Portable FM-CW radar device

Abstract: An FM-CW radar device for frequency-modulating a high frequency signal with a modulation signal, transmitting the modulated high frequency signal, receiving a signal reflected by a target object, and frequency-modulating the reflection signal with a signal generated by a local oscillator signal source provided by branching a portion of the transmitting signal, the FM-CW radar device including a modulation means (3) disposed between a radar transmitting section and a transmitting antenna (4), the modulation means (3) modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by a relative velocity and beat frequency generated by the propagation delay time. The modulation means (3) modulates the transmitting signal, and the received signal reflected by the target object is frequency-modulated by a first frequency converting means (7) with the local oscillator, and further the received signal is frequency-modulated by a second frequency converting means (8) with the second frequency.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a portable FM-CW radar device, more particularly to a device on an automobile which can measure a relative velocity and distance simultaneously with respect to a target object to avoid collision with other automobiles. 2. Description of the Related Art A frequency-modulating continuous-wave (FM-CW) radar device modulates a high frequency signal, transmits the modulated high frequency signal, receives a reflection signal reflected by a target object, and modulates the frequency of the reflected signal with a signal generated by a local oscillator provided by branching a portion of the transmitted signal. In recent years, the number of automobiles owned has increased. The number of traffic accidents, such as collision of automobiles, has increased year by year as a result. To reduce the number of traffic accidents, it is necessary to provide an automobile equipped with a safety device of a reasonable cost, for example, an automobile distance monitoring system which allows a driver to determine a potential for collision in advance. An FM-CW radar system is conventionally used to measure a relative velocity and distance separately with respect to a target object. The above-mentioned radar system can measure the relative velocity and a distance with a simple signal processing circuit, and further a transmitter and receiver for it can be simply constituted. The aforementioned radar system is used to enable general drivers to prevent collisions between automobiles, so the radar system must be made compact, transportable, and low in cost. The principle of an FM-CW radar system is as described for instance in FR.A. 1 227 158. Frequency modulation is conducted on an oscillator by a triangular wave of several hundreds of Hz sent from a modulation signal generator. The frequency-modulated wave is transmitted from a transmitting antenna. A signal reflected from a target object is received by a receiving antenna, and then a frequency changer like a mixer conducts FM-detection on the receiving signal, wherein the frequency-modulation wave is used as a local oscillation signal. At this time, a deviation of frequency (beat) is caused between the reflected wave sent from the object and the transmitting signal in accordance with the distance between the radar apparatus and the object and also in accordance with a Doppler shift caused by the relative velocity. The frequency component f b of this beat is expressed by (Distance frequency f r dependent on distance) ± (velocity frequency f d dependent on speed), so the distance and relative velocity can be measured by the deviation of the frequency. In this case, the following equations are satisfied: f r = (4ΔΩ/Tc)R and f d = (2f 0 /c)v , where ΔΩ is the modulation width, T is the period of a modulation wave, c is the velocity of light, R is the distance from the radar apparatus to an obstacle (a target object), f 0 is the central transmitting frequency, and v is the relative velocity with respect to the obstacle (the object). In the case where this system is used as radar for automobile use, the measured distance is 100 m at most, and the relative velocity is approximately 100 km/h. In order to ensure a sufficient accuracy of measurement of distance, the maximum frequency deviation must be about 100 Mhz. In order to ensure a sufficient accuracy of measurement of relative velocity, a millimeter wave band must be used for the transmitting frequency band. Incidentally, in the case of a conventional millimeter wave band FM-CW radar, frequency modulation of a very large amount of frequency deviation is conducted, so that an AM component having approximately the same frequency component as the modulation signal is overlapped with the frequency modulation wave due to the inclination of the characteristics of oscillating frequency versus electric power of a voltage control type of oscillator. This AM component is detected by a mixer. However, the frequency of a triangular wave to frequency-modulate, and the frequency of a reflected signal after frequency modulation of a reflecting wave has been conducted, are very close to each other. Accordingly, the receiving S/N ratio is deteriorated. Therefore, unless the transmitting output is increased, a necessary searching region can not be ensured. For this reason, expensive Impatt diodes and Gunn diodes must be used for the oscillator under present conditions. US-A-4 115 775 shows cancelling A.M. noise produced by an FM-CW radar transmitter. A pair of sidebands is added to the source. A feedback loop containing a modulator is used to subtract noise from the signal. SUMMARY OF THE INVENTION The present invention has been achieved in view of the aforementioned problems. It is a primary object of the present invention to provide a highly efficient FM-CW radar device, which is low in cost and, which has a high temperature stability even when applied to a millimeter band. In accordance with one aspect of the present invention, there is provided a device as set out in claim 1. Preferred features are set out in dependent claims. In one embodiment of the FM-CW radar device of the invention, a radar signal generated by the voltage control oscillator is modulated by the modulation means (for example, the modulation is performed by means of switching). This output signal is input to a receiver accompanied by a Doppler frequency shift corresponding to the relative velocity between the radar and the target object, and also accompanied by the delay time corresponding to a distance from the radar apparatus the object. In the receiver, a portion of the output signal is utilized as a local oscillator for frequency converting. This output signal is further frequency-converted by the second frequency converting means with the second frequency f L . In a base band (BB) circuit of the radar, the maximum value of the frequency difference ascribed to the Doppler frequency shift corresponding to the relative velocity with regard to the object, and the maximum value of the frequency difference ascribed to the delay time corresponding to the distance to the object, are prescribed, and the frequencies exceeding the maximum value are removed. Accordingly, the signal (IF signal) including distance and velocity information close