JPH05256883A - Digital method and device for analyzing frequency - Google Patents

Abstract

PURPOSE:To accurately detect a frequency by correcting a quantization error produced by sampling based on the frequency and level values of a first spectrum having the highest level and its adjacent second spectrum having the second highest level of spectrum components and drawing real frequency information. CONSTITUTION:After signals to be measured inputted from an input terminal 1 are divided into two parts and the divided signals are respectively subjected to frequency conversion through mixers 2 and 3, the signals are inputted to a discrete Fourier transformation(DFT) circuit 8 through low-pass filters 4 and 5 and D/A converters 6 and 7 and the output of the circuit 8 is inputted to an absolute value circuit 9. The output of a local oscillator 12 is inputted to the mixers 2 and 3, with the output being inputted directly to one of the mixers and through a pi/2-phase shifter 13 to the other mixer. In addition, a frequency correcting section 20 is provided in the succeeding stage of the circuit 9 and a spectrum detector 21 and computing element 22 are incorporated in the section 20. The section 20 calculates real frequency information by correcting a quantization error produced by sampling by using the level difference between the first spectrum having the highest level and its adjacent second spectrum having the second highest level of the spectrum components of the output of the circuit 9 and frequencies of the spectra.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital frequency analyzing method and an apparatus therefor, and the digital frequency analyzing means is, for example, a DFT (Discrete Fourier T
transform: hereinafter referred to as DFT) and the like, and relates to a means for obtaining true frequency information by correcting the quantization error due to sampling.

[0002]

2. Description of the Related Art A DFT is used as one of means for measuring or detecting a frequency of a voice signal or a radio frequency signal in voice synthesis or voice analysis by digital processing. FIG. 6 is a block diagram of a conventional digital frequency analyzer using DFT means. In the circuit block shown in this figure, a signal under measurement input from an input terminal 1 is branched into two and frequency-converted through mixers 2 and 3, respectively, and then low-pass filters 4 and 5 and analog-digital converters 6 and 7 After that, the DFT circuit 8 is input to the DFT circuit 8, and the absolute value circuit 9 and the frequency determination circuit 10 output the DFT circuit output to the output terminal 11 to obtain frequency information.

Further, one of the outputs from the local oscillator 12 is directly input to the mixers 2 and 3 and the other is a π / 2 phase shifter 1.
3 is configured to be input respectively.

As described above, two signals which are phase-shifted by π / 2 are orthogonal to each other in the signal under measurement (one is a real number term,
The other one can be considered as an imaginary number term) and is a method often used in DFT.

Since the operation and processing in this circuit are well known in the art, detailed description thereof will be omitted. However, depending on this processing, the DFT in the DFT may be omitted.
In some cases, the true frequency information of the signal under measurement cannot be obtained due to the quantization error determined by the period, the sampling frequency, the sampling number, and the like.

That is, FIGS. 7 (a) and 7 (b) are diagrams for explaining this state, where (a) is a signal waveform on the time axis and (b) is a frequency spectrum obtained as a result of DFT. FIG.

As shown in FIG. 1 (a), the signal under measurement is set to the period T
When sampling is performed at the sampling frequency fs with the number of samples N, the sampling points 0, 1, ..., (N
If the interval of -1) is T _0, and DFT is performed on it,
As shown in (b), the spectrum component obtained is scattered at any of discrete points fs / N, 2fs / N, ... Fs on the frequency axis.

At this time, the interval between the discrete points is 1 / T, and the longer the DFT period T, the smaller 1 / T and the higher the spectrum analysis capability. That is, when the measured signal frequency is f _X shown by the dotted line on the frequency axis shown in (b), that is, when it is located between the discrete points 2 (fs / N) and 3 (fs / N). , DFT, the spectral components appear as two spectra in solid lines f ′ and f ″ in FIG.

Conventionally, when the frequency resolution is not a problem to the left, the frequency at the position of the maximum spectral component is extracted and used as frequency information.
The T cycle T was lengthened.

However, in each case, a quantization error due to so-called sampling is included, and if the DFT period T is set to be long in order to minimize the error, the number of samples increases, and the digital processing amount correspondingly increases. There is a problem that the processing time naturally becomes long because of the large number, and high-speed processing becomes impossible.

