foreword
Millimeter waves refer to radio waves with a wavelength of 1 ~ 10mm. The propagation speed of our electromagnetic waves in the air is c=3x10^8 m/s, so the frequency of millimeter waves can be calculated as 30~300GHz.
According to different working modes, it can be divided into two types: "pulse" and "continuous wave". The basic principle of pulse-type millimeter-wave radar is similar to that of laser radar, both of which are based on the TOF method. Continuous wave types can be divided into CW constant frequency continuous wave, FSK frequency shift keying continuous wave and FMCW frequency modulation continuous wave. CW is used for speed measurement, FSK can detect the distance and speed of a single target, and FMCW can detect the distance and speed of multiple targets, so FMCW can be widely used in vehicle-assisted driving.
Here, the FMCW frequency modulation continuous wave radar is mainly studied, and the principles of its distance measurement, speed measurement and angle measurement are described below.
Fundamental
Chirp signal
Refers to the sine wave whose signal frequency increases linearly with time, as shown in the figure below
We change it to the frequency-time diagram, and we can see that it is a straight line with a slope S. The initial moment fc=77GHz, spanning the bandwidth of 4GHz in the time of Tc 40μs, B and S are the important parameters that define the system performance.
Let's take a brief look at the working process of FM continuous wave millimeter-wave radar: first, the synthesizer generates a Chirp signal, and then transmits it through the TX antenna, and at the same time sends a piece of data to the mixer, and receives it back through the RX antenna after encountering an object reflection. The wave signal generates an intermediate frequency signal in the mixer.
The mixer works as follows:
① The synthesizer generates a Chirp signal
② The TX antenna transmits
③ The RX antenna receives the reflected chirp pulse
④ The mixer mixes the TX and RX signals to generate an intermediate frequency IF (intermediate frequency) signal
For the two input sine waves, the angular velocity of the output sine wave, that is, the frequency, is the difference between the two input signals, and the phase is also the difference between the two input signals.
IF intermediate frequency signal and single target ranging calculation
In the figure above, TX is the transmit signal, and RX is the receive signal. There is a time delay τ between the two.
It has been said before that the function of the mixer is introduced: the IF signal generated by the mixer is directly subtracted from the two input signals in terms of frequency and phase, so it can be represented by the ft diagram below, that is, a constant frequency straight line.
The time delay τ is the round-trip time from the object to the radar to the object, which can be expressed by
