Siemens sitrans LR 400 Instruction Manual And Users Manual page 28

As the transmission signal has changed its frequency until the reception signal arrives, the
linked signal gives a differential frequency f
the reflecting surface.
The distance d is given by the ratio of the differential frequency f
deviation B and the duration of a frequency modulation phase T :
f
d
B = bandwidth (frequency deviation), d = distance, T = modulation duration,
c = speed of light
Determining the Differential Frequency
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:
f
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.
Page 25
⋅ ⋅
2 B d
----------------- - d ,
=
T c ⋅
fd
B
t
T
8
⋅ ⋅ ⋅
2 2 10
--------------------------------- -
=
3 –
d
⋅ ⋅
10 3 10
SITRANS LR 400 – INSTRUCTION MANUAL
, which is proportional to the distance d from
d
⋅ ⋅
f T c
d
------------------ -
=
2B
10
,
= 13 333kHz
8
to the frequency
d
reception
transmission
signal
signal
time
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