JP5773951B2 - Liquid level measuring device and its VCO predistortion method - Google Patents

Description

  The present invention relates to a radar-type liquid level measuring apparatus that measures the liquid level in a tank that stores liquid such as liquefied gas and the like, and its VCO predistortion method, and in particular, by using a waveguide. This suppresses measurement errors caused by changes in the propagation speed of radio waves in the waveguide.

One of the radar systems used in the liquid level measuring device is an FMCW (Frequency Modulated Continuous Wave) radar system. In this FMCW radar system, as shown in FIG. 10, in a predetermined fixed time (this time is referred to as a sweep time (T)), a predetermined frequency (F) is swept toward a measurement point. It transmits radio waves. As shown in FIG. 11, the transmission frequency is F during a round-trip time t from when a radio wave transmitted at a transmission point is reflected at a measurement point (L is a distance from the transmission point) and returns to the transmission point. -Swept by t / T (Hz). The swept frequency is the difference of the reception frequency F _R and the transmission frequency F _T of the moment of receiving the reflected wave (reception frequency F _R) (beat frequency F _B).

The round trip time t is t = (T / F) × FB
Therefore, if the beat frequency FB can be measured, the time t required for radio waves to reciprocate from the transmission point to the measurement point can be measured. Since the propagation speed of the radio wave in the free space is the same as the speed of light C, the distance L from the transmission point to the measurement point can be expressed by the following formula 1.
Formula 1
L = C × t / 2 = C × T × FB / 2F

  The principle of distance measurement by the FMCW radar method has been schematically described above. FIG. 9 shows a conventional example of an FMCW radar type distance measuring device. In FIG. 9, the FMCW radar type distance measuring device includes a DSP (Digital Signal Processor) 101, a digital / analog converter 103, a VCO (Voltage Controlled Oscillator) 104, a coupling circuit 105, a mixer 110, an AGC (AGC). Automatic gain control circuit) 111 and analog / digital converter 112. The components including the digital / analog converter 103 and the VCO 104 constitute a transmission system, and the components from the mixer 110 to the analog / digital converter 112 constitute a reception system. The DSP 101 has a built-in memory 102. The relationship between the voltage applied to the VCO 104 that determines the oscillation frequency of the VCO 104 with respect to the sweep time T, that is, a voltage-time curve (hereinafter referred to as “VT curve”) is voltage-time. It is stored as a table (hereinafter referred to as “VT table”).

  The DSP 101 reads the VT table from the memory 102, converts the voltage value that continuously changes with the passage of time into an analog signal by the digital / analog converter 103, and sets it as the control voltage of the VCO 104. The oscillation frequency of the VCO 104 changes continuously according to the control voltage. This oscillation signal is transmitted from the appropriate antenna to the measurement point (for example, the liquid level) via the coupling circuit 105. A system under measurement 120 is interposed between the antenna and the measurement point. The radio wave reflected at the measurement point returns to the measured system 120 and is captured by the antenna, and is guided to the receiving system via the coupling circuit 105. In the receiving system, the mixer 110 mixes the received signal and the oscillation signal at the time of reception, and the difference between the reception frequency and the instantaneous oscillation frequency, that is, the beat frequency FB signal is extracted.

  The beat frequency FB signal is controlled to an appropriate amplitude value by the AGC 111, converted to a digital signal by the analog / digital converter 112, and input to the DSP 101. In the DSP 101, the distance L to the measurement point is obtained by performing arithmetic processing by applying the formula 1.

  The FMCW radar system distance measurement principle described above is established in free space. However, in a device that measures the distance of a measurement target having a low reflection coefficient by a radar method, a waveguide is used for a radio wave transmission path. This is because the transmission loss in the transmission line can be greatly reduced by using the waveguide.

  However, in a radar-type liquid level measuring device using a waveguide as a transmission path, the propagation speed in the waveguide depends on the inner diameter of the waveguide and the mode of the radio wave propagating in the waveguide, and varies depending on the frequency. For this reason, the beat frequency of the FMCW radar changes with the frequency sweep, and the measurement accuracy of the liquid level measuring device is lowered. Therefore, in order to increase the measurement accuracy of the liquid level measuring device, it is necessary to consider the frequency characteristics of the radio wave propagating in the waveguide.

