Differential cross division-arc tangent phase generated carrier demodulation method
By adopting the differential cross-phase division-arctangent method in phase generation carrier technology, the influence of phase modulation depth on the demodulation result is solved, and accurate phase signal recovery is achieved in a wider range, improving understanding of modulation accuracy and signal amplitude Value accuracy.
Patent Information
- Application Number
- CN202411262017.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-10
- Publication Date
- 2024-12-13
AI Technical Summary
In the existing phase generation carrier technology, differential cross multiplication and arctangent method have harmonic distortions in the demodulation results under the influence of phase modulation depth, and the working range is limited, making it difficult to work normally at high phase modulation depth.
The differential cross-phase division-arctangent demodulation method is adopted to obtain the square value of the phase modulation depth by cross-phase division, and substitute it into the inverse tangent method to eliminate the influence of the phase modulation depth and achieve undisturbed phase signal recovery.
It effectively eliminates the impact of phase modulation depth on the demodulation result, improves the accuracy and demodulation accuracy of the phase signal, and can accurately demodulate the signal to be measured within a wider phase modulation depth range.
Smart Images
Figure CN119124234A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of phase generated carrier technology, and in particular to a differential cross division-inverse tangent phase generated carrier demodulation method capable of eliminating the influence of phase modulation depth disturbance in the phase generated carrier technology. Background Art
[0002] Fiber optic interferometric sensors, as highly sensitive sensing elements in fiber optic hydrophones, magnetometers, accelerometers, and seismic detectors, have attracted widespread attention over the past three decades. Fiber optic interferometry is a fairly mature technology that is commonly used in fiber optic hydrophones, fiber laser sensors, and high-resolution wavelength demodulators such as fiber Bragg grating sensors. Interferometric demodulation converts the wavelength shift of the fiber optic sensor into a phase shift of the interference signal based on an unbalanced fiber interferometer.
[0003] Among the existing fiber interferometer sensor demodulation technologies, the phase generated carrier technology (PGC) has been widely studied in the fields of interferometric fiber sensors due to its unique advantages of large dynamic range, good linearity, high resolution, and real-time demodulation capability for large-scale fiber sensors. In the demodulation scheme of the phase generated carrier technology, high-frequency sinusoidal phase modulation is applied to one arm of the unbalanced fiber interferometer to generate a phase carrier, and the required phase shift signal is frequency-converted to the sideband of the carrier frequency. By detecting an odd harmonic and an even harmonic of the phase generated carrier interference signal, a pair of orthogonal components containing phase shifts are obtained, thereby overcoming the fading caused by bias. In order to recover the phase shift from the orthogonal components, the traditional phase generated carrier technology uses the differential-and-cross-multiplying method (PGC-DCM) and the arctangent approach (PGC-arctan).
[0004] However, although both the differential cross-multiplication method and the inverse tangent method can recover the phase shift from the orthogonal component, both methods are affected by the phase modulation depth (the phase modulation depth (C)). The differential cross-multiplication method can only work properly when the phase modulation depth (C) is 2.37 rad, and the inverse tangent method can only work properly when the phase modulation depth (C) is 2.63 rad. In addition, 2.37 rad and 2.63 rad are only estimated values. Therefore, the phase information obtained by the differential cross-multiplication method when the phase modulation depth (C) is 2.37 rad and the inverse tangent method when the phase modulation depth (C) is 2.63 rad has certain harmonic distortion in principle. For the demodulation system, how to eliminate the influence of the phase modulation depth on the demodulation result is a technical problem that needs to be solved urgently. Summary of the invention
[0005] The purpose of the present invention is to provide a differential cross division-inverse tangent phase generation carrier demodulation method, which solves the problem that the demodulation result of the existing phase generation carrier technology is affected by the phase modulation depth. By using differential cross division and inverse tangent operation for processing and then dividing, the square value of the phase modulation depth is obtained, and then the value is substituted into the inverse tangent method as a feedback signal to obtain a phase signal that is not affected by the phase modulation depth disturbance. The algorithm is simple and is conducive to improving the accuracy of the phase signal amplitude.
