Multi-frequency simultaneous near-field holographic measurement method using a comb spectrum as the emission source signal

Abstract

The multi-frequency simultaneous near-field holographic measurement method of the present invention uses a comb spectrum as the emission source signal, and includes the following steps: Step 1, obtain the comb spectrum signal according to the sampling frequency, the number of sampling points, and the number of spectral lines of the comb spectrum signal; Step 2, send the comb spectrum signal from the emission source system through the emission beacon source circuit; Step 3, the antenna under test receives the comb spectrum signal sent by the emission source system and performs multi-frequency simultaneous near-field holographic measurement. The present invention uses a comb spectrum signal composed of multiple frequency signals as the emission source signal, which can realize the simultaneous emission of multiple frequency signals, avoid the influence caused by environmental factors when not simultaneous, and further suppress the influence brought by the multipath interference effect. The output signal spectral lines of the comb spectrum signal source are comb-shaped spectral lines with equal frequency intervals. By using a comb spectrum as the emission source signal, compared with using Gaussian white noise uniformly distributed in the full frequency band, the system error brought by it can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of telescopes, and more specifically, relates to a multi-frequency simultaneous near-field holographic measurement method using a comb spectrum as the transmitted source signal. Background Art

[0002] Radio telescopes mainly include single-mirror radio telescopes, aperture synthesis radio telescopes, millimeter-wave radio telescopes, very long baseline interferometry network VLBI, and solar radio telescopes, etc. Nowadays, radio telescopes are developing towards larger apertures. For example, FAST is a spherical reflector radio telescope with an aperture of up to 500 meters. The antenna (main reflector) of such a large-aperture radio telescope is often composed of dozens or even hundreds of composite material panels spliced together. During the installation and maintenance of the telescope, surface shape detection technology is required to measure and adjust to maintain the surface accuracy requirements of the antenna reflector surface.

[0003] Currently, the measurement methods for the surface shape of the antenna reflector mainly include classical measurement methods (such as the theodolite tape method, total station measurement method, etc.), laser tracker method, photogrammetry method, and microwave holographic measurement method, etc. Among them, the measurement accuracy of the classical measurement method is not high and it takes a long time. Some new measurement methods such as photogrammetry method and microwave holography measurement are gradually applied in the adjustment of the reflector surface and the on-site measurement of the antenna reflector surface.

[0004] Near-field holographic measurement is a non-contact new surface shape measurement method. Its core is based on a very important relationship in antenna theory: the far-field radiation pattern (amplitude and phase) of a reflector antenna is the Fourier transform of its aperture field distribution (amplitude and phase). By scanning the antenna, the amplitude and phase distributions of the antenna radiation pattern can be measured within a sufficiently large angular range. Using the Fourier transform, the amplitude and phase distributions of the antenna aperture field can be obtained. The phase of the aperture field is related to the antenna surface shape through the optical path of the light emitted from the main focus and reflected by the parabolic surface to reach the aperture plane. For an ideal parabolic antenna, the phase of its aperture field is a constant, while the optical path difference introduced by the surface shape error of a non-ideal parabolic surface will be reflected in the phase distribution of the aperture field. This method is convenient and fast. As long as the signal-to-noise ratio is sufficient, a measurement accuracy of several micrometers can be obtained.

[0005] In near-field holographic measurement, the transmitting beacon source is generally placed at a height of dozens to hundreds of meters from the antenna under test and is fixed during the measurement process. The electromagnetic wave transmitted by the transmitting beacon source will, during the propagation process from being transmitted to received, due to the influence of the propagation path, cause reflection, scattering, and diffraction of the electromagnetic wave, so that the signal reaching the receiving station does not come from a single path but from many paths, a composite wave of many reflected waves, thus forming multipath propagation. Due to the existence of factors such as mountains and buildings and some human and non-human factors, multipath propagation is inevitable, which leads to the need to improve the accuracy of near-field holographic measurement. Summary of the Invention

[0006] The present invention provides a multi - frequency simultaneous near - field holographic measurement method using a comb - shaped spectrum as the emission source signal to solve the problem of low accuracy in near - field holographic measurement mentioned in the background art.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A multi - frequency simultaneous near - field holographic measurement method using a comb - shaped spectrum as the emission source signal, which includes the following steps:

[0009] Step 1: Obtain the comb - shaped spectrum signal according to the sampling frequency, the number of sampling points, and the number of spectral lines of the comb - shaped spectrum signal.

