PHOTOACUSTIC STREET SENSOR AND METHOD

Description

Technical field

[0001] The invention relates to a photoacoustic gas sensor, a method for determining a measured value using a photoacoustic gas sensor, and a corresponding computer-implemented method. State of the art

[0002] Photoacoustic gas sensors utilize the interaction of electromagnetic radiation ("light") with matter to determine the concentration of a target gas in a carrier gas. Molecules of the target gas absorb light energy when this energy corresponds to an energy difference in the rotational / vibrational states of the molecules. These excited molecules can then distribute the absorbed energy through collisions with neighboring molecules in the carrier gas, creating a pressure fluctuation that propagates at the speed of sound and can be measured, for example, with a microphone. The greater the number of molecules in the gas capable of absorbing light, the greater the pressure fluctuation and, consequently, the microphone signal. If the light intensity is periodically modulated, the microphone signal is also modulated accordingly, and, using, for example, lock-in demodulation of the microphone signal, an offset-free determination of the target gas concentration can be achieved.Pressure fluctuations in the carrier gas, caused by vibrations or noise that happen to have the same periodicity, can impair the determination.

[0003] EP3550286A1 describes a small photoacoustic gas sensor of simple construction, where a measuring cell on a substrate forms a measuring volume that nevertheless provides accurate concentration values ​​of a gas component. A microphone has a bottom port that points towards the substrate and is communicatively connected to the measuring volume.

[0004] EP3859307A1 discloses a method for operating a photoacoustic gas sensor, wherein in a first step an acoustic reference signal is recorded in a measuring chamber, this reference signal is analyzed to obtain noise information, and a filter function is adapted based on the noise information, and then in a second step the light intensity is modulated based on the filter function and the acoustic measurement signal is analyzed based on the filter function to obtain information about the target gas in the measuring chamber.

[0005] US2020 / 300756 A1 discloses a photoacoustic gas sensor wherein a light source is modulated with a frequency that periodically assumes successive values ​​of a sequence.

[0006] One objective of the invention is to provide a simple photoacoustic gas sensor and a corresponding measurement method that are robust against disturbances in the form of vibrations or noise and can deliver measurement data without interruption. Description of the invention

[0007] Against this background, a photoacoustic gas sensor, a method for determining a measured value with a photoacoustic gas sensor, and a corresponding computer-implemented method are presented here according to independent claims 1, 5 and 6; furthermore, a computer program according to claim 7 and a computer-readable storage medium according to claim 8.

[0008] The photoacoustic gas sensor comprises a measuring chamber for receiving a gas, a light source arranged to emit light into the measuring chamber, a microphone arranged to detect sound waves in the measuring chamber and adapted to output a corresponding sound signal, and a light source driver connected to provide a control signal for the light source, according to which the intensity of the emitted light is modulated. The control signal drives the light source in a sequence of time windows with a periodic intensity modulation of a frequency, wherein the frequency of the intensity modulation in one time window differs from the frequency of the intensity modulation in the following time window, and wherein the sequence of frequency values ​​thus defined is periodic with a period of n f ≥ 2 .

[0009] Vibrations and / or acoustic noise caused by motors, compressors, or other rotating or oscillating devices near the photoacoustic sensor can be periodic. They are picked up by the microphone and superimposed as interference on the photoacoustic signal. If one of the frequency components of the interference signal is close to the frequency of the photoacoustic signal, this can lead to a loss of precision (the measurement is noisy) and / or a loss of accuracy (the measurement may not be noisy but biased). This occurs when the frequency difference Δ f The difference between the noise and the desired signal is small. Small in this context means that Δ f ≪ 1 2 τ , where τ the integration time is (the total duration during which data is sampled and from which a concentration value is ultimately determined).

[0010] Let us consider a photoacoustic signal yk with frequency fPA, Amplitude a , phase shift Φ PA , sampled at the frequency fs : y k = a cos 2 πf PA k f s + ϕ PA , k = 0 , 1 , 2 , …

[0011] Let's assume that the ratio f s f PA = N is a natural number (i.e., the acoustic period is an integer multiple of the sampling period), and we perform a discrete Fourier transform of length N: Y j w = ∑ k = wN w + 1 N − 1 y k exp − 2 πijk N = Na 2 e iϕ PA j = 1 Na 2 e − iϕ PA j = N − 1 0 sont where w = 0, 1, 2, ... is the index of the Fourier window. The value of the Fourier coefficient is independent of w because both yk as well as the complex exponential in the above equation being N-periodic.

