JP2002247126A - Digital demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital demodulator that prevents a frequency from being deviated when a received electric field strength level is low and conducts AFC correction at a high-speed when the received electric field strength level is normal and when a received signal is switched. SOLUTION: The digital demodulator comprises a phase error calculation section 18 that calculates a phase error from I and Q signals detected by a digital orthogonal detection section 2, an AFC calculation section 16 that calculates frequency fluctuations on the basis of phase error data, an AFC correction section 4 that automatically corrects the frequency, and an AFC filter section 17 including a level detection section 22 that detects a level of the received signal, a level discrimination section 23 that discriminates a difference between the level of the received signal and a prescribed level threshold, a phase error discrimination means 25 that discriminates whether or not a phase error calculated by the phase error calculation section 18 exceeds a prescribed angular threshold, and a selector means 26 that selects passing or interruption of phase error data to the AFC calculation section 16 on the basis of the result of discrimination by the level discrimination section 23 and the phase error discrimination means 25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-valued QAM (Quadra
The present invention relates to a digital demodulation device of the AMP system, and more particularly to a digital demodulation device that performs high-speed AFC correction that is less likely to malfunction.

[0002]

2. Description of the Related Art Multi-level QA is one of land mobile communication systems.
For example, there is a 16QAM system. As an example of a digital demodulator that receives the 16QAM signal, performs AFC correction, and demodulates the signal, a principle configuration diagram of a conventional quasi-synchronous demodulator is shown in FIG. In the figure, IF is a signal (for example, 455 KHz) obtained by converting a received RF (high frequency) signal into an intermediate frequency by a local oscillation signal having a predetermined frequency.
The signal is input to a quadrature detection unit 51 composed of a multiplier and the like, and quadrature detection is performed. The signals are output as an I-channel (I) signal as an in-phase component and a Q-channel (Q) signal as a quadrature component by quadrature detection. These outputs are AFC-corrected by the AFC correction unit 52 and output from the demodulation unit 5
At 3 the digital data is decoded and output. A
The correction by the FC correction unit 52 is performed by, for example, extracting a reference carrier component by the AFC error detection unit 54 from the output I signal and Q signal by the carrier component detection unit 54a, and calculating the AFC error calculation unit from the extracted carrier component. An error is calculated in 54b, and frequency correction for the deviation from the reference frequency is performed so that the error is reduced.

[0003]

By the way, especially when applied to a mobile object, there is no problem if the normal radio wave intensity can be obtained, but the radio wave is rapidly weakened when moving and the predetermined radio wave intensity cannot be obtained. There is a case. In such a case, the conventional AFC correction circuit erroneously detects a frequency shift in an unstable reception state, and performs AFC correction based on a result of the erroneous detection. Even if it returns, there is a problem that it takes time to restore the frequency once shifted. Conversely, when the channel or mode is switched on the receiving side and the received signal is switched, it takes time to perform an averaging operation or the like to detect a frequency shift, and consequently it takes time to follow the received signal. There was also a problem that convergence was difficult. The present invention has been made in view of the above-described problem, and prevents a frequency from being largely shifted even when a reception electric field level is lowered. When the reception electric field level is normal, AFC correction is performed at high speed and the operation is restored. It is another object of the present invention to provide a digital demodulation device which performs AFC correction at high speed and returns when a received signal is switched.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides quadrature detection of a received multilevel QAM signal by a quasi-synchronous method by digital processing, and outputs an I signal having an in-phase component and a Q signal having a quadrature component. , A low-pass filter that extracts required low-frequency components from the signal from the digital quadrature detector, and a reference phase based on the received signal from the I and Q signals output from the LPF A phase error calculator for calculating the phase error of each of the I signal and the Q signal; and phase error data calculated by the phase error calculator. The frequency error is calculated based on the input phase error data. AFC calculation unit that performs
An AFC correction unit that automatically corrects the frequency based on the frequency fluctuation calculated by the FC calculation unit. The AFC correction unit performs automatic frequency correction, and demodulates the corrected I signal and Q signal. In the digital demodulation device for outputting data, between the phase error calculating section and the AFC calculating section, the phase error calculating section calculates and inputs the phase error data exceeding a predetermined angle threshold to be input to the AFC calculating section. An AFC filter unit that cuts off and passes only the phase error data within the predetermined angle threshold is provided.

