CN112763796B - 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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- CN112763796B CN112763796B CN202011578569.8A CN202011578569A CN112763796B CN 112763796 B CN112763796 B CN 112763796B CN 202011578569 A CN202011578569 A CN 202011578569A CN 112763796 B CN112763796 B CN 112763796B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
- G01R23/02—Arrangements for measuring frequency, e.g. pulse repetition rate; Arrangements for measuring period of current or voltage
- G01R23/10—Arrangements for measuring frequency, e.g. pulse repetition rate; Arrangements for measuring period of current or voltage by converting frequency into a train of pulses, which are then counted, i.e. converting the signal into a square wave
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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 the radio frequency signal; step 2: guiding a microwave local oscillator in an analog domain according to the result of the signal received by the frequency measuring machine, and down-converting the measured signal to a low-medium frequency band; step 3: AD digital sampling is carried out; step 4: after AD sampling, weighting the time domain signal by adopting a window function to realize gating and cutting off of the signal; step 5: carrying out phase difference calculation on the tested signal; step 6: performing phase smoothing treatment; step 7: selecting a low intermediate frequency measurement value; step 8: and calculating the carrier frequency of the LFM signal to be detected. 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 central value, can effectively improve the precision of LFM signal carrier frequency measurement, and the resolution of the carrier frequency signal measured by adopting the technical scheme of the application is far superior to that of the carrier frequency measurement method based on Fourier transformation.
Description
Technical Field
The invention relates to the field of radar signal processing, in particular to a high-precision measuring technology of LFM signal carrier frequency.
Background
With the development of modern electronic warfare, the status of electronic countermeasure in modern warfare is increasingly highlighted, and electronic countermeasure is that the opponents use signals and weapon equipment in the electromagnetic wave frequency range to fight against the fight, and the opponents use the main means of electromagnetic spectrum to receive and transmit electromagnetic waves, thus two main aspects of electronic countermeasure are correspondingly formed: electronic reconnaissance and electronic interference.
With the upgrade of modern electronic warfare, the high density, wide frequency band, complex signal pattern of radiation source signal detection in modern electronic warfare makes electronic reconnaissance a key to win in modern electronic warfare. The electronic investigation mainly receives, investigation, sorting and identification of the space non-cooperative radiation electromagnetic signals, reports the signals with threats and potential threats, and releases the appointed interference signals according to different strategies and the selection of battlefield opportunities by the interference master control. The electronic reconnaissance is an eye of electronic countermeasure, is a precondition and a foundation of electronic interference, and is also ramming stone for signal sorting, identification, threat degree discrimination and battle situation discrimination.
The main purpose of electronic investigation is to measure the PDW (pulse description word) of the spatial radiation source signal, which mainly includes the carrier frequency of arrival, angle, time of arrival, pulse width, pulse power and pulse-in modulation characteristics. The carrier frequency parameter is an important parameter of PDW, is an important mark for judging radar power and threat level, and is also an important basis for signal source sorting and identification, so that accurate carrier frequency parameter measurement is particularly important. The radar signal forms are changed from initial single pulse, simple continuous wave and the like into complex signals such as LFM (Linear frequency modulation), frequency clipping, pseudo random frequency modulation and the like. The LFM can acquire high resolution of the distance direction, and is a main transmitting signal form of the multi-body radar for searching, tracking, imaging and the like. The LFM signal has the characteristics of long time width and large bandwidth, the signal acquired by the actual hardware equipment causes the distortion of the amplitude and phase of the received signal due to the nonlinearity of the hardware equipment, and the carrier frequency guide measurement error is large, so that the type judgment of the radiation source and the modulation of partial interference (mainly referred to as deception interference) signals are affected.
In the prior art, two frequency measurement methods exist, one is a carrier frequency measurement method based on phase difference, the schematic block diagram is shown in fig. 1, the core of the technology is that the maximum value and the minimum value of the in-band frequency of the low intermediate frequency are calculated after the phase difference, the median value is calculated, and finally the calculated values are added with the local oscillation value of microwave down-conversion to be used as carrier frequency measurement results. According to the method shown in fig. 1, the carrier frequency measurement implementation steps of the instantaneous frequency measurement method based on phase difference are as follows: step 1: firstly, guiding a microwave local oscillator in an analog domain according to the result of a frequency measuring machine (coarse frequency measuring), and down-converting a measured signal to a low-medium frequency band; step 2: after AD sampling, the phase of the complex signal is obtained in the 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); 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 differential; step 4: obtaining the maximum value and the minimum value of the frequency, wherein the average value of the maximum value and the minimum value is the estimated value of the measured low intermediate frequency signal because the LFM signal is in-band frequency linear change; step 5: and adding the estimated low intermediate frequency signal center value and the down-conversion local oscillation value to obtain the test result of the LFM signal. The carrier frequency measuring method based on phase difference utilizes a mode of directly utilizing phase difference to calculate the frequency of signals in the whole frequency band, and has the defects that the actual acquired data has amplitude-phase distortion due to acquisition hardware and the like, the maximum value and the minimum value of the frequency of the LFM signal can not be accurately measured, and certain calculation error exists.
