Abstract:
An FSK demodulator and a method for detecting an inflection point extract a greater amount of effective inflection points of a frequency detection signal while reducing erroneous detection of the inflection points. The inflection point detector includes an inflection point extraction part to extract the inflection point corresponding to variation of a sample value of an amplitude value of the frequency detection signal, an amplitude determination part to determine if a size between peak values of sample values in front and rear of the inflection point exists in a first predetermined range, a preamble determination part to determine if a difference between initial and final sample values of at least one of a symbol having the extracted inflection point and a right before symbol exists in a second predetermined range, and an AND operation part to determine a normal inflection point.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to an FSK (Frequency Shift Keying) receiver. More particularly, the present invention relates to an FSK demodulator of an FSK receiver. 
         [0003]    2. Description of the Related Art 
         [0004]    In general, as shown in  FIG. 1 , an FSK demodulator constituting a FSK receiver according to the related art includes a frequency detector  11 , a frequency offset cancellation circuit  12 , and a symbol timing regenerator  13 . The frequency detector  11  converts frequency shift information of an FSK-modulated wave of a received IF signal into an amplitude value to generate a frequency detection signal. The frequency offset cancellation circuit  12  cancels frequency offset components, which are generated due to frequency errors between local oscillators of a transmitter and a receiver, from the frequency detection signal. The symbol timing regenerator  13  generates the optimal symbol timing based on a detection signal and performs data decision based on a detection signal obtained after the frequency offset components have been cancelled from the frequency detection signal. 
         [0005]    According to one scheme to realize the frequency offset cancellation circuit  12 , frequency offset components are calculated by extracting points (inflection points), at which the second derivative of a frequency detection waveform is zero, from the frequency detection waveform, and averaging the points (see Patent Literature 1). 
         [0006]    When the frequency offset cancellation circuit  12  employs the above scheme of extracting the inflection points, the frequency offset cancellation circuit  12  may include, for example, an inflection point detector  21 , an averaging circuit  22 , and a subtraction circuit  23  as shown in  FIG. 2 . The inflection point detector  21  receives a frequency detection signal S 0  which is an output signal of the frequency detector  11  to generate inflection point timings of the frequency detection signal S 0 . The averaging circuit  22  averages amplitude values of the inflection point timings which are output from the inflection point detector  21 . The subtraction circuit  23  subtracts the average amplitude information (frequency offset signal) of the inflection point timings, which is an output signal of the averaging circuit  22 , from the output signal of the frequency detector  11  to generate a frequency detection signal after the frequency offset components are canceled. 
         [0007]    For example, the inflection point detector  21  has a structure shown in  FIG. 3 . The inflection point detector  21  of  FIG. 3  has a circuit structure of detecting inflection points at an operating clock rate which is  16  times greater than a symbol rate. The inflection point detector  21  includes a 16-stage shift register  31  to store sample values corresponding to amplitude values of the frequency detection signal S 0  for one symbol after sampling the frequency detection signal S 0  according to the operating clock, a subtracter C 1  to subtract a first output of the 16-stage shift register  31  from an eighth output of the 16-stage shift register  31 , a subtracter C 2  to subtract a ninth output of the 16-stage shift register  31  from a 16th output of the 16-stage shift register  31 , a subtracter C 3  to perform subtraction with respect to outputs of the subtracters C 1  and C 2 , a subtracter C 4  to subtract the first output of the 16-stage shift register  31  from the 16th output of the 16-stage shift register  31 , an absolute value circuit C 5  to calculate an absolute value of an output of the subtracter C 3 , an absolute value circuit C 6  to calculate an absolute value of an output of the subtracter C 4 , a comparator C 7  to compare an output value of the absolute value circuit C 5  with threshold values A and B in size, a comparator C 8  to compare an output value of the absolute value circuit C 6  with a threshold value C in size, an AND circuit C 9  to perform an AND operation with respect to outputs of the comparators C 7  and C 8 , an edge detector C 10  to detect the rising edge of an output of the AND circuit C 9 , and a pre-frequency offset generator C 11  to extract a frequency detection value at the inflection point timing from an inflection point timing signal, which is an output of the edge detector C 10 , and the output signal of the frequency detector  11 . In addition, the subtracters C 1  to C 3 , the absolute value circuit C 6 , and the comparator C 8  constitute an inflection point extraction circuit  32 , and the subtracter C 4 , the absolute value circuit C 5 , and the comparator C 7  constitute an amplitude monitoring circuit  33 . 
