CN115407126A - Frequency estimation device - Google Patents
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Abstract
The invention relates to a frequency estimation device, one embodiment of the device comprises: the device comprises a frequency comb generating unit, a pre-sampling unit and a signal acquisition and processing unit; the frequency comb generating unit is used for generating a pre-sampling frequency comb; the pre-sampling unit pre-samples a signal to be detected in an analog domain by using a pre-sampling frequency comb to obtain a pre-sampling signal; the pre-sampled signal includes: a plurality of signal copies located in respective nyquist zones of a pre-sampling frequency comb; the signal acquisition and processing unit performs analog-to-digital conversion on the pre-sampled signal, and performs fast Fourier transform on the pre-sampled signal in a digital domain formed by the analog-to-digital conversion to obtain a frequency measurement value of a signal copy and a frequency measurement value of the signal copy on a signal to be detected; and determining the average value of the frequency measurement values of the signal copies to the signal to be measured as the frequency estimation value of the signal to be measured. The embodiment can obviously improve the signal frequency estimation precision through pre-sampling and multi-measurement value averaging at the front end of an FFT processing link.
Description
Technical Field
The invention relates to the technical field of microwave frequency measurement, in particular to a frequency estimation device.
Background
Accurate estimation of frequency is an important issue in signal processing in the fields of communications, radar, sonar, electronic countermeasure, and the like. A Fast Fourier Transform (FFT) algorithm is widely used to acquire frequency information of a signal. Usually, the frequency value of the signal can be obtained by reading the frequency grid point where the peak power of the signal is located directly from the spectrum information obtained by FFT calculation. However, the frequency estimation accuracy of this method depends on the time window length for FFT computation, and is limited by the application requirements of the actual application scenario and the signal processing cost of the detection end, and the window length is usually not very long, which leads to the frequency lattice accuracy of the FFT being rough and difficult to meet the practical requirements. In order to improve the frequency estimation accuracy based on FFT, various high-accuracy frequency estimation methods have been proposed to further improve the frequency estimation accuracy on the FFT spectrum result, such as polynomial fitting, zero padding, interpolation Fast Fourier Transform (IFFT), weighted Phase Averaging (WPA), and band-pass fourier transform (zofft). Although these methods have significant effects, they are all based on digital signal processing, that is, after the FFT processing stage, the estimation accuracy of the signal frequency is further improved by a supplementary algorithm for the already digitally quantized signal. The complexity of these algorithms may occupy a large amount of computational resources, limiting the timeliness of frequency estimation. Therefore, it is necessary to provide a method for improving the accuracy of signal frequency estimation by signal preprocessing at the front end of the FFT processing element.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a frequency estimation device which can remarkably improve the signal frequency estimation precision through pre-sampling and multi-measurement-value averaging at the front end of an FFT (fast Fourier transform) processing link and is compatible with the existing post-processing high-precision frequency estimation method based on FFT.
The frequency estimation device of the present invention includes: the device comprises a frequency comb generating unit, a pre-sampling unit and a signal acquisition and processing unit; wherein, the frequency comb generating unit is used for generating a pre-sampling frequency comb; the pre-sampling unit pre-samples the signal to be detected in the analog domain by using the pre-sampling frequency comb to obtain a pre-sampled signal; the pre-sampled signal comprises: a plurality of signal copies of the signal under test located at each nyquist zone of the pre-sampling frequency comb; the signal acquisition and processing unit performs analog-to-digital conversion on the pre-sampled signal, performs fast Fourier transform on the pre-sampled signal in a digital domain formed by the analog-to-digital conversion, and obtains frequency measurement values of the multiple signal copies; the signal acquisition and processing unit selects a preset number of Nyquist zones from the Nyquist zones, calculates the frequency measurement value of the signal copy to the signal to be measured by using the frequency measurement value of the signal copy of each selected Nyquist zone, and determines the average value of the frequency measurement value of the signal copy of each selected Nyquist zone to the signal to be measured as the frequency estimation value of the signal to be measured.
The signal acquisition and processing unit calculates the frequency measurement value of the selected signal copy in any Nyquist zone to the signal to be measured through the following formula:
at the frequency f of the signal to be measured in At the Nyquist zone of 2n + 1:
when the number of i is an odd number,
when the number i is an even number, the number i,
at f in At the Nyquist zone of 2n + 2:
when the number of i is an odd number,
when i is an even number, the number of bits is,
wherein n is a nonnegative integer, i is the serial number of any selected Nyquist zone, f inM，i Frequency measurement of said signal to be measured for a signal replica of any selected Nyquist zone, f M，i Frequency measurement of signal copies for any selected Nyquist zone, f c Is the repetition frequency of the pre-sampling frequency comb.
The preset number is equal to the number of signal copies used for calculating the frequency estimation value of the signal to be detected; the frequency of each signal copy can be expressed as a linear combination expression of the frequency of the signal to be tested and the repetition frequency; and the frequencies of the signal copies of any two Nyquist zones selected by the signal acquisition and processing unit satisfy: the difference between the coefficients of the repetition frequencies in the linear combination expression is not equal to an integer multiple of the predetermined number.
