STC Filter Error Compensation in FMCW Level Measurement
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Solution Overview
Problem
Level measuring devices using the frequency modulated continuous wave (FMCW) principle face accuracy issues due to high dynamic range echoes, particularly in the short range of the antenna, where spurious echoes can distort the measuring signal, affecting the accuracy of the filling level determination.
Innovation Solution
A level measuring device and method that compensate for errors in the receiving branch by generating a reference signal to calculate correction values, using an inverse filter function to correct the measuring signal processed through a Sensitivity Time Control (STC) filter, thereby improving accuracy in the short range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a high-pass filter is used to attenuate large signals in the short range of the antenna, then the dynamic range of the IF signal is reduced, but the measuring accuracy in the short range deteriorates due to distance-dependent attenuation of echo signals
Solution Approach 1:
The patent applies preliminary action by measuring the filter function of the receiving branch before actual level measurement. The system characterizes the STC filter's distance-dependent attenuation by transmitting test signals at known distances, storing the measured attenuation characteristics, and then using these pre-measured data to correct subsequent measurements. This preliminary characterization enables the system to compensate for the filter's effect on short-range echoes.
Solution Approach 2:
The patent implements feedback by using the measured filter function to calculate correction values that are applied to the received signals. The system continuously monitors the attenuation characteristics and uses this information to adjust the interpretation of echo amplitudes, particularly for short-range measurements. This feedback mechanism allows the system to account for the distance-dependent attenuation introduced by the STC filter.
2Device complexity
If STC function is used to attenuate large signals in the short range, then the processing of all echoes in the IF channel becomes feasible, but the course of the measuring signal curve is influenced affecting accuracy
Solution Approach 1:
The patent introduces an intermediary correction mechanism that mediates between the STC filter's signal attenuation and the measurement accuracy requirement. By measuring the filter function and using it to calculate correction values, the system creates an intermediate step that compensates for the filter's effect. This intermediary correction layer allows the STC filter to perform its dynamic range compression function while simultaneously enabling accurate short-range measurements through mathematical compensation.
Data Source
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AI summary
Level measuring device which can compensate the distortions, caused by an STC filter, of the received signal, by measuring a reference signal which passes through the receiving branch and also through the STC filter, during the ongoing operation of the level measuring device or during manufacture. For example, after passing through the receiving branch, this reference signal can be fed to a microprocessor which can calculate the correction values of the IF signal therefrom. A switch can be provided which can switch over between the reference signal and the IF signal.