[0005] The AFC filter section includes a phase error determining section that determines whether the phase error calculated by the phase error calculating section exceeds a predetermined angle threshold or is within a predetermined angle threshold. According to the determination result, the AF
Selector means for selecting whether to block or pass the phase error data input to the C calculator.

A level detector for detecting the signal level of the received signal from the I signal and the Q signal after the automatic correction of the received frequency by the AFC corrector, and a signal level of the received signal detected by the same level detector. A level discriminating section for discriminating a level from a predetermined level threshold value, wherein the level discriminating section determines that the signal level of the received signal detected by the level detecting section is smaller than the predetermined level threshold value;
The FC filter section cuts off the phase error data calculated by the phase error calculation section, and the level determination section determines that the signal level of the reception signal detected by the level detection section is equal to or higher than the predetermined level threshold. The AFC filter section passes the phase error data calculated by the phase error calculation section.

The predetermined angle threshold is composed of a predetermined first angle threshold and a second angle threshold wider than the first angle threshold. Whether the phase error exceeds each of the predetermined first angle threshold and the second angle threshold, or is within each angle threshold, the selector means, the phase error, The AFC is determined based on the determination result of the determination unit and the determination result of the level determination unit.
When it is possible to select the passage or cutoff of the phase error data output to the calculation unit, when the level determination unit determines that the signal level of the received signal detected by the level detection unit is smaller than the predetermined level threshold, The AFC filter unit passes the phase error data within the predetermined first angle threshold calculated by the phase error calculation unit, and the signal level of the reception signal detected by the level detection unit by the level determination unit Is determined to be equal to or greater than the predetermined level threshold, the AFC filter section allows the phase error data within the predetermined second angle threshold calculated by the phase error calculation section to pass.

[0008] A pilot symbol detecting section for detecting a pilot symbol transmitted every predetermined frame, and a symbol counter reset by a received input signal or a reset signal generated at the time of mode switching and counting the pilot symbols are provided. When the level determination unit determines that the signal level of the reception signal detected by the level detection unit is equal to or higher than the predetermined level threshold, the phase error calculation is performed while the symbol counter after reset is smaller than a predetermined count value. The phase error data calculated by the section passes through the AFC filter section.

When the level determination section determines that the signal level of the reception signal detected by the level detection section is smaller than the predetermined level threshold value, the symbol counter after reset by the reset signal is set to a value smaller than a predetermined count value. While it is small, only the phase error data within the predetermined second angle threshold calculated by the phase error calculator passes through the AFC filter.

[0010] The AFC calculating section includes averaging means for accumulating and averaging the phase error data calculated and input from the phase error calculating section for a predetermined period.

The content of the averaging means is cleared by the reset signal.

[0012] The predetermined level threshold is set based on a reception sensitivity level for ensuring a predetermined bit error rate for the data output.

[0013] The level detector detects a peak level of a pilot symbol transmitted for each predetermined frame of the received signal.

The level detector detects an average value of peak levels of absolute values of amplitudes of the I signal and the Q signal.

The level detector detects an average value of the absolute values of the amplitudes of the I signal and the Q signal.

[0016]

Embodiments of the present invention will be described below with reference to the drawings based on embodiments. FIG. 1 is a main block diagram showing an embodiment of a digital demodulator according to the present invention. Hereinafter, the configuration and operation of the present invention will be described in 16Q.
An embodiment of a digital demodulator which detects and demodulates an AM signal in a quasi-synchronous manner will be described with reference to FIG. In FIG. 1, IF is an intermediate frequency signal of a predetermined frequency (for example, 455 KHz), and is obtained by frequency-converting an RF signal received in a preceding stage (not shown) by a local oscillation signal (hereinafter, referred to as an IF signal). The A / D converter 1 converts the IF signal from an analog signal to a digital signal.
That is, the IF signal is sampled by a sampling clock (CK) having a predetermined frequency (Fck). In this sampling, the frequency Fck of the sampling clock (CK) is set as follows. Fck = intermediate frequency (IF) × (4 / m) (m: any odd number equal to or greater than 5) As can be seen from the above equation, the sampling in the A / D converter 1 is undersampling by the Fck sampling clock lower than the intermediate frequency. It is.