The fractional part of the ratio of the repetition frequency to the frequency resolution of the fast fourier transform satisfies: equal to the inverse of said preset number.
The signal acquisition and processing unit comprises: the analog-to-digital converter and the signal processing module; wherein the analog-to-digital converter is configured to perform the analog-to-digital conversion; the signal processing module is used for determining the sequence number of the Nyquist zone where the frequency of the signal to be detected is located, executing the fast Fourier transform, and calculating the frequency measurement value of the signal copy of each selected Nyquist zone to the signal to be detected and the frequency estimation value of the signal to be detected.
The frequency comb generating unit has a first structure, a second structure or a third structure; in a first structure, the frequency comb generating unit is an optical frequency comb generating device, and comprises a mode-locked laser or an electro-optical modulation optical frequency comb generating device, and the pre-sampling frequency comb is an optical frequency comb; in a second structure, the frequency comb generating unit is an electrical frequency comb generating device, and comprises a nonlinear transmission line or a step recovery diode, and the pre-sampling frequency comb is an electrical frequency comb; in a third structure, the frequency comb generating unit includes the optical-frequency comb generating device and a first photodetector, and the pre-sampling frequency comb is an electrical-frequency comb.
When the frequency comb generating unit has a first structure, the pre-sampling unit includes: the photoelectric modulator, the optical amplifier, the optical filter and the second photodetector; the electro-optical modulator is used for pre-sampling the signal to be detected, and the optical amplifier, the optical filter and the second photoelectric detector are used for processing the pre-sampled signal; and when the frequency comb generating unit is of a second structure or a third structure, the pre-sampling unit comprises an electric mixer for pre-sampling the signal to be detected.
The device further comprises a first signal conditioning unit connected with the pre-sampling unit and a second signal conditioning unit respectively connected with the pre-sampling unit and the signal acquisition and processing unit; the first signal conditioning unit comprises a first electric filter, a first electric amplifier and a first electric attenuator and is used for processing the signal to be detected and transmitting the processed signal to be detected to the pre-sampling unit; the second signal conditioning unit comprises a second electric filter, a second electric amplifier and a second electric attenuator and is used for processing the pre-sampling signal and transmitting the processed pre-sampling signal to the signal acquisition and processing unit.
The signal acquisition and processing unit is further configured to: after calculating the frequency estimation value of the signal to be detected, further calculating on the basis of the calculated frequency estimation value of the signal to be detected by using a preset frequency estimation algorithm; wherein the frequency estimation algorithm comprises at least one of: polynomial fitting, zero padding, interpolated fast fourier transform, weighted phase averaging, band-selected fourier transform.
The apparatus further comprises: the control unit is respectively connected with the frequency comb generating unit, the pre-sampling unit, the signal acquisition and processing unit, the first signal conditioning unit and the second signal conditioning unit; wherein the control unit is configured to: setting the repetition frequency and the power of the pre-sampling frequency comb, setting the parameters of a first electric filter, a first electric amplifier, a first electric attenuator, a second electric filter, a second electric amplifier and a second electric attenuator, setting the parameters of each device in the pre-sampling unit, setting the sampling rate of the analog-to-digital conversion, the number of calculation points of the fast Fourier transform and the preset number, and providing the reference clock frequency for the frequency comb generating unit and the signal collecting and processing unit.
The frequency estimation device of the invention has the following beneficial effects: the signal is pre-sampled in the analog domain by a frequency comb with a specific repetition frequency, and the signal to be tested is expanded into a plurality of pre-sampling Nyquist zones. And then, in a digital domain, extracting signal copy frequency values of a plurality of Nyquist zones in the FFT spectrum of the pre-sampled signal, and further realizing the remarkable improvement of the signal frequency estimation precision through a simple average algorithm.
Drawings
Fig. 1 is a schematic structural diagram of a frequency estimation device according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a pre-sampling of an embodiment of the present invention;
FIG. 3 is a schematic illustration of a measurement error of an embodiment of the present invention;
FIG. 4 is another schematic illustration of measurement error for an embodiment of the present invention;
FIG. 5 is a schematic structural diagram of a first signal conditioning unit according to an embodiment of the present invention;
FIG. 6 is a schematic structural diagram of a frequency comb generating unit according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a pre-sampling unit according to an embodiment of the present invention;
FIG. 8 is another schematic diagram of a pre-sampling unit according to an embodiment of the present invention;
fig. 9 is a schematic structural diagram of a signal acquisition and processing unit according to an embodiment of the present invention.