to the second frequency f L is processed. However, demodulation waves (which will be a cause of noise) of the AM component ascribed to the inclination of the frequency versus output power characteristics of oscillator close to DC, and demodulation wave components not less than 2f L , are removed by the filter. Consequently, the S/N ratio of the receiver can be improved. In the case where a radar signal generated by the voltage control oscillator is phase-modulated by the modulation means, the spectrum of the output signal is a suppressed carrier signal component f e . Advantageously, in the FM-CW radar apparatus of the invention, a radar signal generated by the voltage control oscillator hits the object and is received by the receiving antenna, accompanied by a Doppler frequency shift corresponding to the relative velocity between the radar source and the object, and also accompanied by a frequency difference ascribed to the delay time corresponding to the distance from the radar to the object. After that, the signal is modulated by the modulation means provided between the receiving antenna and the first frequency converting means (for example, this modulation is performed by means of switching). A spectrum of the received signal modulated by the modulation means can be expressed in the same manner. After that, image processing is conducted in the same manner as the first aspect of the invention described above. Consequently, the S/N ratio of the receiver can be improved. When the signal is phase-modulated by the modulation means in this case, a spectrum of the output signal is also expressed by the spectrum in which the carrier signal component f t is suppressed. Advantageously, in the FM-CW radar apparatus of the invention, a radar signal generated by the voltage control oscillator is modulated by the modulation means provided between the voltage control oscillator and the transmitting antenna (for example, the modulation is performed by means of switching). Then, this output signal hits the target object and is input into the receiver, along with a Doppler frequency shift corresponding to the relative velocity between the radar apparatus and the target object and along with a frequency difference ascribed to the delay time corresponding apparatus to the distance from the radar to the object. Between the receiving antenna and the first frequency converting means, a switching means is provided, which can conduct switching synchronously with a modulation signal on the transmission side each time a portion of the signal of the modulation signal generator, which generates a modulation signal sent to the modulation means on the transmission side, is branched. Therefore, the switching means on the reception side functions as a gating circuit which is operated synchronously with a transmission signal. That is, the switching means receives the signal only when a signal is sent from the transmission side. It does not receive other external signals. Accordingly, signal components entering from the outside are not detected as noise. Therefore, the S/N ratio of the receiver can be further improved. A modulation signal composed of a rectangular wave of a 50% duty can be also used, and a rectangular FM wave can be used for the output of the modulation signal generator. Further, in the case where the rectangular FM wave is used for the output of the modulation signal generator, the frequency (the period) of the frequency modulation wave can be changed. Unnecessary frequency components can be removed from a signal input into the second frequency converting means by a band pass filter, which allows the second frequency to pass and is provided between the first frequency converting means which conducts frequency conversion on a receiving signal by a local oscillator, and the frequency converting means which carries out frequency conversion by the second frequency. Further, it is also possible to allow only the fundamental frequency component of a signal generated in the modulation signal generator to pass through the band pass filter provided between the modulation signal generator and the second frequency converting means. When an isolator is inserted between the voltage control oscillator and the modulation means, the impedance on the output side of the voltage control oscillator is maintained constant. Further, when the isolator is inserted between the modulation means and the first frequency converting means, the input impedance of the first frequency converting means is maintained constant. When the switching means is structured to be a balance type, the input and output impedance of the switching means can be maintained constant. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are waveform views showing the operational principle of a prior art FM-CW radar system; Fig. 3 is a view showing the configuration of a conventional FM-CW radar apparatus; Fig. 4 is a block diagram showing the operational principle of an embodiment of the present invention; Fig. 5 is a block diagram showing the operation of another embodiment of the present invention; Fig. 6 is a block diagram showing the operation of still another embodiment of the present invention; Fig. 7 is a view showing a spectrum of output signals; Fig. 8 is a view showing a spectrum of an output of first frequency converter; Fig. 9 is a block diagram showing an embodiment of the present invention; Fig. 10 is a block diagram showing another embodiment of the present invention; Fig. 11 is a block diagram showing still another embodiment of the present invention; Fig. 12 is a view showing a switching means using a PIN diode; Fig. 13 is a view showing a phase modulation means using a circulator; Fig. 14 is a waveform diagram of a modulating signal; Fig. 15 is a waveform diagram of a modulating signal by means of a second frequency; and Fig. 16 is a block diagram showing a balanced-switching means. DESCRIPTION OF THE PREFERRED EMBODIMENT Before describing the preferred embodiments, a prior art FM-CW radar system will be described with reference to Figs. 1 to 3. Figures. 1 and 2 show the principle of a prior art FM-CW radar system, and Fig. 3 shows the structure of a conventional FM-CW radar apparatus. The principle of an FM-CW radar system is as follows. That is, as shown in Fig. 3, frequency modulation is conducted on an oscillator 2 by a triangular wave of several hundred Hz sent from a modulation signal generator 1, so that a frequency-modulation wave is transmitted from a transmitting antenna 4. A reflected signal sent from an object is received by a receiving antenna 6, and then a frequency changer 7 like a mixer conducts FM-detection on the received signal, wherein the frequency-modulation wave is used as a local oscillation signal. At this time, as shown in Figs. 1 and 2, deviation of frequency (beat) is caused between the reflected wave sent from the target object and the transmitting signal in accordance with the distance between the radar apparatus and the object , and also in accordance with a Doppler shift caused by relative velocity. Since the frequency component f b of this beat is expressed by (Distance frequency f r dependent on distance) ± (velocity frequency f d dependent on speed), the distance and relative velocity can be measured by the deviation of the frequency. In this