Patent Publication Number: US-11650283-B2

Title: Detection device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-023864 filed on Feb. 13, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of the embodiments is related to a detection device. 
     BACKGROUND 
     There is known a detection device that transmits a high-frequency signal to a target and detects the target based on a reflection signal. It is known that a frequency obtained by frequency analysis is divided into a frequency range used for measuring a distance to a target and a relative velocity of the target and a frequency range unused for measurement, and an interference signal is detected based on a signal intensity in the frequency range unused for measurement (e.g. see Patent Document 1: Japanese Laid-open Patent Publication No. 2008-107280). 
     SUMMARY 
     According to an aspect of the present invention, there is provided a detection device including: a transmitter that transmits a high-frequency signal as a transmission signal; a receiver that receives a reception signal including a reflection signal formed by reflecting the transmission signal at a target; and a controller that detects the target based on a frequency of the reflection signal, and changes a frequency of the transmission signal based on a frequency of the reception signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating the periphery of a detection device; 
         FIG.  2    is a block diagram illustrating the detection device; 
         FIG.  3    is a diagram illustrating a transmission signal and an interference signal; and 
         FIGS.  4  to  11    are flowcharts illustrating process to be executed by a control unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     When another device close to a detection device emits a signal, particularly a signal having a frequency close to a frequency of a transmission signal of the detection device, the detection device receives the signal, so that the signal might affect the detection of the target by the detection device. 
     The detection device according to a present embodiment can suppress the influence of the signal from another device. 
     Hereinafter, a description will be given of the present embodiment with reference to drawings. 
     First Embodiment 
       FIG.  1    is a block diagram illustrating the periphery of a detection device according to a first embodiment. A detection device  10  outputs a transmission signal  15 . When the transmission signal  15  is reflected by a target  12 , a reflection signal  16  is generated. The transmission signal  15  and the reflection signal  16  are high-frequency signals having frequencies ftx and fre, respectively. The frequency fix of the transmission signal  15  is about 24 GHz, for example. When the target  12  is moving, the frequency fre of the reflection signal  16  changes by the Doppler effect. When the target  12  comes close to the detection device  10 , the frequency fre is greater than the frequency ftx. When the target  12  is away from the detection device  10 , the frequency fre is less than the frequency ftx. When the target  12  is stationary with respect to the detection device  10 , the frequency fre is equal to the frequency ftx. Therefore, a relative speed of the target  12  with respect to the detection device  10  can be detected by calculating a difference between the frequencies ftx and fre. Further, it is possible to determine whether a moving object exists nearby by calculating the difference between the frequencies ftx and fre. 
     On the other hand, when a device  14  emits a signal  17  (hereinafter referred to as “interference signal”) having a frequency fin close to the frequency of the reflection signal  16 , the interference signal  17  interferes with the reflection signal  16  and affects the detection of the target  12  by the detection device  10 . For example, the device  14  is the same type of device as the detection device  10 . Even if the frequency of the signal emitted from the device  14  is different from the frequency ftx of the transmission signal  15  of the detection device  10 , the frequency of the signal emitted from the device  14  comes close to the frequency ftx due to the influence of an ambient temperature or the like, so that the signal emitted from the device  14  may be the interference signal  17 . In recent years, devices using radio waves in the 24 GHz band have increased, and a possibility that signals transmitted from the respective devices interfere with each other has also increased. Therefore, the detection device  10  according to the first embodiment detects the signal that may become the interference signal  17 , and changes the frequency of the transmission signal  15 . 
       FIG.  2    is a block diagram illustrating the detection device  10  according to the first embodiment. A transceiver  20  includes a transmission unit  22  and a reception unit  24 . The transmission unit  22  transmits the transmission signal  15  that is the high-frequency signal from an antenna  50 . The reception unit  24  receives a reception signal  18  from an antenna  52 . The antennas  50  and  52  may be a common antenna. The reception signal  18  includes the reflection signal  16  and the interference signal  17  of  FIG.  1   . The reception unit  24  includes a mixer that mixes the transmission signal  15  output from the transmission unit  22  and the reception signal  18 , and outputs an intermediate signal  19 . A frequency fif of the intermediate signal  19  is calculated using the frequency ftx of the transmission signal  15  and a frequency frx of the reception signal  18 , and the following equation is satisfied: fif=|ftx−frx 1|. 