[0012] The duration of the Fourier transform is N f s = 1 f PA The photoacoustic signal therefore has exactly one period within the Fourier window. For this reason, the target quantity, namely the photoacoustic amplitude, is a , exclusively in the Fourier coefficients Y 1 w and Y N − 1 w They contain coefficients that form a complex conjugate pair because the photoacoustic signal is real-valued. These two Fourier coefficients correspond to the frequencies ± f PA. In the state of the art, only Y 1 w calculated because it contains all the information about the photoacoustic signal. Of course, the Fourier window can also be chosen to be longer. A duration equal to an integer multiple of N However, it is preferred. Is the duration the same? qN (q (a natural number), then the information regarding the amplitude of the photoacoustic signal is located on the Fourier coefficients. Y q and Y qN-q .

[0013] Now let us consider the same photoacoustic signal as before, but superimposed with a noise signal of amplitude b, frequency f intf and phase shift ϕ intf . The overall signal is then given by y k = a cos 2 πf PA k f s + ϕ PA + b cos 2 πf intf k f s + ϕ intf .

[0014] A discrete Fourier transform of length N yields Y j w = ∑ k = wN w + 1 N − 1 y k exp − 2 πijk N = Na 2 e iϕ PA + R intf w j = 1 Na 2 e − iϕ PA + R intf w ∗ j = N − 1 unabhängig von a sonst

[0015] where R intf w a term that depends solely on the interference signal and may vary from one Fourier window to the next. The present invention provides a device and a method that, on the one hand, detects the presence of interfering signals and, on the other hand, mitigates the influence of such interference signals on the concentration measurement.

[0016] In the current state of the art, the light source is modulated with a fixed frequency, f PA = f s N (N a natural number); this ensures that the amplitude of the photoacoustic signal, as described above, depends only on the Fourier coefficient. Y 1 w and Y N − 1 w lies.

[0017] In the present invention, the frequency of the photoacoustic signal is periodically changed and cycles through n f ≥ 2 different values, where nf is a natural number. The measurement time is thus divided into time windows, here called modulation windows, whereby during each modulation window the intensity of the light source is modulated at a fixed frequency. This modulation frequency changes from one modulation window to the next in a periodic manner: The sequence of modulation windows corresponds to a sequence of modulation frequencies, and this sequence of modulation frequencies is n f-periodic ( fm = f m mod n f). The intensity of the light source is thus increased with a frequency f 0 for a specific duration D 0, then with a different frequency f 1 for a duration D 1, and so on until finally reaching the frequency f n f-1 for a duration D n f-1 modulated, and then again with the frequency f 0 and so on. All D mThey can be the same. Furthermore, it is advantageous if all selected frequencies meet the condition. f m = f s N m m = 0 , … , n f − 1 fulfill, whereby N m natural numbers are (here and in the following, m should be in the subscript of f and N always as m modulo n f can be interpreted). Then the information about the amplitude of the photoacoustic signal with frequency is present. f m = f s N m only on the first (and last) Fourier coefficient of a Fourier transform of length N m The modulation windows advantageously have a length of several Fourier windows, i.e., the number of samples in a modulation window is much larger than the N m (i.e., the photoacoustic signal has multiple periods within each modulation window). It is advantageous if the duration of the modulation windows D mis chosen so that a whole number of periods of the photoacoustic signal to be measured always fit into the corresponding modulation window, i.e. D m = q m f m m = 0 , … , n f − 1 , where square meter These are natural numbers. It goes without saying that the duration of the modulation windows does not have to be constant and that the modulation windows do not have to follow each other immediately, e.g., to save energy.

[0018] It can then, for example, be defined as a union of per measurement period. n For each successive modulation window, f (number of modulation frequencies), a measurement value is output. The frequency changes make it likely that at least one frequency per measurement period is not, or not significantly, affected by the interference signal. Advantageously, the measurement period is short compared to the timescale on which the frequency nature of the interference does not change significantly.

[0019] In the invention, the photoacoustic gas sensor further comprises a signal processor connected for recording and processing the sound signal and for determining and outputting a measured value in a measurement period, defined as n f successive time windows of the sequence. The signal processor demodulates the sound signal in each time window of the measurement period using the frequency of the control signal in that time window to determine a useful value, and using the frequency of the control signal in the following time window to determine a noise value. The determination of the measured value takes both the useful and the noise values ​​into account.

