User Manual

TECHNICAL DATA 8
87
OPTIWAVE 7400-24 C
www.krohne.com12/2015 - 4004901901 - MA OPTIWAVE7400-24 R01 fr
8.1 Measuring principle
A radar signal is emitted via an antenna, reflected from the product surface and received after a
time t. The radar principle used is FMCW (Frequency Modulated Continuous Wave).
The FMCW-radar transmits a high frequency signal whose frequency increases linearly during
the measurement phase (called the frequency sweep). The signal is emitted, reflected on the
measuring surface and received with a time delay, t. Delay time, t=2d/c, where d is the distance
to the product surface and c is the speed of light in the gas above the product.
For further signal processing the difference Δf is calculated from the actual transmitted
frequency and the received frequency. The difference is directly proportional to the distance. A
large frequency difference corresponds to a large distance and vice versa. The frequency
difference Δf is transformed via a Fourier transformation (FFT) into a frequency spectrum and
then the distance is calculated from the spectrum. The level results from the difference between
the tank height and the measured distance.
Figure 8-1: Measuring principle of FMCW radar
1 Transmitter
2 Mixer
3 Antenna
4 Distance to product surface, where change in frequency is proportional to distance
5 Differential time delay, Δt
6 Differential frequency, Δf
7 Frequency transmitted
8 Frequency received
9 Frequency
10 Time