KROHNE BM 70 A_P EN - Manual (Page 7)

Installation and operating instructions BM 70 A/P Page: 7 11/00
As the transmit frequency is changed during a short time
interval by FMCW radar the transmitted frequency has to
follow the nominal frequency within micro seconds. This
ensues via modern, fast electronic and processors.
2.2 Signal processing (digital)
The radar signals are digitally processed by a microprocessor. The processing system is based on
intelligent procedures that allow for plausibilities and history.
The following diagram describes the path of internal signal processing:
V
Fourier
spectrum
interpolation
spectrum
plausi-
bility
transform
filter
analog to
digital
spectrum
measurement
signal
gain
switch
gain
control
conversion
corrected
empty tank spectrum
distance
After automatic gain control, the measuring signal is filtered, digitized and converted into a frequency
spectrum by means of the Fourier transform. Plausibility of the value obtained is assessed with the
aid of the so-called empty-tank spectrum, which contains information about constant interference
reflectors. Interpolation of the spectrum helps to improve resolution of the measured value.
2.3 Modularity (signal converter, flange, antenna)
The measuring system consists of the flange system and the signal converter.
The flange system contains the microwave window separating product from pressure, and the
antenna system.
The compact signal converter contains the microwave generator and the entire signal processing
system, including provision of a standardized output signal (4 - 20 mA or digital interface).
The signal converter can be separated from the flange system under process conditions, without loss
of pressure or escape of product.
Converter
Flange with micro-
wave window
Antenna system
O O
O O
Microprocessor
control
Reference
oscillator
Phase
detector
Microwave
oscillator
Loop
filter
Antenna
PLL structure