case, the following equations are satisfied: f r =(4ΔΩ/Tc)R and f d =(2f 0 /c)v , where ΔΩ is the modulation width, T is the period of a modulation wave, c is the apparatus velocity of light, R is the distance from the radar to an obstacle (an object), f 0 is the central transmitting frequency, and v is the relative velocity with respect to the obstacle (the object). In the case where this system is used as a radar apparatus for automobile use, the measured distance is 100 m at most, and the relative velocity is approximately 100 km/h. In order to ensure a sufficient accuracy of measurement of distance the maximum frequency deviation must be about 100 MHz, and in order to ensure a sufficient accuracy of measurement of relative velocity, a millimeter wave band must be used for the transmitting frequency band. Embodiments of the present invention will be explained in detail with reference to the attached drawings. Figure 4 is a block diagram showing the principle of an embodiment of the present invention described in claim 2. The FM-CW radar apparatus according to the second aspect of the invention comprises: a modulation means 3 disposed between a radar receiving section and a receiving antenna 6, modulation means 3 modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by the relative velocity and a beat frequency generated by the propagation delay time, wherein the modulation means 3 modulates the receiving signal, and a modulated output signal of the modulation means 3 is frequency-modulated by the first frequency converting means 7 with the local oscillator and further the receiving signal is frequency-modulated by the second frequency converting means 8 with the second frequency. In this case, the isolator 10 is provided between the modulation means 3 and the first frequency converting means 7 in order to maintain the impedance of the circuit constant. Figure 5 is a block diagram showing another embodiment of the present invention described in claim 1. In Fig. 5, numeral 1 is a modulation signal generator. This modulation signal generator 1 generates a triangular wave signal for frequency modulation. Reference numeral 2 is a voltage control oscillator. This voltage control oscillator 2 generates a frequency-modulation signal of a millimeter band which is output from the radar. Reference numeral 3 is a modulation means. This modulation means 3 modulates a signal given by the voltage control oscillator 2 with a modulation signal generated by a modulation signal generator 5. In more detail, the modulation means modulates the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by the relative velocity and a beat frequency generated by the propagation delay time. This modulation means 3 is composed as a switching means which is switched by the second frequency. Refernce numeral 4 is a transmitting antenna. This transmitting antenna 4 effectively emits a signal output by the modulation means 3 into the air. Reference numeral 5 is a modulation signal generator. This modulation signal generator 5 generates a modulation signal which is used for modulating a signal in the modulation means 3. Reference numeral 6 is a receiving antenna. This receiving antenna effectively receives a signal reflected by an object. Reference numeral 7 is a first frequency converting means. This first frequency converting means 7 generates a necessary signal when a signal of the voltage control oscillator 2 and a signal received by the receiving antenna 6 are mixed. Reference numeral 8 is a second frequency converting means. This second frequency converting means 8 mixes a signal output from the first frequency converting means 7 and a signal generated in the modulation signal generator 5 and generates a signal component containing the relative velocity and distance between the object and the radar. Reference numeral 9 is a band pass filter. This band pass filter 9 allows to pass the frequency which is the same as the modulation signal frequency (the second frequency) generated by the modulation signal generator 5. Reference numeral 10 is an isolator (a circulator). This isolator 10 is provided between the voltage control oscillator 2 and the modulation means 3 in order to maintain the impedance of the circuit constant. Figure 6 is a block diagram showing still another embodiment of the present invention described in claim 3. The FM-CW radar apparatus according to the third aspect of the invention comprises: a modulation means 3 disposed between a radar transmitting section and a transmitting antenna 4, the modulation means 3 modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by a relative velocity and a beat frequency generated by the propagation delay time, wherein the modulation means 3 modulates the transmitting signal; and a switching means 3' disposed between a radar receiving section and a receiving antenna 6, the switching means 3' conducting a switching operation with the same frequency as the second frequency on the transmission side, wherein the receiving signal is subjected to gating by said switching means 3', and the output signal of the switching means 3' subjected to gating is frequency-modulated by a first frequency converting means 7 with the local oscillator and further the receiving signal is frequency-modulated by a second frequency converting means 8 with the second frequency. An isolator 10 may be inserted between the modulation means 3 and the transmitting section, and the isolator 10 may be inserted between the switching means 3' and the first frequency converting means 7. Other means which are shown by numerals 1, 2, 5, and 7 to 9 function in the same manner as shown in Figs. 4 and 5. Accordingly, the detailed explanations are omitted here. In each of the aforementioned aspect of the invention, it is preferable that the second frequency of a rectangular wave of about 50% duty is used as a modulation signal. Further, it is preferable that a rectangular frequency-modulated wave is used as a modulation signal. Further, it is possible that the period of the rectangular frequency-modulated wave used as the second frequency be changed. Furthermore, it is preferable the switching means be structured to be a balance type of switching means. As shown in Fig. 5, in the FM-CW radar apparatus of the aforementioned first aspect of the invention, a radar signal generated by the voltage control oscillator 2 is modulated by the modulation means 3 (for example, the modulation is performed by means of switching). Figure 7 shows the spectrum of the output signal. This output signal is input into a receiver along with a Doppler frequency shift corresponding to the relative velocity between the radar and the object and also along with the delay time corresponding to a distance from the radar apparatus to the object. In the receiver, a portion of the output signal is utilized as a local oscillator for frequency converting, so that the output spectrum of the first frequency converting means 7 is shown in Fig. 8. This output signal is further frequency-changed by the second frequency converting means 8 with the second frequency f L . In a base band (BB) circuit of the radar, the maximum value of the frequency difference ascribed to the Doppler frequency shift corresponding to the relative velocity with regard to the object, and the maximum value