FMCW Radar Level Gauge Single Conductor Probe Impedance Matching
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Solution Overview
Problem
FMCW-type radar level gauges face challenges in achieving high accuracy due to external and internal reflections, which can cause deviations in distance measurements, especially when using high-frequency systems and traditional waveguides like still-pipes are costly and not always effective.
Innovation Solution
The use of a single conductor probe as a surface waveguide with an electrical feed-through and a matching arrangement that provides an impedance match between the probe and the feed-through, reducing reflections and improving measurement accuracy, while using a frequency modulated continuous wave system with a bandwidth of at least 1 GHz and a relative bandwidth less than 2.5, and an upper frequency limit below 4 GHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single conductor probe is used as a surface waveguide instead of a still-pipe, then cost is reduced and the system is simplified, but impedance mismatch occurs causing reflections that degrade measurement accuracy
Solution Approach 1:
An impedance matching arrangement is introduced as an intermediary component between the single conductor probe and the feed-through. This matching arrangement mediates the impedance transition, reducing reflections caused by the impedance mismatch while maintaining the cost benefits of using a single conductor probe instead of an expensive still-pipe waveguide.
Solution Approach 2:
The impedance matching arrangement modifies the impedance parameter along the signal path. By transforming the impedance from the high impedance of the single conductor probe to the lower impedance of the feed-through, the system achieves better impedance matching and reduced reflections, thereby improving measurement accuracy while maintaining cost efficiency.
2Measurement precision
If the upper frequency limit is kept below 4 GHz with a relative bandwidth less than 2.5, then impedance matching is improved and reflections are reduced, but distance resolution capability is limited
Solution Approach 1:
The system operates within a controlled frequency range (upper limit below 4 GHz, relative bandwidth less than 2.5) to optimize impedance matching conditions. This parameter selection reduces reflections and improves measurement stability, while the matching arrangement further enhances the impedance match to compensate for the reduced frequency bandwidth.
3Manufacturing precision
If a bandwidth of at least 1 GHz is used, then distance resolution is improved, but the system becomes more sensitive to external and internal reflections
Solution Approach 1:
The impedance matching arrangement serves as a mediator that reduces the strength of internal reflections at the probe-feed-through interface. By minimizing these spurious reflections, the system can utilize the full 1 GHz bandwidth for improved distance resolution without being overly sensitive to harmful reflections from the transmission path.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces reflections and improves measurement accuracy by minimizing impedance mismatch, allowing for accurate distance determination with reduced power losses and component costs, and is suitable for use in varying tank environments without the need for calibration pulses.
Implementation Method 1
a frequency modulator configured to modulate a frequency of the transmit signal to vary within a frequency range
Implementation Method 2
a mixer configured to mix the transmit signal and the return signal to provide an intermediate frequency signal
Implementation Method 3
a single conductor probe mechanically suspended in a top of the tank and extending into the product in the tank, the single conductor probe being electrically connected to the transceiver and adapted to guide the electromagnetic transmit signal towards the surface and to guide the electromagnetic return signal to the transceiver circuitry
Implementation Method 4
a matching arrangement between the first and second input impedance providing an electrically matched connection between the electrical feed-through and the single conductor probe, a reflection factor of the matched connection being less than -10dB
Data Source
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AI summary
FMCW-type radar level gauge (2) for determining the distance to the surface (7) of a product (6) in a tank (5) comprising a transceiver (11) to emit an electromagnetic transmit-signal and to receive an electromagnetic return-signal reflected by the surface (7), the electromagnetic transmit-signal having a bandwidth of at least 1 GHz, a relative bandwidth (max frequency / min frequency) of less than 2.5 and an upper frequency-limit smaller than 4 GHz. The gauge (2) comprises a single conductor probe (9) mechanically suspended and extending into the product (6) in the tank (5), and a matching arrangement providing an electrically-matched connection between a sealed electrical feed-through (21) and the single conductor probe (9). The relatively expensive still-pipe of the prior-art is here replaced with a relatively inexpensive single conductor probe (9) acting as a surface wave-guide (SWG).