  As in the example shown in FIG. 9, the transmitter of the FMCW radar generally uses the VCO 104. The VCO 104 oscillates when a control voltage is applied, and the oscillation frequency continuously changes during the sweep time T by continuously changing the control voltage corresponding to the sweep frequency. In FIG. 10, the time from the start of the sweep to the stop of the sweep is represented by T, and the change width of the oscillation frequency from the start of the sweep to the stop of the sweep is represented by F. Hereinafter, a graph representing a change in the oscillation frequency F of the VCO 104 with respect to the control voltage V applied to the VCO 104 is referred to as a frequency-voltage characteristic (hereinafter referred to as “FV characteristic”), and represents a change in the oscillation frequency of the VCO 104 during the sweep time T. The graph is referred to as a frequency-time characteristic (hereinafter referred to as “FT characteristic”).

  The VCO is generally used in a PLL (Phase Locked Loop) circuit that can oscillate a signal having a stable frequency, and the use of sweeping the frequency like the VCO 104 is a special use. The linearity of the frequency-voltage characteristic (FV characteristic) output from the VCO 104 is insufficient to obtain the measurement accuracy required by the FMCW radar. Therefore, in the FMCW radar, in order to guarantee the linearity of the FT characteristic output from the VCO 104, a corrected voltage-time table (hereinafter referred to as "corrected VT Curve table ”) and the oscillation voltage of the VCO 104 is controlled using this table.

  However, even if the linearity of the FV curve output from the VCO 104 is guaranteed, the liquid level measuring device using the waveguide in the transmission path, as described above, the frequency of the radio wave propagating in the waveguide. Measurement accuracy deteriorates depending on the characteristics.

  In the liquid level measurement device using the waveguide as described above, a prior art for improving the measurement accuracy in consideration of the frequency characteristics of the radio wave propagating in the waveguide has not been found, but a technique related to the present invention. There exists patent document 1 as what discloses this. Patent Document 1 describes a frequency calibration structure including a VCO having two modes of a normal operation mode and a calibration (calibration) mode, which can be applied to an FMCW radar. The VCO is incorporated in the PLL circuit in the calibration mode to replace the calibration VCO, and has a technical idea different from that of the present invention.

  Further, in Patent Document 2, in order to avoid the influence of the dielectric constant of the gas above the liquid on the calculated liquid level, the transmitter is a group of microwave signals in a propagation mode in the waveguide. A liquid level measuring device adapted to transmit a microwave signal in a frequency band within a predetermined dielectric constant range in which speed basically does not affect the dielectric constant is disclosed. Although the technical idea described in Patent Document 2 focuses on the frequency characteristics of radio waves propagating in the waveguide, the technical idea for avoiding measurement accuracy deterioration due to the frequency characteristics is different from the present invention.

US Pat. No. 7,804,369 JP-T-2006-506639

  The present invention relates to a liquid level measuring device and a liquid level measuring device capable of preventing the measurement accuracy from deteriorating due to the frequency characteristics of radio waves propagating in the waveguide when a waveguide is used in the transmission line. An object of the present invention is to provide a predistortion method for a VCO included in the above.

The present invention
While sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface through the waveguide, and the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is An FMCW radar type liquid level measuring device that measures the distance to the liquid level with a certain beat frequency,
A VCO that generates a sweep frequency signal;
A voltage-time table (hereinafter referred to as “VT table”) in which a voltage-time curve (hereinafter referred to as “VT curve”) for sweeping the frequency of the signal generated by the VCO is stored ;
A spectrum analyzer for analyzing the difference between the beat frequency and the sweep frequency;
A predistortion table generating unit that generates a predistortion table for correcting the VT table in accordance with the degree of change in the difference in beat frequency from the analysis result by the spectrum analyzing unit;
A predistortion VT curve table generating unit that generates the predistortion VT curve table by applying the predistortion table to the VT curve;
A memory for storing the predistortion VT curve table and storing a control voltage to be applied to the VCO from the predistortion VT curve table to cause the VCO to generate the sweep frequency signal ;
It has the most important feature.

  Even if the radio wave propagating in the waveguide has frequency characteristics, the oscillation frequency of the VCO can be controlled in accordance with the frequency characteristics, so that the spread of the beat frequency between the oscillation frequency and the reception frequency is suppressed, Degradation of measurement accuracy can be prevented.