The digital signal output from the A / D converter 1 is input to a digital quadrature detector 2, where the A / D output is multiplied by "1" and "-1" to obtain a digital quadrature detector. I do. By the digital quadrature detection, the digital quadrature detection unit 2 outputs an in-phase component I signal and a quadrature component Q signal. The I signal and the Q signal from the digital quadrature detector 2 are input to an LPF (low-pass filter) 3 having a root Nyquist characteristic, and the input I signal and the Q signal are filtered while avoiding intersymbol interference (required). ). Although not shown, this LPF 3 is composed of an LPF for an I signal, an LPF for a Q signal, and a ROM.
Several types of data for setting filter characteristics are stored in the OM in advance, and required filter characteristics are set respectively. For this reason, a signal for zero-cross point detection on the I-axis is input to the ROM from a zero-cross point detection unit 11 described later, and the ROM sets the above characteristics based on the input signal. The zero-crossing point detection signal sent from the zero-crossing point detection unit 11 is a signal representing the time lag between the actual sample point and the ideal sample point. The points are moved equivalently, and proper baseband signal conversion is performed.

The output signal from the LPF 3 is input to an AFC (automatic frequency control) correction unit 4 where the carrier (that is, the carrier)
Correct the frequency shift of IF). The cause of the frequency deviation of the IF is, for example, a frequency fluctuation of a local oscillation signal. If the frequency deviation occurs, the symbol data rotates in phase, resulting in a phase error. At the time of the frequency correction, A
The phase rotation angle data indicating the phase rotation angle calculated by the FC calculation unit 16 is input to the AFC correction unit 4. AFC
The correction unit 4 stores a predetermined number of control data for frequency correction corresponding to the phase rotation angle data in a ROM in advance. Accordingly, the AFC correction unit 4 obtains required frequency correction control data for the input phase rotation angle data from the ROM, and corrects the frequency of each of the I signal and the Q signal using the control data. The output signal from the AFC correction unit 4 is input to a first phase error calculation unit 18.

In the 16QAM system, for example, one frame is formed by 16 symbols, and a pilot symbol is inserted first to synchronize. This pilot symbol is farthest from the origin on the 16QAM signal point arrangement diagram. Both the I signal and the Q signal are symbols at positive positions. This pilot symbol is a symbol that is inserted first in a signal that forms one frame with, for example, 16 symbols, and is a reference for detecting the phase error, the amplitude in the I-axis direction and the amplitude in the Q-axis direction in QAM, and the like. It is what becomes. In the case of a frame configuration in which one symbol is composed of 16 symbols, 15 symbols after the pilot symbol are symbols as information data, and each symbol is appropriately inserted according to information content. The first phase error calculator 18 calculates a phase error from the input I signal and Q signal based on the pilot symbol. Pilot symbol detection data, which is used as a reference for calculation, is detected by pilot symbol detection unit 12. The first phase error calculator 18 calculates a phase error from the detected pilot symbols and outputs the same as phase error data. In this embodiment, the phase error data is output every 16 symbols.

The phase error data from the first phase error calculator 18 is branched and input to an AFC filter 17 and a first phase corrector 19, which will be described later. One AFC
The filter 17 sorts out the phase error data according to the received signal level and the phase error data, and outputs the correct phase error data to the AFC calculator 16. AFC calculator 16
In the above, the input phase error data is sequentially accumulated and averaged, and based on the averaged phase error data, phase rotation angle data representing a phase rotation angle to be used for frequency correction in the next stage AFC correction unit 4 is calculated. calculate. The calculated phase rotation angle data is sent to the AFC correction unit 4 to correct the frequency. By calculating the phase rotation angle data using the phase error average data, it follows the input signal while absorbing the frequency fluctuation. Also, in the other first phase correction unit 19, the first phase error calculation unit 18
, And the calculated phase error data is input to the first phase correction unit 19, and the phase correction is performed on the detected I signal and Q signal.

The I phase corrected by the first phase corrector 19
The signal and the Q signal are input to the maximum vector level calculator 20. The maximum vector level calculator 20 calculates the I vector level and the Q vector level of the pilot symbol based on the input I signal and Q signal. The pilot symbol is a symbol having a positive maximum vector level on both the I axis and the Q axis. Therefore, each of the calculated I vector level and Q vector level means the maximum vector level. The timing of the calculation of the maximum vector level is the input time point of the detection data of the pilot symbol transmitted from the pilot symbol detection unit 12 described later. I calculated by the maximum vector level calculator 20
Vector level and Q vector level,
I signal and Q that have been phase corrected by the phase
The signal is input to the offset cancellation level calculation unit 21. Further, data representing the vector level of the minimum vector level is also input to the offset cancellation level calculation unit 21 from the minimum vector level calculation unit 15 described later. The offset canceling level calculator 21 extracts the I vector level and the Q vector level that are the minimum vector levels from the input I signal and the Q signal in accordance with the timing at which the data of the minimum vector level is input, and extracts the extracted minimum I level. , Q vector level data and the maximum vector level calculation unit 20
After calculating the levels of the DC offsets of the I and Q vectors (deviation of the DC components of the I axis and the Q axis) based on the maximum I vector level and the Q vector level calculated in
The level required to cancel the DC offset is calculated.