of the frequency difference ascribed to the delay time corresponding to the distance to the object, are prescribed, and the frequencies exceeding the maximum value are removed. Accordingly, the signal (IF signal) including distance and velocity information close to the second frequency f L is processed. However, demodulation waves (which will be a cause of noise) of the AM component ascribed to the inclination of the frequency versus output power characteristics of the oscillator close to DC, and demodulation wave components not less than 2f L , are removed by the filter. Consequently, the S/N ratio of the receiver can be improved. In the case where a radar signal generated by the voltage control oscillator 2 is phase-modulated by the modulation means 3, the spectrum of the output signal is shown in Fig. 7, in which the carrier signal component f t is suppressed. As shown in Fig. 4, in the FM-CW radar apparatus of the second aspect of the invention, a radar signal generated by the voltage control oscillator 2 strikes the object and is received by the receiving antenna 6, along with a Doppler frequency shift corresponding to the relative velocity between the radar apparatus and the object and along with a frequency difference ascribed to the delay time corresponding to the distance from the radar apparatus to the object. After that, the signal is modulated by the modulation means 3 provided between the receiving antenna 6 and the first frequency converting means 7 (for example, this modulation is performed by means of switching). The spectrum of the received signal modulated by the modulation means 3 can be expressed in the same manner as shown in Fig. 7. After that, image processing is conducted in the same manner as the first aspect of the invention described above. Consequently, the S/N ratio of the receiver can be improved. When the signal is phase-modulated by the modulation means 3 in this case, spectrum or the output signal is also expressed by the spectrum shown in Fig. 7, in which the carrier signal component f t is suppressed. Further, in the case of the FM-CW radar apparatus of the third aspect of the invention, as shown in Fig. 6, a radar signal generated by the voltage control oscillator 2 is modulated by the modulation means 3 provided between the voltage control oscillator 2 and the transmitting antenna 4 (for example, the modulation is performed by means of switching). Then, this output signal strikes the target object and is input into the receiver, along with a Doppler frequency shift corresponding to the relative velocity between the radar apparatus and the object and along with a frequency difference ascribed to the delay time corresponding to the distance from the radar apparatus to the object. Between the receiving antenna 6 and the first frequency converting means 7, the switching means 3' is provided which can conduct switching synchronously with a modulation signal on the transmission side at each time a portion of the signal of the modulation signal generator 5, which generates a modulation signal sent to the modulation means 3 on the transmission side is branched. Therefore, the switching means 3' on the reception side functions as a gating circuit which is operated synchronously with a transmission signal. That is, the switching means 3' receives the signal, only when a signal is sent from the transmission side. It does not receive other external signals. Accordingly, signal components entering from the outside are not detected as noise. Therefore, the S/N ratio of the receiver can be further improved. A modulation signal composed of a rectangular of wave a 50% duty can be also used, and a rectangular FM wave can be used for the output of the modulation signal generator 5. Further, in the case where the rectangular FM wave is used for the output of the modulation signal generator 5, the frequency (the period) of the frequency modulation wave can be changed. Unnecessary frequency components can be removed from a signal input into the second frequency converting means 8 by a band pass filter 9 which allows the second frequency to pass and is provided between the first frequency changing means 7 which conducts frequency changing on a received signal by a local oscillator, and the frequency converting means 8 which conducts frequency converting by the second frequency. Further, it is also possible to allow only the fundamental frequency component of a signal generated in the modulation signal generator 5 to pass through the ban pass filter 9 provided between the modulation signal generator 5 and the second frequency converting means 8. When an isolator 10 is inserted between the voltage control oscillator 2 and the modulation means 3, the impedance on the output side of the voltage control oscillator 2 is maintained constant. Further, when the isolator 10 is inserted between the modulation means 3 and the first frequency converting means 7, the input impedance of the first frequency converting means 7 is maintained constant. When the switching means is structured to be a balance type, the input and output impedance of the switching means can be maintained constant. The details of embodiments of the present invention will be explained as follows. (a) Explanation of the first embodiment Figure 10 is a block diagram showing the first embodiment of the present invention. The transmission system of the FM-CW radar apparatus shown in Fig. 10 includes a modulation signal generator 1, voltage control oscillator 2, directional coupler 12, and transmission antenna 4. The reception system includes a receiving antenna 6, low noise amplifier 13, first frequency converter 7, band pass filter 9-1, and second frequency converter 8. The control system includes a modulation signal generator 5 and a band pass filter 9-2. The modulation signal generator 1 generates a triangular wave signal for frequency modulation. The voltage control oscillator 2 generates a frequency-modulated signal of a millimeter band which is output from the radar. The directional coupler 12 takes out a signal sent from the voltage control oscillator 2 to the transmitting antenna 4 as well as to the first frequency converter 7 so as to be used for a local oscillating wave. The transmitting antenna 4 effectively emits a signal output from the modulation means 3, and the receiving antenna 6 effectively receives a signal reflected by an object. The low noise amplifier 13 is an amplitude-modulation means which conducts amplitude-modulation with a signal having the second frequency which is far lower than the radar transmission frequency sent from the modulation signal generator 5, and which is not less than twice as much as the frequency of the sum or difference of a Doppler frequency generated by the relative velocity and a beat frequency generated by the propagation delay time. The low noise amplifier 13 also functions as an isolator (a circulator) which maintains the impedance of the circuit constant. The first frequency converter 7 generates a necessary signal when a signal of the voltage control oscillator 2 taken out by the directional coupler 12 and a signal received by the receiving antenna 6 are mixed. The second frequency converter 8 mixes a signal output from the first frequency converter 7 and a signal generated by the modulation signal generator 