Radar has been around
for a long time:
in 1904, German
an early implementation
of radar technology
that could detect ships
up to 3000 m away.
Fast forward a hundred
years. Radar has been
used for decades to
calculate the velocity, range, and angle of objects on land,
sea, and in the air. On the road, radar is playing a key role
of Advanced Driver Assistance Systems (ADAS), which
constitute an intermediate stage in the development of
Radar is particularly useful in two ADAS technologies:
automatic emergency braking system (AEBS) and
adaptive cruise control (ACC). These applications use
long-range radar (LRR) systems with ranges of 80 m
to 200 m or greater. Current LRR systems operate
in the 77 GHz frequency band (76 – 81 GHz). This
frequency has several advantages for automotive use: the
wide bandwidth available improves accuracy and object
resolution; with a wavelength of 3. 9 mm, the antenna can
be small; and atmospheric absorption limits interference
with other systems.
Principles of FMCW Radars
The broadband Frequency Modulated Continuous Wave
(FMCW) radar has become the dominant technology for
automotive use because it combines high resolution in
range and depth perception, with the detection of objects
like pedestrians and bicycles in a small radar cross-section.
In an FMCW radar, both transmitter and receiver
operate continuously; the transmitter deploys a sinusoidal
carrier with a frequency that increases then decreases
periodically over time–a sequence known as a chirp.
The TX signal travels to the target object and is
reflected back to the receiver; the difference in frequency
between the RX and current TX signal is proportional
to travel time, can be used to measure distance, and
distinguish between different objects.
Figure 2 shows the received FMCW signals. Each one
is a delayed and attenuated reflection of the transmission
corresponding to a different object: in the frequency
domain, each object is a tone with a frequency, fb,
proportional to the object’s distance from the source, R.
Figure 3: Simplified block diagram of the FMCW architecture.
(Image Source: Altera)
Figure 3 shows a conceptual block diagram of an
FMCW radar. The radar detects multiple objects and
their respective distances by performing a fast Fourier
transform (FFT) on the IF beat-frequency signal, and
identifying the tones corresponding to each object.
Figure 4 shows a representative example from Infineon;
Texas Instruments, NXP, and STMicroelectronics are
also focusing heavily on semiconductors for ADAS and
Three components form the radar transceiver’s heart
as seen in Figure 4: the RTN7735PL three-channel
transmitter and local oscillator; RRN7745PL/46PL four-
By Paul Pickering, Technical Contributor
Figure 2: The FMCW IF signal contains multiple tones with
frequencies proportional to distance. (Image Source: Texas