Introduction to mmwave Sensing: FMCW
Radars
Sandeep Rao, Texas Instruments
1
• Basics of FMCW radar operation
• Using the radar to measure range of multiple
objects in front of the radar
• Concept of IF signal and IF bandwidth
• Range Resolution
2
INTRODUCTION TO MMWAVE SENSING: FMCW RADARS
Module 1 : Range Estimation
3
radar object
How does the radar estimate the range of an object?
How close can two object get and still be resolved as two objects?
What if there are multiple objects?
radar object
What determines the furthest distance a radar can see?
d
d
1
d
2
d
max
d
d+d
res
In this module we’ll try to answer the following questions……
What is a chirp?
4
An FMCW radar transmits a signal called a “chirp”. A chirp is a
sinusoid whose frequency increases linearly with time, as shown in
the Amplitude vs time (or ‘A-t’ plot) here.
t
t
f
• A frequency vs time plot (or ‘f-t plot’) is a convenient way to represent a
chirp.
• A chirp is characterized by a start frequency (f
c
), Bandwidth(B) and duration
(T
c
).
• The Slope (S) of the chirp defines the rate at which the chirp ramps up. In
this example the chirp is sweeping a bandwidth of 4GHz in 40us which
corresponds to a Slope of 100MHz/us
f
c
=77GHz
81GHz
T
c
=40us
A
B=4GHz
S
A 1TX-1RX FMCW radar
1. A synthesizer (synth) generates a chirp
2. The chirp is transmitted by the TX antenna
3. The chirp is reflected off an object and the reflected chirp is received at the RX
antenna.
4. The RX signal and TX signal are ‘mixed’ and the resulting signal is called an ‘IF signal’.
We’ll analyze the IF signal in more detail in the next slide
5
1
2
3
4
IF signal
Synth
TX ant.
RX ant.
mixer
A mixer is a 3 port device with 2 inputs and 1 output. For our purposes, a mixer can be
modelled as follows. For two sinusoids x1 and x2 input at the two input ports, the output
is a sinusoid with:
• Instantaneous frequency equal to the difference of the instantaneous frequencies of
the two input sinusoids.
• Phase equal to the difference of the phase of the two input sinusoids
x
1
x
2
x
out
What is a mixer?