     An amplifier  26  amplifies the intermediate signal  19 . A low-pass filter (LPF)  28  passes a low-frequency signal  19   a  among the amplified intermediate signal  19 . A high-pass filter (HPF)  30  passes a high-frequency signal  19   b  among the intermediate signal  19 . Amplifiers  32  and  34  amplify the signals  19   a  and  19   b , respectively. 
     The frequency fif of the intermediate signal  19  is a frequency difference between the transmission signal and the reception signal. When the frequency fif of the intermediate signal  19  is 10 kHz and 100 kHz, the moving speed of the target  12  corresponds to about 223 km/h and 2234 km/h, respectively. Assuming that the moving speed of the target  12  is about 100 km/h at most, a cutoff frequency of the LPF  28  is set to 10 kHz, for example, and a cutoff frequency of the HPF  30  is set to 100 kHz, for example. The signal  19   a  output from the LPF  28  is highly likely to be an intermediate signal (frequency fifa) based on the reflection signal  16  from the target  12  moving within a range of a normal moving speed. On the other hand, when the signal  19   b  is output from the HPF  30 , it is unlikely thought that the reflection signal  16  from the target  12  moving at the normal moving speed has been received, and the signal  19   b  is highly likely to be be an intermediate signal (frequency fifb) based on the interference signal  17 . Here, the cutoff frequencies of the LPF  28  and the HPF  30  can be set appropriately depending on an assumed moving speed of the target  12 . 
     A control unit  40  is a processor such as a microcomputer or a central processing unit (CPU), for example, and includes analog/digital converters (ADCs)  42  and  44 . The ADC  42  converts the amplified signal  19   a  into a digital signal and outputs the digital signal. The control unit  40  detects a state of the target  12  based on the frequency of the digital signal output from the ADC  42 . 
     The ADC  44  converts the amplified signal  19   b  into a digital signal. A power detector  36  detects the intensity of the signal  19   b . A comparator  38  compares a voltage Vdet output from the power detector  36  with a reference voltage Vref. The comparator  38  turns on the ADC  44  when the voltage Vdet is equal to or more than the reference voltage Vref, and turns off the ADC  44  when the voltage Vdet is less than the reference voltage Vref. The ADC  44  is used to detect the frequency of the interference signal. When the intensity of the signal  19   b  is equal to or more than a predetermined power, the ADC  44  converts the signal  19   b  into a digital signal and outputs the digital signal. The control unit  40  instructs the transmission unit  22  to change the frequency of the transmission signal  15  via a digital/analog converter (DAC)  48  based on the frequency of the reception signal  18  determined from the digital signal from the ADC  44 . 
     A memory  46  is a semiconductor memory such as a Dynamic Random Access Memory (DRAM) or a Static Random Access Memory (SRAM), and stores information to be used for the change of the frequency of the transmission signal  15 . 
       FIG.  3    is a diagram illustrating the transmission signal and the interference signal according to the first embodiment. A vertical axis of  FIG.  3    indicates a signal intensity, for example. In  FIG.  3   , the intensity of the transmission signal  15  is illustrated to be larger than the intensity of the interference signal  17 , but the intensity of the transmission signal  15  may be smaller than the intensity of the interference signal  17 . The detection device  10  has a plurality of channels (for example, 11 channels) as frequencies used for the transmission signal  15 . For example, the frequency of a channel Ch1 is 24.05 GHz, and the frequency of a Ch11 is 24.25 GHz. In the example of  FIG.  3   , the detection device  10  uses a channel Ch6, and the transmission unit  22  outputs the transmission signal  15  having the frequency of the channel Ch6. When determining that the frequency of the interference signal  17  comes close to the frequency of the channel Ch6, the control unit  40  changes a transmission channel of the transmission signal  15  to a channel Ch10, for example. Thereby, the transmission unit  22  outputs the transmission signal  15  having the frequency of channel Ch10. Therefore, it is possible to suppress the interference signal  17  from affecting the detection of the target. Although an example in which the number of channels is 11 is described, the detection device  10  needs to have at least the plurality of channels. 
