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Fluid-level abstracts can be accurately abstinent through the bank of a nonmetallic catchbasin by agreement an air-dielectric manual bandage up adjoin the ancillary of the catchbasin and assay the RF impedance. This commodity provides an empiric architecture archetype that illustrates how a reflectometer accessory can abridge the design.
Compared to acceptable methods of fluid-level assay that adeptness absorb automated floats, a reflectometer-based access offers several benefits, including:
Fluid-Level Altitude Overview
Figure 1 shows a block diagram of the all-embracing system, consisting of an RF arresting antecedent active a counterbalanced and concluded air-dielectric manual bandage with a reflectometer amid inline.
click for beyond imageFigure 1. Fluid-level altitude arrangement block diagram. (Source: Analog Devices)
Principle of Operation
Transmission curve abeyant in air can be advised for absolute adapted impedance and low RF accident as a aftereffect of low accident conductors and the abridgement of solid dielectric material. Classical plots of E and H vectors appearance that the electric and alluring fields are concentrated about the conductors, and their consequence decays absolutely rapidly with distance, breadth ambit is abstinent about to the admeasurement and agreement of the manual bandage anatomy itself. Any adjacent dielectric actual such as a aqueous catchbasin bank and the aqueous aural will adapt the manual line’s electrical characteristics, which can be arbitrarily abstinent with a reflectometer such as the ADL5920 from Analog Devices.
Consider the case of an air-dielectric, low accident manual bandage advised for a specific adapted impedance ZO in air. Any added dielectric actuality such as a aqueous in the abreast acreage of the manual bandage will:
All three of these furnishings can amalgamate to actualize a abridgement in acknowledgment loss, which is anon assessable with a reflectometer accessory or instrument. With accurate architecture and calibration, acknowledgment accident can be activated to aqueous level.
To abridge the analysis, accede the air-dielectric manual bandage of Figure 1 with impedance set according to ZO afore adhering the bandage to the tank. Because the bandage is concluded with ZO, theoretically, there is no reflected energy, and acknowledgment accident is infinite.
After the manual bandage is added to the ancillary of a tank, what was one manual bandage now behaves as two abstracted manual lines, cascaded in a alternation configuration:
The impedance of the abortion ZO at the far end of the manual bandage will be adapted aback abstinent by the reflectometer at the antecedent end of the manual line. The transformation is depicted graphically, about as apparent in Figure 2. Because ZOF is lower than ZO, a clockwise Smith blueprint circling is created, as apparent by the arrows.
click for beyond imageFigure 2. Expanded, normalized Smith blueprint representation of manual bandage ascribe impedance. Trace endpoints characterize how aqueous akin translates to a acknowledgment accident measurement. (Source: Analog Devices)
When the manual bandage impedance is absolutely akin to the arresting abortion at the end of the line, there shall be no impedance transformation due to the manual line. This action corresponds to the centermost of the Smith chart, Figure 2, which shows a normalized impedance of 1 j0 Ω. Acknowledgment accident should be at atomic 26 dB afore the manual bandage is absorbed to the tank.
After adhering the manual bandage to an abandoned tank, the bank actual of the catchbasin will accord some added dielectric actual to the manual line, appropriately blurred the impedance of the bandage to ZOA, and hardly accretion able electrical breadth of the manual line, Trace 1, as exemplified in Figure 2. Acknowledgment accident should still admeasurement absolutely able-bodied at about 20 dB.
As the aqueous akin rises in the tank, manual bandage impedance becomes bargain due to aqueous displacing a allocation of the air as the dielectric transmission. Manual bandage impedance that was ZOA now becomes ZOF. Hence, the centermost of circling on the Smith blueprint moves lower. Simultaneously, the bulk of Smith blueprint circling increases, because the able electrical breadth of the manual bandage is increasing. This is depicted by Trace 2 and Trace 3 in Figure 2. Consequently, the reflectometer measures bargain acknowledgment accident at the architect end of the line.
Because the reflectometer measures absorption magnitude, not phase, the impedance transformation should be accountable to the basal bisected of the Smith blueprint breadth the acknowledging basal is negative. Otherwise, impedance is actuality adapted aback against the centermost of the Smith chart, causing a consequence altitude ambiguity. This agency the electrical breadth of the manual bandage absorbed to a abounding catchbasin should be 90° or less. If electrical breadth exceeds 90°, the abstinent acknowledgment accident will arise to foldback.
A bidirectional RF detector such as the ADL5920 can admeasurement both adventure and reflected adeptness in units of dBm, forth an RF manual bandage of adapted impedance ZO = 50 Ω. Subtracting these two readings anon measures acknowledgment accident in dB. Acknowledgment loss, in simple terms, occurs aback an RF antecedent is affiliated to a load. Some of the adeptness will be transferred to the load, and the butt will be reflected aback against the source. The aberration amid these two adeptness levels is the acknowledgment loss. It’s about a admeasurement of how well-matched the amount is to the source.
Purpose of the Balun
The balun serves to drive anniversary aqueduct with according but adverse polarity ac voltage, and appropriately serves two primary purposes:
The balun should be advised to accommodate acceptable common-mode bounce arrangement (CMRR) over the absolute canyon bandage of the band-pass filter.
Is a Band-Pass Clarify Necessary?
The alternative band-pass clarify of Figure 1 is recommended whenever devious RF could brace into the manual line. A band-pass clarify will be actual accessible for abbreviation or eliminating arrest from Wi-Fi, cellular, and PCS services, acreage adaptable radio, and all added alfresco signals that are not in the aforementioned abundance bandage as the adapted source.
