Measuring Principle; Appendix Iv - Siemens sitrans LR 400 Instruction Manual

Appendix IV

Measuring Principle

SITRANS LR 400 operates according to the FMCW (Frequency Modulated Continuous
Wave) method. Its antenna sends microwaves to the surface of the material. The wave
frequency is modulated continuously (see Determining the Differential Frequency on page
94). A receiver registers the reflection at the surface of the measuring medium and links it
with the simultaneously radiated signal.
The propagation speed of microwaves in gases corresponds to the speed of light. The
d
distance is proportional to the propagation time
d= distance, t = measured time, c = speed of light
As the transmission signal has changed its frequency until the reception signal arrives, the
linked signal gives a differential frequency
the 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
7ML19985JC02
SI TRA N S LR 4 0 0
d
d =
⋅ ⋅
2 B d
----------------- - d ,
= =
T c ⋅
SITRANS LR 400 (Solids) – INSTRUCTION MANUAL
t
:
c t ⋅
--------
2
f
, which is proportional to the distance
d
f
T
:
⋅ ⋅
f T c
d
------------------ -
2B
d
from
to the frequency
d
Page 93