CN112763796A - Method for measuring LFM carrier frequency with high precision - Google Patents
Method for measuring LFM carrier frequency with high precision Download PDFInfo
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Abstract
The invention provides a method for measuring LFM carrier frequency with high precision, which comprises the following steps: step 1: the frequency measuring machine receives a radio frequency signal; step 2: guiding a microwave local oscillator in an analog domain according to a signal receiving result of a frequency measuring machine, and carrying out down-conversion on a measured signal to a low-middle frequency band; and step 3: carrying out AD digital sampling; and 4, step 4: after AD sampling, weighting processing is carried out on the time domain signal by adopting a window function, and gating truncation of the signal is realized; and 5: carrying out phase difference calculation on the tested signal; step 6: carrying out phase smoothing treatment; and 7: selecting a low intermediate frequency measurement value; and 8: and calculating the carrier frequency of the tested LFM signal. The invention carries out time domain window limitation and phase smoothing processing on the sampling signal, further simplifies the calculation method of the carrier frequency center value, can effectively improve the accuracy of LFM signal carrier frequency measurement, and the resolution of the carrier frequency signal measured by adopting the technical scheme of the invention is far superior to that of the carrier frequency measurement method based on Fourier transform.
Description
Technical Field
The invention relates to the field of radar signal processing, in particular to a high-precision measurement technology for LFM signal carrier frequency.
Background
Along with the development of modern electronic wars, the position of electronic countermeasures in the modern wars is increasingly prominent, the electronic countermeasures are countermeasures of both opposing parties for winning the war by using signals and weapon equipment in the frequency range of electromagnetic waves, the two opposing parties use the main means of electromagnetic spectrum for receiving and transmitting electromagnetic waves, and two main aspects of the electronic countermeasures are correspondingly formed: electronic reconnaissance and electronic interference.
With the upgrading of modern electronic warfare, the radiation source signal detection of high density, wide frequency band and complex signal patterns in the modern electronic warfare makes electronic reconnaissance increasingly become the key point for winning in the modern electronic warfare. The electronic investigation mainly receives, investigates, sorts and identifies the non-cooperative radiation electromagnetic signals in the space, reports the signals with threats and potential threats, and releases the specified interference signals by the interference master control according to different strategies and selection of battlefield occasions. The electronic reconnaissance is an electronic countermeasure eye, is a precondition and a foundation of electronic interference, and is a rammer for signal sorting, identification, threat degree judgment and battle situation judgment.
The main purpose of electronic detection is to measure the PDW (pulse description word) of the spatial radiation source signal, which mainly includes the carrier frequency of arrival, angle, arrival time, pulse width, pulse power and intra-pulse modulation characteristics. The carrier frequency parameter is an important parameter of the PDW, is an important mark for judging the power and threat degree of the radar, and is an important basis for sorting and identifying a signal source, so that accurate carrier frequency parameter measurement is particularly important. The radar signal is converted from an initial single pulse, a simple continuous wave and the like into a complex signal such as LFM (Linear frequency modulation), frequency truncation, pseudorandom frequency modulation and the like. The LFM can obtain high resolution of distance direction, and is the main transmission signal form of multi-system radars such as searching, tracking, imaging and the like. The LFM signal has the characteristics of wide and long time and large bandwidth, amplitude and phase distortion of a received signal caused by nonlinearity of the actual hardware device, and a large carrier frequency pilot measurement error affect the type judgment of a radiation source and the modulation of a partial interference (mainly referred to as deception interference) signal.