[0006] The above-mentioned object of the present invention is achieved by the following technical solutions:
[0007] The phase generation carrier demodulation method of differential cross division-inverse tangent comprises the following steps:
[0008] Step 1: The light output by the narrowband light source is modulated externally by the piezoelectric ceramics in turn, and the interference light signal is obtained after the external signal to be measured is obtained by the interferometer, and then converted into an electrical signal I(t) by the photodetector; the electrical signal I(t) is expressed as follows:
[0009]
[0010] Among them, A and B are constants related to the attenuation of light intensity, C is the phase modulation depth, ω0 is the frequency of the high-frequency carrier, is the phase to be measured; the electrical signal I(t) is expanded by Bessel function to obtain:
[0011]
[0012] Step 2: Mix the electrical signal I(t) with the single-frequency carrier signal Gsin(ω0t) and the double-frequency carrier signal Hcos(2ω0t) and perform low-pass filtering to obtain a pair of mutually orthogonal sine terms S1(t) and cosine terms S2(t):
[0013]
[0014] Among them, G and H are the amplitudes of the single-frequency signal of the carrier signal and the double-frequency signal of the carrier signal respectively;
[0015] Step 3: Differentiate the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S3(t) and S4(t) respectively:
[0016]
[0017] Step 4: Cross-divide the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering and S3(t) and S4(t) obtained by differentiation, that is, the product of the sine term S1(t) obtained after low-pass filtering and S4(t) obtained by differentiation and the product of the cosine term S2(t) obtained after low-pass filtering and S3(t) obtained by differentiation are divided to obtain S5(t). The formula is as follows:
[0018]
[0019] Step 5: Divide the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S6(t). The calculation formula is as follows:
[0020]
[0021] Step 6: Square S6(t) to get S7(t), the formula is as follows:
[0022]
[0023] Step 7: Divide the result S5(t) after differential cross division by S7(t) to obtain S8(t) related to the phase modulation depth. The formula is as follows:
[0024]
[0025] From the Bessel function curve, it can be obtained that J1(C) and J2(C) are both greater than 0 when the phase modulation depth C is in the range of 0-3.5, and S9(t) can be obtained, and the formula is as follows:
[0026]
[0027] Step 8: Divide S6(t) by S9(t) to obtain the external phase to be measured that is not affected by the modulation depth. The signal S 10 (t), the formula is as follows:
[0028]
[0029] Step 9: S obtained in step 8 10 (t) Perform an inverse tangent operation, then phase unwrap the inverse tangent result, and finally pass a high-pass filter to realize the signal The demodulation formula is as follows:
[0030]
[0031] Among them, HPF|| represents high-pass filtering of the signal, and unwrap() represents phase unwrapping of the signal. At this point, the accurate demodulation of the phase generation carrier-differential cross division-inverse tangent demodulation algorithm is completed.
[0032] Another object of the present invention is to provide a differential cross-phase division-inverse tangent phase generation carrier demodulation system, in which the light output by the narrow-band light source is modulated externally by piezoelectric ceramics in turn, and the interference light signal is obtained after the external signal to be measured is obtained by the interferometer, and then converted into an electrical signal I(t) by a photodetector, and the output end of the electrical signal I(t) is divided into two paths and connected to the input ends of the first multiplier 2 and the second multiplier 3 with the carrier frequency-doubled sine signal 1 and the carrier frequency-doubled cosine signal 4 respectively; the outputs of the first multiplier 2 and the second multiplier 3 are connected to the input ends of the first low-pass filter 5 and the second low-pass filter 6 respectively, and the output of the first low-pass filter 5 is divided into three paths and connected to the input ends of the first divider 7, the first differentiator 9, and the fourth multiplier 12 respectively; the output of the second low-pass filter 6 is divided into three paths and connected to the input ends of the first divider 7, the second differentiator 10, and the third multiplier 11 respectively.