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 obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The invention provides a high-precision frequency estimation device based on pre-sampling and multi-measurement-value averaging, namely, a frequency comb with a specific repetition frequency is used for pre-sampling a signal in an analog domain, and the signal to be measured is expanded into a plurality of pre-sampling Nyquist zones to form a plurality of signal copies. And then in a digital domain, carrying out FFT (fast Fourier transform) on the pre-sampling signal, extracting frequency measurement values of signal copies in a plurality of Nyquist zones from an FFT spectrum, and remarkably improving the estimation accuracy of the frequency of the signal to be measured by a simple frequency averaging algorithm. The pre-sampling process and the processing algorithm of the device are very simple and reliable, and other complex algorithms do not need to be introduced. In addition, the device is used as a representative signal processing means and is compatible with other existing FFT-based high-precision frequency estimation algorithms. The detailed structure of the present invention can be seen in fig. 1, and the technical concept and principle of the present invention will be first explained below.
For a frequency measurement system based on FFT (Fast Fourier Transform), the sampling rate is F s If the number of the calculation points is X, the frequency resolution is f res ＝F s /X, maximum frequency estimation error of f res /2. Suppose the center frequency of the signal to be measured is f in As shown in fig. 2, it can be expressed as
f in ＝(m+δ)f res (1)
Where m is a positive integer and δ is a fraction lying in the interval [0, 1). In the FFT spectrum, a signal frequency measurement is represented as
f inM ＝m×f res +[δ]×f res (2)
Wherein [. Cndot. ] represents the rounding in the near. The measurement error of the signal frequency can be obtained from the equations (1) and (2)
Δ＝f inM -f in ＝{[δ]-δ}f res (3)
Since [ δ ] = int (δ) + [ rmod (δ) ], where int (x) and rmod (x) denote the integer part and the fractional part of x, respectively, equation (3) is simplified to
Δ＝{[rmod(δ)]-rmod(δ)}f res (4)
It is apparent that since 0. Ltoreq. Delta. Ltoreq.1, so-0.5. Ltoreq. Delta. 0.5, the maximum estimation error of the original signal is f res And/2, the FFT frequency detection system is caused by the dispersion of the frequency spectrum distribution.
If it is used heavilyComplex frequency of f c ＝(α+ε)f res The frequency comb of (fig. 2) pre-samples the signal under test, and the spectrum of the sampled signal is as shown in fig. 2, and the signal is copied into each pre-sampling nyquist zone of the pre-sampling frequency comb, i.e. each nyquist zone of the pre-sampling frequency comb will contain a copy of the signal under test. If the signal to be measured is located between the nth (n is a non-negative integer) comb tooth and the n +1 comb tooth, i.e. the 2n +1 or 2n +2 Nyquist zone, the signal to be measured is located in the ith pre-sampling Nyquist zone after being pre-sampled
Frequency f of the internal signal replica i Can be expressed as:
(1) At signal f in At the Nyquist zone of 2n + 1:
or
(2) At signal f in At the Nyquist zone of 2n + 2:
or
Where f is c ＝(α+ε)f res Is the repetition frequency of the pre-sampling frequency comb, nf c Denotes the frequency of the nth comb, α is a positive integer, and ε is the fraction lying in the interval [0,1 ]. Measured values f of frequency values (5-1) to (5-4) in the FFT spectrum M，i Expressed as:
(1) At signal f in At the Nyquist zone of 2n + 1:
or
(2) At signal f in At the Nyquist zone of 2n + 2:
or
Obviously, if the frequency f of the signal under test is known in The Nyquist zone where the comb order n or n +1 of the adjacent pre-sampling frequency is known, and the signal acquisition and processing unit is used to obtain the frequency measurement value f of the signal copy in the ith pre-sampling Nyquist zone M，i Then, the frequency measurement value f of the signal to be measured can be easily calculated from the frequency measurement value by the equations (6-1) to (6-4) inM，i I.e. the frequency measurement of the signal replica to the signal under test.
(1) At signal f in At the Nyquist zone of 2n + 1:
or
(2) At signal f in At the Nyquist zone of 2n + 2:
or
Further, the frequency measurement error of the signal to be measured can be derived as follows:
(1) At signal f in At the Nyquist zone of 2n + 1:
or
(2) At signal f in At the Nyquist zone of 2n + 2:
or
Obviously,. DELTA. i The expression of (a) is similar to Δ, the values of which depend on δ and ε, and the range of values is (-0.5, 0.5)]. Without loss of generality. For a signal to be measured with stable frequency, the determined delta is provided, the repetition frequency of the pre-sampling frequency comb is variable, the epsilon of the repetition frequency comb is also settable, and fractional parts of combs with different orders have the rmod (n epsilon) property. Delta of i The relationship between the value of (d) and epsilon is shown in FIG. 3, delta is 0.2f res A.u. denotes dimensionless, Δ max Is Δ i Of (c) is calculated.
Obviously, depending on the different values of ε, Δ i Distributed over (-0.5, 0.5)]The range, and the value of rmod (n ε) may cover the entire [0,1 ] range. Therefore, the resolution f according to the current FFT can be considered res A specific value of epsilon is set so that epsilon x N =1, N is a predetermined number, which is a positive integer, representing the number of signal copies selected for calculating the frequency estimate of the signal under test (i.e., the frequency value of the signal under test that is finally obtained by averaging multiple measurements). Acquiring signal copies of a signal to be measured in N different pre-sampling Nyquist zones to obtain frequency measurement values f of the signal to be measured M，i (i.e., the frequency measurements of each signal copy to the signal under test) and further calculating the average of these frequency measurements will likely help reduce frequency measurement errors. The specific analysis process is as follows.
