Level measuring device with adjustable timing and corresponding compensation method

Abstract

FMCW-based level measuring device for measuring a compensated level value (L) of a fill material (2) in a container (3), comprising: - An antenna arrangement (10) by means of which a high-frequency signal (S HF ) to the filling material (2) can be sent and, after reflection at the filling material surface, as a received signal (E HF ) is receivable, - a signal generation unit (11) designed to generate the high-frequency signal (S HF ) according to the FMCW principle with reference to a defined clock rate clk1, - a receiving unit (12) designed to ◯ to receive the signal (E HF ) to downmix into a low-frequency base signal (IF) according to the FMCW principle, and ◯ to sample the base signal (IF) at a defined sampling rate clk2, - an evaluation unit (13) designed to ◯ to use the sampled base signal (IF) d) to determine a distance value (d) to the filling material (2), ◯ to compensate the distance value (d) using a first compensation factor k1 and / or a second compensation factor k2, and ◯ to determine the compensated level value (L) based on the compensated distance value (d), and - a diagnostic unit (14) designed to ◯ to determine the first compensation factor k1 by comparing the clock rate clk1 with the sampling rate clk2, according to R 2,1 = clk 2 clk 1 a ratio R 2,1 the rates clk1, clk2 are determined relative to each other, and the first compensation factor k1 is determined according to k 1 = R 2.1 * clk 1, shallclk 2, shall is calculated, whereby clk 1,2soll to determine the respective target values ​​clk 1 / 2,soll the clock rate clk1 or the sampling rate clk2, and / or o to compare the clock rate clk1 and / or the sampling rate clk2 with a reference frequency clk ref and / or to determine the second compensation factor k2 from the first compensation factor k1, according to R 1, ref = clk 1 clkref ; R 2, ref = clk 2 clkref Relationships R 1,ref , R 2,ref the rates clk1, clk2 to the reference frequency clk ref to be determined, whereby the second compensation factor k2 according to k 2 = ( clk 1, shall R 1, ref * clkref ) 2 ⋅ R 2, ref * clkrefclk 2, shall is calculated, and ◯ to transfer the first compensation factor k1 or the second compensation factor k2 to the evaluation unit (13) after determination.

Description

[0001] The invention relates to a level measuring device whose clocking rate is compensable.

[0002] In automation technology, particularly in process automation, a multitude of different measured variables need to be determined, depending on the process. These can include, for example, fill level, flow rate, pressure, temperature, pH value, redox potential, conductivity, or dielectric constant of a medium in a process plant. Field devices specifically designed for this purpose are used to acquire the corresponding measured values. These devices are based on suitable sensors and / or measurement principles. The Endress+Hauser Group manufactures and distributes various types of field devices.

[0003] Radar-based measurement methods have become established for measuring the fill level of contents in containers. In the context of this patent application, the terms "radar" and "high frequency" refer to corresponding signals with frequencies between 0.03 GHz and 300 GHz. Common frequency bands used for level measurement are 2 GHz, 26 GHz, 79 GHz, and 120 GHz. In addition to its robustness and ease of maintenance, radar-based level measurement offers the advantage over other methods of being able to measure the fill level almost continuously.

[0004] In the case of radar, the FMCW principle (Frequency Modulated Continuous Wave) is the most common measurement principle for distance and level measurement. This principle is based on continuously transmitting a high-frequency signal with a modulated frequency. The frequency of the transmitted high-frequency signal lies within a defined frequency band in the range of a standardized center frequency. The frequency change over time is typically linear in FMCW and therefore exhibits a ramp or triangular waveform. However, a sinusoidal change is also possible in principle. The high-frequency signal is usually generated within the level gauge's signal generation unit by a PLL (Phase Locked Loop).The PLL generates the sawtooth-shaped high-frequency signal with reference to a constant internal clock frequency clk1, such as a quartz oscillator.