       FIG.  4    is a flowchart illustrating processing to be executed by the control unit  40  according to the first embodiment. The control unit  40  causes the transmission unit  22  to transmit the transmission signal  15  of the channel ChA (the frequency is ftx) (S 10 ). The channel ChA is the channel Ch6 in  FIG.  3   , for example. When detecting the signal  19   b  output from the HPF  30 , the control unit  40  detects the frequency fifb of the signal  19   b  (S 12 ), and determines whether the frequency fifb comes close to the frequency of ChA (S 14 ). For example, when the frequency of the interference signal  17  comes close to the frequency of the channel Ch6 as illustrated in  FIG.  3   , the control unit  40  determines that the answer to the determination of S 14  is Yes. 
     When the answer to the determination of S 14  is Yes, the control unit  40  selects a new channel ChB (S 16 ). The channel ChB is the channel Ch10, for example. The control unit  40  causes the transmission unit  22  to transmit the transmission signal  15  of the channel ChB (S 18 ). When the answer to the determination of S 14  is No, the control unit  40  does not change the channel of the transmission signal  15  (S 20 ). In  FIG.  4   , when the signal  19   b  is not output, the channel is not changed. 
     According to the first embodiment, the control unit  40  determines whether the interference signal  17  is received based on the frequency fin of the reception signal  18 , and changes the frequency ftx of the transmission signal  15  according to a result of the determination. Thereby, when the frequency of the interference signal  17  comes close to the frequency of the transmission signal  15 , the frequency of the transmission signal  15  can be changed, and the influence on the detection of the target  12  based on the reflection signal  16  can be suppressed. 
     The LPF  28  and the HPF  30  (filter) separate the intermediate signal  19  having the frequency corresponding to the frequency difference frx−ftx between the reception signal  18  and the transmission signal  15  into the signal  19   a  (first signal) having the frequency lower than a predetermined frequency and the signal  19   b  (second signal) having the frequency higher than the predetermined frequency. The control unit  40  detects a state of the target  12  based on the frequency fifa of the signal  19   a . In addition, the control unit  40  changes the frequency ftx of the transmission signal  15  based on the frequency fifb of the signal  19   b . Thereby, since the content of the reception signal  18  is distinguished using the intermediate signal  19  for detecting the target  12 , the circuit configuration can be reduced in size. 
     First Variation of First Embodiment 
       FIG.  5    is a flowchart illustrating processing to be executed by the control unit  40  according to a first variation of the first embodiment. When the frequency fifb is detected in S 12 , the control unit  40  causes the transmission unit  22  to transmit the transmission signal  15  having a frequency ftx  30  (S 22 ). The frequency Δf is a frequency smaller than a frequency interval to an adjacent channel. When detecting the signal  19   b  after the transmission signal  15  having the frequency ftx+Δf is transmitted, the control unit  40  detects a frequency fifb′ of the signal  19   b  (S 23 ). The steps S 22  and S 23  are performed in order to determine whether the frequency fin of the interference signal  17  is higher or lower than the frequency ftx of the transmission signal  15 . After the detection of the frequency fifb′, the control unit  40  generates a random number i (S 24 ). The random number i is an integer such as 1, 2, or 3. 
     The control unit  40  determines whether the frequency fifb′ is more than the frequency fifb (S 26 ), and calculate a channel number “B” for selecting a next channel based on the result of the determination. When the answer to the determination of S 26  is Yes, the frequency of the interference signal  17  is lower than the frequency of the transmission signal  15 . Therefore, the control unit  40  sets “A+i” to the channel number B (B=A+i) (S 28 ), and selects a channel ChB having a frequency higher than a frequency of a channel ChA. When the answer to the determination of S 26  is No, the frequency of the interference signal  17  is higher than the frequency of the transmission signal  15 . Therefore, the control unit  40  sets “A−i” to the channel number B (B=A−i) (S 30 ), and selects the channel ChB having a frequency lower than the frequency of the channel ChA. 