For any integer k and l, due to [ rmod (δ - (n ± k ± lN) epsilon ]]- rmod(δ-(n±k±lN)ε)＝[rmod(δ-(n±k)ε)]-rmod (δ - (n ± k) epsilon), so the following value should be satisfied when selecting a signal replica: in expressions (6-1) to (6-4) and (8-4) to (8-4) of any two signal copies, the difference between the coefficients of epsilon cannot be an integer multiple of N, otherwise the measured values of the two signal copies are equivalent, thus affecting the measurement accuracy. Since the coefficient of epsilon is f c That is, for the frequencies of the signal copies of any two selected nyquist zones, the difference between the coefficients of the repetition frequencies in the linear combination expression is not equal to the integer multiple of the preset number N.
Given that the FFT spectral range of the pre-sampled signal is typically limited, it is typicalFrom DC to a finite cut-off frequency, the low-order nyquist zone in the pre-sampled signal is mainly contained. Therefore, from these nyquist zones, N signal copies satisfying the above-described value conditions should be selected for the average calculation. A typical average value calculation method is to select the 1 st to nth continuous pre-sampling nyquist zone signal copies in the FFT spectrum of the pre-sampled signal, and further average the frequency of the signal to be measured by using the frequency measurement values of the signal to be measured obtained by these copies. For simplicity, assume signal to be measured f in Located at 2n +1 pre-sampling Nyquist zone [ nf c ，(n+1/2)f c ]In which N is even number, the number of the pre-sampling Nyquist zone i belongs to [1, N ]]Based on the average value of the frequency measurement values of the 1 st to Nth pre-sampled Nyquist zone signal copies (i.e. the frequency estimation value of the signal to be measured) as
Equation (9) can be simplified to
Where Δ a Is the error of the measured mean, expressed as
Delta and Delta a The values for different δ are shown in fig. 4, where e =0.1, e × N =1,
is Δ a Is measured. Can see a a The ratio delta is obviously reduced, and the value range is (-0.5, 0.5)]Reduced to (-0.05,0.05,05)]I.e. the frequency estimation accuracy is improved by 10 times. On the basis of the pre-sampling and average value calculation method, other types of FFT-based high-precision frequency can be appliedThe rate estimation algorithm further reduces the measurement error (i.e., measurement differentiation in fig. 4).
For the signal f to be measured, in addition to the conditions set for simplicity above in The method is located in any pre-sampling Nyquist zone, N is any natural number, and as long as the condition of epsilon multiplied by N =1 is met, and the difference of the coefficients of epsilon of any two signal copies in a plurality of signal copies selected for calculating the frequency average value of the signal to be measured is not an integral multiple of N, the effect that the formula (11) improves the estimation precision of the frequency of the signal to be measured can be achieved. For example, the frequency average value of the signal to be measured is calculated by using N copies of the pre-sampling Nyquist zone signals of continuous odd ordinal numbers i.
According to the above technical idea, the frequency estimation apparatus shown in fig. 1 can be designed. As shown in fig. 1, the frequency estimation apparatus according to the embodiment of the present invention may include: the device comprises a frequency comb generating unit, a pre-sampling unit and a signal acquiring and processing unit.
The frequency comb generating unit is connected with the pre-sampling unit, and the pre-sampling unit is connected with the signal acquisition and processing unit. The frequency comb generating unit generates a pre-sampling frequency comb for pre-sampling, and in practical application, the pre-sampling frequency comb may be an optical frequency comb or an electrical frequency comb. The frequency comb generating unit can adopt any generating mode capable of meeting the requirement of the pre-sampling frequency comb, typical modes include an optical frequency comb generating device (such as a mode-locked laser, an electro-optical modulation optical frequency comb generating device and the like which can generate an optical frequency comb signal), an electrical frequency comb generating device (such as a nonlinear transmission line, a step recovery diode and the like which can generate an electrical frequency comb signal), and the optical frequency comb generating device can generate an optical frequency comb firstly and then input the optical frequency comb into the electro-optical detector to form the electrical frequency comb. The fundamental requirement that the presampling frequency comb needs to meet is that the repetition frequency f can be set specifically c The comb spacing is strictly equal to the repetition frequency, and the comb on the frequency spectrum has a relatively flat power distribution. The pre-sampling frequency comb generated by the frequency comb generating unit is input into the pre-sampling unit to pre-sample the signal to be detected.
