Millimeter wave ranging radar principle

Millimeter-wave radar ranges by transmitting a continuous wave with varying frequency in the sweep cycle, the echo reflected by the object has a specific frequency difference with the transmitted signal. The distance information between the target and the radar can be obtained by measuring the frequency difference²⁴. There are two general ways of transmitting wave modulation: triangle wave and sawtooth wave modulation. Triangular wave modulation consists of two symmetrical linear FM continuous waves divided into upper and lower sweeps. Using the nature of the spectral symmetry of the upper and lower swept differential beat signals, the distance and speed are decoupled, and the distance to the target can be determined in one cycle using the differential beat Fourier approach. The way processing is simple and easy to implement. For sawtooth wave modulation, the Doppler shift and the frequency at relative rest cannot be solved in one cycle, and several cycles of continuous observation are needed for the solution. Therefore, the sensor module selected in this paper is HLK-LD303-24G ranging radar module, which adopts the triangular wave modulation method.

Figure ​Figure22 shows the principle diagram of millimeter wave ranging with triangular wave modulation. When the target is at rest, there is a time delay between the transmit signal and the return signal, and the time delay time is τ . Its value is τ=2Dc, D is the distance of radar from the target object c is the propagation speed of the electromagnetic wave. The radar mixes the transmit signal with the echo signal to get the differential beat signal, and the differential beat signal gets the differential beat frequency through signal processing, and the frequency of the differential signal between the transmit signal and the echo signal is fb during the mixing process. When the target is in motion, due to the existence of the Doppler effect, the echo signal of the moving target will produce a Doppler shift fd. At this time, the frequency of the output differential signal after mixing is shown in equations:

In Fig. ​Fig.2,2, B denotes the FM bandwidth, fb+ denotes the difference frequency at the rising edge of the triangular wave, fb- denotes the difference frequency at the falling edge of the triangular wave, and fd denotes the Doppler frequency shift. According to the principle, the following relationship exists for the distance D of the detected target.

Combining equations (1), (2) and (3), the formula for the distance can be obtained as follows.

Principle diagram of millimeter wave distance measurement.