[0020] During each modulation window, the microphone signal can be demodulated first at the current photoacoustic frequency and second at the photoacoustic frequency of the next modulation window, e.g., by explicitly calculating the above sums for selected Fourier coefficients or all Fourier coefficients simultaneously using a common FFT algorithm. The first demodulation yields the amplitude of the photoacoustic signal (useful value, as in the prior art), and the second demodulation, the noise value, yields the noise power at the next photoacoustic frequency (high noise power means that an interfering signal is present at this frequency and will interfere with the photoacoustic measurement in the next modulation window). If the noise power is high, the photoacoustic amplitude measured in the next modulation window is ignored, as it is expected to be noisy and / or biased. Thus, in each measurement period, 2 nf intermediate values, namely n f utility values α m (m = 0, ..., nf - 1) and n f predictive disturbance values β m (m = 0, ..., n f - 1). The 2 n Intermediate values ​​are aggregated into a single measurement, e.g. ∑ m = 0 n f − 1 α m g β m − 1 mod n f , where the utility values α Each modulation window is weighted by weighting functions of the disturbance values ​​β in the preceding modulation window. These weighting functions are monotonically decreasing functions, such as step functions, which vanish above a certain threshold. This simple measurement scheme has the advantage, among others, that a continuous gas concentration series can be output because it is not necessary to wait for an acoustic reference measurement and noise analysis, during which no useful signal is generated.

[0021] In the modulation window m, when the photoacoustic signal is at the frequency fm When the signal is modulated, we calculate the first Fourier coefficient: Y 1 m w f m f m = ∑ k = wN m w + 1 N m − 1 y k f m ⋅ exp − 2 πik N m = N m a 2 e iϕ m + R intf m w f m , which provides a measure for the photoacoustic amplitude a , which also includes a contribution from a potential perturbation term R intf m w f m contains. The superscripts m and w in Y The numbers 1 and 2 denote the modulation window and the Fourier window within the modulation window. The first parameter in square brackets is the modulation frequency within the modulation window and therefore belongs to... yk [ fm ], y k f m = a cos 2 πf m k f s + ϕ m + b cos 2 πf intf k f s + ϕ intf .

[0022] The second parameter in the square brackets denotes the demodulation frequency, i.e., the frequency to which this Fourier coefficient corresponds and belongs to the complex exponential term (as a reminder: N m = fs / fm ), exp − 2 π i k N m .

[0023] This calculation is state of the art. However, we also additionally calculate the Fourier coefficient, which is related to the frequency. f m+1 corresponds to (the modulation frequency of the light source in the following modulation window), while the modulation frequency is still fm is: Y 1 m w f m f m + 1 = ∑ k = wN m + 1 w + 1 N m + 1 − 1 y k f m exp − 2 πik N m + 1 = Q m w f m f m + 1 + R intf m w f m + 1 , where Q (m,w)< [ fm , f m+ 1 ] the contribution of the photoacoustic signal at fm measured at spectral amplitude f m +1 (spectral leakage effect). This complex quantity should average out to approximately zero over the modulation window (assuming that fm and f m +1 are sufficiently different). Alternatively, the duration of the modulation window can be chosen so that Q averages out exactly. This is the case, for example, if the duration of the modulation window is a multiple of N m as also from N m +1 is the contribution of the error term. R intf m w f m + 1 could also average out (for example, if f intf sufficiently different from fm is), but there fSince intf is generally unknown, this is not the case. Therefore, the following applies: Y 1 m ¯ f m f m + 1 ≃ R intf m ¯ f m + 1 , where … ¯ : = M − 1 ∑ w = 0 M − 1 … Averaging over all Fourier windows w = 0, ... , M - 1 in the modulation window means.

[0024] In the following modulation window, m + 1, the photoacoustic signal has a frequency of f m +1, and the measurement is Y 1 m + 1 , w f m + 1 f m + 1 = ∑ k = wN m + 1 w + 1 N m + 1 − 1 y k f m + 1 exp − 2 πik N m + 1 = N m + 1 a 2 e iϕ m + 1 + R intf m + 1 , w f m + 1 .

