Emerson Rosemount 3408, Reference Manual, Page: 9 / 104

2 Transmitter overview
2.1 Measurement principle
The Rosemount 3408 is a two-wire transmitter for continuous level measurements using
fast-sweep Frequency Modulated Continuous Wave (FMCW) technology.
The transmitter continuously emits signal sweeps with a constantly varying frequency
towards the product surface. Since the transmitter continuously changes the frequency of
the transmitted signal, there will be a difference in frequency between the transmitted
and the reflected signals (see
Figure 2-1 ).
The frequency of the reflected signal is subtracted from the frequency of the signal
transmitted at that moment, resulting in a low frequency signal which is proportional to
the distance to the product surface. This signal is further processed to obtain fast, reliable,
and highly accurate level measurements.
Figure 2-1: FMCW-method
d
f
in
f
max
f
min
f
out
f
in
f
out
t
f
A
C
D
B
Δf
∼
d=distance
A. Frequency (GHz)
B. Time (s)
C. Transmitted signal
D. Reflected signal
2.2 Process characteristics
2.2.1 Dielectric constant
A key parameter for measurement performance is reflectivity. A high dielectric constant of
the media provides better reflection and enables a longer measuring range.
Reference Manual Transmitter overview
00809-0100-4418 September 2022
Rosemount 3408 Level Transmitter 9