The data calculated by the offset cancellation level calculator 21 is sent to the I signal adder 5 and the Q signal adder 6, where the data is added to the I signal and the Q signal input from the AFC corrector 4. It is processed. This addition cancels the DC offset level of each of the input I signal and Q signal. The I signal and Q from these adders 5 and 6
Each signal is input to the second phase error calculator 8 and the second phase corrector 9. The second phase error calculator 8 and the second phase corrector 9 have the same functions as the first phase error calculator 18 and the first phase corrector 19 described above. The error calculator 8 calculates a phase error from each of the input I signal and the Q signal, and the second phase corrector 9 corrects the calculated phase error. Here, the difference between the system including the first phase error calculator 18 and the first phase corrector 19 and the system including the second phase error calculator 8 and the second phase corrector 9 will be described. The former is a DC system for correcting the DC component, and the latter is the A system after the DC component is removed.
This is an AC system for further correcting the C component. Each of the I signal and the Q signal whose phase has been corrected by the second phase corrector 9 is branched and input to various processing blocks as shown in FIG.
The timing shift detecting unit 10 detects a timing shift of the baseband signal (= symbol data) based on the I signal, that is, a shift in the cycle of the baseband signal.

Further, the zero-cross point detecting section 11 uses the data of the timing deviation detected by the timing deviation detecting section 10 and each of the phase-corrected I signal and Q signal to make a point (zero) on the I axis. By detecting the zero-cross point, a time lag between the actual sample point and the ideal sample point is detected. Further, pilot symbol detection section 12 detects a pilot symbol indicating the maximum level in the frame. The detected pilot symbols are used for counting the number of occurrences by a symbol counter described later.
In addition, the threshold level calculation unit 13 calculates a threshold level for determining data. This calculation is performed by performing an averaging operation using the pilot symbol I and Q vectors after the DC offset processing. The threshold level data thus obtained is stored in the area determination unit 1.
4 The area determination unit 14 sets another threshold level based on the threshold level data transmitted from the threshold level calculation unit 13, and uses these to determine the area for each symbol. The data is decoded by this area determination. Further, the minimum vector level calculation unit 15 calculates the minimum I, 16 out of 16 symbols based on the area determination data from the area determination unit 14.
The Q vector level is calculated and output as the minimum vector level. A level detecting unit for detecting a signal level of the received signal; and a level determining unit for determining whether the signal level of the received signal detected by the level detecting unit is larger than a predetermined level threshold. The level discrimination result is subjected to filtering of the phase error data according to the signal level output from the AFC filter unit 17, which will be described later in detail.

FIG. 2 is a block diagram of a main part for explaining the operation of the first embodiment of the AFC filter unit. AFC
The filter unit 17 includes a filter threshold ROM 24 that stores a predetermined angle threshold, for example, ± 3 °, and a phase error that determines whether a phase error exceeds a predetermined angle threshold or is within a predetermined angle threshold from input phase error data. A determination unit 25;
And a selector 26 for turning on / off and selecting the phase error data to the AFC calculation unit 16 based on the determination result. The phase error is determined by the phase error determination unit 24 by comparing the absolute value of the phase error with a predetermined phase angle, and if the absolute value of the phase error is within the range of the predetermined phase angle or not. It is determined whether the value exceeds the range of the angle threshold. The level determination unit 23 outputs 0 if the received signal level is smaller than a predetermined level threshold, and outputs 1 if the received signal level is equal to or larger than the predetermined level threshold,
The data is input to the selector 26. The selector 26 is, for example, as shown in FIG.
As shown in the figure, it is composed of an AND circuit and has a predetermined angle threshold,
For example, phase error data exceeding ± 3 ° is cut off and ± 3 °
It is configured to pass only phase error data within the range. As described above, data exceeding a predetermined angle threshold value from the average phase error data is removed as suspicious data. As a result, it is possible to remove a fluctuation due to a short-time phase shift due to noise, radio wave disturbance, or the like, and prevent unstable disturbance from affecting as a fluctuation of a frequency component. The phase error data from which the variation due to the short-time phase shift has been removed is AF
The AFC calculator 16 outputs the averaged phase error data input by the averaging unit 16a for a predetermined period of time and averages them. Is calculated.

