Emerson Rosemount 1408A, Reference Manual

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2 Transmitter overview
2.1 Measurement principle
The Rosemount 1408A is a 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
The Rosemount 1408A can also calculate the volume flow rate in an open channel.
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
00809-0200-4480 May 2021
Reference Manual 7