This article reports on

Millimeter-Wave FMCW Radar Transceiver/Antenna for Automotive Applications A summary of the design and performance of a 77 GHz radar unit David D. Li, Sam C. Luo and Robert M. Knox Epsilon Lambda Electronics Corp. This article reports on the development of an FMCW forwardlooking radar transceiver. The transceiver uses a flat antenna and is capable of acquiring the range and angular information of all obstacles in its field of view, with or without mechanical scanning. This transceiver/antenna is fully integrated for small size and low cost manufacturing. The FM sweep linearity of this system is better than 0.5%, and the range resolution is better than one meter. We will discuss design tradeoffs and present radar front-end performance. There continues to be significant interest in the field of intelligent autonomous cruise control (ACC) and collision warning (CW) radar systems for in-vehicle applications. Rapid detection of hazards by the radar warning system can give the driver time to avoid a collision. Studies have shown that 60% of forward collisions can be avoided with an extra 0.5 second of warning time. With 1 second of extra warning time, the driver can avoid 90% of forward collisions. Although the basic concept of radar is relatively simple, in many instances its practical implementation is not. Radar operates by radiating electromagnetic energy and detecting the echo returned from reflecting targets. The nature of the echo signal provides information about the Figure 1. FMCW radar system block diagram. target. The range, or distance, to the target is found from the time it takes for the radiated energy to travel to the target and back. The angular location of the target is found with either a directive or switching antenna. If the target is moving, a radar can ascertain its track and predict the future location. With sufficiently high resolution, a radar can also discern something about the nature of a target s size and shape. In recent years, various radar sensors have been proposed and applied in the intelligent vehicle systems, such as the switching beam pulse radar system, the switching beam FMCW radar system [1], the mechanical scanning 58 APPLIED MICROWAVE & WIRELESS

FMCW system [2], the switching beam spread spectrum system and the FSK monopulse radar system [3]. This article discusses the development of a fully integrated FMCW radar front-end design for small size and low cost manufacturing, which is capable of acquiring the range and angular information of all obstacles in its field-of-view with single balanced mixer receiver, I/Q mixer receiver and monopulse receiver options. FMCW radar principle The most useful description of the factors influencing radar performance is the radar equation that gives the range of a radar in terms of the radar characteristics. The radar equation is P r 2 PG t tgrσλ = 3 4 ( 4π ) RL s (1) Figure 2. Front view of the radar transceiver. and R where: 2 PG Grσλ = 3 ( 4π ) ktbf ( S / N ) Ls 4 1 1 n o o (2) P r = Minimum receiving power P t = Transmitting power G t = Transmitter antenna gain G r = Receiver antenna gain s = Radar cross section λ = Free space wavelength at operating frequency R = Detect distance ktb = White noise level L s = System loss F n = System noise figure S 0 /N 0 = Signal to noise ratio The radar equation shows that the range of radar is proportional to the fourth root of the transmitter power. Thus, to double the range requires that the power be increased by 16. This means that there often is a practical and economical limit to the amount of power that should be employed to increase the range of radar. According to the above equations, we noticed that the radar system can get a 200-meter-away target in clear weather (3.5 db/km attenuation) and a 100-meter-away target (150 mm/hr rain, 50 db/km attention) in a heavy rain at 76.5 GHz if the target cross-section is 10 dbsm, the transmitting power is higher than 10 dbm, the gain of the antenna is better than 27 dbi and the system noise figure is less than 18 db. System description The millimeter-wave FMCW radar front-end subsystem presented here is a forward-looking, radar based, detection system for the ACC and CW applications. Figure 1 shows the system block diagram. This system operates in the 76-77 GHz frequency band. It generates a FMCW waveform using a voltage controlled oscillator (VCO) with an InP Gunn diode that is phase-locked to a harmonic of a dielectric resonator-oscillator (DRO) by using a linearizer to stabilize the RF frequency and improve system phase noise. The frequency of