In the prior art, two frequency measurement methods exist, one is a carrier frequency measurement method based on phase difference, and a schematic block diagram of the method is shown in fig. 1. As shown in fig. 1, the carrier frequency measurement of the instantaneous frequency measurement method based on phase difference includes the following steps: step 1: firstly, guiding a microwave local oscillator according to a result of a frequency measuring machine (rough frequency measurement) in an analog domain, and down-converting a measured signal to a low-middle frequency band; step 2: after AD sampling, the phase of the complex signal is solved in a digital domain, and the phase difference result of the detected signal is obtained according to different difference modes (forward finite difference, backward finite difference or central finite difference); and step 3: because the frequency is the differential of the phase, the whole frequency of the detected signal in the signal band can be estimated according to the phase difference; and 4, step 4: the maximum value and the minimum value of the frequency are obtained, and the average value of the maximum value and the minimum value is an estimated value of the low-intermediate frequency signal to be detected because the frequency of the LFM signal in the band is linearly changed; and 5: and adding the estimated low-intermediate frequency signal center value and the down-conversion local oscillator value to obtain the test result of the LFM signal. The carrier frequency measurement method based on the phase difference calculates the signal frequency in the whole frequency band by directly utilizing the phase difference, has simple and convenient method, and has the defects that the amplitude and phase distortion exists in the actually acquired data due to the acquisition hardware and other reasons, the maximum value and the minimum value of the frequency of the LFM signal cannot be accurately measured, and certain calculation errors exist.
Another carrier frequency measurement method is a carrier frequency measurement method based on fourier transform, and its functional block diagram is shown in fig. 2. The core of the method is that the low and medium frequency signals are firstly changed into a frequency domain, the position of a 3dB drop point of an in-band frequency spectrum is searched in the frequency domain, the maximum value and the minimum value of the low and medium frequency are calculated according to the 3dB position, the low and medium frequency estimated value is calculated according to the calculation, and finally the estimated value is added with a local oscillation value of microwave down-conversion to be used as a carrier frequency measurement result. As can be seen from fig. 2, the carrier frequency measurement steps of the fourier transform-based measurement method are as follows: step 1: firstly, guiding a microwave local oscillator according to a result of a frequency measuring machine (rough frequency measurement) in an analog domain, and down-converting a measured signal to a low-middle frequency band; step 2: after AD sampling, converting the low and medium frequency signals into a frequency domain by using fast Fourier transform, and searching the positions of the frequency points at two sides which are reduced by 3dB according to the estimated in-band signal power value; and step 3: the maximum value and the minimum value of the frequency are obtained, and the average value of the maximum value and the minimum value is an estimated value of the low-intermediate frequency signal to be detected because the frequency of the LFM signal in the band is linearly changed; and 4, step 4: and adding the estimated low-intermediate frequency signal center value and the down-conversion local oscillator value to obtain the test result of the LFM signal. Compared with an instantaneous frequency measurement method based on phase difference, the carrier frequency measurement method based on Fourier transform transforms low and medium frequency signals to a frequency domain, and improves the signal-to-noise ratio accumulation to a certain extent, but the frequency resolution of the frequency domain is reduced to a certain extent compared with the prior art, in addition, the actually acquired data cause the signal to have larger and asymmetric power fluctuation in the frequency domain band due to the reasons of acquisition hardware and the like, the position of a 3dB drop point is difficult to capture, and the actual measurement precision is poor.
Disclosure of Invention
The invention provides a method for measuring LFM carrier frequency with high precision, aiming at the problems that amplitude-phase distortion exists in the actually acquired LFM signal time domain and in-band fluctuation exists in the frequency domain, so that the LFM signal carrier frequency measurement error is larger, and the method comprises the following steps:
step 1: the frequency measuring machine receives radio frequency signals, and the expressions of the radio frequency signals received by the frequency measuring machine in a detection mode are as follows: y (t) ═ x (t) + n (t); where n (t) is a noise signal, x (t) is a radiation source signal under test, which can be expressed as:
where A (t) is the received signal amplitude, fcFor the signal to be tested, KrIs the chirp rate of the LFM signal, t is the transmit signal time,
for the time-varying phase of the signal, without taking noise into account
Is a constant value.