     The control unit  40  determines whether the channel number B satisfies “1≤B&lt;M” (S 32 ). The parameter “M” is a maximum number of channels, and 11 in  FIG.  3   , for example. When the answer to the determination of S 32  is No, the channel ChB is not within the range of all channels. Therefore, the control unit  40  replaces the channel number B with a channel number C (S 34 ), and selects a specific channel ChC. The channel ChC is the center channel of all channels for example, and is the channel Ch6 in  FIG.  3   . When the answer to the determination of S 32  is Yes, the processing of S 34  is not performed. Then, the control unit  40  changes the channel in S 16  and S 18  based on the channel number “B” obtained in S 28  or S 30 . Thereby, the transmission signal  15  is set to the channel ChB. Since other processing is the same as that in the first embodiment, a description thereof is omitted. 
     When the control unit  40  changes the frequency of the transmission signal  15  regularly (for example, at a fixed interval), and the device  14  that emits the interference signal  17  regularly changes the frequency of the transmission signal as with the detection device  10 , there is a possibility to eternally repeat the processing for changing the channel in  FIG.  4   . For this reason, in the first variation, the control unit  40  randomly selects one of the plurality of channels Ch1 to Ch11 using the random number i when changing the frequency ftx of the transmission signal  15 . Thereby, the detection device  10  and the device  14  can be suppressed from repeating the same channel change. 
     The control unit  40  determines whether the frequency of the interference signal  17  is higher or lower than the frequency of the transmission signal  15  as illustrated in S 26 . When the frequency of the interference signal  17  is lower than the frequency of the transmission signal  15 , the control unit  40  changes the frequency of the transmission signal  15  so as to increase it as illustrated in S 28 . When the frequency of the interference signal  17  is higher than the frequency of the transmission signal  15 , the control unit  40  changes the frequency of the transmission signal  15  so as to decrease it as illustrated in S 30 . Thus, the control unit  40  changes the frequency ftx of the transmission signal  15  so that the frequency ftx of the transmission signal  15  is further away from the frequency fin of the interference signal  17 . Thereby, the frequency ftx of the transmission signal  15  is away from the frequency fin of the interference signal  17 , and the influence of the interference signal  17  can be eliminated. 
     Second Variation of First Embodiment 
       FIG.  6    is a flowchart illustrating processing to be executed by the control unit  40  according to a second variation of the first embodiment. The device  14  uses the same channels Ch1 to Ch11 in  FIG.  3    as those of the detection device  10 . When the control unit  40  detect the signal  19   b , the control unit  40  calculates the sum of the frequencies ftx and fifb as a frequency α of the interference signal  17  (ftx+fifb=α) (S 40 ), and calculates the channel ChB based on the frequency α (S 42 ). For example, when the control unit  40  determines that the device  14  uses a channel closest to the frequency α, the control unit  40  sets a channel unused by the device  14  as the channel ChB. For example, when the frequency α is the closest to the channel Ch6, the control unit  40  can determine that the device  14  uses the channel Ch6. In this case, the control unit  40  sets a channel Ch4 or Ch8 as the channel ChB. Then, the processing of S 16  and S 18  is performed. Thereby, the control unit  40  can select the channel ChB which is away from the channel used by the device  14 . Since other processing is the same as that in the first embodiment, a description thereof is omitted. 
     Third Variation of First Embodiment 
       FIG.  7    is a flowchart illustrating processing to be executed by the control unit  40  according to a third variation of the first embodiment. The memory  46  stores a channel number N of a channel ChN close to the frequency of the interference signal  17  in advance. When the device  14  is installed for example, the channel number N of the channel ChN close to the frequency of the signal emitted from the device  14  is stored in the memory  46 . After calculating the frequency α by the processing S 10  to S 40 , the control unit  40  acquires the channel number N from the memory  46  (S 44 ). It should be noted that the control unit  40  does not select the channel number N of the channel ChN when calculating the channel ChB in S 42 . Then, the processing of S 16  and S 18  is performed. Since other processing is the same as that in the second variation, a description thereof is omitted. 
     According to the third variation, the memory  46  stores in advance the channel number N which is information on a frequency having a possibility of causing interference with the reflection signal  16 . The control unit  40  changes the frequency of the transmission signal  15  by calculating the channel ChB based on the channel number N as illustrated in S 42 . Thereby, it is possible to suppress the frequency fix of the transmission signal  15  from matching the frequency of the channel ChN. The number of channel numbers N to be stored in the memory  46  may be one or more. 