In the example of FIG. 2, the selector 26 has three inputs, and the signal of the result of the determination of the received signal level from the level determination unit 23 is input. As a result, the level determination unit 23
When the selector 26 determines that the signal level of the received signal detected by the level detector 22 is smaller than a predetermined level threshold, the selector 26 blocks the phase error data calculated by the phase error calculator. The predetermined level threshold is set based on a reception sensitivity level for ensuring a predetermined bit error rate of data output, for example, a bit error rate of 1%. This is an example of removing the phase error data below the reception sensitivity level, especially when the reception level is low, since there is a high possibility that a phase shift will occur in a short time due to noise or radio wave disturbance. The AFC correction absorbs a relatively long-time frequency fluctuation, and the AFC calculation unit 16 in the next stage performs an averaging operation for a predetermined period over several frames. As described above, by performing filtering according to the reception level, more reliable phase error data can be secured.

FIG. 3 is a configuration diagram of a main part for explaining the operation of the AFC filter unit in the second embodiment. The difference from the first embodiment is that a predetermined angle threshold stored in the filter threshold ROM 24 is provided with two angle thresholds of ± 3 ° as a first angle threshold and ± 10 ° as a second angle threshold. , The selector 26 and the AND circuits 26a and 26b
This is an example in which a circuit 26e is combined. In the example of FIG. 3, when the level determination unit determines that the signal level of the received signal is lower than the reception sensitivity point, it passes the phase error data within a predetermined first angle threshold (for example, ± 3 °), and Is determined to be equal to or higher than the reception sensitivity point, phase error data within a predetermined second angle threshold (for example, ± 10 °) is passed. As described above, according to the signal level of the received signal, when the radio wave is weak, the angle threshold is reduced, and when the radio wave is strong, the angle threshold is increased. It is possible to maintain the degree and secure a wider range of phase error data.

FIG. 4 is a main part configuration diagram for explaining the operation of the third embodiment of the AFC filter section. The difference from the second embodiment is that a symbol counter 27 for counting pilot symbols is provided.
Is reflected in the selector 26. The symbol counter 27 counts pilot symbols transmitted for each predetermined frame detected by the pilot symbol detection unit 12, and is reset by a received input signal or a reset signal generated at the time of mode switching. The symbol counter 27 starts counting after resetting.
Is output for 10 or more times. This means that 0 is output for 9 frames after switching. The output of the symbol counter 27 is input to the AND circuit of the selector 26, and when the signal level of the received signal is determined to be equal to or higher than a predetermined level threshold (for example, the reception sensitivity point), the symbol counter 27 after reset is reset. Is smaller than the count value (10), the phase error data calculated by the phase error calculator passes through the selector 26, that is, the AFC filter 17. This is for performing AFC correction rapidly after resetting, that is, after switching the receiving channel, and allows all the phase error data when the radio wave condition is good to pass quickly.

When the level determining section 23 determines that the signal level of the received signal is smaller than a predetermined level threshold,
While the symbol counter 27 after the reset is smaller than the predetermined count value, only the phase error data within the predetermined second angle threshold calculated by the phase error calculation unit passes through the selector 26, that is, the AFC filter unit 26. I have. This is because, even when the radio wave condition is poor, the phase error data within the predetermined angle threshold is passed, and the AFC correction is performed as soon as possible after the reception channel is switched. An appropriate angle may be selected from the above. When the symbol counter 27 is reset, A
The contents of the average calculation unit 16a of the FC calculation unit 16 are also cleared. As a result, the old phase error data is cleared and the AFC is calculated based on the newly input phase error data, so that it is possible to quickly respond to the input signal.

As a method of detecting the reception level of the reception signal by the level detection unit 22, the signal level of the reception signal is detected by detecting the peak level of pilot symbols included at predetermined intervals in the reception signal. The pilot symbol appears only once in one frame, and its appearance frequency is low, but stable detection is possible and erroneous detection is small. As another method, the signal level of the received signal may be obtained by detecting the average value of the peak levels of the absolute values of the amplitudes of the I signal and the Q signal. Thereby, the frequency of detection can be increased. Further, the level detection unit 22 may detect the average value of the absolute values of the amplitudes of the I signal and the Q signal to obtain the signal level of the received signal. Thereby, the frequency of detection can be further increased.