The pre-sampling unit uses the pre-sampling frequency comb generated by the frequency comb generating unit to the analog domain to be testedAnd pre-sampling the signal to obtain a pre-sampled signal. The above pre-sampled signal may include: a plurality of signal copies of the signal under test located in each nyquist zone of the pre-sampling frequency comb. FIG. 2 is a pre-sampling schematic diagram of an embodiment of the present invention, as shown in FIG. 2, where A shows an input signal f in I.e. the signal to be measured. B shows the frequency spectrum of the pre-sampling frequency comb, the repetition frequency f C Integral multiple frequencies of e.g. 0, f C 、2f C 、3f C 823060, 8230and is a comb tooth of frequency comb. From the comb teeth with the frequency of zero, two Nyquist zones are arranged between the adjacent comb teeth, the serial numbers of the Nyquist zones are increased from 1, the serial number of the left Nyquist zone is an odd number, and the serial number of the right Nyquist zone is an even number. C in fig. 2 shows the result of the pre-sampling, i.e. the frequency spectrum of the pre-sampled signal, the principle of which is to multiply the signal to be measured with the pre-sampling frequency comb, so that the signal to be measured is copied into the pre-sampling nyquist zones of the pre-sampling frequency comb, i.e. there is one signal copy of the signal to be measured in each nyquist zone of the pre-sampling frequency comb. The pre-sampling unit can be any device that meets the requirements of the pre-sampling process, and typical devices include, but are not limited to, various electro-optical modulators (corresponding to optical frequency combs), electrical mixers (corresponding to electrical frequency combs), and the like.
The signal acquisition and processing unit performs analog-to-digital conversion on the pre-sampling signals output by the pre-sampling unit, and performs fast Fourier transform on the pre-sampling signals of a digital domain formed by the analog-to-digital conversion, so as to obtain frequency measurement values of a plurality of signal copies. Then, the signal acquisition and processing unit selects a preset number of Nyquist zones from the Nyquist zones of the pre-sampling frequency comb, calculates the frequency measurement value of the signal copy to the signal to be measured by using the frequency measurement value of the signal copy of each selected Nyquist zone, and finally determines the average value of the frequency measurement value of the signal copy of each selected Nyquist zone to the signal to be measured as the frequency estimation value of the signal to be measured, thereby realizing the accurate estimation of the frequency of the signal to be measured.
In practical application, after obtaining the signal copies of each nyquist zone through FFT, the signal acquisition and processing unit needs to select N (i.e. a preset number) firstQuantity) for subsequent calculations. As can be seen from the foregoing equations 5-1 to 5-4, the frequency of each signal copy can be expressed as a linear combination expression of the frequency of the signal to be measured and the repetition frequency, and then in the above selection process, the frequencies of the signal copies in any two nyquist zones that are selected need to satisfy: the difference between the coefficients of the repetition frequencies in the linear combination expression is not equal to the integer multiple of the preset number N, only then the highest measurement accuracy can be obtained, otherwise the best effect cannot be achieved. In addition, as mentioned above, the repetition frequency f c Frequency resolution f from fast Fourier transform res The fraction part epsilon of the ratio of (c) satisfies: equal to the inverse of the preset number N.
Thus, after N signal copies have been selected, the aforementioned equations 7-1 through 7-4 can be used to derive the frequency measurement f for the signal copies M，i Calculating the frequency measured value f of the signal copy to the signal to be measured inM，i Namely:
at the frequency f of the signal to be measured in At the Nyquist zone of 2n + 1:
when the number of i is an odd number,
when i is an even number, the number of bits is,
at f in At the Nyquist zone of 2n + 2:
when the number of i is an odd number,
when i is an even number, the number of bits is,
finally, the frequency estimation value of the signal to be measured can be calculated according to the foregoing formula 9, that is, an average value of the frequency measurement values of the signal copy of each selected nyquist zone to the signal to be measured is calculated and determined as the frequency estimation value of the signal to be measured.
Optionally, in practical applications, the frequency estimation apparatus according to the embodiment of the present invention may further include a first signal conditioning unit and a second signal conditioning unit. The first signal conditioning unit is connected between the input port of the signal to be detected and the pre-sampling unit, and can perform signal processing such as filtering, amplification, attenuation and the like on the signal to be detected, so as to meet the signal processing requirements of each subsequent unit of the system. The second signal conditioning unit is connected between the pre-sampling unit and the signal acquisition and processing unit, has the function similar to that of the first signal conditioning unit, and is used for performing signal processing such as filtering, amplification, attenuation and the like on the pre-sampling signal output by the pre-sampling unit so as to meet the acquisition and processing requirements of the signal acquisition and processing unit.
In a specific application, the frequency estimation apparatus according to the embodiment of the present invention may further include a control unit, configured to perform overall control on each unit of the system, and mainly include: setting and controlling the frequency comb repetition frequency of the frequency comb generating unit, the filtering, amplifying and attenuating parameters of the first signal conditioning unit and the second signal conditioning unit, the device parameters of the pre-sampling unit, the sampling rate of the signal acquisition and processing unit, the number of FFT (fast Fourier transform) calculation points, the number (namely the preset number) required by the copies in high-precision frequency estimation and other related parameters. In fig. 1, a solid line represents an analog link, and a dotted line represents a control link, and it can be seen that the control unit is connected with the frequency comb generating unit, the pre-sampling unit, the signal collecting and processing unit, the first signal conditioning unit, and the second signal conditioning unit through the control link.