         [0008]    In the inflection point detector  21  having the above structure, the levels of the input frequency detection signal S 0  are shifted from a first shift register to a 16th shift register one by one in synchronization with the operating clock of the 16-shift register  31  while the levels of the input frequency detection signal S 0  are being retained in the 16-stage shift register  31 . In this case, the first output to the 16th output of the 16-stage shift register  31  for the frequency detection signal S 0  having the waveform of  FIG. 4  have signal levels as shown in  FIG. 4 . In the inflection point extraction circuit  32 , an operation result S 1  of the subtracter C 1  and an operation result S 2  of the subtracter C 2  are obtained as “b−a” and “d−c”, and the gradient of the frequency detection signal S 0  at the duration corresponding to  8  operating clock pulses is calculated at each operating clock. In addition, the difference of the differential values (i.e., S 2 −S 1 =(d−c)−(b−a)) is made by the subtracter C 3 , and the absolute value (|(d−c)−(b−a)|) of the difference is calculated by the absolute value circuit C 6 . Since the difference of two differential values S 2  and S 1  correspond to the value of a second derivative, a point having a value less than or equal to the threshold C is regarded as an inflection point. Therefore, the inflection point can be obtained from the comparator C 8 . 
         [0009]    In addition, in order to prevent inflection points from being erroneously detected due to noise, the amplitude monitoring circuit  33  is provided. The amplitude monitoring circuit  33  regards an amplitude S 3  of the frequency detection signal S 0  of the received IF signal as noise if the amplitude S 3  (value between peaks) of the frequency detection signal S 0  is greater than or equal to the threshold value A, or less than or equal to the threshold value B. An output representing the presence of noise is obtained from the comparator C 7 . 
         [0010]    The AND circuit C 9  negates an inflection point if the inflection point is detected by the inflection point extraction circuit  32  at the timing in which the amplitude S 3  of the frequency detection signal S 0  is regarded as noise due to the condition of S 3 ≧A or S 3 ≦CB. Meanwhile, the AND circuit C 9  outputs an inflection point if the inflection point is detected by the inflection point extraction circuit  32  in the state that a condition of B&lt;S 3 &lt;A is satisfied. 
         [0011]    An inflection point timing signal S 4  is obtained by detecting the rising edge of the output of the AND circuit C 9  in the edge detector C 10 . In addition, the pre-frequency offset generator C 11  extracts the center value of the frequency detection signal S 0  from the inflection point timing signal S 4  and generates a pre-frequency offset signal by using the center value. A final frequency offset signal representing offset components is calculated by performing an averaging operation with respect to the pre-frequency offset signal by the averaging circuit  22  provided at a next stage. 
         [0012]    Patent Literature 1: Japanese Patent Kokai No. 2006-325127 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0013]    However, in the FSK demodulator of the FSK receiver according to the related art, inflection points may be erroneously detected when noise is received or under a low C/N environment. In other words, the effectiveness of an inflection point is determined by determining if the size S 3  between peak values d and a of the frequency detection signal S 0  is in the predetermined range (range formed by thresholds A and B). Accordingly, if the allowable range of the size S 3  is widened in order to detect effective inflection points with noise, a number of inflection points caused by noise are extracted in proportion to the enlarged degree of the range, so that the erroneous detection of inflection points may occur. Therefore, frequency detection values at inflection point timings that have been erroneously detected, so that frequency offset values out of the expected value are calculated. Since the change of a frequency offset value from the expected frequency offset value exerts serious influence upon a receive characteristic (at least receive sensitivity), it is necessary to reduce the erroneous detection, such as an error regarding noise as inflection points. 
