Abstract:
In a Doppler radar system for measuring the velocity of a vehicle, such as an automobile, in order to lessen interference with external electric wave appliances, such as radios and televisions, caused by transmitting microwaves, one of the higher harmonics generated from a mixer diode driven by the fundamental waves of a local oscillator is selected by a filter for transmission and is used as the transmission output wave, whereby the power of the transmission waves is remarkably reduced in comparison with the mixer driving power.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a short-range Doppler radar; and more particularly, it relates to a Doppler radar system of the type which may be carried by a vehicle, such as an automobile, or any vehicle in a broad sense so as to measure the velocity thereof by radiating a microwave signal and detecting the Doppler frequency shift undergone by the reflected signal. Especially, it relates to the construction of a transmitting and receiving device for microwave signals. 
     2. Description of the Prior Art 
     In short-range radars, such as a Doppler radar system for measuring the true ground speed of an automobile, the transmission power should be kept to a very low power level (1-10 μW or less) from the viewpoint of the possible influence of such signal on other radio equipment which may be within the range of the radar signal. On the other hand, the receiving sensitivity of the system ought to be made as high as possible to ensure a high measuring accuracy. 
     A Doppler radar utilizing microwave signals is so constructed that in order to detect the Doppler frequency shift of received reflected waves, a mixing circuit which mixes the received waves and local oscillation waves is provided so as to detect the difference component of the two frequencies from the output thereof. In order to make a device for use in an automobile as small in size as possible, a transmitter/receiver for the microwave signal is constructed of a microwave integrated circuit, and a mixer diode is used as the mixing circuit. 
     Usually, in order to operate the mixer diode at a high sensitivity, approximately 1 mW needs to be applied as a driving power. In this case, a special balanced mixer or the like comprising a pair of diodes may be employed for the purpose of reducing the microwave leakage power. However, there has been the problem that when the characteristics of the two diodes which constitute the balanced mixer degrade at an unequal rate, the transmission power varies sharply. 
     SUMMARY OF THE INVENTION 
     It is accordingly the principal object of this invention to provide a Doppler radar system which produces a microwave signal with a low level transmission power and simultaneously exhibits a high receiving sensitivity. 
     Another object of this invention is to provide a Doppler radar system which produces a microwave signal having a stable transmission power even when the characteristics of the mixer diode and other circuit constituent elements thereof have degraded with time. 
     In order to accomplish these objects, the microwave Doppler radar system according to this invention is constructed of an oscillator, a frequency multipler device which is driven by a fundamental frequency component of said oscillator, a filter circuit which selects one of a plurality of higher harmonic components generated by said frequency multiplier device, an antenna which radiates the selected frequency component to the outside and which receives reflected waves of the radiated frequency component, and a circuit which takes out a low frequency component generated when the received reflected waves are applied to said frequency multiplier device. 
     More specifically, the frequency multiplier device is constructed of a mixer diode or a Gunn oscillator device. A plate including a microwave integrated circuit which is formed of elements, such as the mixer diode and micro strip lines for applying the output of the oscillator to the element and for taking out the low frequency component in the form of a mixing output from the element is installed inside a metallic package; and, a window is provided in a part of the package so as to couple the window to a horn antenna. 
     According to the Doppler radar system of this invention, notwithstanding the fact that the mixer diode forming the frequency multiplier device is driven with a high power as is necessary for its operation, the power of the microwave signal to be radiated can be remarkably reduced in comparison with with the driving power of the mixer because one of the higher harmonics of the mixer diode is selected by the filter and radiated rather than the fundamental frequency. 
     The above-mentioned and other objects and features of this invention will become more apparent from the following description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing the construction of an embodiment of a short-range Doppler radar according to this invention; and 
     FIG. 2 is a constructional view of the essential portions of another embodiment of the short-range Doppler radar according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a package (Doppler radar module) 1 made of a metallic wall in which a dielectric substrate 4 forming a micro-integrated circuit is installed, the wall being partially broken away in order to display the interior. 
     A fundamental wave oscillator portion is constructed of a Gunn diode 2 which is disposed within the package, and a bias circuit which consists of a direct current power supply DC, a connector 9 and a strip line 5 formed on the substrate 4, the three components being connected in series in order to apply a bias voltage as necessary for the oscillation of the Gunn diode. The oscillation signal of the Gunn diode 2 has its fundamental frequency f o  determined by the strip line 5 and a half wavelength bending type strip line resonator 6. The power of the fundamental wave oscillation signal is applied to a mixer diode 11 via a strip line 10. 
     A low-pass filter bias circuit 7 and a no-reflection terminating ciruit 8 for the fundamental wave prevent higher harmonic components and the fundamental wave component from passing onto the DC bias side. In addition, the back surface of the substrate 4 is covered with a conductive layer, except for a cutout portion 13. 
     The mixer diode 11 functions as a frequency multiplier device, and when driven by the fundamental wave f o , it generates a higher harmonic signal which is an integral number of times higher in frequency than f o . In general, the power of the higher harmonic signal lowers as the order of the harmonic increases. 
