The principle and application of millimeter wave FMCW radar ranging and speed measurement

Millimeter wave (millimeter wave) Electromagnetic waves with a wavelength of 1 to 10 millimeters are called millimeter waves. They are located in the overlapping wavelength range of microwaves and far-infrared waves, so they have the characteristics of both spectrums.

It has the following main features:

? Extremely wide bandwidth: It is generally considered that the millimeter wave frequency range is 26.5～300GHz, and the bandwidth is as high as 273.5GHz. More than 10 times the full bandwidth from DC to microwave. Even if atmospheric absorption is considered, only four main windows can be used when propagating in the atmosphere, but the total bandwidth of these four windows can reach 135 GHz, which is 5 times the sum of the bandwidth of each band below the microwave. This is undoubtedly very attractive today when frequency resources are tight.

? Narrow beam: The beam of millimeter wave is much narrower than that of microwave under the same antenna size. For example, a 12cm antenna has a beam width of 18 degrees at 9.4GHz, and a beam width of only 1.8 degrees at 94GHz. As a result, small targets that are closer together can be distinguished or the details of the target can be observed more clearly.

? Compared with lasers: the propagation of millimeter waves is much less affected by climate, and can be considered to have all-weather characteristics.

? Compared with microwave: the size of millimeter wave components is much smaller. Therefore, the millimeter wave system is easier to miniaturize.

For this reason, they are of great significance in communications, radar, guidance, remote sensing technology, radio astronomy, and spectroscopy. Using the millimeter wave frequency of the atmospheric window can realize large-capacity satellite-ground communication or ground relay communication. The narrow beam and low sidelobe performance of the millimeter wave antenna can be used to realize low-elevation precision tracking radar and imaging radar. When a long-range missile or spacecraft returns to the atmosphere, it is necessary to use millimeter waves that can smoothly penetrate the plasma to achieve communication and guidance. The high-resolution millimeter wave radiometer is suitable for remote sensing of meteorological parameters. Using millimeter-wave and sub-millimeter-wave radio telescopes to detect the radiation spectrum of space can infer the composition of interstellar matter.

Millimeter Wave Radar (Millimeter Wave Radar) application

On the surface, it seems that the application range of the millimeter wave system and the microwave system are roughly the same. But in fact the performance of the two is very different, the advantages and disadvantages are just the opposite. Therefore, millimeter wave systems often form complementary systems with microwave systems. The following describes the progress of various applications. The advantages of millimeter-wave radar are high angular resolution, wide frequency bandwidth, which facilitates the use of pulse compression technology, large Doppler and small system size. The disadvantage is that due to the large atmospheric absorption, when a large operating distance is required, the required transmit power and antenna gain are higher than those of a microwave system. The following are some typical application examples.

Space Target Recognition Radar: They are characterized by the use of large antennas to obtain the required angular resolution and sufficiently high antenna gain for imaging, and the use of high-power transmitters to ensure range. For example, a space target recognition radar operating at 35GHz has an antenna diameter of 36m. With a traveling wave tube to provide 10kw transmission power, you can take pictures of satellites as far away as 16,000km. The antenna diameter of a space target recognition radar operating at 94GHz is 13.5m. When the echo tube is used to provide 20kw transmission power, it can take high-resolution imaging of targets at a distance of 14,400km.

Automobile anti-collision radar: Because its operating distance does not need to be very far, the output power of the transmitter does not need to be high, but it requires a high range resolution (up to meter level), and it must be able to measure speed, and the volume of the radar must be As small as possible. Therefore, a millimeter wave pulse Doppler radar with a solid-state oscillator as a transmitter is used. The pulse compression technology is used to compress the pulse width to the nanosecond level, which greatly improves the range resolution. Velocity value obtained by using the relatively large characteristic of millimeter wave Doppler.

Helicopter Prevention and Control Radar: In modern helicopter accidents, accidents caused by collisions between aircraft and high-voltage overhead cables account for a very high rate. Therefore, the helicopter prevention and control radar must be able to detect high-voltage overhead cables with a thinner wire diameter, and a short-wavelength radar with a higher resolution is required. In practice, a 3mm radar is often used.

Precision tracking radar: The actual precision tracking radar is mostly a dual-frequency system, that is, a radar can simultaneously work in the microwave frequency band (long range but poor tracking) and millimeter wave band (high tracking but shorter range), both Complementarity has achieved better results. For example, the dual-frequency precision tracking radar developed by the U.S. Navy has a 9GHz, 300kw transmitter and a 35GHz, 13kw transmitter and corresponding receiving system, sharing a 2.4m parabolic antenna, and has successfully tracked 30m above the water. The target has a range of up to 27km. The double amount also brings an additional benefit: the millimeter wave frequency can be used as a concealed frequency to improve the anti-jamming capability of the radar.

The working principle of the car's active anti-collision

The automobile anti-collision system is very important to improve the safety of automobile driving, and the research of this system has always been paid much attention. Since 1971, there have been many kinds of active vehicle anti-collision systems such as ultrasonic, laser, infrared, microwave, etc., but the above systems have some shortcomings, and they have not been widely promoted and applied in automobiles. With the rapid development of highway networks in various countries, vicious traffic accidents continue to increase. In order to reduce accidents, protective measures such as driving seat belts and airbags have been adopted. However, these technologies are passive protection and cannot fundamentally solve the problem. The millimeter-wave RF bandwidth is large, the resolution is high, the antenna component size is small, and it can adapt to harsh environments. Therefore, the millimeter-wave radar system has the characteristics of light weight, small size and all-weather. The "active automotive millimeter-wave anti-collision radar system" has become an international The research and development hotspots have been launched, and existing products have begun to be put into the market. The prospects are very promising.