[0005] The distance d to the contents or the fill level is determined based on the instantaneous frequency difference f when implementing the FMCW method. IF The difference between the currently received high-frequency signal and the currently transmitted high-frequency signal is determined by generating a base signal through mixing the corresponding electrical high-frequency signals. The difference d can be determined based on the frequency f. IF of the base signal can be determined, since the frequency f IF The frequency of the base signal changes proportionally to the distance d. Here, c is the speed of light, and k represents the ramp rate of the frequency change. The frequency f IFThe base signal can be determined, for example, by digitizing it at a defined sampling rate clk2 and then subjecting it to a Fast Fourier Transform. The FMCW-based measurement principle for level measurement is described in more detail, for example, in "Radar Level Detection," Peter Devine, 2000.

[0006] Ideally, level gauges based on the FMCW method can achieve high level resolution with relatively little circuitry complexity, thus reaching the required sub-millimeter resolution depending on the application. However, in practice, the resolution is often limited by various component tolerances. In the case of FMCW, it is primarily component tolerances regarding internal clock frequencies that lead to corresponding measurement errors in level measurement. Compensating for these measurement errors, caused by deviations of the clock frequencies from the target frequencies, is difficult to implement within the instrument itself. This is because direct frequency measurement is not possible from a circuit design perspective; it can only be achieved indirectly by measuring the ratio of the actual frequency to a known reference frequency.However, integrating a reference source whose reference frequency exhibits virtually no component-related tolerances is not economically feasible.

[0007] In publication DE 102006058852 A1, a method is described in connection with FMCW-based level measurement in which the non-linear VCO characteristic is corrected by adjusting the VCO output frequency to the respective ramp frequency.

[0008] Publication WO 001998038525 A1 also describes a method for straightening nonlinear VCO characteristics by adjusting the transmission frequency to the desired ramp frequency at each point in time during the frequency ramp.

[0009] The invention is based on the objective of providing a level measuring device whose tolerances with regard to internal clocking can be compensated in order to minimize measurement errors.

[0010] The invention solves this problem by means of an FMCW-based level measuring device for measuring a compensated level value of a product in a container according to the features of claim 1. Accordingly, the level measuring device comprises the following components: - An antenna arrangement by means of which a high-frequency signal can be sent towards the contents and, after reflection at the surface of the contents, can be received as a receiving signal, - A signal generation unit, designed for example as a PLL, to generate the high-frequency signal according to the FMCW principle with reference to a defined clock rate, - A receiving unit designed to ◯ to downmix the received signal into a low-frequency base signal according to the FMCW principle, and ◯ to sample the base signal at a defined sampling rate, - an evaluation unit designed to ◯ to determine a distance value to the contents, for example using a Fourier transform, based on the sampled basic signal, ◯ to compensate for the distance value using a first compensation factor and / or a second compensation factor, and ◯ to determine the compensated fill level value based on the compensated distance value, and - a diagnostic unit designed to ◯ to determine the first compensation factor by comparing the clock rate with the sampling rate, according to R2,1=clk2clk1 a ratio R 2,1 the rates clk1, clk2 are determined relative to each other, and the first compensation factor k1 is determined according to k1=R2,1∗clk1,sollclk2,soll ◯ is calculated, where clk is 1,2soll to determine the respective target values ​​clk 1 / 2,soll the clock rate clk1 or the sampling rate clk2, and / or ◯ to determine the second compensation factor by comparing the clock rate and / or the sampling rate with a reference frequency and / or the first compensation factor, according to R1,ref=clk1clkref;R2,ref=clk2clkref Relationships R 1,ref , R 2,ref the rates clk1, clk2 to the reference frequency clk ref to be determined, whereby the second compensation factor k2 according to k2=(clk1,sollR1,ref∗clkref)2⋅R2,ref∗clkrefclk2,soll is calculated, and ◯ to transfer the first compensation factor or the second compensation factor to the evaluation unit after determination.

[0011] The level measuring device may include a suitable (manufacturing) interface for the possible transmission of the first compensation factor, the second compensation factor, the clock rate, the sampling rate and / or a reference frequency.