[0025] If we assume that the disturbance is in the modulation window m If +1 was already present in the preceding modulation window m, then we can estimate the noise term. R intf m + 1 ¯ f m + 1 2 ≃ R intf m ¯ f m + 1 2 ≃ Y 1 m ¯ f m f m + 1 2 .

[0026] If this value is large compared to the power of the pure photoacoustic signal, then we can assume that the current measurement Y 1 m ¯ f m + 1 f m + 1 This is significantly biased and should be ignored. The above equality usually does not hold true without absolute values.

[0027] Otherwise, the measurement could simply be corrected by subtraction: Y 1 m ¯ f m + 1 f m + 1 − Y 1 m ¯ f m f m + 1

[0028] If, therefore, the disturbance value Y 1 m ¯ f m f m + 1 2 , i.e., demodulation in the modulation window m with frequency f m +1 where the photoacoustic frequency fm If the value exceeds a certain threshold, then the photoacoustic utility value Y 1 m + 1 ¯ f m + 1 f m + 1 , i.e., demodulation in the modulation window m + 1 with frequency f m +1 where the photoacoustic frequency f m +1 is ignored (or is included in the measurement value of the measurement period with a smaller weight).

[0029] In one embodiment of the invention, the acquisition of the raw data, i.e., the microphone signal, and the determination of the useful values, interference values, and consequently the measured value, can be performed asynchronously: The raw data can be buffered locally on a storage medium, e.g., on a memory chip that is part of the signal processor, and / or sent to a cloud for further data processing. The determination of the measured values ​​and ultimately the gas concentration values ​​can then take place in the cloud, where more computing power and / or processing time is available. The advantage of this embodiment is that the signal processor can be chosen more simply and, in particular, more cost-effectively the smaller the requirements for its computing and storage power.

[0030] In an unclaimed embodiment, the photoacoustic gas sensor comprises a measuring chamber for receiving a gas, a light source arranged for emitting light into the measuring chamber, a microphone arranged for detecting sound waves in the measuring chamber and adapted for outputting a corresponding sound signal, a light source driver connected for providing a control signal for the light source according to which the intensity of the emitted light is modulated, and a signal processor connected for receiving and processing the sound signal and for determining and outputting the measured value.The control signal drives the light source in a sequence of time windows with a periodic intensity modulation of a specific frequency. The signal processor demodulates the sound signal in the first time window using the frequency of the control signal to determine a useful value, and in the second time window using a different frequency to detect noise. The frequency of the intensity modulation in the subsequent second time window is based on the noise, and the measurement in the first time window takes the useful value into account.

[0031] After the interference measurement, i.e., the demodulation at frequency f m +1 in the modulation window m, especially before the photoacoustic measurement at frequency f m +1 in the modulation window m + 1 occurs, and, if the disturbance exceeds a certain threshold, the useful value at frequencyf m If +1 is ignored, the measurement can be omitted entirely. In one embodiment, the photoacoustic signal can therefore be measured in the modulation window m. Y 1 m ¯ f m f m and the disturbances Y 1 m ¯ f m f i 2 all other frequencies fi , i ≠ calculate m (not only from f m +1 ), and then change the modulation frequency in the following modulation window m + 1 to the frequency that has the smallest noise value. Brief description of the drawings

[0032] Figure 1 shows the influence of periodic disturbances on the useful signal as a function of the disturbance frequency. Figure 2 shows the mean and variance of the averaged first Fourier coefficient as a function of the difference between the excitation frequency and the disturbance frequency. Figure 3 shows measurement data series from a photoacoustic gas sensor that is operated sequentially at three different frequencies. Figure 4shows a comparison of the determination of the measured values ​​according to the present invention with moving averages. Figure 5 shows a photoacoustic gas sensor. Figure 6 shows, in a schematic way, a time course of the control signal of the light source. Ways to implement the invention in detail

