TEST, SAFETY & SYSTEMS
Figure 1
beat signal, the distance to an object and its velocity can be determined. Figure 1 shows the signal modulation schemes for automotive radar. Unlike pulse radar, FMCW offers
benefits such as low transmission power and high signal-to-noise ratio. Furthermore, the relatively low response frequency of the transceiver circuit enables a simple design, which reduces costs. Consequently, the FMCW method is widely used in automotive radar.
FMCW BASICS A signal for which the frequency increases linearly with time is known as a chirp, shown in Figure 2 (a), and is key to the performance of the FMCW signal. From the chirp signal shown in Figure 2 (b), with the vertical axis replaced by frequency, the range or distance resolution and maximum distance range are obtained. These are the main performance characteristics of the FMCW radar. The range resolution Dres is expressed by Dres = c/2B = c/2STc, where c is speed of light, B is the chirp bandwidth (end stop frequency - start frequency), S is the chirp slope, and Tc is the chirp duration. From the equation, the wider the chirp bandwidth, the higher the
Figure 2
resolution that can be detected. For example, the range resolution is about
7.cm for a chirp bandwidth of 2GHz, and the range resolution is about 3.8cm for a chirp bandwidth of 4GHz. The maximum detection range is inversely proportional to the chirp slope S, which represents the rate of increase in frequency. This means that the smaller the chirp slope the greater the maximum detection range. For a fixed chirp duration, a wider bandwidth B will result in a higher resolution. However, this results in a trade-off as the maximum detection range is reduced because the chirp slope increases with bandwidth. This trade-off requires caution when designing automotive radar systems. Automotive millimetre-wave
radar will prioritise detection range or resolution depending on the application. For example, in adaptive cruise control the ability to detect a vehicle at long range is important, while high resolution is not so necessary. On the other hand, collision avoidance requires high resolution as the vehicle needs to respond rapidly to sudden changes at close range. From the chirp signal, the radar
velocity resolution Vres and the maximum detection velocity Vmax can also be calculated using Vres =
/2Tc and Vmax = /4Tc where radar wavelength = c/f. The maximum detection velocity
Vmax is inversely proportional to the chirp duration Tc. Reducing chirp duration increases maximum detection velocity. However, shortening chirp duration adversely affects the range resolution. A radar frame comprises a few to several hundred chirps. A chirp frame is depicted in Figure 3. The frame time Tf is calculated by
multiplying the number of chirps by the sum of the chirp duration and the waiting (idle) time until the next chirp signal is sent out: Tf = (Tc + Twait) x N where Twait is the wait or idle time until the next chirp is sent, and N is the number of chirps. The reason for using multiple chirps within a frame is to obtain Doppler information from the object to accurately ascertain its velocity. There is also a variable off time between each frame that can be used to improve the power efficiency of the chipset.
FMCW SIGNAL MEASUREMENT Factors that make measuring chirp signals difficult include chirp frequency changes in an extremely short time, ultra-wideband modulation,
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