That is, the reduction of processing time and the improvement of frequency resolution are contradictory requirements, and a means for satisfying both requirements has been desired.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and is provided with a means for correcting a quantization error when a frequency is analyzed by digital processing or when frequency information is calculated, like a DFT. Accordingly, it is an object of the present invention to provide a digital frequency analysis method and a device therefor capable of detecting a frequency extremely accurately without increasing the processing time.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above object, the first invention of the present application is to sample a frequency of a signal under measurement at a required frequency and to extract a spectrum component corresponding to the frequency of the signal under measurement by Fourier transform to obtain the frequency. In the method of measuring or detecting its frequency information,
Of the spectral components, the quantization error due to sampling is corrected based on the frequency and the level value of the first spectrum of the maximum level and the second spectrum of the second level value adjacent thereto, and the true frequency information is derived. It is characterized by doing.

The second invention of the present application measures the frequency or detects the frequency information by suppressing the measured frequency at a required frequency and extracting the spectrum component corresponding to the measured signal frequency by Fourier transform. In the method, the maximum level of the spectral components, the frequency of the adjacent second spectrum of the second level value adjacent thereto, and the magnitude relationship between the levels, and the respective values are substituted into the single carrier spectrum calculation formula. By doing so, the true frequency information in which the quantization error due to sampling is corrected is derived.

In a third aspect of the present invention, the frequency to be measured is supplemented with a required frequency and the frequency is measured or the frequency information is detected by extracting a spectrum component corresponding to the frequency of the signal to be measured by Fourier transform. In the device, a local signal generation source having substantially the same frequency as the signal under measurement, a means for mixing the local signal and the signal under measurement to generate a real number component and an imaginary number component, and the two components Means for performing DFT processing based on D,
Means for obtaining absolute value of FT output, and quantization by sampling from frequency information and level value of first spectrum of maximum level and second spectrum of second level adjacent to the first spectrum among spectrum components of output of the absolute value means And a means for correcting an error.

In a fourth aspect of the present invention, the measured frequency is supplemented with a required frequency and the frequency component is measured or the frequency information is detected by extracting a spectrum component corresponding to the measured signal frequency by Fourier transform. In the device, a local signal generation source having substantially the same frequency as the signal under measurement, a means for mixing the local signal and the signal under measurement to generate a real number component and an imaginary number component, and the two components Means for performing DFT processing based on D,
The means for obtaining the absolute value of the FT output, and the level difference between the first spectrum having the maximum level and the second spectrum having the second level adjacent to the first spectrum among the spectrum components of the output of the absolute value means and the respective frequencies are sampled. And means for calculating true frequency information in which the quantization error is corrected.

The fifth aspect of the present invention is characterized in that the local signal frequency is set so that the first and second spectral levels of the DFT output are substantially equal to each other.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings. FIG. 1 is a DF to which the present invention is applied.
FIG. 1 is a block diagram showing an embodiment of a T-type digital frequency analysis device, and the principle and specific example of the present invention will be explained with reference to this diagram.

In FIG. 1, reference numerals 1 to 9 and 12, 1
Since 3 is the same as the conventional circuit shown in FIG. 6, the duplicated description will be omitted. The characteristic of the device shown in this figure is that a frequency correction unit 20 is provided at the subsequent stage of the absolute value circuit 9,
The correction unit 20 includes a spectrum detector 21 and a calculator 22.

The device operates and is controlled as follows. That is, the measured signal frequency of the input terminal 1 is f _1, and the output signal frequency f ₂ of the local signal generator 12 is f ₂ .
_{When 1} = f ₂ , the outputs of the aliasing low-pass filters 4 and 5 are baseband signals, which correspond to the complex number signals of the real part and the imaginary part of the spectral component, respectively.

Assuming now that the frequency difference between the measured signal frequency f ₁ and the local signal frequency f ₂ is Δf _I , the frequency difference Δf is output to the mixers 2 and 3.
_I is included. At this time, the real term component signal I (t) and the imaginary term component signal Q of the baseband signal of each mixer output
From (t), the real number component value X (k) and the imaginary number component value Y (k) of the frequency component are obtained, and the absolute value calculator (ABS) 9
The frequency spectrum is calculated by the following equation (1).

[0022]

[Equation 1] Here, the value obtained as a result of the DFT is a discrete value determined by the sampling frequency of the A / D converter, but the frequency shift value Δfd is not a multiple of fs / N, but is, for example, the frequency shift value Δ. When fd is a frequency fa between 2fs / N and 3fs / N, the absolute value | A (k) | is affected by the DFT sampling frequency and corresponds to the received signal as shown by the dotted line in FIG. Since the true frequency spectrum component cannot be obtained, as shown by the solid line, 2fs / N and 3fs / N
The spectrum components of N frequencies are separated.