The circuits below the digital quadrature detector 2 except for the A / D 1 are all constituted by the same digital signal processor including the AFC filter 17.
As a result, the circuit configuration is simplified, and it is possible to cope with various communication systems only by changing software, thereby suppressing an increase in cost.

[0031]

As described above, according to the present invention, a received multi-level QAM (quadrature amplitude modulation) signal is quadrature-detected by a quasi-synchronous system by digital processing, and an in-phase component I signal and a quadrature component Q signal are detected. , A low-pass filter that extracts required low-frequency components from the signal from the digital quadrature detector, and a reference phase based on the received signal from the I and Q signals output from the LPF A phase error calculator for calculating the phase error of each of the I signal and the Q signal, and an AFC for inputting the phase error data calculated by the phase error calculator and calculating a frequency variation based on the input phase error data. A calculation unit, and an AFC correction unit that automatically corrects the frequency based on the frequency fluctuation calculated by the AFC calculation unit. In the digital demodulation device for demodulating the I signal and the Q signal and outputting the data, a predetermined angle threshold value calculated by the phase error calculation unit and input to the AFC calculation unit is provided between the phase error calculation unit and the AFC calculation unit. An AFC filter unit that blocks the phase error data exceeding a predetermined angle threshold value and passes only the phase error data within a predetermined angle threshold value;
A level detector for detecting a signal level of the received signal from the I signal and the Q signal after the automatic correction of the received frequency by the AFC corrector, a signal level of the received signal detected by the same level detector, and a predetermined level threshold; And a level discriminator for discriminating the magnitude of the signal. If the level discriminator discriminates that the signal level of the received signal detected by the level detector is smaller than a predetermined level threshold, the AFC filter section calculates the phase error calculated by the phase error calculator. When the level error detector determines that the signal level of the received signal detected by the level detector is equal to or greater than a predetermined level threshold, the AFC filter unit detects the phase error data calculated by the phase error calculator. Since the signal is passed, it is possible to prevent the phase from being largely shifted even when the reception electric field level is lowered, and to increase the AFC correction when the reception electric field level becomes normal. It is possible to provide a digital demodulating apparatus adapted to return to go to.

[Brief description of the drawings]

FIG. 1 is a main block diagram showing an embodiment of a digital demodulation device according to the present invention.

FIG. 2 shows A in the digital demodulator according to the present invention.
FIG. 3 is a main part configuration diagram for explaining an operation of the first embodiment of the FC filter unit.

FIG. 3 shows a digital demodulator according to the present invention,
FIG. 9 is a main part configuration diagram for explaining an operation of a second embodiment of the FC filter unit.

FIG. 4 shows a digital demodulator according to the present invention,
FIG. 9 is a main part configuration diagram for explaining an operation of a third embodiment of the FC filter unit.

FIG. 5 is a main block diagram showing an example of the principle configuration of AFC correction of a conventional demodulation device.

[Explanation of symbols]

 Reference Signs List 1 A / D 2 Digital quadrature detector 3 LPF 4 AFC corrector 5, 6 Adder 8 Phase error calculator 9 Phase corrector 10 Timing shift detector 11 Zero cross point detector 12 Pilot symbol detector 13 Threshold level calculator 14 Area determination unit 15 Minimum vector level calculation unit 16 AFC calculation unit 16a Average calculation unit 17 AFC filter unit 18 Phase error calculation unit 19 Phase correction unit 20 Maximum vector level calculation unit 21 Offset cancellation level calculation unit 22 Level detection unit 23 Level determination unit 24 Filter threshold ROM 25 Phase error discriminator 26 Selector 27 Symbol counter

Claims (12)

