1. Field of the Invention
The present invention relates to a vehicle mounted radar apparatus capable of measuring a distance between vehicles. More particularly, the present invention is directed to such a vehicle mounted radar apparatus capable of preventing electromagnetic waves from being returned from a transmitter circuit to a receiver circuit.
2. Description of the Related Art
FIG. 17 is a block diagram conceptually representing a circuit arrangement of a conventional a vehicle mounted radar apparatus.
FIGS. 2 to 4 conceptually indicate operating characteristics of the conventional a vehicle mounted radar apparatus. Moreover, the operating characteristics shown in FIG. 2 may be commonly applied to a vehicle mounted radar apparatus according to the present invention (described later).
The radar apparatus depicted in FIG. 17 is a vehicle mounted radar apparatus of which the entire construction is made compact by using a common transmitter/receiver antenna. Thus, this type of radar apparatus can be readily mounted on a vehicle.
The conventional vehicle mounted radar apparatus of FIG. 17 includes an oscillator 1, a power divider 2, a transmitter amplifier 3 and a transmitter/receiver changeover switch 5. Also, the apparatus further comprises a common transmitter/receiver antenna 6 for transmitting electromagnetic waves toward a target (object) 7 and receiving them reflected therefrom, a receiver amplifier 8, a mixer 9, a filter 10, an automatic gain control amplifier 11, an A/D converter 12, and a signal processing circuit 13.
Next, a description will be made of the electromagnetic wave transmitting operation of the conventional a vehicle mounted radar apparatus with the above arrangement.
First, electromagnetic waves having a transmission frequency of, for example, ftx=76.5 GHz are output from the oscillator 1. The electromagnetic waves pass through the power divider 2 and are then amplified by the transmitter amplifier 3. Since the transmitter/receiver changeover switch 5 connects the transmitter amplifier 3 with the common transmitter/receiver antenna 6, the electromagnetic wave amplified by the amplifier 3 passes through the transmitter/receiver changeover switch 5, and thereafter output from the antenna 6.
Next, the electromagnetic wave receiving operation of this conventional vehicle mounted radar apparatus will be explained.
At the time elapsed from the commencement of the electromagnetic wave transmission by a pulse time width "Tg", for instance, Tg=33.3 ns (=1/30 MHz, equal to a distance of 5 m), the transmitter/receiver changeover switch 5 is switched to the reception side (namely, the position indicated in FIG. 17) so as to connect the antenna 6 with the receiver amplifier 8.
Also, the electromagnetic waves output from the antenna 6 to the surrounding environment are reflected from the target 7 existing at a position separated from the vehicle mounted radar apparatus by a distance "R". Hence, the reflected electromagnetic waves enter into the antenna 6 with a delay time "t" depending upon the distance R with respect to the transmitted electromagnetic waves (see FIG. 2).
When the target has a relative speed, the frequency of the received electromagnetic waves is Doppler-shifted by "fb" with respect to the frequency "ftx" of the transmitted electromagnetic waves, and the Doppler-shifted electromagnetic waves enter the antenna 6. Then, the electromagnetic waves entering from the antenna 6 are amplified by the receiver amplifier 8, and the amplified electromagnetic waves are mixed with electromagnetic waves supplied from the power divider 2 to the LO (local oscillator) 13 by the mixer 9. In consequence of the mixing, a beat signal corresponding to the Doppler shift fb (see FIG. 2) is output from the mixer 9. The beat signal output from the mixer 9 is filtered by the filter 10, the cut-off frequency of which is selected to be 30 MHz. The filtered beat signal is amplified by the AGC amplifier 11, and then the amplified beat signal is input into the A/D converter 12.
Next, a description will be made of a method for calculating the distance and relative speed of the target 7 with the signal processing circuit 13 based upon the data (beat signal) entered from the A/D converter 12.
Assuming that a speed resolution of, for instance, 1 km/h is desired, the resolution ".DELTA.f" of the Doppler frequency is calculated based upon the transmission frequency "ftx"=76.5 GHz whereas ##EQU1## Consequently, a measurement time of 7.06 ms is required.
In this case, when the maximum measurement distance is selected to be, for example, 150 m, the pulse repetition period becomes 33.3 ns.times.30=1 .mu.s. To obtain a speed resolution of 1 km/h with the vehicle mounted radar apparatus shown in FIG. 17, when 7060 pulses of the beat signal are acquired at every distance gate, as shown in FIG. 3, and all of the acquired data is processed using a fast Fourier transform (FFT) every distance gate, the Doppler shift "fb" is output at a certain distance gate (see FIG. 4).
In this case, both the distance and the relative speed can be calculated based upon the following formulae (2) and (3): EQU Distance=tg.times.n.times.C (2), EQU Relative speed=fb.times.C/2'f0 (3),
wherein the symbol "tg" indicates a distance gate time width (pulse time width); symbol "n" indicates a distance gate number; symbol "C" represents the light velocity; symbol "fb" represents a beat frequency; and symbol "f0" denotes the transmission frequency (76.5 GHz).
However, when electromagnetic waves are received by the conventional vehicle mounted radar apparatus, even though the transmitter/receiver changeover switch 5 connects the antenna 6 with the receiver amplifier 8, the electromagnetic waves which are produced from the oscillator 1 and further amplified by the transmitter amplifier 3 are transferred to the receiver amplifier 8 while these electromagnetic waves are attenuated by the transmitter/receiver changeover switch 5.
Generally, isolation of the transmitter/receiver changeover switch 5 is on the order of 20 dB. Consequently, electromagnetic waves having very high levels, as compared with the input levels of the waves reflected from the target 7 into the receiver amplifier 8, continuously enter the receiver amplifier 8.
As previously described, with the conventional vehicle mounted radar apparatus, the electromagnetic waves output from the oscillator 1 and leaking from the transmission system to the receiver system during reception of the electromagnetic waves adversely influence the electromagnetic waves entering the conventional vehicle mounted radar apparatus. Also, the input levels of the leaked electromagnetic waves are very high compared with the input levels of the incoming electromagnetic waves reflected from the target 7. Consequently, there is a problem in that the waves reflected from the target 7 which are originally required in the radar measurement can not be detected.
For example, the frequency spectrum of the waves reflected from the target 7, with a relative speed near zero, is completely overlapped by the frequency spectrum of the electromagnetic waves which are transmitted from the transmitter amplifier 3 to the receiver amplifier 8 via the transmitter/receiver changeover switch 5, so that the frequency spectrum of the waves reflected from the target 7 cannot be detected.
Also, in order to detect the waves reflected from the target 7, with a high relative speed, if the number of bits for the A/D converter 12 is increased to secure the desirable dynamic range, then the target 7 can be detected. However, in general, when the number of bits for the A/D converter 12 is increased, the cost thereof increases exponentially. Accordingly, there is a problem in that this is not a suitable means for the vehicle mounted radar apparatus.