[0012] Within the scope of the invention, the term "unit" is understood to mean, in principle, any electronic circuit suitable for the intended purpose. Depending on the requirements, this could be an analog circuit for generating or processing corresponding analog signals. However, it could also be a digital circuit such as an FPGA or a storage medium in conjunction with a program. The program is designed to execute the corresponding process steps or to apply the necessary arithmetic operations of the respective unit. In this context, various electronic units of the measuring device, as defined by the invention, can potentially also access a common physical memory or be physically operated by means of the same digital circuit.

[0013] Analogous to the level measuring device according to the invention, the problem underlying the invention is also solved by a corresponding measuring method for determining the level. This method comprises the following steps: - Determining the first compensation factor by comparing the clock rate with the sampling rate, and / or - Determination of the second compensation factor by comparing the clock rate and / or the sampling rate with the reference frequency.

[0014] The invention thus takes advantage of the fact that, in case of doubt, the level measuring device can be compensated for potential internal clock deviations, at least to a certain extent, even without an external reference source, by comparing the clock rate with the sampling rate. Within the scope of the invention, the clock rate (clk1) and the sampling rate (clk2) can, for example, be compared with each other by... R2,1=clk2clk1 a ratio (R 2,1 ) the rates (clk1, clk2) relative to each other are determined. In this case, the first compensation factor k1 can be calculated according to k1=R2,1∗clk1,sollclk2,soll to be calculated, whereby clk 1,2soll This concerns the respective target values ​​of the clock rate or the sampling rate.

[0015] Similarly, the clock rate (clk1) and the sampling rate (clk2) can each also be set using an external reference frequency (clk ref ) are compared by following R1,ref=clk1clkref;R2,ref=clk2clkref

[0016] relationships (R 1,ref , R 2,ref ) of the rates (clk1, clk2) to the reference frequency (clk ref ). In this case, the second compensation factor (k2) can be determined according to k2=(clk1,sollR1,ref∗clkref)2⋅R2,ref∗clkrefclk2,soll will be calculated.

[0017] An advantage of the method according to the invention is that the first compensation factor can be continuously updated. This means that, in this design variant, the analysis unit repeatedly determines the first compensation factor during or between continuous level measurements. With such a design of the level measuring device, it can be classified as functioning correctly as long as the first compensation factor does not exceed an initially defined minimum change value over the continuous level measurements. Otherwise, a warning signal could be generated, for example, to signal the malfunction to a process control center. A further advantage of continuously recalculating the first compensation factor is that, with a corresponding design of the level measuring device, the determined level value is considered traceable, for example, according to the EN ISO 9001:2015 DIN series of standards.

[0018] In particular, if the level gauge does not have an integrated reference source for generating a high-precision reference frequency, the necessary second compensation factor can be determined during manufacturing based on an external reference frequency. This can be done, for example, by comparing the clock rate and / or sampling rate with the reference frequency of the external reference source. The second compensation factor can then be calculated by an external unit, in which case it must be transferred to the evaluation unit via the manufacturing interface. Alternatively, the reference frequency can be transferred to the analysis unit via the manufacturing interface, in which case the second compensation factor is calculated within the level gauge by the analysis unit.

[0019] The invention is explained in more detail using the following figures. They show: Fig. 1: An FMCW-based level gauge on a container, Fig. 2: a block diagram of the level measuring device according to the invention, and Fig. 3: A phase-locked loop for generating the high-frequency signal.

[0020] For a fundamental understanding of radar-based level measurement, see in Fig. Figure 1 shows a container 3 with a substance 2, the fill level L of which is to be determined. Depending on the type of substance 2 and the application, the container 3 can be more than 100 m high. To determine the fill level L, a level gauge 1 is mounted at a known installation height h above the substance 2 at a corresponding opening on the container 3. The level gauge 1 is typically connected to a higher-level unit 4, such as a process control system, via an interface, such as "PROFIBUS", "HART", or "Wireless HART". The determined fill level value L can be transmitted via this interface, for example, to control the inflow or outflow of the container 3. Other information about the general operating status of the level gauge 1 can also be communicated.