5 and generates a signal component which contains the relative velocity and distance with respect to the object and the radar. The band pass filter 9-1 allows to pass the same frequency as the modulation signal frequency (the second frequency) which is generated by the modulation signal generator 5. This band pass filter 9-1 is provided between the first frequency converter 7 which changes the frequency of a receiving signal by a local oscillator, and the second frequency converter 8 which changes the frequency by the second frequency. The modulation signal generator 5 generates a modulation signal (a signal having the second frequency) which is used for modulating a signal by the low noise amplifier 13. The band pass filter 9-2 also allows to pass the same frequency as the modulation signal frequency (the second frequency) generated by the modulation signal generator 5. This band pass filter 9-2 is inserted between the modulation signal generator 5 and the second frequency converter 8. The FM-CW radar apparatus having the aforementioned structure is operated in the following manner. First, in order to conduct frequency modulation, a triangular wave generated by the modulation signal generator 1 is input into the voltage control oscillator 2. The output waveform of the generated signal wave is expressed as follows. (1 + A)sin[ω r t + ∫ s(t)dt] where (1 + A) is an amplitude, ω r = 2πf 0 , and ∫ s(t)dt is a triangular modulation amount. This output signal is emitted from the transmitting antenna 4. When the relative velocity is zero, the waveform which has returned from the object is delayed correspondingly to the distance between the radar apparatus and the object, further attenuated in the air, and input into the receiving antenna 6. The input reflecting wave is processed in such a manner that a drain current of the low noise amplifier 13 corresponding to the AM modulation means is modulated with a rectangular wave generated by the amplitude modulation signal generator 5. The waveform output at that time is expressed as follows. α(1 + A)(1 + B)sin[ω r (t - τ) + ∫ s(t - τ)dt] where α is a constant which is determined when consideration is given to the gain of the transmitting and receiving antennas, the attenuation in the air, and the gain of the low noise amplifier, and (1 + B) is an amplitude modulation component. Suppose that ω r t = Ω 1 t, ω r (t - τ) = Ω 2 t, ∫ s(t)dt =  1 , and ∫ s(t - τ)dt =  2 . Then, the formulas (1) and (2) are expressed as follows. (1 + A)sin(Ω 1 t +  1 ) α(1 + A)(1 + B)sin(Ω 2 t +  2 ) In the first frequency converter 7, an output wave of the low noise amplifier 13 of the amplitude modulation means and an frequency-modulated output wave of the voltage control oscillator 2 are mixed. Accordingly, the frequency component can be expressed as follows. {(1 + A)sin(Ω 1 t +  1 ) + α(1 + A)(1 + B)sin(Ω 2 t +  2 )} 2 From formula (5), the output signal of the first frequency converter 7 is expressed as follows. (1 + A) 2 /2 + α 2 (1 + A) 2 (1 + B) 2 /2 + α 2 (1 + A) 2 × (1 + B)/2 × cos{(Ω 1 - Ω 2 )t + ( 1 +  2 )} When only the signals close to the second frequency component B are allowed to pass by the band pass filters 9-1, 9-2 and the signals are frequency-modulated by the second frequency changer 8 with the second frequency B, only the component expressed by the following formula can be taken out as a signal. Sdet = α 2 B(1 + A) 2 /2 + [α 2 B(1 + A) 2 /2]cos{(Ω 1 - Ω 2 )t + ( 1 +  2 )} In the above formula, the first term corresponds to a noise component of a detection signal, and the second term corresponds to a signal component. When an investigation is made into a conventional FM-CW radar apparatus, the frequency component of the first frequency converter 7 is expressed as follows. {(1 + A)sin(Ω 1 t +  1 ) + α(1 + A)sin(Ω 2 t -  2 )} 2 From formula (8), a frequency component output by the first frequency converter 7 is expressed as follows. Sdet = (1 + A) 2 (1 + α 2 )/2 + [α(1 + A) 2 ]cos{(Ω 1 - Ω 2 )t + ( 1 +  2 )} In the above formula, the first term corresponds to a noise component of a detection signal, and the second term corresponds to a signal component. When a comparison is made between the noise component of formula (7) and that of formula (9), the noise component of formula (7) is much smaller than that of formula (9) since α << 1 in general. Consequently, according to the circuit of this embodiment, the FM-CW radar apparatus comprises: a low noise amplifier 13 disposed between the radar receiving section and the receiving antenna 6, the low noise amplifier 13 modulating the transmitting signal with the second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by the relative velocity and the beat frequency generated by the propagation delay time, wherein the low noise amplifier 13 modulates the receiving signal, and the modulated output signal of the low noise amplifier 13 is frequency-modulated by the first frequency converting means 7 with the local oscillator and further the receiving signal is frequency-modulated by the second frequency converting means 8 with the second frequency. Accordingly, a radar apparatus, with as S/N ratio better than that of a conventional FM-CW radar apparatus can be realized. The band pass filter 9-1 which allows the second frequency to pass is provided between the first frequency converter 7 which changes the frequency of a receiving signal by the local oscillator and the frequency converter 8 which changes the frequency of the receiving signal by the second frequency. By the aforementioned band pass filter 9-1, unnecessary frequency components are removed from the signal input into the second frequency converter 8. Due to the foregoing, noise which is caused by unnecessary frequency components when frequency changing is conducted by the second frequency converter 8 can be reduced. Further, only the fundamental frequency component of the signal generated by the modulation signal generator 5 is allowed to pass through the band pass filter 9-2 provided between the modulation signal generator 5 and the second frequency changer 8. Therefore, the occurrence of noise which is caused by frequency components other than the fundamental wave generated by the modulation signal generator 5 can be conduced. (b) Explanation of the second embodiment Figure 9 is a block diagram showing the second embodiment of the present invention. The transmission system of the FM-CW radar apparatus shown in Fig. 9 includes a modulation signal generator 1, voltage control oscillator 2, directional coupler 12, electric power amplifier 13', and transmission antenna 4. The reception system includes a receiving antenna 6, first frequency converter 7, band pass filter 9-1, and second frequency converter 8. The control system includes a modulation signal generator 5 and band pass filter 9-2. In this second embodiment, between the radar transmitting section and the transmitting antenna 4, there is provided the electric power amplifier 13' which is modulated by the second frequency and by which the transmitting signal is modulated, wherein the second frequency is far lower than the radar transmitting frequency and not less than twice as much as the sum or difference of a Doppler frequency caused by the relative velocity and a beat frequency caused by the propagation delay time. The transmitting