[0033] The output of the first divider 7 is divided into two paths and connected to the input ends of the squarer 8 and the fourth divider 16 respectively. The output of the squarer 8 is connected to the input end of the third divider 14; the outputs of the first differentiator 9 and the second differentiator 10 are connected to the input ends of the third multiplier 11 and the fourth multiplier 12 respectively. The output of the third multiplier 11 is connected to the input end of the second divider 13; the output of the fourth multiplier 14 is connected to the input end of the second divider 13, and the output of the second divider 13 is connected to the input end of the third divider 14; the output of the third divider 14 is connected to the input end of the opposite square root operator 15, the output of the opposite square root operator 15 is connected to the input end of the fourth divider 16, the output of the fourth divider 16 is connected to the input end of the inverse tangent operator 17, the output of the inverse tangent operator 17 is connected to the input end of the phase unwrapping operator 18, the output of the phase unwrapping operator 18 is connected to the input end of the high-pass filter 19, and the high-pass filter 19 outputs the phase to be measured.
[0034] The beneficial effects of the present invention are:
[0035] The present invention aims at the problem that both the differential cross multiplication algorithm and the inverse tangent algorithm in the existing phase generation carrier technology are affected by the phase modulation depth. According to the signal characteristics of the classical phase generation carrier technology under the influence of the phase modulation depth, the invention improves the classical phase generation carrier technology, changes the differential cross multiplication into a differential cross division, and obtains a phase matrix containing only the signal to be measured. The inverse of the square value of the inverse tangent algorithm is used to obtain the coefficients affected by the phase modulation depth and the measured signal. The square value of the product; the differential cross-division algorithm obtains only the signal to be measured The inverse number of the square value is squared with the inverse tangent algorithm to obtain the coefficient affected by the phase modulation depth and the measured signal The square of the product is fed into the divider to eliminate the signal to be measured. Only keep the signal to be tested The opposite of the square of the coefficient related to the phase modulation depth mentioned above, according to the characteristics of the first-order and second-order Bessel function curves that are positive numbers in the phase modulation depth range of 0-3.5rad, is used to measure the signal The opposite of the square of the coefficient related to the phase modulation depth is taken and then squared to obtain the coefficient affected by the phase modulation depth. The coefficient affected by the phase modulation depth is fed back to the inverse tangent algorithm and then phase unwrapping and high-pass filtering are performed to obtain the signal to be measured. The signal to be tested is accurately restored. BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The drawings described herein are used to provide further understanding of the present invention and constitute a part of this application. The illustrative examples of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.
[0037] Figure 1 It is the first-order and second-order Bessel function curve graph of the present invention;
[0038] Figure 2 It is a principle block diagram of the differential cross-division-inverse tangent phase generation carrier demodulation algorithm of the present invention;
[0039] Figure 3 It is a principle block diagram of the differential cross-division-inverse tangent phase generation carrier demodulation algorithm of the present invention.
[0040] In the figure: 1. carrier frequency one-time sine signal; 2. first multiplier; 3. second multiplier; 4. carrier frequency two-time cosine signal; 5. first low-pass filter; 6. second low-pass filter; 7. first divider; 8. squarer; 9. first differentiator; 10. second differentiator; 11. third multiplier; 12. fourth multiplier; 13. second divider; 14. third divider; 15. square root operator for finding the opposite number; 16. fourth divider; 17. inverse tangent operator; 18. phase unwrapping operator; 19. high-pass filter. DETAILED DESCRIPTION
[0041] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.
[0042] See also Figure 1 and Figure 2 As shown, the differential cross division-inverse tangent phase generation carrier demodulation method (where "-" represents the meaning of combination) of the present invention, which can eliminate the influence of phase modulation depth disturbance, obtains only the tangent square value of the relevant phase signal by cross division of the sine component and the cosine component, and then obtains the product containing the relevant modulation depth value and the tangent square value of the relevant phase signal by squaring the result obtained by the inverse tangent algorithm. The specific value of the modulation depth can be obtained by dividing the two, and this value is fed back to the inverse tangent algorithm to demodulate the phase information not affected by the phase modulation depth. The present invention adaptively eliminates the influence of the phase modulation depth on the signal and accurately restores the signal to be measured. The method is simple and is conducive to improving the accuracy of the phase signal amplitude.