Step 2: guiding a microwave local oscillator in an analog domain according to a signal receiving result of a frequency measuring machine, and carrying out down-conversion on a measured signal to a low-middle frequency band; suppose that the down-conversion local oscillator is fc-fIAfter analog down-conversion processing, the signal to be measured is represented as:
wherein f isIIs the intermediate frequency of the signal to be tested. At this time, the phase of the signal to be tested is represented as:
and step 3: performing AD digital sampling on the signal processed in the last step, wherein the AD sampling rate is fs；
And 4, step 4: after AD sampling, firstly, weighting processing is carried out on a time domain signal by adopting a window function, and gating truncation of the signal is realized; after AD digital sampling, the phase of the signal to be tested is represented as:
and 5: performing phase difference calculation on the tested signal by adopting a classical phase difference algorithm; the classical phase difference algorithm comprises: forward finite difference, backward finite difference, and center finite difference.
The method for obtaining the phase difference by using the forward finite difference comprises the following steps:
ΔΞ(n)＝{[Ξ(n+1)-Ξ(n)]}fs/(2π)
step 6: carrying out phase smoothing processing on a tested signal; because the phase of the actual acquisition signal is polluted, smooth filtering processing is required, specifically, the phase difference result is subjected to smooth filtering according to a multipoint smooth averaging mode, and the signal-to-noise ratio of the signal phase is poor due to low signal-to-noise ratio of the actual acquisition signal, so that the signal-to-noise ratio can be effectively improved by filtering processing in a multipoint smooth mode. At this time, the phase difference of the measured signals is divided into:
and 7: selecting a low intermediate frequency measurement value; since the LFM signal varies linearly in frequency in the band, the frequency value corresponding to the center pixel of the time domain window is selected as the low-if measurement value, which is:
and 8: calculating the carrier frequency of the tested LFM signal, wherein the carrier frequency of the tested LFM signal is fc-fI+fIc(ii) a Operator in the above formula
Rounding operation is performed.
The frequency variation of the LFM signal at a unit sampling point in the effective window is the frequency resolution of the LFM signal, and the analytical expression can be characterized as follows: rhoB＝Kr/fs。
The technical scheme of the invention is based on the carrier frequency measurement method based on the phase difference, the time domain window limitation and the phase smoothing processing are carried out on the sampling signal, the calculation method of the carrier frequency center value is further simplified, the carrier frequency measurement precision of the LFM signal can be effectively improved, and the resolution of the carrier frequency signal measured by the method is far better than that of the carrier frequency measurement method based on the Fourier transform.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.
Fig. 1 is a schematic block diagram of a carrier frequency measurement based on phase difference;
FIG. 2 is a schematic block diagram of a carrier frequency measurement based on Fourier transform;
FIG. 3 is a schematic block diagram of the present application for high-precision measurement of LFM carrier frequency;
FIG. 4 is a time domain waveform of a measured low IF signal;
FIG. 5 shows the forward finite difference phase analysis of the signal under test;
FIG. 6 is a frequency domain waveform of the measured low IF signal;
FIG. 7 is a measured signal acquisition window;
FIG. 8 is a filtered differential phase;
fig. 9 shows the test results for different frequencies.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Specific parameters of the LFM signal of the device under test are shown in table 1, the AD sampling rate is 100MHz, the frequency guide frequency is 250MHz, and the time domain waveform after AD sampling is shown in fig. 4, and it can be seen from fig. 4 that the envelope of the signal has a certain degree of distortion.
TABLE 1 LFM Signal parameters of a device under test
The forward finite difference is performed on the measured signal, and the result is shown in fig. 5, and it is obvious from fig. 5 that, due to the existence of noise, the differential phase has obvious fluctuation, which will reduce the measurement accuracy of the carrier frequency signal measurement result.
The frequency domain waveform of the signal is shown in fig. 6, and it is obvious that the fluctuation in the band is large, the left and right peak points of the band are asymmetric, the theoretical 3dB sampling point positions should be 40966 points and 122885 points, the actually estimated 3dB sampling point positions are about 40960 points and 122844 points, the difference from the theoretical value is about 47 sampling points, the frequency difference is about 28.687kHz, and the error of the frequency measurement precision is large.
