Radar Technology for Level Gauging - Krohne

3. Radar-Füllstandsmesssysteme
3.6 FMCW radar
3.6.1 Principle
FMCW radar uses a linear frequency-modulated high-frequency signal; the transmission
frequency rises e.g. linearly in a time interval (frequency sweep).
transmitter
mixer
f
a
antenna
reflector
Due to the time delay during signal propagation, the transmitted frequency changes so
that from the difference between the momentary transmitted frequency and the received
frequency a low-frequency signal (typically up to a few kHz) is obtained. The frequency f
of that signal is proportional to the reflector distance a; in this method, therefore, the
delay t is transformed into a frequency (df/dt is the sweep velocity):
f = df/dt · t
transmitter
delay time
t = 2a/c
Technically, the differential frequency is formed by mixing. If the frequency sweep is
linear, the frequency of the low-frequency mixed signal remains constant during the sweep
procedure. Because the resultant signal frequencies 8 are low, further signal processing is
technically simple and very accurate. Normally evaluation is by means of digital signal
processing.
3.6.2 Type model
Fig. 8 shows an example of an FMCW radar system 9 .A variable oscillator VCO is controlled
byamicroprocessor so that the desired frequency sweep is obtained. This signal is amplified
and fed via a pin coupler into the transmitting antenna.
The instantaneous frequency needs to be measured in order to ensure good sweep linearity.
This is done by counting the frequency after it has been mixed with a known frequency
(DRO).
The received signal is decoupled via a directional coupler, mixed with the transmission
signal, and processed by the microprocessor.
8 As opposed to the pulse radar method
9 The individual electronic components are described in Section 4.
receiver
differential frequency f
frequency
Fig. 7:
Functional principle
and signal trend in
FMCW radar
Radar handbook 13
time
3