier circuit directly or by controlling the attenuation amount of the variable attenuation circuit with a variable attenuation circuit included in the amplifier circuit and thereby changing the amplification degree of the amplifier circuit as a whole. This embodiment is explained with reference to the variable attenuation circuit used in the latter configuration.  
         [0070]      FIGS. 8, 9  are connection diagrams showing a configuration of the variable attenuation circuit. In  FIGS. 8, 9 , reference numerals C 41 , C 42  denote capacitors for DC blocking electrically between the variable attenuation circuit and external circuits. Reference numeral R 41  denotes a resistor for input impedance matching. Numeral R 43  denotes a load resistor. Numerals D 41 , D 42 , D 43  denote PIN diodes for adjusting the attenuation amount.  
         [0071]     In  FIGS. 8, 9 , PIN diodes D 41 , D 43  are connected in cascade between an input terminal (In) and an output terminal (Out) with the input terminal (In) of N polarity, and a PIN diode D 42  is connected between the junction point of the PIN diodes D 41 , D 43  and a common terminal (ground) with the common terminal at P polarity. The resistor R 41  is connected between the input terminal (In) and the common terminal, and the resistor R 43  between the output terminal and the control power terminal.  
         [0072]     In  FIG. 8 , a voltage positive with respect to the common terminal is applied to the control power terminal connected with an end of the resistor R 43 . The current flows from the resistor R 43  through the PIN diodes D 43 , D 41  to the resistor R 41 . By increasing the applied voltage and thus increasing the current flowing through the PIN diodes D 43 , D 41 , the impedance of the PIN diodes D 41 , D 43  is decreased. The most part of the signal input to the input terminal (In) is loaded on the resistor R 43  and the signal level at the output terminal (Out) is not attenuated. Therefore, the attenuation amount of the variable attenuation circuit is decreased.  
         [0073]     By decreasing the applied voltage, on the other hand, the current flowing through the PIN diodes D 43 , D 41  is decreased, while the impedance of the PIN diodes D 41 , D 43  increases. The signal input from the input terminal (In) is mainly loaded on the PIN diodes D 41 , D 43 , and the signal level of the output terminal (Out) is attenuated, and therefore the attenuation amount of the variable attenuation circuit increases.  
         [0074]     In  FIG. 9 , the impedance of the PIN diode D 43  is further increased by applying the voltage negative with respect to the common terminal to the control power terminal connected with an end of the resistor R 43 . At the same time, a slight current flows through the PIN diodes D 42 , D 43  and the resistor  43 . The impedance of the PIN diode D 42  is decreased, while the impedance of the PIN diode D 43  is increased. Thus, the most part of the signal input from the input terminal (In) is loaded on the PIN diodes D 41 , D 43 , and the signal level at the output terminal (Out) is further attenuated. Therefore, the attenuation amount of the variable attenuation circuit further increases.  
         [0075]     This configuration can make up a variable attenuation circuit in which the attenuation amount with a large variation can be achieved while securing the input impedance matching at the resistor R 41 . Further, it is possible to reduce power consumption because there is only one current path. Further, for lack of a reactance element having a frequency characteristic as a component element, a wide operation range is secured.  
         [0076]     The receiving circuit having the above-mentioned variable attenuation circuit can control the voltage applied to the control power terminal and thereby control the amplification degree of the receiving circuit.  
         [0077]     The output impedance matching can be assured by rendering the impedance of the resistor R 43  coincide with the output resistance. Also, to secure the output impedance matching alone, the input terminal (In) is connected to the output, while the output terminal (Out) is connected to the input.  
         [0078]     As explained above, a pulse wave radar device having a high distance resolution can be provided, in which the receiving pulses having a large receiving level difference including the receiving pulse wave of a large signal level from an object at a short distance and the receiving pulse wave of a small signal level from an object at a long distance can be accurately received to measure the distance.  
         [0079]     Also, a pulse wave radar device can be provided having a receiving circuit capable of variable gain amplification and detection with a simple circuit configuration and small in power consumption.  
         [0080]     The pulse wave radar device according to the invention can find an application as an in-vehicle device aimed at the prevention of vehicle collision or auto cruise, and as a stationary pulse wave radar device as well.