It is a flowchart which shows in order the process of the VCO predistortion process in the Example of the liquid level measuring apparatus which concerns on this invention. It is a block diagram which shows the Example of the liquid level measuring apparatus which concerns on this invention. It is a schematic diagram which shows notionally the mode of measurement by the liquid level measuring apparatus which concerns on this invention. It is a perspective view which shows the example of a waveguide whose cross section is a square. It is a perspective view which shows the example of a waveguide with a circular cross section. It is a graph which shows the example of the propagation speed with respect to the frequency of the electromagnetic wave in a waveguide. It is a graph which shows the relationship of the beat frequency with respect to the sweep frequency of the liquid level measuring apparatus using a waveguide. It is a graph which compares and shows the spectrum waveform obtained by carrying out FFT processing of the beat signal by free space reflection, and the beat signal by reflection in a circular waveguide. It is a block diagram which shows the example of the conventional liquid level measuring apparatus. It is a graph which shows the example of the frequency sweep by FMCW radar. It is a graph which shows the distance measurement principle by the frequency sweep of FMCW radar.

  Embodiments of a liquid level measuring device and its VCO predistortion method according to the present invention will be described below with reference to the drawings.

  FIG. 2 shows an embodiment of the liquid level measuring device by the FMCW radar system according to the present invention. In FIG. 2, the liquid level measuring device includes a DSP 11, a digital / analog converter 13, a VCO 14, a coupling circuit 15, a switch 16, a mixer 20, an AGC 21, and an analog / digital converter 22. The components including the digital / analog converter 13 and the VCO 14 constitute a transmission system, and the components from the mixer 20 to the analog / digital converter 22 constitute a reception system. The DSP 11 has a built-in memory 12, and the memory 12 stores the relationship of the voltage applied to the VCO 14 that determines the oscillation frequency of the VCO 14 with respect to the sweep time T, that is, the VT curve as a VT table. The memory 12 also stores a predistortion table, a predistortion VT curve table, etc., which will be described later.

  The portion described so far is the same as the configuration of the conventional distance measuring apparatus using the radar system shown in FIG. 9 except that the switch 16 is added. Therefore, the operation at the time of measurement is also the same as that in the conventional example. That is, the DSP 11 reads from the VT table stored in the memory 12 a voltage that controls the operation of the VCO 14 in synchronization with the clock frequency determined by the sweep time and the number of sampling points. The read control voltage value is converted into an analog signal by the digital / analog converter 13 and applied to the VCO 14 as a control voltage. The VCO 14 outputs a signal having a frequency corresponding to the control voltage, that is, a high-frequency signal whose frequency continuously changes with time. This high frequency signal is transmitted from the appropriate antenna to the system under test 25 via the coupling circuit 15 and the switch 16.

  The system under test 25 has a liquid level that is a measurement point, and the high-frequency signal is reflected by the liquid level. The reflected high-frequency signal returns to the system to be measured 25 and is captured by the antenna, and is guided to the receiving system through the switch 16 and the coupling circuit 15. In the mixer 20 of the reception system, the reception signal and the oscillation signal at the time of reception are mixed, and a difference signal between the reception frequency and the oscillation frequency at that moment, that is, a beat signal is extracted. The beat signal is adjusted to an appropriate amplitude by the AGC 21, converted to a digital signal by the analog / digital converter 22, and input to the DSP 11.

  In the DSP 11, during the sweep time T, VT curve data is read and beat signals are captured, and unnecessary noise is removed by performing filtering processing on the captured beat signal data group on the time axis. Further, the DSP 11 performs FFT (Fast Fourier Transform) processing on the beat signal data group to convert it to frequency axis data, and extracts the beat frequency FB from the result. The beat frequency FB is input to the equation 1 and subjected to calculation processing to obtain the distance L to the measurement point. Therefore, the DSP 11 includes the noise removal filter and the FFT processing unit as the beat frequency FB extraction unit.

  An installation example of the liquid level measuring apparatus described above is schematically shown in FIG. In FIG. 3, reference numeral 10 denotes a liquid level measuring device including a radar unit, and reference numeral 40 denotes a tank in which the liquid level measuring device 10 is installed. The tank 10 stores a liquid 41 such as liquefied gas. In the tank 40, a cylindrical waveguide 30 is vertically installed from the top plate of the tank 40 toward the bottom of the tank 40. The liquid 41 can enter the waveguide 30, and the liquid level in the tank 40 matches the liquid level in the waveguide 30. On the top plate of the tank 40, the liquid level measuring device 10 including a radar unit is installed above the waveguide 30. The liquid level measuring apparatus 10 is an FMCW radar type liquid level measuring apparatus configured by each block shown in FIG. 2, and a converter 32 is interposed between the liquid level measuring apparatus 10 and the upper end of the waveguide 30. doing. The converter 32 is an antenna that converts a signal output from the unit 10 into a radio wave and radiates it into the waveguide 30 toward the liquid surface.