     Since the wall of the package 1 and the conductor formed on the rear surface of the substrate 4 at a location corresponding to the position of the mixer diode 11 are removed as indicated by the dotted line 13, a dielectric window is formed, and a rectangular waveguide 14 having a section indicated by the dotted line 13 is coupled thereto. A strap 12 short-circuits one terminal of the mixer diode 11 for the fundamental wave, and straps 15 and 16 short circuit both terminals of the mixer diode 11 to the conductor on the rear surface of the substrate 4 at a desired transmitting harmonic frequency n·f o  (where n denotes an integer) generated by the mixer diode 11. A strap 17 matches the input impedance of the mixer diode 11 to the fundamental wave f o . 
     The output waveguide 14 is cut off for frequencies lower than the transmitting wave n·f o , and operates as a high pass filter. The window 13 is selectively matched to the transmitting harmonic frequency n·f o , and attenuates unwanted harmonics in a higher frequency band. Accordingly, the output waveguide 14 transmits only the intended harmonic output, and it radiates microwaves of low power to the outside through a horn antenna 23. The radiated microwaves are reflected by a reflector, such as the road surface, and undergo a Doppler frequency shift Δf to become a frequency component (n·f o  +Δf). The reflected signal is returned to the mixer diode 11 via the horn antenna 23 and the waveguide 14. 
     The reflected waves from the reflective object have been subjected to the so-called Doppler frequency shift, and the frequency difference Δf of the transmitted and received waves proportional to the velocity of the microwave transmitting and receiving portion with respect to the stationary reflective object is detected by the mixer diode 11. The received signal is applied from an output signal terminal 20 to a load resistor 21 by means of a direct-current return circuit 18 and a strip line 19 and is detected as a Doppler signal. 
     In the present embodiment, the fundamental wave oscillation frequency f o  is set at 12 GHz, and the 24 GHz transmitting wave output, at the time when the input power of the mixer diode is 0.5 mW and the load resistance is 1 kΩ, is approximately 10 μW. The fine adjustment of the transmitting power can be made, not only by adjusting the driving power of the mixer or the bias voltage of the Gunn diode 2, but also by adjusting the resistance of the load resistor 21. The sharp adjustment of the transmitting power depends upon the selection of the multiplication order n, and the reduction of the transmitting power is facilitated by utilizing a high-order high frequency. 
     On the other hand, when the multiplication order n is made very great, the receiving conversion loss of the harmonic mixer increases. Therefore, it is usually suitable to set the multiplication order in the range of n=2 to 5. 
     The Doppler radar system of this invention utilizes the conversion loss of the multiplier for the reduction of the transmitting power from the mixer driving power, and is therefore characterized in that the variation of the transmitting power with time is much smaller than the fluctuation of the leakage power in the conventional balanced mixer system employing the same frequency for transmission. 
     A further advantage of the Doppler radar system of this invention is that the oscillation frequency can be made low with respect to the intended transmitting frequency. In general, an oscillator device of lower frequency is easier to manufacture and more stable in operation than one of higher frequency. 
     That is, according to this invention, a millimeter wave Doppler radar system can be constructed by employing an inexpensive microwave oscillator device without any expensive millimeter wave oscillator device or any special varactor diode for multiplication. 
     In addition, according to this invention, a microwave or millimeter wave Doppler radar system of high frequency can be constructed by employing an inexpensive transistor of low frequency band as the oscillator device. 
     FIG. 2 is a diagram of a circuit structure showing another embodiment of the short-range Doppler radar system according to this invention. The present embodiment is simplified in such a way that, in place of the mixer diode, the Gunn diode 2 which serves to generate the oscillation signal also carries out the multiplication of the fundamental wave for transmission and the harmonic mixing for reception. The remaining parts are the same as in the embodiment of FIG. 1. 
     The harmonic waves n·f o  of the Gunn diode 2, which operates at the fundamental wave oscillation frequency f o  determined by the strip line resonator 6, are transmitted from the output waveguide 14 as the transmitting power. Received waves are directly applied to the Gunn diode 2, and a Doppler signal detected by the nonlinearity of the Gunn diode 2 is provided across the load resistor 21. For the load resistor 21, a resistance which is not very high in comparison with the DC resistance of the Gunn diode 2 ought to be selected, and a value on the order of 10Ω is suggested. For a capacitor 22, a capacitance of at least 100 μF is employed in order to make the impedance low for the Doppler signal of low frequency. 
     As set forth above, according to this invention, a Doppler radar system having a transmission power which is low in comparison with the mixer driving power can be constructed with a simple circuit structure, and it is easy to stablize the transmission output. 
     The Doppler radar system of this invention has its oscillation frequency harmonically coupled with the transmitting output frequency. Therefore, the coupling between the output circuit and the oscillator is low, and the oscillator is extraordinarily stable against external signals or load fluctuations. 
     While we have shown and described several embodiments in accordance with the present invention, it is to be understood that the invention is not limited thereto but is susceptible of numerous changes and modifications as are known of those of ordinary skill in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to those skilled in the art.