[0010] Step 2: Send the comb - shaped spectrum signal from the emission source system through the emission beacon source circuit.

[0011] Step 3: The reference antenna and the antenna under test simultaneously receive the comb - shaped spectrum signal emitted by the emission source system and perform multi - frequency simultaneous near - field holographic measurement.

[0012] Furthermore, the expression of the comb - shaped spectrum signal is as follows:

[0013]

[0014] In the formula, f(t) is the comb - shaped spectrum signal, A 0 is the amplitude coefficient corresponding to each spectral line of the signal, b n is the frequency coefficient corresponding to each spectral line, f 0 is the frequency interval, φ n is the initial phase corresponding to each spectral line, N f is the number of spectral lines, and t is the period time.

[0015] The antenna under test realizes real - time FFT sampling transformation and conjugate correlation of the comb - shaped spectrum signal through a digital acquisition correlator, and then obtains the surface shape error distribution of the reflecting surface of the antenna under test. To avoid FFT sampling spectrum leakage, the frequency interval of the comb - shaped spectrum signal is set to an integer multiple of the FFT spectrum interval, that is:

[0016] Δf = nf s / N

[0017] Δf is the frequency interval of the comb - shaped spectrum signal, f s is the FFT sampling rate, and N is the FFT data length.

[0018] After the antenna under test receives the comb - shaped spectrum signal, the comb - shaped spectrum signal is reflected to the aperture field of the antenna under test and received by the dual - channel broadband receiver, and the dual - channel broadband receiver is connected to the digital acquisition correlator.

[0019] Further, the specific content of Step 1 is as follows:

[0020] Step 1.1: Write a Matlab program according to the sampling frequency, the number of sampling points, and the number of spectral lines parameters to obtain the data of the comb-shaped spectrum time-domain signal; build a high-speed data acquisition system of NI, and use Labview to load the comb-shaped spectrum time-domain data.

[0021] Step 1.2: Build a high-speed data acquisition system of NI, and use Labview to load the comb-shaped spectrum time-domain data.

[0022] Step 1.3: Use Labview to process and sort the time-domain data according to the settings of the amplitude and the acquisition frequency, and output the processed comb-shaped spectrum signal on the analog port.

[0023] The output signal spectrum lines of the comb-shaped spectrum signal are comb-shaped spectrum lines with equal frequency intervals.

[0024] The beneficial effects of the present invention are as follows:

[0025] 1. The multi-frequency simultaneous near-field holographic measurement system of the present invention uses a comb-shaped spectrum signal composed of multiple frequency signals as the emission source signal, which can realize the simultaneous emission of multiple frequency signals, avoid the influence caused by environmental factors when not simultaneous, and thus suppress the influence brought by the multipath interference effect.

[0026] 2. The output signal spectrum lines of the comb-shaped spectrum signal source adopted by the present invention are comb-shaped spectrum lines with equal frequency intervals. By using the comb-shaped spectrum as the emission source signal, compared with using Gaussian white noise uniformly distributed in the full frequency band, the system error brought by it can be reduced. Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of the near-field holographic measurement system;

[0028] Figure 2 It is a time-domain waveform diagram of the comb-shaped spectrum;

[0029] Figure 3 It is a frequency-domain waveform diagram of the comb-shaped spectrum;

[0030] Figure 4 It is a waveform diagram generated by loading a txt file in the program block diagram;

[0031] Figure 5 It is a waveform diagram of loading two-way waveform signals through the AWG;

[0032] Figure 6 It is a front panel waveform amplitude and sampling rate setting interface diagram;

[0033] Figure 7It is the diagram of outputting waveforms to AO0 and AO1 interfaces by the Download Waveforms 2chSame control;

[0034] Figure 8 It is the block diagram of the transmitting beacon source system;

[0035] Figure 9 It is the diagram of the generation process of the comb spectrum signal. Specific implementation manners

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention.