[0021] The level gauge 1 is aligned and attached to container 3 in such a way that it receives 10 high-frequency signals S via an antenna arrangement. HF emits approximately along a horizontally oriented axis towards the surface of the fill material 2. The high-frequency signal S exhibits HF According to the FMCW principle, within a defined frequency band of, for example, 79 GHz to 81 GHz, a constant frequency change occurs, resulting in a sawtooth or triangular frequency profile within the frequency band.

[0022] After reflection from the surface of the contents, the level measuring device 1 receives the reflected radar signals E HF again via the antenna arrangement 10. The frequency difference f is... IF between the currently emitted high-frequency signal s HF and the instantaneous received signal E HFDue to the frequency change inherent in the FMCW principle, the frequency is proportional to the distance d between level sensor 1 and the contents 2. Accordingly, level sensor 1 can, for example, determine the measured frequency difference f based on a corresponding calibration. IF to assign the respective distance d. The level measuring device 1 can then be used according to this. d=h−L Determine the fill level L, provided that the installation height h is stored in the level gauge 1.

[0023] A circuit by means of which the level gauge 1 can implement the FMCW principle for level measurement is shown as a block diagram in Fig. 2 shown in more detail: For the generation of the high-frequency signal S HF The level measuring device 1 includes a signal generation unit 11, which is designed to generate the high-frequency signal (s HF) according to the FMCW principle in the corresponding frequency band with a ramp- or triangular frequency change. The high-frequency signal s is used in this process. HF The frequency ramp is generated in such a way that the ramp-like, i.e., constant, frequency change repeats periodically within the frequency band. By default, the frequency ramp repeats with a periodicity of a few hundred milliseconds. The duration of each individual frequency ramp is between 100 microseconds and 100 milliseconds. The position of the frequency band must be set in accordance with regulatory requirements, which is why the ISM bands at 6 GHz, 26 GHz, 79 GHz, or 120 GHz are preferably implemented. The bandwidth, depending on the position of the frequency band, is generally between 0.5 GHz and 10 GHz. Higher frequency bands are generally preferred because they allow for a higher absolute bandwidth. This, in turn, increases the potential accuracy of the measurement.

[0024] According to the state of the art, the signal generation unit 11 is implemented as a standard PLL (Phase Locked Loop, PLL), as described in Fig. Figure 3 shows the core of the circuit. A controllable, high-frequency electrical oscillator 111 (implemented as a "Voltage Controlled Oscillator" by default) is used to generate the high-frequency electrical signal s HF generated. The frequency of the VCO or the high-frequency signal s HF In the signal generation unit 11 shown, the signal is controlled by feedback and thus stabilized against fluctuations in the ambient temperature; on the other hand, the constant frequency change of the high-frequency signal s is maintained. HF Set up: The feedback is achieved by extracting the high-frequency signal s HF of the high-frequency oscillator 111 a control signal s cThe signal is branched off and fed to a phase comparator 112. The phase comparator 112 compares the instantaneous phase shift of the control signal s. c to a frequency-constant clock rate clk1. A crystal oscillator 113, which typically generates a clock rate clk1 of between 10 MHz and 100 MHz, can be used as the source for the clock rate clk1. Depending on the phase difference between the control signal s c The phase comparator 112 generates a control signal s from the quartz oscillator 113. DC , which is connected to a corresponding control input of the high-frequency oscillator 111. Provided that the high-frequency oscillator 111 is designed as a VCO and is therefore used to control the frequency of the high-frequency signal s HFIf a DC voltage is required, a charge pump can be connected downstream of the digital phase comparator 112, which performs a corresponding digital / analog conversion of the control signal s DC carries out.

[0025] The ramp-shaped frequency change of the high-frequency signal, typical for FMCW radar, s HF will be during the in Fig. The variant of the high-frequency generation unit 11 described in section 3 is set on a frequency divider 114, which is arranged in the signal path between the high-frequency oscillator 111 and the phase comparator 112: For this purpose, this frequency divider 114, known as a "fractional-N divider," is controlled according to the prior art such that its division factor N changes constantly over time, i.e., in a ramp-like manner. The smallest resolvable frequency resolution of fractional-N dividers depends on the word width N and is between 20 and 32 bits. Due to the operating principle of the in Fig. The frequency ramp of the resulting high-frequency signal is shown in the PLL 3. HF thus generated in reference to the clock frequency clk1 of the quartz oscillator 113.

