Introduction to the Principle of Frequency Modulated Continuous Wave (FMCW) Radar

Pulse radar and frequency modulated continuous wave (FMCW) radar are two classifications of radar. The following is a technical analysis of the working principle of frequency modulated continuous wave (FMCW) radar. 1. Radar classification Radar is divided into pulse radar and continuous wave radar according to the type of transmitted signal. Conventional pulse radar transmits periodic high-frequency pulses, and continuous wave radar transmits continuous wave signals. The working principle of pulse Doppler radar can be described as follows: When the radar transmits a fixed frequency pulse wave to scan the air, if it encounters a moving target, the frequency of the echo and the frequency of the transmitted wave appear frequency difference, which is called Doppler frequency. According to the size of the Doppler frequency, the relative radial movement speed of the target to the radar can be measured; according to the time difference between the transmitted pulse and the received, the distance of the target can be measured. At the same time, the Doppler frequency spectrum line of the target is detected by the frequency filtering method, and the spectrum line of the interference clutter is filtered out, so that the radar can distinguish the target signal from the strong clutter. Therefore, the pulse Doppler radar has stronger anti-clutter ability than ordinary radar, and can detect moving targets hidden in the background. The signal transmitted by the continuous wave radar can be single frequency continuous wave (CW) or frequency modulated continuous wave (FMCW). Among them, single-frequency continuous wave radar can only be used for speed measurement, but cannot measure distance, while FMCW radar can measure both distance and speed, and its advantages in short-range measurement are becoming more and more obvious. 2. Frequency modulation continuous wave radar The signal transmitted can be single frequency continuous wave (CW) or frequency modulation continuous wave (FMCW), and there are many kinds of frequency modulation methods, such as triangular wave, sawtooth wave, code modulation or noise frequency modulation. Among them, single-frequency continuous wave radar can only be used for speed measurement, but cannot measure distance, while FMCW radar can measure both distance and speed, and its advantages in short-range measurement are becoming more and more obvious. 3. Working principle FMCW radar transmits a continuous wave whose frequency changes during the frequency sweep period. The echo reflected by the object has a certain frequency difference with the transmitted signal. The distance information between the target and the radar can be obtained by measuring the frequency difference. The frequency of the frequency signal is relatively low, generally KHz, so the hardware processing is relatively simple, suitable for data acquisition and digital signal processing. 4. Advantages and disadvantages of FMCW At the same time of transmitting and receiving FMCW radar, in theory, there is no ranging blind spot that pulse radar exists, and the average power of the transmitted signal is equal to the peak power, so only low-power devices are needed, thereby reducing the probability of interception and interference. FMCW radar has the advantages of easy implementation, relatively simple structure, small size, light weight and low cost, and has been widely used in civil/military fields. The disadvantage is that the ranging range is short, the distance Doppler coupling and the transceiver isolation are difficult. 5. FMCW radar block diagram If the FM continuous wave radar is composed of a transceiver and a control unit with a microprocessor, if the transceiver uses a single antenna for simultaneous transmission and reception, the FMCW radar needs a ferrite circulator to separate the transmit and receive signals , which requires higher isolation. Of course, if a patch antenna with separate transceivers is used, the cost will be relatively lower. The high-frequency signal is generated by a voltage-controlled oscillator (VCO), part of which is additionally amplified by the power divider and fed to the transmitting antenna, and the other part is coupled to the mixer, mixed with the received echo, and low-pass filtered to obtain the baseband. The difference frequency signal is sent to the microprocessor for processing after analog-to-digital conversion. 6. The principle of FMCW radar ranging/velocity measurement The principle of ranging/velocity measurement of radar is briefly introduced by taking the triangular wave frequency modulated continuous wave as an example. As shown in the figure below, red is the frequency of the transmitted signal, green is the frequency of the received signal, the frequency sweep period is T, and the frequency sweep bandwidth is B. The transmitted signal is transmitted through the target, and the echo signal will have a delay. In the frequency change of the triangle, it can be Distance measurements are made on both rising and falling edges. If there is no Doppler frequency, the frequency difference during the rising edge is equal to the measurement during the falling edge. For a moving target, the frequency difference during the rising/falling edge is different, and we can measure distance and speed through these two frequency differences. The beat signal is sent to the digital signal processor after low-pass filtering and amplification to complete the FFT and detection of the beat signal, and the target data is calculated and then sent to the display and control terminal for display. The triangular-wave frequency-modulated continuous wave radar uses positive and negative frequency-modulated slopes to eliminate the coupling between distance and speed, and then estimate the target speed.