[0038] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installed", "connected", "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0039] The near-field holographic measurement system mainly includes three parts: a transmitting beacon source, a two-channel broadband receiver, and a digital acquisition correlator. As Figure 1 shown.

[0040] The specific method of near-field holographic measurement is that the transmitting beacon source emits a comb spectrum line signal, the two-channel broadband receiver receives the comb spectrum line signal, and the digital acquisition correlator uses Fourier transform to invert the far-field radiation pattern (amplitude and phase) of the reflector antenna to obtain the aperture field amplitude and phase information, and finally obtains the surface shape error distribution of the antenna reflector surface. Multi-frequency simultaneous near-field holographic measurement is to perform near-field holographic measurement simultaneously at multiple frequencies.

[0041] The transmitting beacon source in these three parts, that is, the generation of radio frequency signals, is a very crucial link in the entire system. As described in the background art, the transmitting beacon source is generally placed at a height of dozens to hundreds of meters from the antenna under test, and its position is fixed during the measurement process. The electromagnetic wave transmitted by the transmitting beacon source will, during the propagation process from being transmitted to being received, due to the influence of the propagation path, cause reflection, scattering, and diffraction of the electromagnetic wave, resulting in the signal reaching the receiving station not coming from a single path but from many paths and being a composite wave of many reflected waves, thus forming multipath propagation. Due to the existence of factors such as mountains and buildings and some human and non-human factors, multipath propagation is inevitable.

[0042] To suppress the influence brought by the multipath interference effect, the present invention designs and establishes a multi-frequency simultaneous near-field holographic measurement system, which uses a broadband signal to replace the traditional narrowband signal as the transmitting source signal, and can realize the simultaneous measurement of multiple frequency signals, avoiding the influence brought by environmental factors due to non-simultaneity.

[0043] The transmitting source signal can adopt two signal sources, namely a noise source and a comb spectrum signal. The output signal spectrum lines of the comb spectrum signal source are comb spectrum lines with equal frequency intervals. The output spectrum lines of the noise signal source are Gaussian white noise uniformly distributed in the whole frequency band. In order to suppress the multipath effect in this application, a comb spectrum is adopted as the transmitting source signal to reduce the system error brought by it.

[0044] The expression of the comb spectrum signal is as follows:

[0045]

[0046] In the formula, A 0 is the amplitude coefficient corresponding to each spectrum line of the signal, b n is the frequency coefficient corresponding to each spectrum line, f 0 is the frequency interval, φ n is the initial phase corresponding to each spectrum line, N f is the number of spectrum lines, and t is the time period.

[0047] The following uses a specific example to explain how the present invention sets the comb spectrum:

[0048] In practical applications, a set of comb spectrum is built as the transmitting system using NI instruments and equipment. The chassis uses NI PXIe1092, the controller uses NI PXIe8861, and the intermediate frequency (IF) transceiver module uses the NI PXIe-5785 board. Labview is used to program and control the NI PXIe-5785 board. The NI PXIe-5785 board is designed with two input and output channels, namely two output channels AO0 and AO1, and two input channels DI0 and DI1, with a sampling frequency of 3.2 GHz and a sampling period of 0.3125 ns. In programming, the FlexRIO (reconfigurable I / O) driver can be called, which supports waveform acquisition and generation, provides flexible and definable I / O to the Labview FPGA module, and facilitates custom algorithms and secondary development to achieve real-time signal processing.