[0026] As in Fig. As shown in Figure 2, the signal generation unit 11 carries out the high-frequency signal S to be transmitted. HF The antenna array 10 is connected via a signal divider 15 and a subsequent transmit / receive switch 16. The design of the transmit / receive switch 16 is not fixed in principle; it can, for example, be implemented as a duplexer. The design of the antenna array 10 is primarily determined by the frequency band. For frequencies in the single-digit GHz range, the antenna array 10 can be configured as shown in Fig. As schematically indicated in Figure 2, a horn antenna, for example, can be used. Particularly in the higher double-digit GHz range, a more compact planar antenna, such as a patch antenna or a fractal antenna, can also be used.

[0027] The received radar signal E HF , which is reflected from the surface of the filling material, is converted by the antenna arrangement 10 into a purely electrical received signal e HF converted back and, if necessary, amplified by a receiver amplifier (not in Fig. 2 shown). The received signal e is then HF by means of a mixer 17 with the high-frequency signal to be transmitted s HF downmixed, whereby the high-frequency signal s HFFor this purpose, a signal is tapped from the signal divider 15. This generates a base signal IF typical of the FMCW method, which can be used to determine the distance d or the fill level L. The FMCW principle is used, according to which the resulting frequency f IF of the base signal IF according to d=c⋅fIF2⋅k⋅clk12 proportional to the distance d.

[0028] To determine the frequency f IF An analog-to-digital converter 12 digitizes the base signal IF. To comply with the Nyquist-Shannon sampling theorem, the analog-to-digital converter 12 preferably samples the base signal IF at a sampling frequency clk2, which is at least twice the frequency f corresponding to the interval d. IFof the basic signal IF. Thus, a suitably designed evaluation unit 13 can subject the digitized basic signal to a (Fast) Fourier Transform, or FFT for short. Ideally, the frequency of the global maximum of the corresponding FFT spectrum corresponds to the distance d.

[0029] As can be seen from the previous formula, to correctly determine the distance d, it is necessary that the clock rate clk1 of the signal generation unit 11 exactly matches its target value clk 1,soll This corresponds. In addition, a deviation of the sampling rate clk2 from its target value clk also leads to... 2,soll This results in a corresponding error in determining the distance value d' calculated by evaluation unit 13. An internal device adjustment of the clock rate clk1 or sampling rate clk2 to the respective target value clk 1,2sollHowever, this is not commercially feasible, as direct frequency measurement is not possible with a reasonable level of technical effort. Instead, the only hardware-based option is to measure individual frequencies as ratios V. 1,2 V 1,ref in relation to fixed reference frequencies clk ref to determine. However, it is not technically feasible to integrate a separate reference source with the desired accuracy into the level measuring device 1.

[0030] According to the invention, the in Fig. 2 level measuring device 1 shown is therefore a diagnostic unit 14, by means of which the clock rate clk1 of the signal generation unit 11 and the sampling rate clk2 of the analog / digital converter 12 are compared with each other, by means of R2,1=clk2clk1 the ratio R 2,1 The clock rate clk1 to the sampling rate clk2 is determined. The ratio R can be calculated in this process. 2,1for example, based on at least one digital counter.

[0031] Based on the determined ratio R 2,1 The diagnostic unit 14 can be used according to k1=R2,1∗clk1,sollclk2,soll Calculate a first compensation factor k1. This is clk 1,2soll to the respective known target values ​​clk 1 / 2,soll the clock rate clk1 or the sampling rate clk2. The determined first compensation factor k1 can be transmitted to the evaluation unit 13, so that it can calculate the distance value d' determined by FFT according to d=k1∗d' This can be used to compensate for the resulting deviations in order to determine the compensated fill level value L. According to the invention, the clock rate clk1 and the sampling rate clk2 are compared relative to each other for compensation purposes. This allows any deviations of the rates clk1 and clk2 from their target values ​​clk1 to be corrected. 2,sollat least partially compensated, thus minimizing measurement errors caused by this.