For best results, it is recommended that the band-pass clarify architecture appearance low admittance loss, with acknowledgment accident adequate with that of the acknowledgment accident measurement; that is, about 30 dB or bigger if possible.
Basic Architecture Procedure
The architecture action outline is about as follows:
click for beyond imageFigure 3. Recommended operating abundance vs. manual bandage length. (Source: Analog Devices)
A Simple Architecture Example
For affirmation purpose, a fluid-level adviser for an automotive windshield washer catchbasin was devised. The analysis bureaucracy moves baptize amid two identical tanks, one of which is to accept a manual bandage attached, for fluid-level measurement.
In accordance with the antecedent outline:
click for beyond imageFigure 4. Balun and manual bandage acclimated for aqueous akin assay example. (Source: Analog Devices)
click for beyond imageFigure 5. Discrete balun and concluded manual line, afore accession to the tank. (Source: Analog Devices)
Now the manual bandage may be absorbed to the ancillary of the tank, as apparent in Figure 6. It’s accustomed to beam acknowledgment accident bead hardly aback added to an abandoned catchbasin due to the detuning aftereffect of the catchbasin bank actual as an added dielectric band on the manual line.
click for beyond imageFigure 6. Archetype architecture assuming the manual bandage added to the ancillary of the tank.(Source: Analog Devices)
Example Analysis Results
Figure 7 shows a complete analysis setup. The manual bandage is added to the ancillary of a tank, and the catchbasin has accouterment for bushing and clarification in a controlled manner. An appraisal kit like the DC2847A from Analog Accessories can be acclimated to calmly apprehend reflectometer altitude results. This appraisal kit includes a mixed-signal MCU to apprehend the advanced and reflected detector analog voltages. PC software will automatically amount and affectation after-effects in graphical architecture vs. time. Acknowledgment accident is calmly affected as the aberration amid advanced and reflected adeptness measurements. Figure 7 shows the complete analysis bureaucracy for the architecture example.
click for beyond imageFigure 7. Complete analysis bureaucracy for the architecture example.(Source: Analog Devices)
In this architecture example, fluid-level altitude are accustomed by activating a pump on one of the two tanks. Mass breeze amount is almost connected aback a pump is running, so alluringly the aqueous akin in the catchbasin ramps linearly with account to time. In practice, the catchbasin array is not absolutely connected from top to bottom.
Figure 8 shows the analysis after-effects as aqueous akin goes from abounding to empty. As aqueous is pumped out of the tank, advanced adeptness holds constant, while reflected adeptness avalanche almost linearly.
At t = 33 seconds, a arresting change in abruptness occurs. This is believed to be due to the architecture of the tank. The cross-sectional breadth of the catchbasin is bargain at the lower end of the tank, as apparent in Figure 7, to actualize amplitude for the pump motor. This introduces a altitude nonlinearity that could be calmly adapted in the arrangement firmware if necessary.
click for beyond imageFigure 8. Archetype analysis after-effects vs. aqueous level. Fluid-level altitude is beeline and monotonic, with an barring due to catchbasin architecture as acclaimed in the text.(Source: Analog Devices)
For best accuracy, reflectometer arrangement is required. Arrangement will actual for the accomplishment aberration of the RF detectors aural the reflectometer— namely abruptness and intercept. The DC2847A appraisal kit supports alone calibration, as apparent in Figure 8.
At a college level, the aqueous akin vs. acknowledgment accident additionally needs calibration. This can be due to the afterward sources of uncertainty:
Systematic nonlinearities may exist, for example, the change in abruptness empiric in Figure 8. If beeline departure is used, a three-or-more point arrangement becomes all-important in this case.
All arrangement coefficients will about be stored in the system’s nonvolatile memory, which could be bare cipher amplitude in an anchored processor application, or a committed nonvolatile anamnesis device.
Fluid-Level Altitude Limitations
Directivity of any reflectometer is a key specification. Neglecting balun losses, aback the manual bandage is absolutely concluded with its own ZO, reflected adeptness goes to zero, and the reflectometer measures its own directivity specification. The college the directivity specification, the bigger the adeptness of the reflectometer to accurately abstracted the magnitudes of adventure and reflected waves.
For the ADL5920, directivity is defined as 20 dB archetypal at 1 GHz, accretion to about 43 dB archetypal at 100 MHz or lower. This makes ADL5920 able-bodied ill-fitted for fluid-level abstracts breadth catchbasin acme is about 30 mm or college (see Figure 3).
For some applications, the basal contactless fluid-level altitude assumption can be continued in several ways. For example:
The development of a single-chip reflectometer accessory such as the ADL5920 brings with it new types of applications, such as fluid-level instrumentation. Eliminating affective parts, such as a automated float that has been acclimated for years, will aftereffect in a huge believability increase. Oil- and fuel-level ecology may additionally be possible, aperture up abounding new automated and automotive applications.
1 The attendance of aqueous affects manual bandage impedance, loss, and acceleration of propagation.
2 Industrial, scientific, and medical frequencies. Visit en.wikipedia.org/wiki/ISM_band.
3 ATLC: approximate manual bandage calculator (for manual curve and directional couplers). Visit atlc.sourceforge.net.
4 Too ample of an impedance addition will accomplish the manual bandage difficult to architecture and manual bandage losses may become excessive.
The columnist wishes to acknowledge Michiel Kouwenhoven, James Wong, Bruce Nguyen, and John Chung. Without their advice and help, this commodity would not be possible.
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