         [0014]    Accordingly, the present invention has been made in view of the above problems occurring in the related art, and an object of the present invention is to provide an FSK demodulator and a method for detecting an inflection point, capable of extracting a greater amount of effective inflection points of the frequency detection signal while reducing the erroneous detection of the inflection point caused by the noise. 
       Solution to the Problem 
       [0015]    In order to accomplish the object of the present invention, according to an aspect of the present invention, there is provided an FSK demodulator including a frequency detector to generate a frequency detection signal representing an amplitude value according to frequency shift of a received FSK modulation wave, a frequency offset cancellation part to cancel a frequency offset component from the frequency detection signal according to the frequency detection signal provided at a detection time point of an inflection point after detecting the inflection point of the frequency detection signal by an inflection point detector, and a data demodulation part to acquire demodulation data according to a frequency detection signal from which the frequency offset component is cancelled by the frequency offset cancellation part. The inflection point detector includes an inflection point extraction part to extract the inflection point according to variation of a sample value after sampling an amplitude value of the frequency detection signal at each predetermined operating clock, an amplitude determination part to determine if a size between peak values of sample values of the frequency detection signal, which are provided in front and rear of the inflection point extracted from the inflection point extraction part, is in a first predetermined range, a preamble determination part to determine if a difference between initial and final sample values of at least one of a symbol having the extracted inflection point and a symbol right before the symbol having the extracted inflection point is in a second predetermined range, and an AND gate part to determine the inflection point, which is extracted from the inflection point extraction part, as a normal inflection point if the amplitude determination part determines that the size between the peak values is in the first predetermined range, and if the preamble determination part determines that the difference between the initial and final sample values is in the second predetermined range. 
         [0016]    According to another aspect of the present invention, there is provided a method for detecting an inflection point in an FSK demodulator including a frequency detector to generate a frequency detection signal representing an amplitude value according to frequency shift of a received FSK modulation wave, a frequency offset cancellation part to cancel a frequency offset component from the frequency detection signal according to the frequency detection signal provided at a detection time point of an inflection point after detecting the inflection point of the frequency detection signal, and a data demodulation part to acquire demodulation data according to a frequency detection signal from which the frequency offset component is cancelled by the frequency offset cancellation part. The method includes an inflection point extraction step to extract the inflection point according to variation of a sample value after sampling an amplitude value of the frequency detection signal at each predetermined operating clock, an amplitude determination step to determine if a size between peak values of sample values of the frequency detection signal, which are provided in front and rear of the inflection point extracted from the inflection point extraction step, is in a first predetermined range, a preamble determination step to determine if a difference between initial and final sample values of at least one of a symbol having the extracted inflection point and a symbol right before the symbol having the extracted inflection point is in a second predetermined range, and an AND operation step to determine the inflection point, which is extracted in the inflection point extraction step, as a normal inflection point if existence of the size between the peak values in the first predetermined range is determined in the amplitude determination step, and if existence of the difference between the initial and final sample values in the second predetermined range is determined in the preamble determination step. 
       Advantageous Effects 
       [0017]    As described above, according to the FSK demodulator and the method for detecting the inflection point, it is determined that the difference between initial and final sample values of at least one of both a symbol having an extracted inflection point and a symbol right before the symbol having the inflection point is in the second predetermined range, thereby determining a preamble pattern. Accordingly, even if the allowable range of the first predetermined range used to extract the inflection point of the frequency detection signal is widened, the erroneous detection of the inflection point caused by noise frequency can be blocked. Therefore, a greater amount of inflection points of the frequency detection signal can be extracted while reducing the extraction of the inflection points caused by pure noise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram schematically showing the structure of a conventional FSK demodulator; 
           [0019]      FIG. 2  is a block diagram showing the structure of a frequency offset cancellation circuit provided in a circuit of  FIG. 1 ; 
           [0020]      FIG. 3  is a block diagram showing the structure of an inflection point detector provided in the frequency offset cancellation circuit of  FIG. 2 ; 
           [0021]      FIG. 4  is a view showing the relation between the frequency detection signal and inflection points; 
           [0022]      FIG. 5  is a block diagram schematically showing the structure of an inflection point detector according to a first embodiment of the present invention; 
           [0023]      FIG. 6  is a view showing the relation between a frequency detection signal and inflection points in the inflection point detector of  FIG. 5 ; 
           [0024]      FIG. 7  is a block diagram schematically showing the structure of an inflection point detector according to a second embodiment of the present invention; 
           [0025]      FIG. 8  is a block diagram showing the structure of a consecutive inflection point generation detector provided in the inflection point detector of  FIG. 7 ; and 
           [0026]      FIG. 9  is a timing chart showing the operation of the consecutive inflection point generation detector of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    Hereinafter, embodiments of the present invention will be described in detail with reference to accompanying drawings. 