Fig. 5 is a schematic structural diagram of a first signal conditioning unit according to an embodiment of the present invention, and as shown in fig. 5, the first signal conditioning unit may include a first electrical filter, a first electrical amplifier, and a first electrical attenuator, and be configured to perform filtering, amplifying, and attenuating processes on a signal to be measured, respectively, and further include a signal input port and a signal output port. And the processed signal to be detected is transmitted to the pre-sampling unit from the signal output port. The second signal conditioning unit has a similar structure and comprises a second electric filter, a second electric amplifier and a second electric attenuator and is used for processing the pre-sampling signal and transmitting the processed pre-sampling signal to the signal acquisition and processing unit. The specific constitution, parameter selection and connection relation of the first signal conditioning unit and the second signal conditioning unit can be adjusted according to actual needs.
In the embodiment of the present invention, the frequency comb generating unit may use any frequency comb generating manner capable of generating a repetition frequency and satisfying a pre-sampling requirement. Illustratively, the frequency comb has a first structure, a second structure, or a third structure. In the first structure, the frequency comb generating unit is an optical frequency comb generating device, and includes a mode-locked laser or an electro-optically modulated optical frequency comb generating device, and the generated pre-sampling frequency comb is an optical frequency comb.
In the second structure, the frequency comb generating unit is an electrical frequency comb generating device, and the generated pre-sampling frequency comb is an electrical frequency comb, and the electrical frequency comb generating device comprises a nonlinear transmission line or a step recovery diode, and the devices can generate the pre-sampling electrical frequency comb with the required repetition frequency under the excitation of a high-power dot frequency signal with the required repetition frequency of the frequency comb.
In a third structure, the frequency comb generating unit includes the above optical frequency comb generating device and the first photodetector, and has a structure as shown in fig. 6, and generates a frequency comb as an electrical frequency comb signal. The optical frequency comb generating device is used for generating an optical frequency comb, and the first photoelectric detector is used for converting the optical frequency comb into an electrical frequency comb. The optical frequency comb generating device can use a mode-locked laser or an electro-optical modulation optical frequency comb generating device, and the first photoelectric detector can adopt any photoelectric detector capable of converting an optical frequency comb signal into an electrical frequency comb signal.
Preferably, when the frequency comb generating unit has the first structure, the pre-sampling unit may include: the electro-optical modulator, the optical amplifier, the optical filter and the second photodetector may further include an optical-frequency comb input port and a signal output port, as shown in fig. 7. The electro-optical modulator is a necessary device and is used for pre-sampling an input signal to be detected, namely modulating the signal to be detected to the optical frequency comb. The electro-optic modulator may be of any type capable of up-converting the signal under test to a pre-sampled optical frequency comb, including but not limited to an intensity modulator, a polarization modulator, a phase modulator, etc. The optical amplifier and the optical filter are optional devices and are respectively used for signal amplification and filtering, the connection relationship of the two devices can be adjusted according to actual requirements, whether the two devices are used, which type of devices are used and which connection relationship is used depends on the quality requirement of the pre-sampling signal output by the pre-sampling unit in the system design process. The second photodetector is optional and any photodetector that can adequately down-convert the pre-sampled signal from the optical domain to the electrical domain may be used.
When the frequency comb generating unit is of a second structure or a third structure, the pre-sampling unit comprises an electric mixer for pre-sampling the signal to be detected. As shown in fig. 8, the pre-sampling unit includes an electrical mixer, a signal input port, an electrical frequency comb input port, and a signal output port. The signal input port is used for receiving a signal to be detected, the electric frequency comb input port is used for receiving the pre-sampling electric frequency comb, the signal output port is used for outputting the pre-sampling signal, and the electric mixer is used for modulating the signal to be detected onto the electric frequency comb. The electrical mixer may be any electronic device that meets the above modulation requirements. When necessary, an electric amplifier and an electric filter can be added between the input port of the electric frequency comb and the electric mixer, so that the regulation and control of the power and the number of the comb teeth of the electric frequency comb are realized, and the pre-sampling of a signal to be detected is facilitated.
As shown in fig. 9, the signal acquiring and processing unit according to the embodiment of the present invention may include: the analog-to-digital converter and the signal processing module further comprise a signal input port and a data output port. The signal input port is used for inputting a pre-sampling signal, the data output port is used for outputting a calculated frequency estimation value of the signal to be detected, the analog-to-digital converter is used for executing the analog-to-digital conversion of the pre-sampling signal, and the signal processing module is used for determining the Nyquist zone serial number of the frequency of the signal to be detected, executing FFT (fast Fourier transform) and calculating the frequency estimation value of the signal to be detected. The signal processing module has the following four functions.
