Emerson Rosemount 5408, Reference Manual

Page: 11 / 274

2 Transmitter overview
2.1 Measurement principle
The Rosemount 5408 and 5408:SIS are two-wire transmitters for continuous level
measurements over a broad range of liquids, slurries, and solids. The measurement
principle is fast-sweep Frequency Modulated Continuous Wave (FMCW).
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. See
Figure 2-2 for a schematic overview of the
signal processing.
Figure 2-1: FMCW-method
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PD[
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PLQ
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RXW
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RXW
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%
Δf≈d=distance
A. Frequency (GHz)
B. Time (s)
C. Transmitted signal
D. Reflected signal
Reference Manual Transmitter overview
00809-0100-4408 February 2020
Rosemount 5408 and 5408:SIS Level Transmitters 11