User's Manual

7ML19985FH01 SITRANS LR 400 – INSTRUCTION MANUAL DRAFT Page 17
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Operation
Since the transmission signal has changed its frequency until the reception signal arrives,
the link 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 (see“Determining the
Differential Frequency” on page 17):
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 which are caused by fixed targets such as struts inside the vessel.
f
d
2 Bd⋅⋅
Tc⋅
------------------
d,
f
d
Tc⋅⋅
2B
-------------------==
fd
time
Frequency
T
B
transmission
signal
reception
signal
t
f
d
2210
8
10⋅⋅ ⋅
10
3–
310
8
⋅⋅
---------------------------------- 13 333kHz,==
chap6.fm Page 17 Tuesday, October 9, 2001 2:26 PM