Coarse frequency estimation: carrying out frequency rough discrimination on the signal to be measured to determine the signal to be measuredWhich Nyquist zone of the pre-sampling frequency comb the sign lies in, i.e. the specific value of n and f in the preceding equations 7-1 to 7-4 are determined in Is located in the 2n +1 Nyquist zone or the 2n +2 Nyquist zone, so that subsequent calculations can be performed using equations 7-1 through 7-4. In practical applications, a FFT with a small number of points (e.g. 128 points) may be performed on the signal to be measured, or a spectrometer or other instrument may be used to perform a coarse frequency estimation.
FFT: an FFT power spectrum of the pre-sampled signal is calculated.
Multi-signal frequency estimation: and calculating the frequency measurement values of the signal copies in a plurality of Nyquist zones of the power spectrum of the pre-sampled signal and the frequency measurement values of the signal copies to the signal to be tested.
Frequency averaging: and calculating the average value of the frequency measurement values of the signal copies to the signal to be measured, and determining the average value as the frequency estimation value of the signal to be measured.
As a preferred solution, the signal processing module may add other sub-functions besides the above four functions, such as digital down-conversion, down-sampling, re-sampling, etc. In particular, the signal processing module may be further configured to: after the frequency estimation value of the signal to be measured is calculated, a preset frequency estimation algorithm is used for further calculation on the basis of the calculated frequency estimation value of the signal to be measured, so that the frequency estimation precision is further improved. Wherein the frequency estimation algorithm comprises at least one of: polynomial fitting, zero padding, interpolated fast fourier transform, weighted phase averaging, band-selected fourier transform.
In the embodiment of the present invention, the frequency estimation apparatus may further include a control unit, which is respectively connected to the frequency comb generating unit, the pre-sampling unit, the signal collecting and processing unit, the first signal conditioning unit, and the control unit of the second signal conditioning unit, and configured to perform overall control on the system. Specifically, the method can be used for setting the repetition frequency and power of the pre-sampling frequency comb, setting the parameters of the first electric filter, the first electric amplifier, the first electric attenuator, the second electric filter, the second electric amplifier and the second electric attenuator, setting the parameters of each device in the pre-sampling unit, setting the sampling rate of analog-to-digital conversion, the number of calculation points of fast fourier transform and the preset number, and providing the reference clock frequency for the frequency comb generation unit and the signal acquisition and processing unit.
In summary, in the technical solution of the embodiment of the present invention, a high-precision frequency estimation apparatus based on pre-sampling and multi-measurement value averaging is provided, that is, a frequency comb with a specific repetition frequency is used to pre-sample a signal in an analog domain, and the signal to be measured is extended into a plurality of pre-sampling nyquist zones. And then, in a digital domain, extracting frequency measurement values of signal copies of a plurality of Nyquist zones in the FFT spectrum of the pre-sampled signal, and further realizing the remarkable improvement of the signal frequency estimation precision through a simple averaging algorithm. The key features are two points:
first, a signal to be measured is pre-sampled with a frequency comb having a specific repetition frequency. The pre-sampling frequency comb repetition frequency preferably satisfies the following relationship, i.e., the spectral resolution f according to the system FFT res Setting a specific pre-sampling repetition frequency value, so that the decimal part epsilon of the repetition frequency satisfies epsilon multiplied by N =1, wherein N is the number of signal copies used for calculating the frequency of the signal to be measured in the FFT spectrum of the pre-sampling signal (namely the above preset number).
And secondly, extracting frequency measurement values of signal copies in a plurality of pre-sampling Nyquist zones in the FFT spectrum of the pre-sampling signal, and calculating a frequency estimation value of the signal to be measured through an average value.
The super-resolution measurement of the signal frequency is realized by pre-sampling processing of a signal to be measured on hardware and a simple signal processing method based on FFT spectrum data, and the main problem of improving the frequency estimation precision based on hardware and algorithm level at present is solved. The pre-sampling process and the processing algorithm of the device are very simple and reliable, and other complex algorithms do not need to be introduced. In addition, the device is used as a representative signal processing means and is compatible with other existing FFT-based high-precision frequency estimation algorithms.
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 (10)
1. A frequency estimation apparatus, comprising: the device comprises a frequency comb generating unit, a pre-sampling unit and a signal acquisition and processing unit; wherein,
the frequency comb generating unit is used for generating a pre-sampling frequency comb;
the pre-sampling unit pre-samples the signal to be detected in the analog domain by using the pre-sampling frequency comb to obtain a pre-sampled signal; the pre-sampled signal comprises: a plurality of signal copies of the signal under test located in each nyquist zone of the pre-sampling frequency comb;
the signal acquisition and processing unit performs analog-to-digital conversion on the pre-sampling signal, and performs fast Fourier transform on the pre-sampling signal of a digital domain formed by the analog-to-digital conversion to obtain frequency measurement values of the plurality of signal copies;
the signal acquisition and processing unit selects a preset number of Nyquist zones from the Nyquist zones, calculates the frequency measurement value of the signal copy to the signal to be measured by using the frequency measurement value of the signal copy of each selected Nyquist zone, and determines the average value of the frequency measurement value of the signal copy of each selected Nyquist zone to the signal to be measured as the frequency estimation value of the signal to be measured.