         [0028]      FIG. 5  is a block diagram showing the structure of an inflector point detector applied to an FSK demodulator according to a first embodiment of the present invention. The inflection point detector includes a 32-stage shift register  51 , an inflection point extraction circuit  52 , an amplitude monitoring circuit  53 , preamble detectors  54  and  55 , an AND gate  56 , an edge detector C 10 , a pre-frequency offset generator C 11 , and a delay circuit C 16 . 
         [0029]    The 32-stage shift register  51  retains sample values corresponding to amplitude values of a frequency detection signal S 0  by two symbols after sampling the frequency detection signal S 0  at an operating clock rate that is  16  times greater than a symbol rate. In addition, as shown in  FIG. 5 , the 32-stage shift register  51  has retained-outputs of a first shift register or a 32th shift register. The 32-stage shift register  51  outputs sample values of the frequency detection signal S 0 , which are sequentially input from the first shift register, while retaining the sample values. In addition, the sampling rate is not limited to 16 times greater than the symbol rate. For example, the sampling rate may be 32 times greater than the symbol rate. If the sampling rate is 32 times greater than the symbol rate, the shift register  51  must have 64 stages. The inflection point extraction circuit  52  corresponds to an inflation point extraction part, and includes subtracters C 1  to C 3 , an absolute value circuit C 6 , and a comparator C 8  similarly to the structure of the inflection point extraction circuit  32  shown in  FIG. 3 . In this case, the subtracter C 1  subtracts a ninth output of the 32-stage shift register  51  from a 16th output of the 32-stage shift register  51 , and the subtracter C 2  subtracts a  17 th output of the 32-stage shift register  51  from a  24 th output of the 32-stage shift register  51 . 
         [0030]    The amplitude monitoring circuit  53  corresponds to an amplitude determination part, and includes a subtracter C 4 , an absolute value circuit C 5 , and a comparator C 7  similarly to the structure of the amplitude monitoring circuit  33  shown in  FIG. 3 . The subtracter C 4  subtracts a first output of the 32-stage shift register  51  from the 16th output of the 32-stage shift register  51 . 
         [0031]    The preamble detectors  54  and  55  correspond to a preamble determination part. The preamble detector  54  includes a subtracter C 12 , an absolute value circuit C 13 , and a comparator C 14 . The subtracter C 12  subtracts the  17 th output of the 32-stage shift register  51  from a 32th output of the 32-stage shift register  51 . The abstract value circuit C 13  calculates the absolute value of an output of the subtracter C 12 . The comparator C 14  compares an output of the absolute value C 13  with a threshold D in size. The preamble detector  55  includes a comparator C 15 . The comparator C 15  compares an output of the absolute value circuit C 5  provided in the amplitude monitoring circuit  53  with a threshold value D in size. The delay circuit C 16  delays an output of the comparator C 7  provided in the amplitude monitoring circuit  53 . 
         [0032]    The AND gate  56  constitutes an AND gate part together with the delay circuit C 16 , and performs an AND operation with respect to outputs of the comparators C 8 , C 14 , and C 15  and an output of the delay circuit C 16 . 
         [0033]    The edge detector C 10  and the pre-frequency offset generator C 11  are the same as those of the inflection point detector shown in  FIG. 3 . 