Generally, in frequency analysis by DFT or the like, the frequency is obtained as a discrete value determined by the sampling period and the like. However, in order to reduce the discrete width, it is necessary to increase the DFT period T and select a large number of samples. ,
As described above, it is disadvantageous in increasing the processing speed.
Therefore, in this embodiment, the following is performed.

First, the spectrum detector 34 of the demodulator W
Therefore, the spectral component having the maximum absolute value | A (k) | (hereinafter referred to as the first spectral component) and the spectral component adjacent to the component having the larger absolute value | A (k) | (Hereinafter, referred to as the second spectrum component) respectively, and the absolute value | A (k ₁ ) | of the first spectrum component and the value k ₁ corresponding to the frequency value f ₁ thereof are detected.
And the absolute value | A (k ₂ ) | of the second spectral component and the value k ₂ corresponding to the frequency value thereof, respectively.

Here, the single carrier spectrum equation used when obtaining the frequency spectrum of the baseband signal in the DFT 29, that is, the equation representing the frequency component obtained at the output of the DFT, is the following equation (2).

[0026]

[Equation 2] The absolute value, that is, the spectrum value is expressed by the following equation (3).

[0027]

[Equation 3] In this equation (3), a is a value corresponding to the frequency of the signal input to the DFF. Therefore, in the equation (3), the first
The absolute value | A (k ₁ ) | of the spectrum component and the value k ₁ are substituted to obtain the value a corresponding to the frequency shift value Δfd. Here, the above equation (3) is an objective function for k = a, and the value a obtains two solutions 1 and 2, so that the computing unit 2
2 compares the above k ₁ and k ₂ with each other, and when k ₁ is large, selects either a1 or a2 where the value a satisfies k ₂ ≦ a <k _1, and when k ₂ is large Is selected from either a1 or a2 where the value a satisfies k ₁ ≦ a <k ₂ .

In this device, as described above, the antialiasing filters 6 and 7 remove the frequency components higher than fs / 2, and the frequency components higher than fs / 2 become negative frequency components according to Nyquist's theorem. Correspondingly, either the value a1 or a2 selected by the arithmetic unit, for example, when the value a1 is larger than the value N / 2 corresponding to the frequency fs / 2, the value a1 minus the sample number N The frequency corresponding to the frequency shift value f, or the frequency corresponding to the value a1 when the value a1 is smaller than the value N / 2.
Calculate as d. The frequency shift value Δfd obtained here can be obtained from the absolute value of the first spectrum component and its frequency value, the absolute value of the second spectrum component, its frequency value, and the single carrier spectrum equation as described above. By adding this value to the local signal frequency f ₂ , the true frequency value can be obtained.

Further, the present invention can be modified as follows. That is, in the above-described embodiment, the frequency information was calculated by substituting each value obtained from the DFT spectrum into each of the above equations. As a modification, for example, | A (k) | obtained by the above equation (3) and Adjacent spectrum | A (k +
1) | is calculated by the following equation (4),

[0030]

[Equation 4] The difference between the two, that is, the following equation (5),

[0031]

FIG. 5 is a method for obtaining true frequency information from these values.

That is, the horizontal axis represents the value a corresponding to the frequency shift value Δfd, and the vertical axis represents the absolute value | A obtained by the above equation (5).
If (k) |-| A (k + 1) |, these relationships can be drawn as a graph as shown in FIG. Therefore,
If the absolute values | A (k) |, | A (k + 1) | and the value k are solved, the value a can be obtained from the above equation (5). Utilizing this fact, the arithmetic unit compares the values k ₁ and k ₂ with each other, and when the value k ₁ is large, the absolute value | A
_{(K 1) |, | A} (k 2) | and the value k ₁ respectively the formula
(5) is substituted into | A (k) |, | A (k + 1) | and k to obtain the value a corresponding to the frequency shift value Δfd. On the other hand, when the value k ₂ is large, the absolute value | A (k ₂ )
|, | A (k ₁ ) | and the value k ₂ are respectively | A of the above equation (5).
(K) |, | A (k + 1) | and k are substituted to obtain the value a.