[0043] The phase generation carrier demodulation method of phase generation carrier differential cross division-inverse tangent comprises the following steps:
[0044] 1. The light output by the narrow-band light source is modulated externally by piezoelectric ceramics, and then the interference light signal is obtained by the interferometer to obtain the external signal to be measured, and then converted into an electrical signal I(t) by the photodetector; the electrical signal I(t) is expressed as follows:
[0045]
[0046] Among them, A and B are constants related to the attenuation of light intensity, C is the phase modulation depth, ω0 is the frequency of the high-frequency carrier, is the phase to be measured; the electrical signal I(t) is expanded by Bessel function to obtain:
[0047]
[0048] 2. After mixing the electrical signal I(t) with the single-frequency carrier signal Gsin(ω0t) and the double-frequency carrier signal Hcos(2ω0t) and performing low-pass filtering, a pair of mutually orthogonal sine terms S1(t) and cosine terms S2(t) are obtained:
[0049]
[0050] Among them, G and H are the amplitudes of the single-frequency signal of the carrier signal and the double-frequency signal of the carrier signal respectively;
[0051] 3. Differentiate the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S3(t) and S4(t) respectively:
[0052]
[0053] 4. The sine term S1(t) and cosine term S2(t) obtained after low-pass filtering are cross-divided with S3(t) and S4(t) obtained by differentiation. That is, the product of the sine term S1(t) obtained after low-pass filtering and S4(t) obtained by differentiation is divided by the product of the cosine term S2(t) obtained after low-pass filtering and S3(t) obtained by differentiation to obtain S5(t). The calculation formula is as follows:
[0054]
[0055] 5. Divide the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S6(t). The calculation formula is as follows:
[0056]
[0057] 6. Square S6(t) to get S7(t), the calculation formula is as follows:
[0058]
[0059] 7. Divide the result S5(t) after differential cross division by S7(t) to obtain S8(t) related to the phase modulation depth. The calculation formula is as follows:
[0060]
[0061] From the Bessel function curve, it can be obtained that J1(C) and J2(C) are both greater than 0 when the phase modulation depth C is in the range of 0-3.5, and S9(t) can be obtained. The calculation formula is as follows:
[0062]
[0063] 8. Divide S6(t) by S9(t) to get the external phase to be measured that is not affected by the modulation depth. The signal S 10 (t), the calculation formula is as follows:
[0064]
[0065] 9. For S obtained in step 8 10 (t) Perform an inverse tangent operation, then phase unwrap the inverse tangent result, and finally pass a high-pass filter to realize the signal The demodulation formula is as follows:
[0066]
[0067] Among them, HPF|| represents high-pass filtering of the signal, and unwrap() represents phase unwrapping of the signal. At this point, the accurate demodulation of the phase generation carrier-differential cross division-inverse tangent demodulation algorithm is completed.
[0068] The differential cross-division-inverse tangent phase generation carrier demodulation system of the present invention includes a carrier frequency-doubled sine signal 1, a first multiplier 2, a second multiplier 3, a carrier frequency-doubled cosine signal 4, a first low-pass filter 5, a second low-pass filter 6, a first divider 7, a squarer 8, a first differentiator 9, a second differentiator 10, a third multiplier 11, a fourth multiplier 12, a second divider 13, a third divider 14, a square root operator for finding the opposite number 15, a fourth divider 16, an inverse tangent operator 17, a phase unwrapping operator 18, and a high-pass filter 19. The light output by the narrowband light source is modulated externally by piezoelectric ceramics in turn, and obtains the external signal to be measured after passing through the interferometer to obtain an interference light signal, and then converted into an electrical signal I(t) by a photodetector. The output end of the electrical signal I(t) is respectively connected to the input ends of the first multiplier 2 and the second multiplier 3 through the carrier frequency-doubled sine signal 1 and the carrier frequency-doubled cosine signal 4; the outputs of the first multiplier 2 and the second multiplier 3 are respectively connected to the input ends of the first low-pass filter 5 and the second low-pass filter 6, and the output of the first low-pass filter 5 is divided into three paths and respectively connected to the input ends of the first divider 7, the first differentiator 9, and the fourth multiplier 12; the output of the second low-pass filter 6 is divided into three paths and respectively connected to the input ends of the first divider 7, the second differentiator 10, and the third multiplier 11.