Another carrier frequency measurement method is a carrier frequency measurement method based on fourier transform, and its schematic block diagram is shown in fig. 2. The method is characterized in that a low intermediate frequency signal is 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 intermediate frequency are calculated according to the position of the 3dB, a low intermediate frequency estimated value is calculated, and finally the low intermediate frequency estimated value is added with a local oscillation value of microwave down-conversion to be used as a carrier frequency measuring 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 in an analog domain according to the result of a frequency measuring machine (coarse frequency measuring), and down-converting a measured signal to a low-medium frequency band; step 2: after AD sampling, the low intermediate frequency signal is converted into a frequency domain by utilizing fast Fourier transform, and the positions of frequency points at two sides which are reduced by 3dB are searched according to the estimated in-band signal power value; 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 the estimated value of the measured low intermediate frequency signal because the LFM signal is linearly changed in the band; step 4: and adding the estimated low intermediate frequency signal center value and the down-conversion local oscillation value to obtain the test result of the LFM signal. Compared with the instantaneous frequency measurement method based on phase difference, the carrier frequency measurement method based on Fourier transform transforms the low intermediate frequency signal to the frequency domain, so that the signal-to-noise ratio accumulation is improved to a certain extent, but the frequency resolution of the frequency domain is reduced to a certain extent compared with that of the technology, in addition, the actual acquisition data cause that the power fluctuation of the signal in the frequency domain is large and asymmetric due to acquisition hardware and the like, the position of a 3dB drop point is difficult to capture, and the actual measurement accuracy is poor.
Disclosure of Invention
Aiming at the problems of large amplitude-phase distortion of the actually acquired LFM signal time domain and large in-band fluctuation of the frequency domain, which cause large measurement error of the LFM signal carrier frequency, the invention provides a method for measuring the LFM carrier frequency with high precision, which comprises the following steps:
step 1: the frequency measuring machine receives the radio frequency signal, and the frequency measuring machine detects the received radio frequency signal expression 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, which can be expressed as:
wherein A (t) is the amplitude of the received signal, f c For the carrier frequency, K, of the signal to be tested r For the frequency modulation slope of the LFM signal, t is the time of transmitting the signal,
for the time-varying phase of the signal, without taking into account noise +.>
Is a constant value.
Step 2: guiding a microwave local oscillator in an analog domain according to the result of the signal received by the frequency measuring machine, and down-converting the measured signal to a low-medium frequency band; let the down-conversion local oscillator be f c -f I After analog down-conversion processing, the signal to be measured is expressed as:
wherein f I Is the intermediate frequency of the signal to be tested. At this time, the phase of the signal to be tested is expressed as: />
Step 3: performing AD digital sampling on the signal processed in the previous step, wherein the AD sampling rate is f s ；
Step 4: after AD sampling, firstly weighting the time domain signal by adopting a window function to realize gating and cutting off of the signal; after AD digital sampling, the phase of the signal to be tested is expressed as:
step 5: carrying out 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 phase difference is obtained by utilizing a forward finite difference mode and is as follows:
ΔΞ(n)＝{[Ξ(n+1)-Ξ(n)]}f s /(2π)
step 6: carrying out phase smoothing treatment on the tested signal; because the actual acquisition signal phase has pollution, smoothing filter processing is needed, specifically, smoothing filter is carried out on the phase difference result according to a multipoint smoothing average value obtaining mode, and because the actual acquisition signal to noise ratio is not high, the signal phase signal to noise ratio is poor, and therefore, the signal to noise ratio can be effectively improved by carrying out filter processing in a multipoint smoothing mode. At this time, the phase difference of the measured signal is:
step 7: selecting a low intermediate frequency measurement value; since the LFM signal varies linearly in-band frequency, the frequency value corresponding to the center pixel of the time-domain window is selected as a low intermediate frequency measurement value, which is:
step 8: calculating the carrier frequency of the detected LFM signal, wherein the carrier frequency of the detected LFM signal is f c -f I +f Ic The method comprises the steps of carrying out a first treatment on the surface of the Operator in above
To round up operations.