[0032] An advantage of the compensation according to the invention is that no external, high-precision reference source is required. Accordingly, the level gauge 1 can perform the compensation independently after its manufacture. Such compensation can, for example, be carried out repeatedly in regular cycles during measurement operation. The repeated re-determination of the first compensation factor k1 during or between continuous level measurements can be used, for instance, to check the functionality of the level gauge 1. The level gauge 1 can thus be defined as malfunctioning as soon as the first compensation factor k1 exceeds a defined minimum change Ak1 over the course of the continuous level measurements compared to the value of the first compensation factor k1, which was determined at or before the start of measurement operation.

[0033] Furthermore, traceability of the measured fill level value L, e.g., according to the EN ISO 9001:2015 series of standards, can be enabled by (a-)cyclically recalculating the first compensation factor k1, provided that a factory calibration is performed in addition to the first compensation factor k1. This calibration can be carried out by calibrating the clock rate clk1 and / or the sampling rate clk2 with a high-precision reference frequency clk ref a comparison with an external reference source. In this case, according to R1,ref=clk1clkref; R2,ref=clk2clkref again, relationships R 1,ref , R 2,ref the rates clk1, clk2 to the reference frequency (clk ref ) can be determined. Based on these, according to k2=(clk1,setR1,ref*clkref)2⋅R2,ref*clkrefclk2,set A second compensation factor k2 is calculated. It is essentially irrelevant whether the second compensation factor k2 is calculated internally by diagnostic unit 14 or externally. In the case of internal calculation, the reference frequency clk can be used. ref The diagnostic unit 14, for example, can be programmed via a manufacturing interface 18. Alternatively, the clock rate clk1 and the sampling rate clk2 can be transmitted externally via the manufacturing interface 18, so that the second compensation factor k2 is determined externally at a corresponding compensation station during production and subsequently transmitted to the evaluation unit 13 via the manufacturing interface 18. This ensures the traceability of the compensated distance d during the ongoing measurement operation of the level gauge 1 by continuously comparing the first compensation factor k1 with the second compensation factor k2.

[0034] Alternatively or additionally, the evaluation unit 13 of the level measuring device 1 can use the determined distance value d' ex works according to d=k2*d' Compensation. This additional factory compensation further reduces measurement errors in level measurement. In this context, it is also conceivable that the first compensation factor k1, at least initially during production, is not calculated by the diagnostic unit 14, but also by the external compensation station. Reference symbol list 1 level gauge 2 Filling material 3 containers 4. Higher-level unit 10 Antenna Arrangement 11 Signal Generation Unit 12 analog-to-digital converters 13 Evaluation Unit 14 Diagnostic Unit 15 signal splitters 16 Transmit / receive switch 17 mixers 18 Manufacturing interface d distance clk ref Reference frequency clk1 Clock rate clk2 sampling rate E HF Received signal h Installation height IF, IF d Low-frequency base signal k, k1, k2 compensation factors L level value S HF High-frequency signal V 1,2 V 1,ref Ratios of the rates to each other or to the reference frequency

