User's Manual

Page 25 SITRANS LR 400 – INSTRUCTION MANUAL 7ML19985FH04
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Operation
As the transmission signal has changed its frequency until the reception signal arrives, the
linked signal gives a differential frequency f
d
, which is proportional to the distance d from
the reflecting surface.
The distance d is given by the ratio of the differential frequency f
d
to the frequency
deviation B and the duration of a frequency modulation phase T :
B = bandwidth (frequency deviation), d = distance, T = modulation duration,
c = speed of light
Determining the Differential Frequency
Example
The linear frequency deviation is 200 MHz at a modulation duration of 10 ms. The surface
of the measuring medium is 10 m away from the transmitting antenna. The difference
signal then has a frequency of:
Every reflection at a surface generates a different frequency. The reception signal
therefore consists of a frequency mix from which the disturbance frequencies must be
filtered. These can be caused by fixed targets like struts inside the vessel.
f
d
2 Bd⋅⋅
Tc⋅
------------------
d,
f
d
Tc⋅⋅
2B
-------------------
==
frequency
B
fd
t
T
time
transmission
signal
reception
signal
f
d
2210
8
10⋅⋅ ⋅
10
3–
310
8
⋅⋅
--------------------------------- -
13 333kHz,==