[0033] Measurement results for a specific embodiment are presented in the Figures 3 and 4 A photoacoustic CO₂ gas sensor 1 with measuring chamber 2 is shown schematically in [reference to figure]. Figure 5 , is operated in room air with approximately 400 ppm CO2. A microprocessor 5 is configured to periodically switch the light source 3 on and off for 1 second at frequencies of f 0 = 41.7 Hz, f 1 = 55.6 Hz and f2 = 62.5 Hz. The light source 3 is a heater, i.e., a blackbody radiator, plus a frequency filter at 4.3 µm (where there is an absorption maximum for CO₂, which is simultaneously an absorption minimum for H₂O), which can reach temperatures up to 500°C by means of pulse density modulation (100 kHz) of the operating voltage of 5 V. The control signal is a square wave with a 50% duty cycle at the frequencies fm , m = 0, 1, 2 between two values ​​representing the target temperatures of room temperature and approximately 500°C. The time course of the control signal is shown in Figure 6 A measurement period 7 is divided into three modulation windows 8, 9, and 10. In each modulation window, the control signal has a corresponding modulation period 11, 12, and 13. Due to the non-zero thermal mass of the heater, the resulting time course of the heater's actual temperature is, of course, only approximately a square wave. From time t= 90 s to t For 230 seconds, the gas sensor is subjected to a vibration with a peak acceleration of 1 g at a frequency of 55 Hz. In the upper panel of the Figure 3 is the 1s average of the real part of the measured amplitude Re Y 1 m ¯ f m f m The signal is plotted as a function of time in seconds, for each of the three frequencies. We use the real part because the Fourier transform was rotated so that the useful signal has only a real part. The measured values ​​at 55.6 Hz are strongly affected by the interference and fluctuate around the correct value (normalized to 1).

[0034] In Figure 1 The following are numerical examples illustrating the influence of periodic interference signals on the Fourier coefficient. The y-axis in the upper panel shows the absolute value of the 1-second average of the first Fourier coefficient. Y 1 | / N , where Y 1 ¯ = 1 N p ∑ p Y 1 p and N pThe number of Fourier windows of length N that fit into 1 second is shown in the lower panel, along with the argument. The x-axis displays the time in seconds. The parameters are... a = b = 1, ϕ PA = ϕ intf = 0, f PA = 40 Hz, f s = 1 kHz, N = 25 and five different values ​​of the interference frequency f As the interfering frequency approaches the photoacoustic frequency, the absolute value begins to oscillate with increasing amplitude and decreasing frequency. This is because the two signals alternate between constructive and destructive interference in the different 1-second windows. When the interfering frequency is equal to the photoacoustic frequency, the signals interfere completely constructively, and the absolute value is constant but biased at 2.

[0035] In the middle panel of the Figure 3 is the standard deviation of the real part of the measured amplitude, σ Re Y 1 m f m f m , shown. It is noticeable that the frequency 55.6 Hz, which is most affected by the vibration, exhibits the smallest standard deviation. The reason for this is that during the 1 s period, the photoacoustic signal and the vibration-induced noise signal are coherent. The measured amplitude is therefore approximately constant but biased.

[0036] In Figure 2 This fact will be illustrated using the numerical example of Figure 1 explained in more detail. The top panel of Figure 2 The lower panel shows the mean and the lower panel the standard deviation, each calculated over 10 s of 2| Y 1 | / Nas a function of the frequency difference between the photoacoustic frequency and the interference frequency. Up to a frequency difference of approximately 1 Hz, the mean value is approximately correct (1 in this example). For frequency differences below 1 Hz, the integration time of 1 s does not provide sufficient spectral resolution to distinguish between the contributions of the photoacoustic and the interference signal. The measurement 2| Y 1 | / N It contains non-zero contributions from both signals. The standard deviation generally increases as the frequency difference approaches zero. However, as noted above, there are specific frequency differences where the standard deviation vanishes (the mean is constant, but biased).

[0037] In the lower panel of the Figure 3 is the interference power Y 1 m ¯ f m f m + 1 2 The graph shows that during the interval in which the gas sensor is subjected to vibration, the interference power at 55.6 Hz is several orders of magnitude greater than the interference power at the other frequencies, indicating a strong interference signal near 55.6 Hz. When determining the measured value in the 3-second measurement period, the useful value at the photoacoustic frequency of 55.6 Hz could be ignored or weighted less. The result of different weightings is shown in the graph. Figure 4 shown. The dotted curve shows the ongoing 1-second averaging as in the upper panel of Figure 3The dashed line shows the continuous weighted average over 3 seconds with equal weights of 1 / 3 for the three frequencies. This measurement clearly oscillates strongly around the correct value of 1 as long as the interfering vibration is present. The solid line shows the continuous weighted average over 3 seconds where the weight for the frequency 55.6 Hz was set to zero as soon as the interfering power exceeded the threshold value, e.g., 25,000. The interference from the vibration is then barely perceptible. List of reference symbols

