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The Fundamentals of Millimeter Wave Radar Sensors

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The Fundamentals of Millimeter Wave Radar Sensors The fundamentals of millimeter wave radar sensors Cesar Iovescu Radar Applications Manager Sandeep Rao Radar Systems Architect Texas Instruments Introduction Millimeter wave (mmWave) is a special class of radar technology that uses short- wavelength electromagnetic waves. Radar systems transmit electromagnetic wave signals that objects in their path then reflect. By capturing the reflected signal, a radar system can determine the range, velocity and angle of the objects. mmWave radars transmit signals with a wavelength that is in the millimeter range. This is considered a short wavelength in the electromagnetic spectrum and is one of the advantages of this technology. Indeed, the size of system components such as the antennas required to process mmWave signals is small. Another advantage of short wavelengths is the high accuracy. An mmWave system operating at 76–81 GHz (with a corresponding wavelength of about 4 mm), will have the ability to detect movements that are as small as a fraction of a millimeter. A complete mmWave radar system includes transmit (TX) and receive (RX) radio frequency (RF) components; analog components such as clocking; and digital components such as analog-to-digital converters (ADCs), microcontrollers (MCUs) and digital signal processors (DSPs). Traditionally, these systems were implemented with discrete components, which increased power consumption and overall system cost. System design is challenging due the complexity and high frequencies. Texas Instruments (TI) has solved these challenges and designed complementary metal-oxide semiconductor (CMOS)-based mmWave radar devices that integrate TX- RF and RX-RF analog components such as clocking, and digital components such as the ADC, MCU and hardware accelerator. Some families in TI’s mmWave sensor portfolio integrate a DSP for additional signal-processing capabilities. TI devices implement a special class of mmWave technology called frequency- modulated continuous wave (FMCW). As the name implies, FMCW radars transmit a frequency-modulated signal continuously in order to measure range as well as angle and velocity. This differs from traditional pulsed-radar systems, which transmit short pulses periodically. The fundamentals of millimeter wave radar sensors 2 July 2020 Figure 2 shows the same chirp signal, with frequency as a function of time. The chirp is characterized by a start frequency (fc), bandwidth (B) and duration (Tc). In the example provided in Figure 2, fc = 77GHz, B = 4GHz and Tc = 40µs. Figure 2 shows the same chirp signal, with frequency as a function of time. The chirp is characterized by a start frequency (fc), bandwidth (B) and duration (Tc). In the example provided in Figure 2, fc = 77GHz, B = 4GHz and Tc = 40µs. Range measurement 2 TX ant. 1 The fundamental concept in radar systems is the transmission of an electromagnetic signal that objects reflect in its path. In the signal used in 3 FMCW radars, the frequency increases linearly Figure 2. Chirp signal, with frequencySynth as a function of time. 4 with time. This type of signal is alsoAn FMCWcalled radara chirp. system transmits a chirp signal and captures the signals reflected by objects in its path. Figure 3 Figure 1 shows a representation representsof a chirp asignal, simplified block diagram of the main RF components of a FMCW radar. The radar operates as follows: FiRXgure ant. 2. Chirp signal, with frequencyIF signalas a function of time. with magnitude (amplitude) as a function of time. mixer • FIGURE 1 & 2An FMCWA synthesizer radar system (synth) transmitsFigure generates 3 . aFMCW chirp a radar chirpsignal block. and diagram. captures the signals reflected by objects in its path. Figure 3 represents• The achirp simplified is transmit blockted diagram by a transmit of the main antenna RF components (TX ant). of a FMCW radar. The radar operates as follows: A • The reflection of the chirp by an object generates a reflected chirp captured by the receive antenna (RX ant). • • A synthesizer (synth) generates a chirp. • AA “mixer” synthesizer combines (synth) the generates RX and T aX chirp signals. to produce an intermediate frequency (IF) signal. • The chirp is transmit•ted The by achirp transmit is transmitted antenna (TX by ant) a. transmit antenna • The reflection of the chirp(TX byant). an object generates a reflected chirp captured by the receive antenna (RX ant). • At “mixer” combines the RX and TX signals to produce an intermediate frequency (IF) signal. • The reflection of the chirp by an object generates a reflected chirp captured by the Figure 1. Chirp signal, with amplitude as a function of time. f receive antenna (RX ant). 