2. The apparatus of claim 1, wherein the signal acquisition and processing unit calculates the frequency measurement of the signal replica of any selected nyquist zone for the signal under test by the following formula:
at the frequency f of the signal to be measured in At the Nyquist zone of 2n + 1:
when the number of i is an odd number,
when the number i is an even number, the number i,
at f in At the Nyquist zone of 2n + 2:
when the number of i is an odd number,
when the number i is an even number, the number i,
wherein n is a nonnegative integer, i is the serial number of any selected Nyquist zone, and f inM,i Frequency measurement of said signal to be measured for a signal replica of any selected Nyquist zone, f M,i Frequency measurement of signal replica for any selected Nyquist zone, f c Is the repetition frequency of the pre-sampling frequency comb.
3. The apparatus of claim 2, wherein the predetermined number is equal to the number of signal copies used to calculate the frequency estimate of the signal under test; the frequency of each signal copy can be expressed as a linear combination expression of the frequency of the signal to be tested and the repetition frequency; and (c) a second step of,
the frequency of the signal copies of any two Nyquist zones selected by the signal acquisition and processing unit satisfies the following conditions: the difference between the coefficients of the repetition frequencies in the linear combination expression is not equal to an integer multiple of the predetermined number.
4. The apparatus of claim 2, wherein a fractional part of a ratio of the repetition frequency to a frequency resolution of the fast fourier transform satisfies: equal to the inverse of said preset number.
5. The apparatus of claim 1, wherein the signal acquisition and processing unit comprises: the analog-to-digital converter and the signal processing module; wherein,
the analog-to-digital converter is used for executing the analog-to-digital conversion;
the signal processing module is used for determining the sequence number of the Nyquist zone where the frequency of the signal to be detected is located, executing the fast Fourier transform, and calculating the frequency measurement value of the signal copy of each selected Nyquist zone to the signal to be detected and the frequency estimation value of the signal to be detected.
6. The apparatus of claim 1, wherein the frequency comb generation unit has a first structure, a second structure, or a third structure;
in a first structure, the frequency comb generating unit is an optical frequency comb generating device, and comprises a mode-locked laser or an electro-optical modulation optical frequency comb generating device, and the pre-sampling frequency comb is an optical frequency comb;
in a second structure, the frequency comb generating unit is an electrical frequency comb generating device, and comprises a nonlinear transmission line or a step recovery diode, and the pre-sampling frequency comb is an electrical frequency comb;
in a third structure, the frequency comb generating unit includes the optical frequency comb generating device and a first photodetector, and the pre-sampling frequency comb is an electrical frequency comb.
7. The apparatus of claim 6, wherein when the frequency comb generating unit has the first structure, the pre-sampling unit comprises: the photoelectric modulator, the optical amplifier, the optical filter and the second photodetector; the electro-optical modulator is used for pre-sampling the signal to be detected, and the optical amplifier, the optical filter and the second photoelectric detector are used for processing the pre-sampled signal;
and when the frequency comb generating unit is of a second structure or a third structure, the pre-sampling unit comprises an electric mixer for pre-sampling the signal to be tested.
8. The apparatus according to claim 1, further comprising a first signal conditioning unit connected to the pre-sampling unit, and a second signal conditioning unit connected to the pre-sampling unit and the signal acquisition and processing unit, respectively; wherein,
the first signal conditioning unit comprises a first electric filter, a first electric amplifier and a first electric attenuator and is used for processing the signal to be detected and transmitting the processed signal to be detected to the pre-sampling unit;
the second signal conditioning unit comprises a second electric filter, a second electric amplifier and a second electric attenuator and is used for processing the pre-sampling signal and transmitting the processed pre-sampling signal to the signal acquisition and processing unit.
9. The apparatus of claim 1, wherein the signal acquisition and processing unit is further configured to: after calculating the frequency estimation value of the signal to be detected, further calculating on the basis of the calculated frequency estimation value of the signal to be detected by using a preset frequency estimation algorithm; wherein,
the frequency estimation algorithm includes at least one of: polynomial fitting, zero padding, interpolated fast fourier transform, weighted phase averaging, band-selected fourier transform.
10. The apparatus of claim 1, further comprising: the control unit is respectively connected with the frequency comb generating unit, the pre-sampling unit, the signal acquisition and processing unit, the first signal conditioning unit and the second signal conditioning unit; wherein,
the control unit is used for: setting the repetition frequency and the power of the pre-sampling frequency comb, setting the parameters of a first electric filter, a first electric amplifier, a first electric attenuator, a second electric filter, a second electric amplifier and a second electric attenuator, setting the parameters of each device in the pre-sampling unit, setting the sampling rate of the analog-to-digital conversion, the number of calculation points of the fast Fourier transform and the preset number, and providing the reference clock frequency for the frequency comb generating unit and the signal collecting and processing unit.
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