the mixed signal is compared to a low frequency linear sweep reference to generate an error signal which corrects the VCO sweeping slope. Both the varactor-tuned and the biased-pushing VCOs were investigated for this system. Figure 2 shows the front view of the integrated radar transceiver/antenna with monopulse antenna. The FM modulation technique of the radar front-end may also be used to broadcast a digital radar system identifier. By periodically switching between two transmit frequencies according to an assigned code, the radar identifier can be broadcast as a part of the system modulation by considering two frequencies as binary code. The receiver design in the described system uses a homodyne approach. This transceiver/antenna can be used to determine range, relative velocity and the azimuth angle for multiple targets in three types. Model ELFI71-1A is the radar transceiver with a single balanced mixer and a fixed beam, mechanically scanned patch antenna. It generates a single video output representing the target return from which target range and velocity can be derived. Model ELFI71-1B is the radar transceiver with a I/Q mixer and a fixed beam, mechanically scanned patch antenna. The two IF outputs of this model are related by a quadrature phase relationship. This feature provides a means for discriminating the polarity of the beat signal independently from the range information. Model ELFI71-1C is the radar transceiver with sum 60 APPLIED MICROWAVE & WIRELESS

and delta channel mixer and fixed monopulse patch antenna. It can generate the target s angular information simultaneously with the range information without mechanical scanning. The fixed-beam and monopulse microstrip patch array antenna is designed on a laminated Teflon circuit soft-board substrate. This antenna is mounted on the front face of the radar enclosure. In Model ELF171-1A and Model ELF171-1B, the antenna serves as the transmitting antenna and receiving antenna at the same time through a circulator. For the monopulse type transceiver, the sum channel antenna serves as the transmitting antenna and the sum channel receiving antenna at the same time. The antenna on reception produces sum and delta beams to cover the field-of-view. Target azimuth angle information is determined by comparing the signal strength on the sum channel beat signal with the corresponding frequency on the delta channel. This provides relatively fine azimuth resolution and a broad field-of-view while minimizing the aperture size. Signal analysis The transmitted and received signal of the FMCW radar is shown in Figure 3. The transmitted frequency F t and received frequency F r are and F t = f 0 + (B r /T)t 0 < t < T F t = f 0 + B r (B r /T)(2T t) T < t < 2T (3) Figure 3. Frequency versus time waveform of FMCW radar. f d = 2V d f/c, (6) where V d = Radial velocity of target C = Velocity of propagation f = Operating frequency R = Range of the target The range resolution R of a radar transceiver depends on the sweeping bandwidth B r. Since R = (C/2B r )(f r T) (7) at one period only one frequency is processed (f r T = 1), so that the theoretical range resolution is F r = f 0 + (B r /T)(t + t) + f d 0 < t < T R = C/(2B r ) (8) F r = f 0 + B r (B r /T)(2T t τ) + f d T < t <2T (4) where F t = Transmitted signal frequency F r = Received signal frequency B r = Frequency modulation bandwidth f 0 = Transmitted frequency at time t=0 f d = Doppler frequency shift t = Time since start of sweep T = Sweep repetition interval of the transceiver τ = Time of flight of signal from the transmitter to the target and back. The IF output frequencies are different in period T and 2T for a target moving with a relative velocity V d. The frequency shift f r due to the time delay in range and Doppler frequency fd are: f r = (2R/C)(B r /T), (5) Therefore, to obtain one meter range resolution, the theoretical FM bandwidth required is at least 150 MHz. The actual range resolution also depends on the target beat frequency width and receiver frequency resolution. The nonlinearity of the FM signal will cause the ambiguity of range accuracy. Normally, the FM sweep signal is not ideally linear. For a free running VCO, the linearity is typically about 10%. For a linearized VCO, the linearity can be improved to 0.03% to 0.5%. Generally speaking, the maximum acting range of a radar system depends on the radar equation characteristics. The range resolution R depends on the FM sweep bandwidth and the FFT beat signal sample spacing. The accuracy of the range measurement depends on the FM sweep linearity. System noise discussion From equation (1), we can assume that the system noise is dependent on white noise (ktb). However, the oscillator of radar system is not ideal and contains FM 62 APPLIED MICROWAVE & WIRELESS