In the above embodiment, the spectrum detector 3
The absolute value depending on the magnitude relationship between the value k ₁ and k ₂ in 4 | A (k ₁₎ | and | A (k ₂₎ | value k ₁ or k ₂
The value a is calculated by substituting any of the above into the equation (5), but the present invention is not limited to this. For example, in the equation (5), if the magnitude relationship between the values k ₁ and k ₂ can be determined as described above, the mutual difference | A (k) | of the absolute values as shown in FIG.
Since the value a can be obtained from − | A (k + 1) |, the ROM in which the value a corresponding to the values 1 to −1 that the mutual difference | A (k) | − | A (k + 1) |
May be replaced with the arithmetic part of the value a in the arithmetic unit. In this case, the mutual difference | A (k ₁ ) | − depending on the result of comparing the values k ₁ and k ₂ with each other as described above.
| A (k ₂₎ |, or _{| A (k 2) | -} | A (k 1) |
If the value a to be obtained is read from the ROM in correspondence with any one of the above and any one of the values k ₁ or k ₂ , it is possible to omit the complicated calculation as shown in the above equation (5). It would be convenient to get information quickly.

FIG. 4 is a block diagram showing a concrete example of a receiving apparatus using the present invention. The part surrounded by a broken line is the frequency analyzing part of the present invention described above. In this example, the fluctuation of the received signal is tracked. The frequency f of the local oscillator VCO of the receiver
A so-called A for controlling _L so that the intermediate frequency IF is located at the center of the intermediate frequency band pass filter BPF
The present invention is used as an FC circuit.

That is, in the super-heterodyne receiver, the received signal is mixed with the output of the local oscillator VCO (here, the variable frequency oscillator VCO is used to control the frequency) in the mixer MIX, and the signal of the difference between the two is band-passed. After being selected by the filter BPF, it is generally configured to be input to the demodulator DEM.

However, if the reception frequency fluctuates due to fluctuations in the output of the transmitter oscillator, fading, the Doppler effect, etc., the intermediate frequency deviates from the band of the bandpass filter BPF, and normal demodulation becomes impossible. there were. Therefore, in this embodiment, the above-described frequency analysis means of the present invention is used to detect the difference between the reception frequency and the output of the local oscillator VCO, and the VCO is controlled so that the difference becomes zero.

That is, if the local frequency f _I for DFT is set to be the same as the intermediate frequency IF of the receiver in the AFC section surrounded by the broken line, the local signal frequency inside the DFT is set in the frequency analysis section including the DFT as described above. Since the frequency information of the difference between the intermediate frequency of the receiver and the intermediate frequency of the receiver is obtained, the VCO control voltage is output from the output signal from the arithmetic unit through the D / A converter so that the difference becomes zero, and the local oscillation frequency is output. Control it.

FIG. 5 is a block diagram showing another specific example to which the present invention is applied, in which a frequency analysis means of the present invention is applied to an AFC circuit in a receiver using a spread spectrum system. The spread spectrum communication method is often used for wireless communication using artificial satellites in recent years. However, when the system is mobile communication or when the artificial satellite is not the stationary method but the earth surrounding method, Doppler is used. The frequency shift due to the effect becomes a problem.

In particular, in the spread spectrum system, it is necessary to synchronize the PN sequence (pseudo noise code) for demodulation, and as described with reference to FIG. However, according to the present invention, it is possible to correct the quantization error when digitally extracting the frequency information and control the local oscillation frequency extremely accurately.

In FIG. 5, reference numeral 51 is a data demodulation unit of the spread spectrum receiver, 52 is a PN generator, 53 is a DLL circuit (delay lock loop circuit) forming a part of the sliding correlator, and 54 is. It is a timing generation circuit for controlling a PN generator based on a signal output from the spectrum detector of the DFT.

The above-mentioned blocks newly added in this embodiment are necessary for a general spread spectrum receiver, and the operation and control method thereof are well known, so the description thereof will be omitted. In the spectrum detector, when the spectrum component of the required level or higher is detected as a result of the DFT, the PN series synchronization is established, and the synchronization determination signal SJ is output assuming that the correlation value is obtained.

The PN generator 52 slides the PN series bit by bit until the judgment signal SJ is output, and the slide operation is stopped by the appearance of the signal. This operation is what is called a sliding correlation. Also in this example, the frequency analysis block surrounded by a broken line is not different from the above description. In addition, V after the arithmetic unit
It is necessary to insert a D / A (digital / analog converter) for generating a CO control voltage to generate a DC voltage.

