Principle of Operation
FMCW Radar
PD420 FMCW radars modulate the frequency of the transmit signal in a linear fashion.
The difference between the frequencies of the local oscillator and the signal returned
from the target is proportional to the time delay between these signals and thus is
proportional to the distance to target. In case of a moving target we also take into account
Doppler shift of the return signal. PD420 utilizes double linear ramp modulation, first
increasing and then decreasing the frequency of the signal. Additional information
derived from two ramps allows the radar to measure both range to target and target
velocity.
The PD420 employs advanced target tracking technique based on a proprietary algorithm
that allows it to detect, measure and track multiple targets simultaneously. It also features
advanced “application filters” pre-configured to optimize performance for a variety of
applications. The PD420 radar may be deployed as a complete product without any
further requirements for signal processing.
For a more detailed theoretical description of the principles of FMCW radar operation
please see this
article on the Internet
.
Radar Detection Zone
The radar detection zone has an oval shape and is defined by the beam cone (38ºx45º for
PD420, 20ºx60º for PD420) and incident angle to the road surface. Note that the beam
does not cutoff abruptly at the boundary of the detection zone but rather gradually tapers
off. Thus weak targets near the boundaries may be missed while strong targets outside
may still get detected. The strength of the target is determined by its radar cross-section
(RCS) and depends on the target material, area, shape and incident angle of the radar
beam. Large flat metallic surfaces positioned at exactly 90 degrees to the incident radar
beam make the best targets. Examples are vehicle sides, front and rear ends. Flat metal
surfaces at angles other than perpendicular to the beam tend to reflect the radar signal
away and reduce the signal strength. Two or three metal surfaces joined at 90 degree
angle, for example a corner of a pickup truck bed create perfect reflector and usually
result in a very strong return signal.
As the radar beam diverges with distance the detection zone gets wider. This can be used
to a great advantage if you need to increase the detection area. In this case, move the
radar away from the target location. This may involve, for example, mounting the radar
on the opposite side of the road or increasing setback and/or height. This kind of a setup
is often used in a loop-replacement application for intersections.