[0038] 1 Photoacoustic gas sensor 2 Measuring chamber 3 Light source 4 Microphone 5 Light source driver 6 Signal processor 7 Measuring period 8-9 Modulation window 10-12 Modulation periods

Claims

1. Photoacoustic gas sensor (1), comprising a measurement chamber (2) for receiving a gas, a light source (3) arranged to emit light into the measurement chamber, a microphone (4) arranged to detect acoustic waves in the measurement chamber (2) and adapted to output a corresponding acoustic signal, a light-source driver (5) connected to provide a drive signal for the light source (3), according to which the intensity of the emitted light is modulated, wherein the drive signal, in a sequence of time windows, drives the light source in a time window (8, 9, 10) of the sequence with a periodic intensity modulation at a frequency, wherein the frequency of the intensity modulation in a time window (8, 9, 10) differs from the frequency of the intensity modulation in the subsequent time window, wherein the sequence of frequency values thus defined is periodic and takes on nf different values, wherein nf is a natural number greater than or equal to two, and a signal processor (6) connected to receive and process the acoustic signal and to determine and output a measurement value in a measurement period (7), defined as nf successive time windows (8, 9, 10) of the sequence, characterized in that the signal processor (6) demodulates the acoustic signal in each time window (8, 9, 10) of the measurement period (7) with the frequency of the drive signal in the time window (8, 9, 10) to determine a useful value, and demodulates it with the frequency of the drive signal in the subsequent time window to determine an interference value, and determining the measurement value takes into account the useful values and the interference values.

2. Photoacoustic gas sensor according to claim 1, wherein the measurement value is a function of the useful values and the interference values.

3. Photoacoustic gas sensor according to claim 2, wherein the measurement value is a linear combination of the useful values and wherein the weights of the linear combination comprise monotonically decreasing functions of the interference values.

4. Device according to claim 1, 2 or 3, wherein the difference between two successive frequencies of the frequency sequence is greater than the inverse of the duration of the time window (8, 9, 10) in which the acoustic signal is demodulated with the two frequencies.

5. Method for determining a measurement value with a photoacoustic gas sensor (1), comprising providing a measurement chamber (2) for receiving a gas, a light source (3) arranged to emit light into the measurement chamber (2), and a microphone (4) arranged to detect acoustic waves in the measurement chamber (2) and adapted to output a corresponding acoustic signal, providing a light-source driver (5) connected to provide a drive signal for the light source (3), according to which the intensity of the emitted light is modulated, providing a signal processor (6) connected to receive and process the acoustic signal and to determine and output the measurement value, modulating the light intensity, in a sequence of time windows (8, 9, 10), in a time window (8, 9, 10) of the sequence with a periodic function at a frequency, wherein the frequency in a time window (8, 9, 10) differs from the frequency in the subsequent time window, and wherein the sequence of frequency values thus defined is periodic and takes on nf different values, wherein nf is a natural number greater than or equal to two, characterized by the steps of: demodulating the acoustic signal in a modulation window with the frequency of the drive signal in the modulation window to determine a useful value, and with the frequency of the drive signal in the subsequent modulation window to determine an interference value, determining the measurement value in a measurement period (7), defined as nf successive modulation windows of the sequence, taking into account the useful values and the interference values, outputting the measurement value.

6. Computer-implemented method comprising the steps of receiving an acoustic signal representing the acoustic waves in a measurement cell (2) of a photoacoustic gas sensor (1), where the gas was exposed to light with a light-intensity modulation, wherein the light-intensity modulation in a sequence of time windows in a time window (8, 9, 10) was periodic at a frequency, wherein the frequency in a time window (8, 9, 10) differed from the frequency in the subsequent time window, and wherein the sequence of frequency values thus defined is periodic and takes on nf different values, wherein nf is a natural number greater than two, receiving configuration frequencies corresponding to the frequencies of the frequency sequence, segmenting the acoustic signal into signal segments corresponding to the time windows, demodulating a signal segment with the configuration frequency in the corresponding time window to determine a useful value, and with the configuration frequency corresponding to the subsequent time window to determine an interference value, determining a measurement value per measurement period (7), defined as signal segments of nf successive time windows of the sequence, taking into account the useful values and the interference values.

7. Computer program comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to claim 6.

8. Computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to perform the method according to claim 6.