signal is modulated by this electric power amplifier 13'. The received signal reflected by the object is frequency-converted by the first frequency converter 7 with the local oscillator, and further the signal is frequency-converted by the second frequency converter 8 with the second frequency. As shown in the above structure, this second embodiment is a case in which the amplitude modulation means (the electric power amplifier) 13' is provided on the transmission side. Even when the aforementioned structure is adopted, the same effect as that of the first embodiment, such as improvement in the S/N ratio, can be provided due to the same principle as the first embodiment. (c) Explanation of the third embodiment Figure 11 is a block diagram showing the third embodiment of the present invention. The transmitting system of the FM-CW radar apparatus shown in Fig. 11 includes a modulation signal generator 1, voltage control oscillator 2, direction coupler 12, electric power amplifier 13', and transmitting antenna 4. The receiving system includes a receiving antenna 6, switching means 13'', isolator (circulator) 10, first frequency converter 7, band pass filter 9-1, and second frequency converter 8. The control system includes a modulation signal generator 5 and band pass filter 9-2. In the third embodiment, between the radar transmitting section and the transmitting antenna 4, the electric power amplifier 13' is provided which is modulated by the second frequency, wherein the second frequency is far lower than the radar transmitting frequency and not less than twice as much as the sum or difference of a Doppler frequency caused by the relative velocity and a beat frequency caused by the propagation delay time. The transmitting signal is modulated by this electric power amplifier 13'. The switching means 13'' which conducts switching by the same frequency as the second frequency on the transmission side is provided between the radar receiving section and the receiving antenna 6. A receiving signal is subjected to gating by the switching means 13", and the output signal of the switching means 13'' which has been subjected to gating is input into the first frequency converter 7 through the isolator 10. Then, the signal is frequency-changed by the first frequency converter 7 with a local oscillator, and further the signal is frequency-changed by the second frequency converter 8 with the second frequency. For example, one of the specific elements to compose the switching means 13'' is the PIN diode 20 shown in Fig. 12, or the switching means 13'' is realized when a drain current (a drain voltage) of a three terminal element of GaAs, MESFET (metal semiconductor FET), or HEMT (high electron mobility transistor) is switched. As shown in Fig. 16, a balance type of switching means may be adopted in which two switching means 13'' are connected by hybrids 11 so that the input and output impedance of the switching means 13'' is maintained constant. When the aforementioned structure is provided, the change of impedance of the switching means 13'' does not affect the oscillation characteristics of the voltage control oscillator 2, so that the fluctuation of oscillation frequency of the voltage control oscillator 2, which is caused by the change of impedance of the switching means 13", and the fluctuation of output voltage can be reduced. Further, the fluctuation of frequency changing characteristics of the frequency converter which is caused when the impedance of the switching means 13'' is changed can be reduced. The isolator (circulator) 10 maintains the input impedance of the first frequency converter 7 constant. When the isolator 10 is provided, the fluctuation of frequency changing characteristics of the frequency converter which is caused by the change of impedance of the switching means 13'' can be reduced. Due to the structure described above, a radar signal generated by the voltage control oscillator 2 is subjected to amplitude modulation by the electric power amplifier 13' provided between the voltage control oscillator 2 and the transmission antenna 4. This output signal strikes an object and is input into the receiver, along with a Doppler frequency shift corresponding to the relative velocity between the radar and the object and further along with a frequency shift caused by the delay time corresponding to the distance from the radar to the object. Between the receiving antenna 6 and the first frequency converter 7, there is provided the switching means 13'' which can conduct a switching operation synchronously with a modulation signal on the transmission side each time a portion of signal of the modulation signal generator 5, which generates a modulation signal to be sent to the electric power amplifier 13' on the transmission side is branched. Accordingly, the switching means 13'' on the reception side functions as a gating circuit which is operated synchronously with a transmission signal. That is, a signal is received, only when the signal is sent from the transmission side. Other external signals are not received except for that time. Accordingly, signal components which enter from the outside are not detected as noise. Therefore, the S/N ratio of the receiver can be further improved. In the manner mentioned above, according to the third embodiment, approximately the same effects and advantages as those of the first and second embodiments can be provided, and further the S/N ratio of the receiver can be improved. (d) Others In the aforementioned examples, amplitude modulation means other than the electric power amplifier 13' and the low noise amplifier 13 can be used for a modulation means provided between the radar transmission section and the transmitting antenna, or between the radar reception section and the receiving antenna. Further, a switching means which conducts switching operation by the second frequency may be used. (In this case, the switching means modulates in amplitude into 0 or 1, so that this switching means can be considered to be a special amplitude modulation means.) Furthermore, a phase-modulation means can be used which conducts phase-modulation with the second frequency. For example, one of the specific elements to compose the switching means is the PIN diode 20 shown in Fig. 12, or the switching means is realized when a drain current (a drain voltage) of a three-terminal element of a GaAs, MESFET or HEMT is switched. As shown in Fig. 16, a balance type of switching means may be adopted in which two switching means 3' (13'') are connected by hybrids 11 so that the input and output impedance of the switching means 3' (13'') can be maintained constant. When the aforementioned structure is provided, the change of impedance of the switching means 3' (13'') does not affect the oscillation characteristics of the voltage control oscillator, so that the fluctuation of oscillation frequency of the voltage control oscillator, which is caused by the change of impedance of the switching means 3'(13''), and the fluctuation of output voltage can be reduced. Further, the fluctuation of frequency changing characteristics of the frequency converter which is caused when the impedance of the switching means 3' (13'') is changed can be reduced. As shown in Fig. 13, a circuit in which a PIN diode 20 and circulator 21 are combined