Figure 4. Monopulse antenna array layout showing the feed system. Figure 5. Sum and difference channel patterns of the monopulse antenna. and AM noise. Generally, the influence of the phase noise is much higher than that of the AM noise since the latter could be reduced by a balanced mixer. The actual system noise level may be raised if the phase noise of the VCO is high and the isolation of the circulator is poor. We should notice that the minimum receiving power is the minimum power that can be detected. This means that the system noise level should be limited to a certain level. Since the influence of the phase noise maybe higher than the actual white noise level, this means that although the thermal noise is ktb, the actual noise level may be higher than this value. The extra noise will degrade the system s performance. One of the most important factors that needs to be addressed for a homodyne FMCW radar system is the leakage of the transmitter signal into the receiver. If a common transmit/receive antenna is used, the two dominant sources of leakage will be reflections from the antenna and leakage around the circulator. The combination of these sources of leakage will typically give isolation between transmitter and receiver of about 15-25 db. The problem of noise leakage has been addressed in reference [4, 5]. The contribution of the FM noise can be reduced by identifying the major sources of the leakage and arranging that the path length from the transmitter to the mixer is equalized. The limit to the degree of cancellation that can be achieved will either be due to incidental FM to AM conversion or to the presence of multiple leakage sources. Monopulse transceiver In the monopulse type transceiver, a signal reflected from the target is received by two channels in the receiver in order to obtain the azimuth angular information. The received signal is fed into the sum and difference channels at the same time. By comparing properties of the signals from the two receiver mixers, the azimuth angle to a vehicle target can be calculated. By using the monopulse antenna, complete information on the angular position of the target may be obtained through the processing of a single reflected pulse. Figure 4 shows the layout of the monopulse microstrip antenna. The theory of operation for the monopulse feature consists of combining the return signals from the right half of antenna (A) with the left half of antenna (B) in particular ways. The return signal from antenna A is added to the return signal from antenna B, releasing the sum signal (Σ), which is processed in the sum channel of the system. The return signal from A is also subtracted from the return signal from B, producing the difference signal ( ), which is processed in the difference channel of the system. A hybrid ring circuit, located on the center of the antenna, performs the addition and subtraction of the signals from the A and B sides. The radiation patterns from sum and difference channels are shown in Figure 5. By com- Figure 6. Antenna return loss plot. 64 APPLIED MICROWAVE & WIRELESS

Field-of-view Elevation BW Gain SLL Return loss (degrees) (degrees) (dbi) (db) (db) Antenna 1 12 4 26-20 -15 Antenna 2 20 4 25-20 -15 Antenna 3 8 4 27-20 -15 Table 1. Monopulse antenna performance summary. Radar Model No. ELFI71-1A ELFI71-1B ELFI71-1C Receiver type Single balanced mixer I/Q mixers Monopulse RF frequency 76-77 GHz 76-77 GHz 76-77 GHz Transmitter power 10 dbm 10 dbm 10 dbm FM sweep range 200 MHz 200 MHz 200 MHz Range resolution <1 m <1 m <1 m Linearity <0.5% <0.5% <0.5% Phase noise < 95dBc/Hz@250kHz < 95dBc/Hz@250kHz < 95dBc/Hz@250kHz Receiver noise figure <15 db <15dB (single channel) <15dB (single channel) Antenna Fixed-beam Fixed-beam Monopulse Antenna Type Microstrip Microstrip Microstrip Antenna gain >29 dbi >29 dbi >26 dbi Temperature range 40 C to 85 C 40 C to 85 C 40 C to 85 C Table 2. Front-end specifications. paring the amplitude of the sum and difference channel signals, we can determine the target azimuth angle. The measured return loss of one monopulse antenna with 12 degrees coverage is shown in Figure 6. By using this antenna, the system can measure azimuth angle over a range from approximately 6 degrees on the left to 6 degrees on the right. The elevation beam width is fixed at about 4 degrees to allow for vehicle pitch and variations in road inclines. The measured