When the present invention is used as the Doppler correction means in spread spectrum communication as in this example, a in the above equations (2), (3), (4) and (5) is the reception due to the Doppler effect or the like. It corresponds to the frequency or the difference between the frequency after frequency conversion and the local frequency inside the DFT.

Although the principle and application examples of the present invention have been described above, in carrying out the present invention, it is not necessary to limit to the above examples and various modifications can be made. For example, when the received signal or the signal under measurement contains noise, the DFT output or the VCO control voltage may fluctuate.
Inserting a low-pass filter or a smoothing circuit into a required part in an analog or digital manner is also effective for stabilizing the operation.

Further, when the received level or the signal under measurement is accompanied by level fluctuation, the spectrum value may fluctuate in the same manner. Therefore, an AGC circuit is inserted in a portion that does not affect the data demodulation to stabilize the level. That would also be effective.

[0046]

Since the present invention is constructed or controlled as described above, means for digitally performing frequency analysis or frequency information detection such as DFT or FFD is provided.
It is possible to remove and correct the quantization error that inevitably occurs, and obtain extremely accurate frequency information.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining an example of the present invention.

FIG. 3 is a diagram for explaining a modified example of the present invention.

FIG. 4 is a configuration diagram showing an application example of the present invention.

FIG. 5 is a configuration diagram showing another application example of the present invention.

FIG. 6 is a configuration diagram of a conventional analyzer.

FIG. 7 is a diagram for explaining a conventional analyzer,
(a) is an explanatory view of a case where the signal under measurement is sampled at a sampling frequency fs at a sampling number N, and (b) is an explanatory view of its frequency spectrum.

[Explanation of symbols]

1 input end, 2, 3 mixer, 4, 5 low pass filter, 6, 7 analog-digital converter, 8 DFT,
9 absolute value circuit, 12 local oscillator, 13π / 2 phase shifter, 20 frequency correction unit, 21 spectrum detector,
22 arithmetic unit, 23 D / A converter.

[Equation 5]

Claims (5)

[Claims] 1. A method of measuring a frequency of a signal under measurement at a required frequency, measuring the frequency by extracting a spectral component corresponding to the frequency of the signal under measurement by Fourier transform, or detecting frequency information thereof, Of the spectral components, the quantization error due to sampling is corrected based on the frequency and the level value of the first spectrum of the maximum level and the second spectrum of the second level value adjacent thereto, and the true frequency information is derived. A digital frequency analysis method comprising: 2. A method of measuring a frequency or detecting frequency information thereof by substituting a frequency to be measured at a required frequency and extracting a spectrum component corresponding to the frequency of the signal to be measured by Fourier transform. Sampling is performed by substituting the magnitude relationship between the frequency and level of each of the first spectrum of the maximum level of the spectrum components and the second spectrum of the second level value adjacent thereto and their respective values in the single carrier spectrum calculation formula. A digital frequency analysis method, wherein true frequency information is derived by correcting the quantization error due to. 3. An apparatus for measuring a frequency to be measured or detecting frequency information thereof by substituting a frequency to be measured with a required frequency and extracting a spectrum component corresponding to the frequency of the signal to be measured by Fourier transform. A local signal source having substantially the same frequency as the measurement signal, a means for mixing the local signal and the signal under measurement to generate a real number component and an imaginary number component, and DFT processing based on the two components Means, means for obtaining the absolute value of the DFT output, and sampling from the frequency information and the level value of the first spectrum of the maximum level among the spectral components of the output of the absolute value means and the second spectrum of the second level adjacent thereto. And a means for correcting a quantization error due to the digital frequency analysis apparatus. 4. An apparatus for measuring a frequency to be measured or detecting frequency information thereof by sampling a frequency to be measured at a required frequency and extracting a spectral component corresponding to the frequency of the signal to be measured by Fourier transform. A local signal source having substantially the same frequency as the measurement signal, a means for mixing the local signal and the signal under measurement to generate a real number component and an imaginary number component, and DFT processing based on the two components Means, a means for obtaining an absolute value of the DFT output, a level difference between the first spectrum of the maximum level and the second spectrum of the second level adjacent to the maximum spectrum among the spectrum components of the output of the absolute value means, and respective frequencies. And a means for calculating true frequency information in which the quantization error due to sampling is corrected from the digital frequency analysis device. . 5. The digital frequency analyzer according to claim 3 or 4, wherein the local signal frequency is D
A digital frequency analyzer, wherein the first and second spectral levels of FT output are set to be substantially equal.