is used for the phase modulation means. Incidentally, in each of the aforementioned embodiments, it is possible to use a rectangular wave shown in Fig. 14, the duty of which is approximately 50%, for the second frequency. Due to the foregoing, the attenuation amount of a signal with respect to the electric power of the carrier can be minimized as compared with a case in which another waveform such as a sine wave and a triangular wave is used. In the case where an interference waves exist, it is possible for the apparatus to receive only its own signal, so that an FM-CW radar apparatus which can withstand interference waves is composed. Further, an FM-CW radar apparatus which only receives its own signal and withstands interference waves can be composed in the following manner. A frequency modulation wave formed on a rectangular wave is utilized for the output of the modulation signal generator 5. When an interference wave exists, the frequency (the period) of the frequency modulation wave is changed as shown in Fig. 15. In the case where a modulation means other than the low noise amplifier 13 and the electric power amplifier 13' is used for the modulation means in the aforementioned first and third examples, the impedance on the output side of the voltage control oscillator can be maintained constant when an isolator is inserted between the voltage control oscillator and the modulation means. When the apparatus is structured in the manner mentioned above, the change of impedance of the modulation means does not affect the oscillation characteristics of the voltage control oscillator. Accordingly, the fluctuation of the oscillation frequency of the voltage control oscillator caused by the change of impedance of the modulation means can be reduced, and the fluctuation of output power can be also reduced. In the case where a modulation means other than the electric power amplifier 13' is used in the aforementioned second embodiment, the input impedance of the first frequency converter is maintained constant when an isolator is inserted between the modulation means and the first frequency changer. Due to the foregoing, the fluctuation of frequency changing characteristics of the frequency changer which is caused by the change of impedance of the modulation means can be reduced. As explained in detail above, the present invention provides an FM-CW radar apparatus frequency-modulating a high frequency signal with a modulation signal, transmitting the modulated high frequency signal, receiving a signal reflected by an object, and frequency-modulating the reflection signal with a signal generated by a local oscillation signal source provided by branching a portion of the transmitting signal, the FM-CW radar apparatus comprising: a modulation means (for example, a switching means) disposed between a radar transmitting section and a transmitting antenna, the modulation means modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by a relative velocity and a beat frequency generated by the propagation delay time, wherein the modulation means modulates the transmitting signal, and the received signal reflected by the object is frequency-modulated by a first frequency converting means with the local oscillator and further the received signal is frequency-modulated by a second frequency converting means with the second frequency. Further, the present invention provides an FM-CW radar apparatus comprising: a modulation means (for example, a switching means) disposed between a radar receiving section and a receiving antenna, the modulation means modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by a relative velocity and a beat frequency generated by the propagation delay time, wherein the modulation means modulates the transmitting signal, and a modulated output signal of the modulation means is frequency-modulated by a first frequency changing means with the local oscillator and further the receiving signal is frequency-modulated by a second frequency converting means with the second frequency. Consequently, it becomes possible to remove noise components from the reception circuit, wherein the noise components enters the reception circuit by the amplitude modulation component which is caused by the inclination of the oscillation frequency versus output power characteristics in the voltage control oscillator of the FM-CW radar apparatus. Due to the foregoing, the S/N ratio of the radar apparatus can be improved, and as a result, the present invention greatly contributes to the improvement in the performance of an FM-CW radar apparatus. Further, the present invention provides an FM-CW radar apparatus comprising: a modulation means (for example, a switching means) disposed between a radar transmitting section and a transmitting antenna, the modulation means modulating the transmitting signal with a second frequency which is far lower than the radar transmitting frequency and twice as much as the sum or the difference of a Doppler frequency generated by a relative velocity and a beat frequency generated by the propagation delay time, wherein the modulation means modulates the transmitting signal; and a switching means dispose between a radar receiving section and a receiving antenna, the switching means conducting a switching operation with the same frequency as the second frequency on the transmission side, wherein the received signal is subjected to gating by said switching means, and the output signal of said switching means subjected to gating is frequency-modulated by a first frequency converting means with the local oscillator and further the receiving signal is frequency-modulated by a second frequency converting means with said second frequency. Accordingly, the present invention is advantageous in that the S/N ratio of a receiver can be further improved. It is possible to use a modulation signal of a rectangular wave, with a duty of 50%. In this case, the attenuation amount of a signal with regard to the electric power of the carrier becomes minimum as compared with cases in which other waveforms (such as a sine wave or a triangular wave) are utilized. When a rectangular modulation signal is used as a signal having the second frequency, it becomes possible to receive only its own signal in the case where interference waves exist. Consequently, an FM-CW radar apparatus which is not affected by interference can be composed. When interference wave exist, it becomes possible to receive its own signal by changing the period of the aforementioned modulation wave. Consequently, an FM-CW radar which is not affected by interference can be composed. Further, a balance type of switching means can be employed. In this case, the input and output impedance of the switching means can be maintained constant, so that the change of impedance of the modulation means does not affect the oscillation characteristics of the voltage control oscillator. Therefore, the fluctuation of the oscillation frequency of the voltage control oscillator which is caused by the change of impedance of the modulation means can be reduced, and the fluctuation of output power can be also reduced. Further, the fluctuation of frequency changing characteristics of the frequency converter which is caused by the change of impedance of the modulation means can be reduced.