Active car collision avoidance is based on radar ranging and speed measurement. The anti-collision radar system monitors the front of the vehicle in real time. When there is a dangerous target (such as a stopped or slow-moving vehicle), the radar system sends an alarm to the driver in advance so that the driver can react in time. At the same time, the radar output signal reaches the car control system. Automatic braking or deceleration according to the situation.

There are two types of millimeter wave anti-collision radar systems: frequency modulated continuous wave (FMCW) radar and pulse radar. For pulse radar systems, when the target is very close, the time difference between the transmitted pulse and the received pulse is very small. This requires the system to adopt high-speed signal processing technology. The short-range pulse radar system becomes very complicated and the cost increases significantly. Therefore, the automotive millimeter-wave radar collision avoidance system often adopts a frequency modulated continuous wave radar system that is simple in structure, low in cost, and suitable for short-range detection.

The RF transceiver front end is a component of the radar system. A lot of in-depth research on the front-end has been done at home and abroad, and considerable progress has been made. Various structure front ends have been developed, mainly including waveguide structure front ends, microstrip structure front ends and monolithic integration of front ends. The RF front-end developed in China is mainly a waveguide structure front-end. A typical RF front-end mainly includes three parts: linear VCO, circulator and balanced mixer. The intermediate frequency signal output by the front-end mixing is amplified by the intermediate frequency and sent to the subsequent data processing part. The basic goal of the data processing part is to eliminate unnecessary signals (such as clutter) and interference signals, and process the mixed signal after intermediate frequency amplification, and extract information such as target distance and speed from the signal spectrum.

Millimeter wave FMCW radar ranging and speed measurement principle

The radar system transmits a series of continuous frequency modulation millimeter waves through the antenna and receives the reflected signal from the target. The frequency of the emitted wave changes with time according to the law of the modulation voltage. Generally, the modulation signal is a triangular wave signal. The shape of the reflected wave is the same as the transmitted wave, but there is a delay in time. The frequency difference between the transmitted signal and the reflected signal at a certain moment is the frequency of the intermediate frequency signal output by the mixer, and the target distance is proportional to the intermediate frequency frequency output by the front end . If the reflected signal comes from a relatively moving target, the reflected signal includes a Doppler shift caused by the relative motion of the target. According to the Doppler principle, the target distance and relative speed of the target can be calculated.

Developed active anti-collision millimeter wave radar for vehicles

Bosch recently announced the millimeter wave radar LRR (Long Range Rader) 3 using SiGe technology. The millimeter-wave radar developed this time consists of a 77GHz band MMIC (Monolithic Microwave Integrated Circuits) chipset, 4 patch antennas and a dedicated ASIC. The chipset consists of two chips for sending and receiving, and both chips use SiGe technology. The detectable range of millimeter wave radar is 0.5m～250m. The detection angle range is 30 degrees at a distance of 30m.

Bosch said that through the use of SiGe technology, it can reduce costs compared to the previous MMIC technology. In the future, it is expected to be equipped with two millimeter wave radars on the vehicle, and additional functions can be added. The company is equipped with two millimeter-wave radars in front of the vehicle and announced the vehicle test results-the detection angle range has been expanded to 60 degrees at a distance of 30m.

Compared with only one millimeter wave radar, the two millimeter wave radars improve the detection of sharp turns, and can more accurately capture the vehicle ahead and the guardrail on the roadside. In the actual vehicle test, the vehicle ahead can be accurately identified on a road with a radius of curvature of 35m. The company said that the equipment can improve the accuracy of the ACC (Adaptive Cruise Control System) for low-speed tracking. In addition, various functions can be added in the future, such as detecting the shape of the curve by detecting the guardrail on the roadside, and matching the yaw moment of the vehicle to prevent sideslip.

Hitachi Manufacturing Co., Ltd. has recently developed two smaller vehicle-mounted millimeter-wave radars, which use the 76GHz frequency band and have a detection distance of up to 200m.

is used for long-distance detection (detection range 1m-127m) millimeter wave radar, the size is 100mm wide×80mm long×30mm thick. Compared with the original model, the thickness and volume of the module have been reduced to approximately 1/3 and 1/4 respectively. In addition, the millimeter-wave radar (detection range 0.1m-25m) used for short-distance detection has expanded the detection angle from ±15 degrees for long-distance detection radars to ±35 degrees by improving the antenna.

millimeter wave radar is mainly composed of antenna, high frequency circuit and signal processing part. In order to reduce the thickness of the millimeter wave radar, Hitachi has improved the high-frequency circuit and signal processing part. By encapsulating the MMIC chip on the multilayer printed circuit board, the volume is reduced, which is comparable to the original radar using a single-layer printed circuit board. In comparison, the packaging density of high-frequency components is greatly improved. While improving the performance of the microprocessor, by increasing the storage capacity of the mixed-load memory, all processing is concentrated on one microprocessor. Due to the reduction of the microprocessor, the internal heat produced by the signal processing part is also reduced, thereby increasing the packaging density of the components, which also contributes to the miniaturization of the signal processing part.

Denso announced that it has developed a millimeter-wave radar that is half the size of the original model and greatly reduces the cost. Using high-performance signal processing technology, by reducing the number of antenna receiving channels, miniaturization of antennas and transceiver components are realized. By connecting the antenna and the transmitting and receiving element, the waveguide that transmits the radio wave and the transmitting and receiving antenna are integrated, which realizes the miniaturization and cost reduction of the product.