Siemens sitrans LR 400 Instruction Manual page 26

Since the transmission signal has changed its frequency until the reception signal arrives,
the link gives a differential frequency
reflecting surface.
d
The distance is given by the ratio of the differential frequency
B
deviation and the duration of a frequency modulation phase
f
d
B = bandwidth (frequency deviation), d = distance, T = modulation duration,
c = speed of light
Determining the Differential Frequency
Frequency
B
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 such as struts inside the vessel.
Page 22
f
d
⋅ ⋅
2 B d
----------------- - d ,
=
T c ⋅
f
d
t
T
8
⋅ ⋅ ⋅
2 2 10
--------------------------------- -
=
d
3 – ⋅ ⋅
10 3 10
SITRANS LR 400 – INSTRUCTION MANUAL
, which is proportional to the distance
⋅ ⋅
f T c
d
------------------ -
=
2B
transmission
signal
10
,
= 13 333kHz
8
d
from the
f
to the frequency
d
T
:
reception
signal
time
7ML19985FH03