Claims: An FM-CW radar device for frequency-modulating a high frequency signal with a modulation signal, transmitting the modulated high frequency signal, receiving a signal reflected by a target object, and modulating the reflection signal with a signal provided by branching a portion of the transmitting signal, said FM-CW radar device comprising: a radar transmitting section (1, 2) providing a high-frequency transmitting signal frequency-modulated by a first frequency; and a modulation means (3) modulating the transmitting signal or the receiving signal with a second frequency which is far lower than the radar transmitting frequency; characterised in that said second frequency is twice as much as the sum or the difference of a Doppler frequency generated by relative velocity of the target object and a beat frequency generated by the propagation delay time corresponding to the target object, and in that the receiving signal reflected by the target object or modulated by the modulation means is frequency-modulated by a first frequency converting means (7) with a portion of the signal provided by the radar transmitting section (1, 2) and further the receiving signal is frequency-modulated by a second frequency converting means (8) with said second frequency. The FM-CW radar device of claim 1, which is portable, and wherein the modulation means (3) is disposed between the radar transmitting section (1, 2) and a transmitting antenna (4), said modulation means modulating the transmitting signal. The FM-CW radar device of claim 1, which is portable and, wherein the modulation means (3) is disposed between a radar receiving section and a receiving antenna, said modulation means modulating the receiving signal, and the modulated output signal of the modulation means is frequency-modulated by the first frequency converting means (7). The FM-CW radar device of claim 1, wherein the modulation means (3) is disposed between the radar transmitting section and a transmitting antenna (4), said modulation means modulating the transmitting signal, the device comprising a switching means (3) disposed between a radar receiving section which comprises the first and the second frequency converting means and a receiving antenna (6), said switching means conducting a switching operation with the same frequency as the second frequency on the transmission side, wherein the received signal is subjected to gating by said switching means, and the output signal of said switching means subjected to gating is frequency-modulated by the first frequency converting means (7). The FM-CW radar device according to any one of claim 1 to 4, wherein said modulation means is a switching means that is switched by said second frequency. The FM-CW radar device according to any one of claims 1 to 4, wherein said modulation means is an phase-modulated means that is phase-modulated by said second frequency. The FM-CW radar device according to any one of claims 1 to 4, wherein the second frequency is a rectangular wave having a duty factor of about 50%. The FM-CW radar device according to any of claims 1 to 4, wherein the second frequency is a rectangular frequency-modulated wave. The FM-CW radar device according to claim 7 or 8, wherein the period of the rectangular frequency-modulated wave used for the second frequency is variable. The FM-CW radar device according to claim 2, wherein a band pass filter is inserted between the first frequency converting means and the second frequency converting means. The FM-CW radar device according to any of claims 1 to 4, wherein a band pass filter for passing the second frequency is inserted between a modulation signal generation means for generating said second frequency and said second frequency converting means. The FM-CW radar device according to claim 2, wherein an isolator is inserted between said modulation means and the radar transmitting section. The FM-CW radar device according to claim 3, wherein an isolator is inserted between said modulation means and said first frequency converting means. The FM-CW radar device according to claim 4, wherein an isolator is inserted between said switching means and said first frequency converting means. The FM-CW radar device according to any one of claims 4 to 6, wherein said switching means is constituted as a balanced type switching means. The FM-CW radar device according to claim 4 or any claim depending on claim 4, wherein a PIN diode is used for said switching means.

Inventors: ISAJI OSAMU, KAWASAKI YOSHIHIRO, KURIHARA HIROSHI, NINOMIYA TERUHISA, OHASHI YOJI, OKUBO NAOFUMI, SAITO TAMIO, ISAJI, OSAMU, KAWASAKI, YOSHIHIRO, KURIHARA, HIROSHI, NINOMIYA, TERUHISA, OHASHI, YOJI, OKUBO, NAOFUMI, SAITO, TAMIO, Kawasaki, Yoshihiro, c/o FUJITSU LIMITED, Kurihara, Hiroshi, c/o FUJITSU LIMITED, Ninomiya, Teruhisa, c/o FUJITSU LIMITED, Ohashi, Yoji, c/o FUJITSU LIMITED, Okubo, Naofumi, c/o FUJITSU LIMITED, Saito, Tamio, c/o FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU TEN LTD, FUJITSU LIMITED, FUJITSU TEN, LTD.
Application year: 1992
Main classification: G01S7
IPC codes: G01S13, G01S7