[0049] Theoretically, in the Labview program, the pre-calculated spectrum signal can be loaded and generated. Here, the calculated spectrum signal can be a signal of any waveform. In practical applications in the multi-frequency simultaneous near-field holographic measurement, the comb spectrum signal is loaded by Labview. The NI PXIe-5785 can generate any waveform. First, the host computer sends the time-domain signal data of the comb spectrum signal with the pre-calculated spectrum to the NI PXIe-5785. The calculation of the time-domain signal data can be achieved by programming software such as Matlab. The time-domain signal is calculated and generated at the fixed sampling frequency of 3.2 GHz of the NI PXIe-5785, and its data length is exactly one full period of the comb spectrum frequency interval. In the case of non-integer period sampling, the signal frequency does not fall at the center of the frequency spectrum line but between two frequency spectrum lines, resulting in spectrum leakage. To avoid FFT sampling spectrum leakage, the frequency interval of the comb spectrum signal is set to an integer multiple of the FFT spectrum interval, that is:

[0050] Δf = nf s / N

[0051] f s where f is the FFT sampling rate of 3.2 GHz, and N is the FFT data length, that is, 4096 points are sampled. The frequency resolution is 3.2G / 4096 = 0.78125 MHz, which is an integer multiple of 0.78125M.

[0052] When the highest frequency of the required comb spectrum is 1.25 GHz and the number of spectral lines is 200, the frequency interval is 1.25G / 200 = 6.25 MHz, which is 8 times the FFT spectrum interval.

[0053] The mathematical expression of the comb spectrum signal is as follows:

[0054]

[0055] where A0 is the amplitude coefficient corresponding to each spectral line, b n is the frequency coefficient corresponding to each spectral line, f 0 is the frequency interval, φ n is the initial phase corresponding to each spectral line, N f is the number of spectral lines, and t is the time period.

[0056] Based on the above formula, in the Matlab environment, the above algorithm can be implemented through programming to generate time-domain comb spectrum waveform data. The data length (number of rows) generated by Matlab is:

[0057]

[0058] When fs = 3.2 GHz and Δf takes 6.25 MHz (n = 8), the data generated by Matlab is a txt text format file with 512 rows.

[0059] Set parameters such as the sampling spectrum, number of spectral lines, highest frequency of the comb spectrum, and initial phase in Matlab, and write the code to generate the comb spectrum. The generated time-domain and frequency-domain diagrams are respectively Figure 2 and Figure 3 as shown.

[0060] After generating 512 rows of comb spectrum data, save it on the hard disk for Labview to read. In Labview, call the Arbitrary_waveform_generation.vi module, load the txt data of the comb wave. In addition, this module can also set Fs (sampling frequency) and Amplitude (amplitude) of the waveform, such as Figure 4 . The module first opens the directory where the txt is located, then calls the Read from Text file control to read the values in the txt in sequence, then calls the Delimited String to 1DString Array control to convert 512 values into a one-dimensional character array, and then calls String to Number to convert the character array into a numeric variable. Multiply the above numeric variable by Amplitude in sequence according to the set amplitude (multiple) to get a new one-dimensional array. Then call the Build Waveform control to generate a waveform according to the specified number of intervals between data points (sampling frequency). The settings of the two parameters of amplitude and sampling frequency are completed through the interface on the front panel of Labview.

[0061] On the upper computer interface in Labview, parameters such as the amplitudes and sampling frequencies of two channels can be set, and these two parameters are passed to the program block diagram to configure the waveform.

[0062] The above describes a single Arbitrary_waveform_generation.vi (AWG) arbitrary waveform generation module. Since the PXIe-5785 has two AOs (analog output ports), in order to achieve simultaneous output of two channels, the AWG module is loaded twice in the Labview program framework. As Figure 5 , in order to distinguish the output of the two waveforms, the outputs of the two channels are processed two-dimensionally through the Build Array control, and after the two-dimensional array is output, it is displayed on the graph through the Graph.

