The Krohne Optiwave uses frequency modulated continuous wave (FMCW) technology and has an average transmission frequency of 25 GHz (which is fully crystal-controlled), a bandwidth of 2 GHz and a dynamic range of more than 100 dB. A digital signal processor provides the system with a very high computing capacity. Complex evaluation algorithms ensure reliable and precise calculation of measured values.
Transmission frequency
Using the same size antenna, and compared to the lower operating 9 GHz frequency of Krohne's BM702 and BM70x models, the higher frequency allows improved concentration of the microwave signals. This allows better focusing of the target, and improves accuracy of the measured value, stability of the measurement and insensitivity to tank internals.
Bandwidth
The 2 GHz bandwidth is an important factor for accurate and stable measurements. The bandwidth describes the resolution of the radar system in the axial direction, ie, the direction in which the electromagnetic waves are propagated. A large bandwidth is inversely proportional to the effective pulse width in a pulsed system. Accordingly, a large bandwidth corresponds to short pulses in pulsed systems. This makes it easier to distinguish between unwanted and wanted signals, and allows measured values to be evaluated with greater accuracy.
Dynamic range
The FMCW principle facilitates a dynamic range of more than 100 dB. The dynamic range determines the ratio between the strongest signals that the system can sense and the weakest possible signals in the form of fundamental noise. The dB scale is a logarithmic representation of signal levels. 100 dB signifies a ratio of 1010:1.
According to Krohne, the improvement in dynamic range of Optiwave over the best pulsed systems is more than 30 dB. This is equivalent in signal power to a sensitivity that is 1000 times greater. The larger dynamic range of the Optiwave allows, for example, more stable detection of liquids with very low relative permittivity (εr) and thus very weak reflections. Between an (εr) of 2 and an (εr) of 1,2, in an otherwise identical measurement environment, the difference in signal level is approx. 11,5 dB, which is equivalent to 14 times the power difference on a linear scale. Given a large reserve in the dynamic range, as in the Optiwave, measuring accuracy will remain high even when signals are very weak, whereas in systems with a small dynamic range measuring accuracy will decline rapidly when signals are weak.
Stability of transmission frequency
A decisive factor for measuring accuracy, and in particular stability of measured values, is the generation of a highly stable crystal-controlled transmit frequency. With the aid of two phase-locked loops (PLL) all frequencies occurring in the system are connected to a central crystal-controlled oscillator. This means that, for instance, given a fixed target, the measured value of the radar system will vary less than 50 μm. This high stability is also evident in the display by the fact that the last digit is quite steady - without any averaging occurring. Being able to dispense with long averaging times makes the Optiwave a fast level radar measuring system. Besides very good measured-value stability and accuracy, the fully PLL-based signal generation also allows exact predetermination of frequency settings. Since the frequencies are digitally programmable, the ISM band, for example, can be precisely set for measurements in free space. This possibility of emitting random frequency bands with high accuracy on a software-controlled basis is entirely lacking in pulsed systems. Here, the emitted frequency band can vary within wide limits. Particularly in difficult applications, different devices can also respond very differently.
Digital signal processing
A further point that is becoming increasingly important is signal evaluation by digital signal processing. For this purpose, a digital signal processor with a high computing capacity has been implemented in the Optiwave. Besides the Fourier transform, this signal processor features additional filter and signal processing algorithms which greatly improve signal quality. Special calibration cycles with reference reflectors are used to free the measuring signals from the ever-present systematic measuring errors. The digital signal processor allows the necessary degrees of freedom to develop efficient algorithms which would otherwise not be convertible in devices without a digital signal processor.
For more information contact Mahendra Rajcoomar, Krohne, +27 (0)11 314 1391, [email protected], www.krohne-mar.com
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