[0069] The output of the first divider 7 is divided into two paths and connected to the input ends of the squarer 8 and the fourth divider 16 respectively. The output of the squarer 8 is connected to the input end of the third divider 14; the outputs of the first differentiator 9 and the second differentiator 10 are connected to the input ends of the third multiplier 11 and the fourth multiplier 12 respectively. The output of the third multiplier 11 is connected to the input end of the second divider 13; the output of the fourth multiplier 14 is connected to the input end of the second divider 13, and the output of the second divider 13 is connected to the input end of the third divider 14; the output of the third divider 14 is connected to the input end of the opposite square root operator 15, the output of the opposite square root operator 15 is connected to the input end of the fourth divider 16, the output of the fourth divider 16 is connected to the input end of the inverse tangent operator 17, the output of the inverse tangent operator 17 is connected to the input end of the phase unwrapping operator 18, the output of the phase unwrapping operator 18 is connected to the input end of the high-pass filter 19, and the high-pass filter 19 outputs the phase to be measured.
[0070] The present invention aims at the problem that both the differential cross-multiplication algorithm and the inverse tangent algorithm in the existing phase generation carrier technology are affected by the phase modulation depth. According to the signal characteristics of the classical phase generation carrier technology under the influence of the phase modulation depth, on the basis of the classical phase generation carrier technology, the sine component and the cosine component are cross-divided to obtain the tangent square value containing only the relevant phase signal, and then the result obtained by the inverse tangent algorithm is squared to obtain the product containing the relevant modulation depth value and the tangent square value of the relevant phase signal. The two are divided to obtain the specific value of the relevant modulation depth, and this value is fed back to the inverse tangent algorithm to demodulate the phase information not affected by the phase modulation depth. The present invention adaptively eliminates the influence of the phase modulation depth on the signal and accurately restores the signal to be measured.
[0071] Figure 3 The effect diagram of the present invention is shown, which respectively shows the demodulation effect diagram of the sinusoidal signal with an amplitude of 1.3rad and a frequency of 500Hz when the modulation depth C is 3.0, 2.63, 2.37, and 1.5. Figure 3 It can be seen that the differential cross division-inverse tangent algorithm can accurately demodulate the signal to be tested with an amplitude of 1.3rad when the modulation depth C is 3.0, 2.63, 2.37, and 1.5; while the differential cross multiplication algorithm can only demodulate a sine wave with a frequency of 500Hz, but cannot achieve accurate demodulation in amplitude; the inverse tangent algorithm can more accurately demodulate the signal to be tested when the modulation depth is 2.37 and 2.63, but it is still much worse than the differential cross division-inverse tangent algorithm, and the demodulation result of the inverse tangent algorithm has been slightly distorted when the modulation depth is 1.0. Therefore, the present invention adaptively eliminates the influence of the phase modulation depth on the signal and accurately restores the signal to be tested. The method is simple and is conducive to improving the accuracy of the phase signal amplitude.
[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made to the present invention shall be included in the protection scope of the present invention.