[0063] Four variables, namely the sampling rate (FS) and amplitude (Amplitude) of each channel in the figure, can be changed through the settings on the Labview front panel. As Figure 6 , the amplitudes of the two comb spectra are set to 0.4 volts, and the sampling rate is set to 3.2 GHz.

[0064] On the other hand, the waveform data of the two channels sends the analog waveform signal to the two hardware output ports 0 and 1 through the Download Waveforms 2chSame control. As Figure 7 .

[0065] After generating the waveform of the comb spectrum on the PXIe-5785 board, it is also necessary to convert it into a radio frequency signal and send it out. Here, a transmitting beacon source circuit needs to be designed. The principle block diagram of this part of the circuit is as Figure 8 shown.

[0066] It mainly consists of a local oscillator frequency synthesizer, a fixed attenuator, an integrated transmitting module, an isolator, an adjustable attenuator, and a transmitting corrugated horn. Among them, the integrated transmitting module consists of an octupler, an up-conversion mixer, and an amplifier. The transmitting beacon source can mix the intermediate frequency signal and the local oscillator signal into a double-sideband radio frequency signal, and the radio frequency signal frequency band it can transmit is 88 GHz to 104 GHz. Since only one transmitting beacon source is required for near-field holographic measurement, so as Figure 8 , the multi-frequency comb spectrum signal only needs to be connected from AO0 or AO1 of the PXIe-5785 board, and both outputs can be connected.

[0067] Summarize the generation process of the above comb spectrum signal as Figure 9 .

[0068] The above is only the preferred implementation manner of the present invention. The protection scope of the present invention is not limited to the above embodiments. Any technical solutions falling within the idea of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should be regarded as within the protection scope of the present invention.

Claims

1. A multi-frequency simultaneous near-field holographic measurement method using a comb spectrum as the emission source signal, characterized in that: It includes the following steps: Step 1: Obtain the comb spectrum signal according to the sampling frequency, the number of sampling points, and the number of spectral lines of the comb spectrum signal. Specifically, it includes: Step 1.1, Write a Matlab program according to the sampling frequency, the number of sampling points, and the number of spectral lines to obtain the comb spectrum time-domain signal data; Step 1.2, Build a high-speed data acquisition system and use Labview to load the comb spectrum time-domain data; Step 1.3, Use Labview to process and sort the time-domain data according to the settings of amplitude and acquisition frequency, and output the processed comb spectrum signal on the analog port; the output signal spectral lines of the comb spectrum signal are comb spectral lines with equal frequency intervals; Step 2: Send the comb spectrum signal from the emission source system through the emission beacon source circuit; Step 3: The antenna under test receives the comb spectrum signal emitted by the emission source system and performs multi-frequency simultaneous near-field holographic measurement; The antenna under test realizes the real-time FFT sampling transformation and conjugate correlation of the comb spectrum signal through a digital acquisition correlator, and then obtains the surface shape error distribution of the reflecting surface of the antenna under test. To avoid FFT sampling spectrum leakage, the frequency interval of the comb spectrum signal is set to an integer multiple of the FFT spectrum interval, that is: is the frequency interval of the comb spectrum signal, and f s is the FFT sampling rate, and N is the FFT data length.

2. The multi-frequency simultaneous near-field holographic measurement method using a comb spectrum as the emission source signal according to claim 1, characterized in that: The expression of the comb spectrum signal is as follows: In the formula, is a comb spectrum signal, is the amplitude coefficient corresponding to each spectral line of the signal, is the frequency coefficient corresponding to each spectral line, is the frequency interval, is the initial phase corresponding to each spectral line, is the number of spectral lines, and t is the time period.

3. The multi-frequency simultaneous near-field holographic measurement method using a comb spectrum as the emission source signal according to claim 1, characterized in that: After the antenna under test receives the comb spectrum signal, the comb spectrum signal is reflected to the aperture field of the antenna under test and is received by a dual-channel broadband receiver, and the dual-channel broadband receiver is connected to the digital acquisition correlator.

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