The space from the surface of the liquid 41 in the tank 40 to the ceiling surface is filled with the evaporated gas of the liquid 41. Let L be the distance from the ceiling surface to the surface of the liquid 41. The inner diameter of the waveguide 30 is represented by d. F _T is the frequency of the radio waves emitted toward the liquid surface in the waveguide 30 from the liquid level measuring device 10, F _R is a liquid level measuring device in the waveguide 30 the radio wave is reflected by the liquid surface 10 The frequency of the radio wave returning toward is shown.

  The embodiment of the liquid level measuring device according to the present invention shown in FIG. 2 is different from the conventional radar type distance measuring device in that a near-end reflecting member 26 is provided. The near-end reflecting member 26 is made of a member capable of reflecting radio waves radiated from the liquid level measuring device, that is, the radar unit 10, and is in the waveguide 30 shown in FIG. 3 and is a ceiling surface of the tank 40. At the same time, it is installed at a known length position from the reference plane position which is also the radio wave radiation surface of the converter 32. The near-end reflecting member 26 is a member necessary for performing VCO calibration. VCO calibration refers to correcting the VT curve stored in the memory 12 in accordance with the FV characteristic of the VCO 14 so that the FT curve of the signal output from the VCO 14 becomes a straight line. That is, even if the linearity of the VT curve is maintained, if the linearity of the FV curve of the VCO 14 is broken due to the characteristics of the individual VCO 14 or changes over time, the FT output from the VCO 14 The curve collapses. Therefore, calibration is performed so that the linearity of the FT curve is maintained.

  However, since VCO calibration is not the subject of the present invention, and the subject of the present invention is VCO predistortion, a specific configuration for performing VCO predistortion and a method of VCO predistortion will be described below.

The wavelength λg of the radio wave propagating in the waveguide is expressed by the following formula 2.
Formula 2

Here, λo is the wavelength of the propagating frequency, and λc is the cutoff frequency, that is, the wavelength of the lowest frequency that can be transmitted through the waveguide.

In the case of a waveguide having a rectangular cross section as shown in FIG. 4 and having a long side dimension a and a short side dimension b on the inner surface side, the wavelength λc of the cutoff frequency is It can be expressed by Equation 3.
Formula 3
Here, m and n are the number of modes in the waveguide, respectively. Therefore, in the basic mode, that is, the TE10 mode, since m = 1 and n = 0, λc = 2a.

In the case of a waveguide having a circular cross section as shown in FIG. 5 and a waveguide having an inner diameter d, the wavelength λc of the cutoff frequency can be expressed by the following equation 4.
Formula 4
λc = Knm · d
Here, Knm is a unique value for each transmission mode. That is, Knm = 1.706 in the TE11 mode and Knm = 0.82 in the TE01 mode.

The velocity vg of the radio wave propagating in the waveguide can be expressed by the following formula 5.

Formula 5

As can be seen from Equation 5, the velocity vg of the radio wave in the waveguide is a function of the wavelength λo of the propagating frequency, and the velocity vg changes as the frequency changes. The graph of FIG. 6 represents the frequency characteristic of the in-pipe speed, that is, the change in the in-pipe speed (vg) with respect to the change in the frequency (f), and the in-pipe speed changes nonlinearly with respect to the frequency.

Substituting Equation 5 into Equation 1 yields Equation 6.
Equation 6

From Equation 6, the beat frequency FB can be obtained by the following Equation 7.
Equation 7

  FIG. 7 is a graph showing changes in the beat frequency FB with respect to the sweep frequency when the reflecting surface is fixed. The beat frequency FB does not change linearly with respect to the change in the sweep frequency, and is non-linear. Has changed.

  FIG. 8 shows a spectrum waveform obtained by subjecting the beat signal to FFT processing. Waveform A is a spectrum waveform in the case of free space reflection, and waveform B is a spectrum waveform in the case of reflection in a circular waveguide. Show. The spectrum waveform is broader in the case of reflection in a circular waveguide than in the case of free space reflection. This indicates that the beat frequency is changed by the frequency sweep of the FMCW radar in the circular waveguide even if the measurement point, that is, the reflection point of the radio wave does not change. In the case of frequency sweeping in a circular waveguide, it can be seen that the spectrum waveform obtained by subjecting the beat signal to the FFT calculation process is widened, so that the accuracy of peak detection deteriorates and the measurement accuracy deteriorates.