     Fourth Variation of First Embodiment 
     A fourth variation of the first embodiment indicates an example of detecting the channel number N in the third variation of the first embodiment.  FIG.  8    is a flowchart illustrating processing to be executed by the control unit  40  according to a third variation of the first embodiment, and particularly illustrates processing for confirming the presence or absence of a frequency band that interferes with the reflection signal. The control unit  40  performs the following processing at any timing. The control unit  40  sets the channel number N=0 (S 50 ), and sets a change amount i=1 (S 52 ), as initial values. The control unit  40  increments the channel number N by the change amount i (N=N+i) (S 54 ). In the case of the channel number N=0 and the change amount i=1, the sum of them “N+i” is 1. Therefore, the control unit  40  selects the channel Ch1, and performs the processing of S 10  and S 12  to detect the frequency fifb of the signal  19   b.    
     The control unit  40  determines whether the frequency fifb is greater than a threshold frequency fth based on the frequency fifb detected in S 12  (S 56 ). The threshold frequency fth is 100 kHz, for example. When the answer to the determination of S 56  is Yes, there is a high possibility that the interference signal  17  exists in the channel selected in S 54 . Therefore, the control unit  40  stores the channel number N set in S 54  into the memory  46  (S 58 ). Then, the control unit  40  increments the change amount i by 1 (i=i+1) (S 60 ). When the answer to the determination of S 56  is No, the processing advances to S 60 . The control unit  40  determines whether the change amount i is greater than a maximum channel number imax (i&gt;imax) (S 62 ). The maximum channel number imax of  FIG.  3    is 11. When the answer to the determination of S 62  is Yes, the processing of  FIG.  8    is terminated. When the answer to the determination of S 62  is No, the processing returns to S 54 , and the control unit  40  determines whether the interference signal  17  exists in the channel of a next channel number N+1. 
     According to the fourth variation, the control unit  40  detects the frequency having the possibility of causing interference with the reflection signal  16  by changing the frequency of the transmission signal  15  by scanning, for example, and stores, in the memory  46 , information regarding a frequency and a channel at which the signal that interferes with the reflection signal  16  is received, based on the detection result. Thereby, the use of the transmission signal  15  having the frequency close to the frequency of the interference signal  17  can be avoided in advance, and the interference due to the interference signal  17  can be suppressed. The processing of  FIG.  8    may be executed periodically and/or aperiodically when the detection device  10  is installed, when the detection device  10  is turned on, or when the detection device  10  is used, or the like. 
     Fifth Variation of First Embodiment 
       FIG.  9    is a flowchart illustrating processing to be executed by the control unit  40  according to a fifth variation of the first embodiment. After S 10  and S 12 , the control unit  40  calculates the sum of the frequencies ftx and fifb as the frequency α of the interference signal  17  (ftx+fifb=α) as in  FIG.  6    (S 40 ), and waits for a predetermined period (S 70 ). The predetermined period is 1 second, for example. Then, the control unit  40  detects the frequency fifb′ of the signal  19   b  (S 72 ), and determines whether the control unit  40  determines whether the sum of the frequencies ftx and fifb′ approximates the frequency α (S 74 ). For example, when a difference between the frequency α and the sum of the frequencies ftx and fifb′ is equal to or less than a predetermined frequency, the control unit  40  determines that the sum of the frequencies ftx and fifb′ approximates the frequency α. 
     When the sum of the frequencies ftx and fifb′ approximates the frequency α, it is considered that the frequency fin of the interference signal  17  has hardly changed. For this reason, there is a low possibility that the frequency fin comes close to the frequency ftx of the transmission signal  15 . Therefore, when the answer to the determination of S 74  is Yes, the control unit  40  does not change the channel Ch (S 20 ). 
     On the other hand, when the answer to the determination of S 74  is No, it is considered that the frequency fin of the interference signal  17  has changed. Therefore, the control unit  40  determines whether the frequency fifb′ is less than the frequency fifb (S 76 ). When the answer to the determination of S 76  is No, it is considered that the frequency fin is away from the frequency ftx, and hence the control unit  40  does not change the channel Ch (S 20 ). 