Claims

[1] FMCW-based level measuring device for measuring a compensated level value (L) of a fill material (2) in a container (3), comprising: - An antenna arrangement (10) by means of which a high-frequency signal (S HF ) to the filling material (2) can be sent and, after reflection at the filling material surface, as a received signal (E HF ) is receivable, - a signal generation unit (11) designed to generate the high-frequency signal (S HF ) according to the FMCW principle with reference to a defined clock rate clk1, - a receiving unit (12) designed to ◯ to receive the signal (E HF ) to downmix into a low-frequency base signal (IF) according to the FMCW principle, and ◯ to sample the base signal (IF) at a defined sampling rate clk2, - an evaluation unit (13) designed to ◯ to use the sampled base signal (IF) d) to determine a distance value (d) to the filling material (2), ◯ to compensate the distance value (d) using a first compensation factor k1 and / or a second compensation factor k2, and ◯ to determine the compensated level value (L) based on the compensated distance value (d), and - a diagnostic unit (14) designed to ◯ to determine the first compensation factor k1 by comparing the clock rate clk1 with the sampling rate clk2, according to R2,1=clk2clk1 a ratio R 2,1 the rates clk1, clk2 are determined relative to each other, and the first compensation factor k1 is determined according to k1=R2,1*clk1,sollclk2,soll is calculated, whereby clk 1,2soll to determine the respective target values ​​clk 1 / 2,soll the clock rate clk1 or the sampling rate clk2, and / or o to compare the clock rate clk1 and / or the sampling rate clk2 with a reference frequency clk ref and / or to determine the second compensation factor k2 from the first compensation factor k1, according to R1,ref=clk1clkref; R2,ref=clk2clkref Relationships R 1,ref , R 2,ref the rates clk1, clk2 to the reference frequency clk ref to be determined, whereby the second compensation factor k2 according to k2=(clk1,setR1,ref*clkref)2⋅R2,ref*clkrefclk2,set is calculated, and ◯ to transfer the first compensation factor k1 or the second compensation factor k2 to the evaluation unit (13) after determination. [2] Level measuring device according to claim 1, wherein the level measuring device (1) is used to transmit the first compensation factor k1, the second compensation factor k2, the clock rate clk1, the sampling rate clk2 and / or an external reference frequency clk refcan be connected via a manufacturing interface (18). [3] Level measuring device according to claim 1 or 2, wherein the signal generation unit (11) is implemented as a phase-controlled loop. [4] Level measuring device according to one of the preceding claims, wherein the evaluation unit (13) is designed to determine the distance value (d) by means of a Fourier transform of the sampled base signal (IF). d to determine. [5] Method for compensating a level measuring device (1) according to any of the preceding claims, comprising the following method steps: - Determination of the first compensation factor k1 by comparing the clock rate clk1 with the sampling rate clk2, and / or - Determination of the second compensation factor k2 by comparing the clock rate clk1 and / or the sampling rate clk2 with the reference frequency clk ref , where the clock rate clk1 and the sampling rate clk2 are compared by according to R2,1=clk2clk1 a ratio R 2,1 the rates clk1, clk2 are determined relative to each other, and the first compensation factor k1 is determined according to k1=R2,1*clk1,sollclk2,soll is calculated, whereby clk 1,2soll to determine the respective target values ​​clk 1 / 2,soll the clock rate clk1 or the sampling rate clk2, and / or wherein the clock rate clk1 and the sampling rate clk2 are each associated with the reference frequency clk ref can be compared by following R1,ref=clk1clkref; R2,ref=clk2clkref Relationships R 1,ref , R 2,ref the rates clk1, clk2 to the reference frequency clk ref to be determined, and where the second compensation factor k2 according to k2=(clk1,setR1,ref*clkref)2⋅R2,ref*clkrefclk2,set is calculated. [6] Method according to claim 5, wherein the first compensation factor k1 is determined repeatedly during continuous level measurements, and wherein the level measuring device (1) is classified as functioning as long as the first compensation factor k1 does not exceed a minimum change Ak1 over the continuous level measurements. [7] Method according to claim 5 or 6, wherein the second compensation factor k2 is determined during the manufacture of the level measuring device (1) by comparing the clock rate clk1 and / or the sampling rate clk2 with the reference frequency clk ref is determined from an external reference source. [8] Method according to claim 7, wherein the second compensation factor k2 is calculated by an external unit, and wherein the second compensation factor k2 is transmitted to the evaluation unit (13) via the manufacturing interface (18). [9] Method according to claim 7, wherein the reference frequency clk refis transferred via the manufacturing interface (18) to the analysis unit (14), and the second compensation factor k2 is calculated by the analysis unit (14).

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