After the AD sampling, an amplitude threshold is set for the time domain waveform of the signal, and an initial termination window function of the signal is established, as shown in fig. 7, the signal amplitude outside the window function is set to zero.
The differential phase in fig. 5 is subjected to smoothing filtering, and a specific filtering manner is an averaging manner of 1000-point neighborhood values. The result of the smoothing filtering process is shown in fig. 8, and at this time, after the filtering smoothing, the discrimination of the phase is obviously improved.
Finally, the center value test result (including the pilot frequency) is 249998011.89Hz, the frequency measurement error is-1988.11 Hz, and the frequency measurement resolution is 4166.4 Hz.
Table 2 shows the comparison between the multiple measurement results of different frequency points and the frequency measurement accuracy, and the result shows that the carrier frequency error measured by using the time domain windowing, phase difference and smoothing filtering can be controlled within the frequency measurement accuracy.
TABLE 2 measurement results at different frequency points
Through the above description of the embodiments, those skilled in the art will clearly understand that each embodiment can be implemented by software plus a necessary general hardware platform, and certainly can also be implemented by hardware. With this understanding in mind, the above-described technical solutions may be embodied in the form of a software product, which can be stored in a computer-readable storage medium such as ROM/RAM, magnetic disk, optical disk, etc., and includes instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute the methods described in the embodiments or some parts of the embodiments.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (3)
1. A method for measuring LFM carrier frequency with high precision comprises the following steps:
step 1: the frequency measuring machine receives radio frequency signals, and the expressions of the radio frequency signals received by the frequency measuring machine in a detection mode are as follows: y (t) ═ x (t) + n (t); where n (t) is a noise signal, and x (t) is a radiation source signal under test, expressed as:
where A (t) is the received signal amplitude, fcFor the signal to be tested, KrIs the chirp rate of the LFM signal, t is the transmit signal time,
for the time-varying phase of the signal, without taking noise into account
Is a constant value;
step 2: guiding a microwave local oscillator in an analog domain according to a signal receiving result of a frequency measuring machine, and carrying out down-conversion on a measured signal to a low-middle frequency band; suppose that the down-conversion local oscillator is fc-fIAfter analog down-conversion processing, the signal to be measured is represented as:
wherein f isIThe intermediate frequency of the signal to be tested, at this time, the phase of the signal to be tested is represented as:
and step 3: performing AD digital sampling on the signal processed in the last step, wherein the AD sampling rate is fs；
And 4, step 4: after AD sampling, firstly, weighting processing is carried out on a time domain signal by adopting a window function, and gating truncation of the signal is realized; after AD digital sampling, the phase of the signal to be tested is represented as:
and 5: performing phase difference calculation on a tested signal by adopting a classical phase difference algorithm, wherein the classical phase difference algorithm comprises the following steps: forward finite difference, backward finite difference, and center finite difference;
step 6: carrying out phase smoothing processing on a tested signal;
and 7: selecting a low intermediate frequency measurement value; since the LFM signal varies linearly in frequency in the band, the frequency value corresponding to the center pixel of the time domain window is selected as the low-if measurement value, which is:
and 8: calculating the carrier frequency of the tested LFM signal, wherein the carrier frequency of the tested LFM signal is fc-fI+fIc。
2. The method for measuring LFM carrier frequency with high accuracy according to claim 2, wherein: the step 5 of obtaining the phase difference by using a forward finite difference method is as follows: Δ xi (n) { [ xi (n +1) -xi (n)]}fs/(2π)。
3. The method for measuring LFM carrier frequency with high accuracy according to claim 1, wherein: the step 6 specifically includes: and performing smooth filtering on the result of the phase difference according to a multipoint smooth averaging mode, wherein the phase difference of the measured signals is divided into:
。
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Denomination of invention: A high-precision method for measuring LFM carrier frequency Effective date of registration: 20230719 Granted publication date: 20230509 Pledgee: Beijing first financing Company limited by guarantee Pledgor: Beijing Zhongke Ruixin Technology Co.,Ltd. Registration number: Y2023980049042 |