1. Field of the Invention
The present invention relates to a CW (continuous wave) radar and, more particularly, to a CW radar loadable in a mobile object such as a car to effectively prevent collision thereof.
2. Description of the Related Art
This CW radar is loaded in a mobile object such as a car and detects its relative velocity to a target such as a preceding car or an obstacle by irradiating electric waves of a high frequency band and receiving the reflected waves. There is another type known as an FM (frequency modulation)-CW radar which detects both the relative velocity and the relative distance to a target by irradiating frequency-modulated high frequency waves to the target and receiving the reflected waves therefrom. It is desired that any of such CW radars meets various requirements including low noise, a small-sized structure and low production cost. These requirements are remarkably increasing, particularly in the car-loadable type which has recently indicated urged needs.
FIG. 1 is a block diagram of a conventional CW radar which will now be described below. It is assumed here that this CW radar is loaded in a car. The CW radar shown in this diagram is a type utilizing Doppler shift to measure the relative velocity by detecting a beat frequency (FIG. 4) which represents the frequency difference between a CW transmitted signal of a fixed frequency (FIG. 2) and a reflected wave (FIG. 3) of which frequency has a deviation due to the Doppler shift. FIGS. 2 to 4 graphically show the relationship between the signal frequency f and the time t, where a signal S2 in FIG. 3 is formed with a frequency shift of a signal S1 shown in FIG. 2, and a signal S3 in FIG. 4 is obtained on the basis of the difference between the signals S1 and S2.
The CW radar shown in FIG. 1 comprises an oscillator 1, a hybrid circuit 2, a transmitting antenna 3, a receiving antenna 4, a mixer 5, and a signal processor 6 including an MPU (microprocessor). The oscillator 1 generates an RF signal S1 of a fixed frequency shown in FIG. 2. The RF signal S1 is branched by the hybrid circuit 2, and one signal S1 thereof is outputted forward as a radio wave M1 from the antenna 3 via an unshown transmitter. The radio wave M1 thus outputted is reflected by, e.g., an unshown preceding car and then is returned as a reflected wave M2. The reflected wave M2 is received by the antenna 4 and is inputted to an unshown receiver. An output signal S2 of the receiver having a frequency shown in FIG. 3 is mixed by a mixer 5 with another signal S1' outputted from the hybrid circuit 2. A signal S3 of a frequency shown in FIG. 4 is produced as a result of such mixing and then is outputted to the signal processor 6, which calculates the relative velocity between the radar-loaded car and the preceding car on the basis of the signal S3.
An FM-CW radar will now be described below with reference to FIG. 5. It is supposed in this case also that the FM-CW radar is loaded in a car. The FM-CW radar of FIG. 5 detects the relative velocity and the relative distance by detecting a beat frequency (FIG. 8) which represents the difference between a frequency-modulated transmitted signal (FIG. 6) and a reflected wave thereof (FIG. 7). The reflected wave is a result of combining a signal (FIG. 9), which is produced from the transmitted signal of FIG. 6 with a frequency deviation caused due to the Doppler shift, with a signal (FIG. 10) produced from the transmitted signal with a phase delay due to the propagation distance.
The FM-CW radar shown in FIG. 5 comprises a modulating signal generator 11, a voltage controlled oscillator 12, a hybrid circuit 13, a transmitting antenna 14, a receiving antenna 15, a mixer 16, and a signal processor 17 including an MPU (microprocessor). The modulating signal generator 11 generates a triangular modulating signal S5 of a predetermined frequency and supplies the same to the voltage controlled oscillator 12. The modulating signal S5 is used for frequency-modulating the oscillation signal obtained from the voltage controlled oscillator 12. More specifically, the voltage controlled oscillator 12 produces a signal S6 of FIG. 6 which is a result of frequency-modulating the oscillation signal with the triangular modulating signal S5. The frequency of the output signal S6 obtained from the voltage controlled oscillator 12 is varied by changing the level of the triangular modulating signal.
The signal S6 thus obtained is branched in the hybrid circuit 13, and one signal S6 thereof is outputted forward as a radio wave M3 from the antenna 14. The outputted radio wave M3 is reflected by, e.g., a preceding car and then is returned therefrom as a reflected wave M4. The reflected wave M4 is received by the antenna 15, from which a signal S7 shown in FIG. 7 is obtained. Subsequently this signal S7 is mixed in the mixer 16 with the other signal S6' (FIG. 6) outputted from the hybrid circuit 13. A signal S8 of FIG. 8 obtained through such mixing is outputted to the signal processor 17. Since the signal S8 is a mixture of the Doppler frequency component obtained from the reflected wave M4 and the frequency component outputted from the voltage controlled oscillator 12, it becomes possible to compute the relative velocity and the relative distance between the radar-loaded car and the other car by detecting and calculating the frequency component in the signal processor 17.
In the conventional FM-CW radar shown in FIG. 5, there exists a problem that since the oscillation signal from the voltage controlled oscillator 12 is frequency-modulated, AM (amplitude modulation) noise is generated to consequently fail in achieving proper calculations of the relative velocity and the relative distance. As the oscillation signal of the voltage controlled oscillator 12 has frequency characteristics, it follows that an AM component corresponding to the oscillation signal is produced in the component element of the mixer 16. The frequency of such AM component caused with frequency modulation is approximately equal to the beat frequency, which is hardly cut by means of a filter or the like and is thereby left as a noise component. Meanwhile, if any other means is contrived to suppress such AM noise, the whole structure of the radar is complicated and rendered greater in scale to eventually become unpractical in use.
Since the CW radar shown in FIG. 1 is capable of detecting merely the relative velocity alone, there arises a problem that it is impossible to detect whether the target is relatively separating or approaching. In the CW radar of FIG. 1, the frequency f0 of the transmitted signal S1 has such a frequency spectrum as shown in FIG. 11. In this diagram, f1 denotes the frequency of the received signal S2 when the target is relatively approaching, and f2 denotes the frequency of the received signal S2 when the target is relatively separating. As is obvious from FIG. 11, the frequency f1 of the received signal deviates to be higher when the target is relatively approaching, while the frequency f2 of the received signal S2 deviates to be lower when the target is relatively separating.
However, the frequency f3 of a signal S3 of FIG. 12 obtained by processing the received signal S2 in the mixer 5 represents the mere difference between the frequency f0 of the transmitted signal S1 and the frequency f1 or f2 of the received signal S2. Therefore it is impossible to discriminate between the frequency at the time of relative approach and the frequency at the time of relative separation.