[Claims] 1. A digital quadrature detection section for quadrature detecting a received multilevel QAM (quadrature amplitude modulation) signal by a quasi-synchronous method by digital processing and dividing it into an in-phase component I signal and a quadrature component Q signal. A low-pass filter (hereinafter, referred to as an LPF) for extracting a required low-frequency component from a signal from the digital quadrature detector; an I signal output from the LPF;
A phase error calculator for extracting a reference phase based on a received signal from a signal to calculate a phase error of each of the I signal and the Q signal, and the phase error data calculated by the phase error calculator being input, and An AFC calculator for calculating a frequency variation based on the phase error data; and an AFC corrector for automatically correcting the frequency based on the frequency variation calculated by the AFC calculator. A digital demodulator for performing correction, demodulating the corrected I signal and Q signal, and outputting the data, wherein the phase error calculator calculates the AFC between the phase error calculator and the AFC calculator. Input to the calculator,
Intercepting the phase error data exceeding a predetermined angle threshold,
A digital demodulation device, comprising: an AFC filter unit that passes only the phase error data within the predetermined angle threshold. 2. An AFC filter comprising: a phase error determination unit configured to determine whether the phase error calculated by the phase error calculation unit exceeds a predetermined angle threshold value or falls within a predetermined angle threshold value; According to the determination result of the means,
2. The digital demodulation device according to claim 1, further comprising: selector means for selecting whether to block or pass the phase error data input to the FC calculation unit. 3. A level detector for detecting a signal level of the received signal from the I signal and the Q signal after the automatic correction of the received frequency by the AFC corrector, and a signal of the received signal detected by the same level detector. Providing a level discriminating unit for discriminating a level and a predetermined level threshold, when the level discriminating unit determines that the signal level of the received signal detected by the level detecting unit is smaller than the predetermined level threshold, The AFC filter unit cuts off the phase error data calculated by the phase error calculation unit, and the level determination unit determines the signal level of the reception signal detected by the level detection unit to be equal to or higher than the predetermined level threshold. The case according to claim 1, wherein the AFC filter unit passes the phase error data calculated by the phase error calculation unit. 4. Ijitaru demodulator. 4. The apparatus according to claim 1, wherein the predetermined angle threshold comprises a predetermined first angle threshold and a second angle threshold wider than the first angle threshold. Whether the calculated phase error exceeds the respective angle threshold of the predetermined first angle threshold and the second angle threshold, or it is possible to determine whether it is within each angle threshold, the selector means, The AF result is determined based on the determination result of the phase error determination means and the determination result of the level
A case where the phase error data output to the C calculating unit can be selected to be passed or cut off, and the level determining unit determines that the signal level of the received signal detected by the level detecting unit is smaller than the predetermined level threshold The AFC filter unit passes the phase error data within the predetermined first angle threshold calculated by the phase error calculation unit, and the signal of the reception signal detected by the level detection unit by the level determination unit. When the level is determined to be equal to or higher than the predetermined level threshold, the AFC filter unit passes the phase error data within the predetermined second angle threshold calculated by the phase error calculation unit. The digital demodulation device according to claim 3, wherein 5. A pilot symbol detector for detecting a pilot symbol transmitted every predetermined frame, a symbol counter reset by a received input signal or a reset signal generated at the time of mode switching, and counting the pilot symbol. When the signal level of the received signal detected by the level detector is determined to be equal to or higher than the predetermined level threshold, the phase determination is performed while the symbol counter after reset is smaller than a predetermined count value. 5. The digital demodulation device according to claim 3, wherein all of the phase error data calculated by the error calculator passes through the AFC filter. 6. When the level discriminator judges that the signal level of the received signal detected by the level detector is smaller than the predetermined level threshold, the symbol counter after reset by the reset signal counts a predetermined count. While smaller than the value, only the phase error data within the predetermined second angle threshold calculated by the phase error calculation unit,
The digital demodulation device according to claim 5, wherein the digital demodulation device passes through the AFC filter unit. 7. The AFC calculation unit includes an averaging means for averaging the phase error data calculated and input from the phase error calculation unit for a predetermined period. The digital demodulation device according to claim 1, wherein 8. The digital demodulation device according to claim 7, wherein the content of the averaging means is cleared by the reset signal. 9. The digital demodulation according to claim 1, wherein the predetermined level threshold is set based on a reception sensitivity level for ensuring a predetermined bit error rate for the data output. apparatus. 10. The digital demodulator according to claim 1, wherein the level detector detects a peak level of a pilot symbol transmitted for each predetermined frame of the received signal. 11. The level detector, wherein the I signal and the Q signal
10. The digital demodulator according to claim 1, wherein an average value of a peak level of an absolute value of each amplitude of the signal is detected. 12. The level detecting section according to claim 1, wherein said I signal and
10. The digital demodulator according to claim 1, wherein an average value of absolute values of respective amplitudes of the signal is detected.