Claims
1. A differential cross-division-inverse tangent phase generation carrier demodulation method, characterized in that: The following steps are involved: Step 1: The light output by the narrowband light source is modulated externally by the piezoelectric ceramics in turn, and the interference light signal is obtained after the external signal to be measured is obtained by the interferometer, and then converted into an electrical signal I(t) by the photodetector; the electrical signal I(t) is expressed as follows: Among them, A and B are constants related to the attenuation of light intensity, C is the phase modulation depth, ω0 is the frequency of the high-frequency carrier, is the phase to be measured; the electrical signal I(t) is expanded by Bessel function to obtain: Step 2: Mix the electrical signal I(t) with the single-frequency carrier signal Gsin(ω0t) and the double-frequency carrier signal Hcos(2ω0t) and perform low-pass filtering to obtain a pair of mutually orthogonal sine terms S1(t) and cosine terms S2(t): Among them, G and H are the amplitudes of the single-frequency signal of the carrier signal and the double-frequency signal of the carrier signal respectively; Step 3: Differentiate the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S3(t) and S4(t) respectively: Step 4: Cross-divide the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering and S3(t) and S4(t) obtained by differentiation, that is, the product of the sine term S1(t) obtained after low-pass filtering and S4(t) obtained by differentiation and the product of the cosine term S2(t) obtained after low-pass filtering and S3(t) obtained by differentiation are divided to obtain S5(t). The formula is as follows: Step 5: Divide the sine term S1(t) and cosine term S2(t) obtained after low-pass filtering to obtain S6(t), the formula is as follows: Step 6: Square S6(t) to get S7(t), the formula is as follows: Step 7: Divide the result S5(t) after differential cross division by S7(t) to obtain S8(t) related to the phase modulation depth. The formula is as follows: From the Bessel function curve, it can be obtained that J1(C) and J2(C) are both greater than 0 when the phase modulation depth C is in the range of 0-3.5, and S9(t) is obtained, and the formula is as follows: Step 8: Divide S6(t) by S9(t) to obtain the external phase to be measured that is not affected by the modulation depth. The signal S 10 (t), the formula is as follows: Step 9: S obtained in step 8 10 (t) Perform an inverse tangent operation, then phase unwrap the inverse tangent result, and finally pass a high-pass filter to realize the signal The demodulation formula is as follows: Among them, HPF|| represents high-pass filtering of the signal, and unwrap() represents phase unwrapping of the signal. Thus, accurate demodulation of the phase generation carrier-differential cross division-inverse tangent demodulation algorithm is completed.
2. A system for implementing the differential cross division-inverse tangent phase generation carrier demodulation method according to claim 1, characterized in that: The light output by the narrowband light source is modulated externally by piezoelectric ceramics in sequence, and then obtained by an interferometer to obtain an interference light signal after obtaining an external signal to be measured, and then converted into an electrical signal I(t) by a photoelectric detector. The output end of the electrical signal I(t) is divided into two paths and connected to the input ends of a first multiplier (2) and a second multiplier (3) with a carrier frequency-doubled sine signal (1) and a carrier frequency-doubled cosine signal (4); the outputs of the first multiplier (2) and the second multiplier (3) are connected to the input ends of a first low-pass filter (5) and a second low-pass filter (6) respectively; the output of the first low-pass filter (5) is divided into three paths and connected to the input ends of a first divider (7), a first differentiator (9) and a fourth multiplier (12) respectively; the output of the second low-pass filter (6) is divided into three paths and connected to the input ends of a first divider (7), a second differentiator (10) and a third multiplier (11) respectively.
3. The differential cross division-inverse tangent phase generation carrier demodulation system according to claim 2, characterized in that: The output of the first divider (7) is divided into two paths and connected to the input ends of the squarer (8) and the fourth divider (16) respectively, and the output of the squarer (8) is connected to the input end of the third divider (14); the outputs of the first differentiator (9) and the second differentiator (10) are connected to the input ends of the third multiplier (11) and the fourth multiplier (12) respectively, and the output of the third multiplier (11) is connected to the input end of the second divider (13); the output of the fourth multiplier (14) is connected to the input end of the second divider (13), and the output of the second divider (13) is connected to the input end of the to the input end of a third divider (14); the output of the third divider (14) is connected to the input end of a negative square root operator (15); the output of the negative square root operator (15) is connected to the input end of a fourth divider (16); the output of the fourth divider (16) is connected to the input end of an inverse tangent operator (17); the output of the inverse tangent operator (17) is connected to the input end of a phase unwrapping operator (18); the output of the phase unwrapping operator (18) is connected to the input end of a high-pass filter (19); the high-pass filter (19) outputs the phase to be measured.