The frequency variation of the unit sampling point of the LFM signal in the effective window is the frequency resolution of the LFM signal, and the analytic expression can be characterized as follows: ρ B ＝K r /f s 。
According to the technical scheme, on the basis of the carrier frequency measuring method based on phase difference, time domain window limitation and phase smoothing are carried out on the sampling signals, the calculating method of the carrier frequency center value is further simplified, the accuracy of carrier frequency measurement of the LFM signals can be effectively improved, and the resolution of the carrier frequency signals measured by the method is far better than that of the carrier frequency measuring method based on Fourier transformation.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the following description will briefly explain the drawings used in the embodiments or the description of the prior art, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic block diagram of carrier frequency measurement based on phase difference;
fig. 2 is a schematic block diagram of 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 frequencies;
FIG. 4 is a time domain waveform of a measured low intermediate frequency signal;
FIG. 5 is a diagram showing the result of a forward finite difference phase analysis of a measured signal;
FIG. 6 is a frequency domain waveform of a measured low intermediate frequency signal;
FIG. 7 is a measured signal acquisition window;
FIG. 8 is a filtered differential phase;
fig. 9 shows test results for different frequencies.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Specific parameters of the LFM signal of the tested device are shown in table 1, the AD sampling rate is 100MHz, the frequency guiding frequency is 250MHz, 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 distortion.
TABLE 1 LFM Signal parameters of device under test
The result of forward finite difference is shown in fig. 5, and it is obvious from fig. 5 that the differential phase has obvious fluctuation due to the existence of noise, which reduces the measurement accuracy of the carrier frequency signal measurement result.
As shown in fig. 6, the frequency domain waveform of the signal is obvious that the in-band fluctuation is large, the left and right peak points of the frequency band are asymmetric, the theoretical 3dB sampling point positions are 40966 points and 122885 points, the actual estimated 3dB sampling point positions are about 40960 points and 122844 points, the difference between the actual estimated 3dB sampling point positions and the theoretical value is about 47 sampling points, the difference between the frequency and the theoretical value is about 28.687kHz, and the frequency measurement precision error is large.
After AD sampling, an amplitude threshold is set for the time domain waveform of the signal, and an initial and terminal window function of the signal is established, as shown in fig. 7, the amplitude of the signal outside the window function is set to zero.
The differential phase in fig. 5 is smoothed, and a specific filtering mode adopts a mode of averaging 1000-point adjacent values. The result of the smoothing filter processing is shown in fig. 8, and at this time, the degree of discrimination of the phases is significantly improved after the smoothing filter processing.
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 4166.4Hz.
Table 2 shows that the carrier frequency error measured by means of time domain windowing, phase difference dividing and smoothing filtering can be controlled within the frequency measurement precision.
Table 2 measurement results of different frequency points
From the above description of the embodiments, it will be apparent to those skilled in the art that the embodiments may be implemented by means of software plus necessary general hardware platforms, or of course may be implemented by means of hardware. Based on this understanding, the foregoing technical solution may be embodied essentially or in a part contributing to the prior art in the form of a software product, which may be stored in a computer readable storage medium, such as ROM/RAM, a magnetic disk, an optical disk, etc., including several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute the method described in the respective embodiments or some parts of the embodiments.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the 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 scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (2)
1. A method of measuring LFM carrier frequency with high accuracy, comprising:
step 1: the frequency measuring machine receives the radio frequency signal, and the frequency measuring machine detects the received radio frequency signal expression as follows: y (t) =x (t) +n (t); wherein n (t) is a noise signal, x (t) is a radiation source signal under test, expressed as:
wherein A (t) is the amplitude of the received signal, f c For the carrier frequency of the signal to be tested，K r For LFM signal frequency modulation slope, t is transmit signal time, < ->
For time-varying phase of signal, without taking into account noise
Is a constant value;
step 2: guiding a microwave local oscillator in an analog domain according to the result of the signal received by the frequency measuring machine, and down-converting the measured signal to a low-medium frequency band; let the down-conversion local oscillator be f c -f I After analog down-conversion processing, the signal to be measured is expressed as:
wherein f I The intermediate frequency of the signal to be tested is set, and at this time, the phase of the signal to be tested is expressed as:
step 3: performing AD digital sampling on the signal processed in the previous step, wherein the AD sampling rate is f s ；
Step 4: after AD sampling, firstly weighting the time domain signal by adopting a window function to realize gating and cutting off of the signal; after AD digital sampling, the phase of the signal to be tested is expressed as:
step 5: and carrying out phase difference calculation on the 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 treatment on the tested signal; comprising the following steps: smoothing and filtering the phase difference result in a mode of multipoint smoothing and averaging, wherein the measured signalThe phase difference of the numbers is:
step 7: selecting a low intermediate frequency measurement value; since the LFM signal varies linearly in-band frequency, the frequency value corresponding to the center pixel of the time-domain window is selected as a low intermediate frequency measurement value, which is:
step 8: calculating the carrier frequency of the detected LFM signal, wherein the carrier frequency of the detected LFM signal is f c -f I +f Ic 。
2. The method for measuring LFM carrier frequency with high accuracy according to claim 1, wherein: the step 5 obtains the phase difference by using a forward finite difference mode as follows: ΔΣ (n) = { [ Σ (n+1) - Σ (n)]}f s /(2π)。
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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 |