  As described above, the cause of the deterioration of measurement accuracy is that the beat frequency is changed by the frequency sweep of the FMCW radar. Therefore, in anticipation of a change in the beat frequency due to the frequency sweep, the VT curve characteristic of the control voltage applied to the VCO 14 is reversed with respect to the change in the beat frequency to suppress the change in the beat frequency. When the change in the beat frequency is suppressed, the spread of the spectrum indicated by the waveform B in FIG. 8 is suppressed, and the deterioration of measurement accuracy can be suppressed by sharpening the spectrum waveform. As described above, generating a VT curve in which the control voltage applied to the VCO 14 has reverse characteristics is referred to as “VCO predistortion”.

  FIG. 1 shows the procedure of VCO predistortion. Processing steps are shown in the central vertical direction of FIG. Each processing step is provided with a reference numeral such as S1, S2,. Hereinafter, the procedure of VCO predistortion will be described.

  First, the control voltage of the VCO 14 is read from the VT curve table prepared in advance in accordance with the lapse of the sweep time, and this control voltage is applied to the VCO 14 to generate a signal having a frequency corresponding to the control voltage. . In the VT curve table, the linearity should be maintained originally, but the VCO is maintained so that the linearity of the FT curve output from the VCO 14 (see the graph on the upper left and right in FIG. 1) is maintained. A calibration is being performed. The VT curve shown on the upper left side of FIG. 1 is a non-linear VT curve in which VCO calibration has been executed in advance according to the characteristics of the VCO 14 or the like.

  A frequency sweep is performed in accordance with a non-linear VT curve table in which VCO calibration is performed (S1), and a radio wave generated by the sweep is radiated toward a measurement point through a circular waveguide. The radio wave reflected at the measurement point is received, and as described above, the beat waveform of the reception frequency and the transmission frequency at a certain time point within the sweep time is acquired (S2). The upper right waveform in FIG. 1 shows an image of the beat waveform.

  Next, beat spectrum analysis is performed by subjecting the beat waveform to FFT calculation processing (S3). The waveform diagram in the second stage from the upper right of FIG. 1 shows an image of spectrum analysis of the beat waveform. As shown in the waveform diagram, the beat frequency difference with respect to the center frequency of the sweep frequency is obtained, and the spectrum analysis of the beat waveform is performed by analyzing the degree of change in the beat frequency difference with respect to the sweep frequency. This spectrum analysis is executed as a part of the function of the DSP 11 by a program incorporated in the DSP 11.

  Next, a spectrum correction function: D (x) is generated from the spectrum analysis result of the beat waveform (S4). The waveform diagram in the third stage on the right side of FIG. 1 shows an example of the spectrum correction function: D (x). The spectrum correction function: D (x) corresponds to the degree of change in the beat frequency difference. Yes. The generation of the spectrum correction function: D (x) is executed as a part of the function of the DSP 11 by a program incorporated in the DSP 11.

  Next, a predistortion table is generated based on the spectrum correction function: D (x) (S5). The predistortion table is a table for correcting the VT curve applied to the VCO 14, and the waveform diagram at the bottom right side of FIG. 1 shows the VT correction curve. The VT correction curve has a reverse characteristic with respect to the waveform representing the spectrum correction function: D (x). A predistortion table generation function is also provided in the DSP 11 as a part of the function.

  Next, a predistortion VT curve table is generated by applying the VT correction curve to the VT curve (S6). A predistortion VT curve table generation function is also provided in the DSP 11 as part of the function. The left graph of the two graphs shown in the lower left of FIG. 1 shows the VT curve, the curve a1 shows the initial VT curve before correction, and the curve a2 shows the predistortion VT curve. . The initial VT curve a1 is a VT curve after the VCO calibration is performed.

  The right graph of the above two graphs shows the FT curve output from the VCO 14, the curve b1 shows the FT curve before the predistortion, and the curve b2 shows the FT curve after the predistortion. ing. The FT curve b1 before pre-distortion maintains linearity, but when a sweep frequency signal is generated according to the FT curve b1, a change in the propagation speed of radio waves in the waveguide, that is, a change in frequency as described above. Becomes a factor of deterioration of measurement accuracy. Therefore, a sweep frequency signal is generated according to the FT curve b2 after the predistortion. Further, the pre-distortion FT curve b2 is stored as a pre-distortion VT curve table in a memory built in or attached to the DSP 11.