gain of this antenna is 26 dbi. The isolation between the sum and delta channels is higher than 25 db. A wide angle coverage antenna (20 degrees) and a narrow angle coverage antenna (8 degrees) were also designed and tested for monopulse type transceiver ELFI71-1C. Table 1 shows the performance of these antennas. System performance Table 2 summarizes the performance specifications of three types of radar transceiver/antennas: ELFI71-1A, ELFI71-1B and ELFI71-1C. Figure 7 shows the temperature test data of the transmitter. Notice that the transmitter power variation is about ±0.9 db and the center frequency drift is about 50 MHz from 40 C to +85 C. The RF signal sweep linearity is higher than 0.5%. An H-plane broadband Y-junction circulator at 76.5 GHz was designed and tested in favor of repeatability for the transceiver production. A single junction multi-step pedestal is used in the design to get a broadband matching performance. The isolation and insertion loss of the circulator are shown in Figure 8. At 76.5 GHz, the insertion losses (s 21, s 32 and s 13 ) of the circulator are less than 1 db and the isolations (s 12, s 23 and s 31 ) are about 24 db. The bandwidth of the circulator with 20 db isolation is about 1.5 GHz. The bandwidth of the circulator with 15 db isolation is larger than 5 GHz. The bandwidth of this circulator is wide enough to compensate the fabrication and assembly tolerance variation in a production environment. A 12 GHz DRO is designed and tested for the phase lock loop. The phase noise of the DRO is about 115 Figure 7. System temperature test data. Figure 8. Wideband circulator test data. 66 APPLIED MICROWAVE & WIRELESS

Target range and range resolution are measured by using a spectrum analyzer with this transceiver/antenna. Figure 10 shows the target return signal from 100 meters away, using Model ELFI71-1B transceiver/antenna (the target is a Honda Civic passenger car). The resolution bandwidth of the spectrum analyzer is set to 300 Hz. It is shown that the system noise figure is adequate to achieve the necessary signal to noise ratio. Figure 9. Mixer noise figure test result. Figure 10. Return signal from a target at 100 m distance. dbc/hz at 100 khz offset. The phase noise at 12 GHz is good enough to keep the phase-locked system phase noise at 76.5 GHz better than 95 dbc/hz at 250 khz offset. A harmonic mixer is designed to mix the RF signal at 76.5 GHz and the 12 GHz signal from the DRO. The conversion loss of the harmonic mixer is better than 35 db for the 6th harmonic. The conversion loss of the harmonic mixer is satisfactory in order for a reasonable S/N ratio to drive a high performance linearizer without degradation of the system phase noise. A microstrip type single balanced mixer at 76.5 GHz is designed and tested for the receiver. Figure 9 shows the test data of the noise figure vs. the LO pumping power. A SSB noise figure of 9 db is achieved in this design. Conclusions It was shown that with the described transceiver/ antenna, it is now possible to determine target range, relative velocity and the azimuth angle with or without mechanical scanning. The FM sweep linearity of this system is better than 0.5% and the range resolution is better than 1 meter. With the monopulse option, the azimuth angle accuracy and resolution of this system are of tenth degrees. Compared to a 3-D mechanical scanning or lens transceiver/antenna, the volume of this transceiver is rather small since the 2-D patch array antenna has been used. The simplicity and producibility of this kind of microstrip antenna also helps to reduce the cost of large scale production of the frontend. A second generation FMCW FLR radar transceiver/ antenna design, which will improve the radar performance and keep the transceiver size small, is currently in progress. References 1. Celsius Tech, High Performance Automotive Radar for Automotive ACC, IEEE International Radar Conference, 1995. 2. Mark E. Russell, Millimeter Wave Radar Sensor for Automotive Intelligent Cruise Control (ICC), 1997 IEEE-MTT-S International Microwave Symposium Digest, 1997. 3. Merrill Skolnik, Radar Handbook, McGraw-Hill, 1990. 4. A. G. Stove, Linear FMCW Radar Techniques, IEE Proceeding-F, Vol. 139, No. (1992): 343-350. 5. Jerry D. Woll, 60 GHz Vehicle Radar for Japan, Future Transportation Technology Conference, 1998. Author information David D. Li, Sam C. Luo and Robert M. Knox can be reached at Epsilon Lambda Electronics Corp., 427 Stevens Street, Geneva, IL 60134; tel: 630-232-9611; fax: 630-232-9613; e-mail: epsilon@inil.com. 68 APPLIED MICROWAVE & WIRELESS