81 GHz • A “mixer” combines the RX and TX signals to S Figure 2 shows the same chirp signal, with produce an intermediate frequency (IF) signal. FIGURE 1 & 2 B = 4 GHz frequencyA as a function of time. The chirp is A frequency mixer is an electronic component that characterized by a start frequency (fc), bandwidth (B) f = 77 GHz combines two signals to create a new signal with a c Figure 3. FMCW radar block diagram. and duration (Tc). The slope of the chirp (S) capturest new frequency. Tc = 40 µs the rate of change of frequency. InA frequencythe example mixer is an electronic component that combines two signals to create a 1new signal with a new frequency. t For two sinusoidal inputs x1 and x2 (Equations 1 provided in Figure 2, fc = 77 GHz, B = 4 GHz, Figure 3. FMCW radar block diagram. For two sinusoidal inputs x1and 2): and x2 (Equations 1 and 2): T = 40 µs and S = 100 MHz/µs. c A frequency mixer is an electronic component that combines two signals to create a new signal with a new frequency. = sin ( + ) (1)(1) f For two sinusoidal inputs x1 and x2 (Equation1s 1 and 2):1 1 81 GHz = sin ( + ) (2)(2) S 2 = sin ( 2 + 2 ) (1) The output xout has an instantaneousThe output frequency xout has equal an instantaneous to the difference frequency of the instantaneous frequencies of the two B = 4 GHz 1 1 1 input sinusoids. The phase equalof the outputto the xdifferenceout is equal= sin (toof thethe + differenceinstantaneous ) of the (2) phases of the two input signals (Equation 3): frequencies of the2 two input2 sinusoids.2 The phase The output xout has an instantaneous frequency equal to the difference of the instantaneous frequencies of the two fc = 77 GHz of the output= xsinout [is( equal to) the+ ( difference )] of the (3) input sinusoids.t The phase of the output xout is equal to the difference of the phases of the two input signals (Equation Tc = 40 µs 3): phases of the two input1 signals2 (Equation1 2 3): The operation of the frequency mixer can also be understood− graphically − by looking1 at TX and RX chirp frequency Figure 2. Chirp signal, with frequency as a functionrepresentation of time. as a function of time. = sin[( ) + ( )] (3) (3) The operation of the frequencyThe operation mixer can alsoof the be 1frequencyunderstood− 2 mixergraphically1 − can2 alsoby looking be at TX and RX chirp frequency An FMCW radar system transmitsrepresentation a chirp signal as and a function understood of time. graphically by looking at TX and RX captures the signals reflected by objects in its path. chirp frequency representation as a function of time. Figure 3 represents a simplified block diagram of The upper diagram in Figure 4 on the following the main RF components of an FMCW radar. The page shows TX and RX chirps as a function of time radar operates as follows: for a single object detected. Notice that the RX chirp is a time-delay version of the TX chirp. The fundamentals of millimeter wave radar sensors 3 July 2020 The upper diagram in Figure 4 shows TX and RX chirps as a function of time for a single object detected. Notice that the RX chirp is a time-delay version of the TX chirp. The upper diagram in FigureThe 4 uppershows diagram TX and RX in chirpsFigure as4 shows a function TX and of timeRX chirps for a assingle a function object ofdetected. time for Notice a single that object the detected. Notice that the The time delay (τ) can be mathematically derived as Equation 4: RX chirp is a time-delay versionRX chirp of theis a TXtime chirp.-delay version of the TX chirp. = The time delay (τ) can be mathematicallyThe time delay (derivedτ) can be as mathematically Equation 4: derived(5) as Equation 4: 2 where d is the distance to the detected object and c is the speed of light. = (5) = (5) 2 2 To obtain the frequency representation as a function of time of the IF signal at the output of the frequency mixer, where d is the distance to wherethe detected d is the object distance and to c theis the detected speed of object light. and c is the speed of light. subtract the two lines presented in the upper section of Figure 4. The distance between the two lines is fixed, which meansTo obtain that the the frequency IF signal consis representationTo obtaints of a t tonehe frequencyas with a function a constant representation of time frequency of the as IF. aFig signal functionure 4at shows the of time output that of this theof the frequencyIF signal frequency at is the Sτ mixer .output The, IF of the frequency mixer, signalsubtract is valid the twoonly lines in the presented timesubtract interval in the the where two upper lines both section presented the ofTX Figurechirp in the and 4. upperT thehe distance RX section chirp between overlapof Figure (i.e., the 4. T twothehe distanceintervallines is fixed betweenbetween, which thethe two lines is fixed, which verticalmeans thatdotted the lines IF signal in Figure consismeans 4ts) . ofthat a tone the IFwith signal a constant consists frequencyof a tone with.
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