  A predistortion VT curve table is used for the subsequent liquid level measurement. By controlling the oscillation frequency of the VCO 14 in accordance with the predistortion VT curve table, a frequency signal corresponding to a change in the propagation speed of the radio wave in the waveguide is output, so that the beat frequency changes within the sweep time. Less. As a result, the spectrum waveform of the beat frequency signal is sharpened, and the measurement accuracy is increased.

  As described above, according to the liquid level measuring device and the VCO predistortion method according to the present invention, the oscillation frequency of the VCO is controlled according to the VT curve table in which predistortion is executed. Therefore, even if the radio wave propagating in the waveguide has frequency characteristics, it is possible to control the oscillation frequency of the VCO corresponding to the frequency characteristics, and the spread of the beat frequency between the oscillation frequency and the reception frequency is suppressed. The sharpness of the spectrum analysis result of the beat frequency is increased, and the measurement accuracy can be increased.

  The measurement object according to the present invention may be anything as long as it is a liquid, but is particularly suitable for an application for measuring a highly accurate liquid level.

11 DSP
12 Memory 13 Digital to Analog Converter 14 VCO
15 coupling circuit 16 switch 20 mixer 21 AGC circuit 25 system to be measured

Claims (8)

While sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface through the waveguide, and the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is An FMCW radar type liquid level measuring device that measures the distance to the liquid level with a certain beat frequency,
A VCO that generates a sweep frequency signal;
A voltage-time table (hereinafter referred to as “VT table”) in which a voltage-time curve (hereinafter referred to as “VT curve”) for sweeping the frequency of the signal generated by the VCO is stored ;
A spectrum analyzer for analyzing the difference between the beat frequency and the sweep frequency;
A predistortion table generating unit that generates a predistortion table for correcting the VT table in accordance with the degree of change in the difference in beat frequency from the analysis result by the spectrum analyzing unit;
A predistortion VT curve table generating unit that generates the predistortion VT curve table by applying the predistortion table to the VT curve;
A memory for storing the predistortion VT curve table and storing a control voltage to be applied to the VCO from the predistortion VT curve table to cause the VCO to generate the sweep frequency signal ;
A liquid level measuring device. It includes a spectrum correction function generating unit for generating a spectrum correction function corresponding to the degree of change in the difference in the beat frequency from the analysis result by the spectrum analyzing portion,
The liquid level measurement device according to claim 1, wherein the predistortion table generation unit generates a predistortion table for correcting a VT curve based on the spectrum correction function. The liquid level measurement device according to claim 1, wherein the spectrum analysis unit includes an FFT calculation unit that performs FFT calculation processing on a difference between beat frequencies. The V-T table, VCO carry Bray sucrose emissions is a linear executed Frequency - time characteristic (hereinafter "F-T characteristics" hereinafter) liquid level measuring apparatus according to claim 1, wherein implementing the. While sweeping a predetermined frequency in the sweep time, the radio wave is transmitted toward the liquid surface through the waveguide, and the difference between the transmission frequency at the moment when the radio wave reflected by the liquid surface is received and the reception frequency of the reflected wave is A VCO predistortion method in an FMCW radar type liquid level measuring device that measures the distance to the liquid level with a certain beat frequency,
Sweeping the frequency of a signal generated by the VCO according to a voltage-time table (hereinafter referred to as “ VT table”) in which a voltage-time curve (hereinafter referred to as “VT curve”) is stored ;
Spectrum analysis of the difference of the beat frequency with respect to the sweep frequency;
Generating a predistortion table for correcting the VT curve corresponding to the degree of change in the difference in beat frequency from the spectrum analysis result;
Applying a VT correction curve of the predistortion table to the VT curve to generate a predistortion VT curve table;
Storing the predistortion VT curve table in a memory;
Providing the VCO as a control signal for generating the sweep frequency signal from the predistortion VT curve table;
A VCO predistortion method for a liquid level measuring device. Comprising the step of generating a spectrum correction function corresponding to the degree of change in the difference of the beat frequency from the spectrum analysis results, according to claim 5, wherein generating the predistortion table for correcting the V-T curve based on the above spectrum correction function VCO predistortion method of the liquid level measuring device. The spectrum analysis, VCO predistortion method of the liquid level measuring device according to claim 5 or 6, wherein done by FFT processing the difference beat frequency. The V-T table subjected to the VCO, VCO liquid level measuring apparatus according to claim 5, 6 or 7, wherein VCO carry Bray sucrose emissions as F-T characteristic with linearity from the VCO is obtained is running Predistortion method.