     When the answer to the determination of S 76  is Yes, the frequency fin comes close to the frequency ftx. In this case, the control unit  40  determines whether a difference between the frequencies fifb′ and fifb is greater than the threshold frequency fth (S 78 ). When the answer to the determination of S 78  is Yes, the change in the frequency fin is very fast. For this reason, even if the frequency fin comes close to the frequency ftx, the frequency fin immediately moves away from the frequency ftx, and it is considered that the interference signal interferes with the transmission signal only for an instant and the possibility of affecting the detection of the target is small. Therefore, the control unit  40  does not change the channel Ch (S 20 ). 
     When the answer to the determination of S 78  is No, the frequency fin comes close to the frequency ftx, and there is a possibility that the interference signal interferes with the transmission signal for a long time. Therefore, the control unit  40  changes the channel of the transmission signal to the channel ChB in S 16  and S 18 . The processing for changing to the channel ChB and other processing are the same as those in the first embodiment, and a description thereof is omitted. 
     According to the fifth variation, the control unit  40  changes the frequency ftx of the transmission signal  15  based on a change in the frequency fin of the interference signal  17  and a degree of the change in the frequency fin. Thereby, the interference due to the interference signal  17  can be suppressed. 
     Sixth Variation of First Embodiment 
       FIG.  10    is a flowchart illustrating processing to be executed by the control unit  40  according to a sixth variation of the first embodiment. After transmitting the transmission signal from the channel ChA in S 10 , the control unit  40  acquires the frequency fifb based on the reception signal in S 12 . The control unit  40  acquires the information on a frequency fibbb stored at the time of signal reception in the past from the memory  46  (S 80 ). The control unit  40  determines whether to change the channel for transmitting the transmission signal based on the past frequency fibbb and the frequency fifb detected in S 12  (S 82 ). When the frequency fin of the interference signal  17  gradually comes close to the frequency ftx, the frequency fifb gradually increases with lapse of time. When it is determined that there is such a tendency, the control unit  40  determines that the answer to the determination of S 82  is Yes, and changes the channel in S 16  and S 18 . When the answer to the determination of S 82  is No, the control unit  40  does not change the channel Ch (S 20 ). Then, the control unit  40  stores the frequency fibb detected in S 12  into the memory  46  (S 84 ). The processing of S 84  may be executed at any timing after S 12 . Since other processing is the same as that in the first embodiment, a description thereof is omitted. 
     The control unit  40  stores the past frequency fifbb into the memory  46 , and compares the currently acquired frequency fifbb with the past frequency fifbb to determine the magnitude relationship thereof. Thereby, the control unit  40  can recognize a change speed of the frequency fifb, that is, a change amount of the frequency fifb per unit time. The control unit  40  changes the frequency ftx of the transmission signal  15  based on a change speed of the frequency fin of the interference signal  17 . Thereby, the interference due to the interference signal  17  can be suppressed. The detection of frequency fifb in  FIG.  10    can be performed periodically, for example. 
     Seventh Variation of First Embodiment 
       FIG.  11    is a flowchart illustrating processing to be executed by the control unit  40  according to a seventh variation of the first embodiment. After detecting the signal  19   b  (S 85 ), the control unit  40  acquires an intensity IF of the signal  19   b  (S 86 ). The intensity of the signal  19   b  is substantially proportional to an electric field intensity of the interference signal  17 . The control unit  40  determines whether a threshold intensity Ith is smaller than the intensity IF (S 88 ). When the answer to the determination of S 88  is No, the intensity of the interference signal  17  is small, and the possibility that the interference signal  17  interferes with the reflection signal  16  is small. Therefore, the control unit  40  does not change the channel Ch in S 20 . When the answer to the determination of S 88  is Yes, the intensity of the interference signal  17  is equal to or more than a predetermined value, and the possibility that the interference signal  17  interferes with the reflection signal  16  is large. Therefore, the control unit  40  changes the channel in S 16  and S 18 . Since other processing is the same as that in the first embodiment, a description thereof is omitted. 
     According to the seventh variation, the control unit  40  changes the frequency ftx of the transmission signal  15  based on the intensity of the signal  19   b . Thereby, when the possibility that the interference signal  17  interferes with the reflection signal  16  is small, the frequency ftx can be unchanged. 
     The processing may be executed by appropriately combining any of the first to the seventh variations of the first embodiment. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.