         [0034]    In the inflection point detector having the above structure, the levels of the input frequency detection signal S 0  are shifted from the first shift register to the 32th shift register one by one while the levels of the input frequency detection signal S 0  are being retained in the 32-stage shift register  51  in synchronization with the operating clock. In this case, the first output to the 32th output of the 32-stage shift register  51  for the frequency detection signal S 0  having the waveform of  FIG. 6  have signal levels as shown in  FIG. 6 . 
         [0035]    In the inflection point extraction circuit  52 , the operation results S 1  and S 2  of the subtracters C 1  and C 2  are obtained as “b−a” and “d−c”, respectively, and the gradient of the frequency detection signal S 0  at the duration corresponding to 8 operating clock pulses is calculated at each operating clock. In addition, the difference of the differential values (i.e., S 2 −S 1 =(d−c)−(b−a)) is made by the subtracter C 3 , and the absolute value (|(d−c)−(b−a|) of the difference is calculated by the absolute value circuit C 6 . Since the difference of two differential values S 2  and S 1  correspond to the value of a second derivative, a point having a value less than or equal to the threshold value C is regarded as an inflection point. Therefore, the inflection point can be obtained as a high (H) level value in the output of the comparator C 8 . 
         [0036]    In a state that the operation result (S 1 =b−a) of the subtracter C 1  and the operation result (S 2 =d−c) of the subtracter C 2  are obtained in the inflection point extraction circuit  52 , an operation result (b−e) is obtained from the subtracter C 4  of the amplitude monitoring circuit  53 . The output of the amplitude monitoring circuit  53  is delayed by 8 clock pulses through the timing adjustment of the delay circuit C 16  and supplied to the AND gate  56 . Therefore, since the amplitude monitoring circuit  53  at a time point earlier by 8 clock pulses calculates an operation result (a−d) through the subtracter C 4 , an amplitude S 3  (value between peaks) of the frequency detection signal S 0  of the received IF signal is obtained from the absolute value circuit C 5 . If the detected amplitude S 3  is greater than or equal to the threshold value A or less than or equal to the threshold value B, the comparator C 7  outputs a high (H) level value representing the presence of noise. 
         [0037]    In addition, the output value (S 3 =|b−e|) of the absolute value circuit C 5  is compared with the threshold value D in size by the comparator C 15  of the preamble detector  55 . If the output value (|b−e|) of the absolute value circuit C 5  is less than or equal to the threshold value D, the comparator C 15  generates an H level output. 
         [0038]    In the preamble detector  54 , the operation result (f−c) is calculated by the comparator C 12 . If an absolute value (|f−c|) output from the absolute value circuit C 13  is less than or equal to the threshold value D, the comparator C 14  generates an H level output. 
         [0039]    The logical product for the output of the delay circuit  16  and the outputs of the comparators C 18 , C 14 , and C 15  is found by the AND gate  56 . A rising edge is detected from the edge detector C 10  based on the output logical product. The rising edge becomes an inflection point timing signal S 4 , and is input to the pre-frequency offset generator C 11 . The pre-frequency offset generator C 11  extracts frequency detection values serving as inflection points from the inflection point timing signal S 4  and the frequency detection signal S 0  and outputs the frequency detection values as a pre-frequency offset signal to the averaging circuit  22  placed at a next stage. The pre-frequency offset signal is averaged in the averaging circuit  22  to serve as a final frequency offset signal. 
         [0040]    As described above, according to the first embodiment, an operation equivalent to an operation of monitoring a pattern of “1010” or “0101” is performed by applying conditions of “|b−e|≦threshold value D” and “|f−c|≦threshold value D” to the inflection point detector (see  FIG. 3 ) according to the related art. This means that inflection points are monitored while monitoring the preamble pattern. Therefore, erroneous detection can be reduced, and the stability of frequency offset values can be improved by using an inflection point detector specialized for the preamble pattern. In addition, the preamble pattern can be monitored by constructing only the inflection point detector without using demodulation data and demodulation clock obtained from the symbol timing regenerator  13  of  FIG. 1 . 
         [0041]    In addition, according to the first embodiment, since the satisfaction of conditions of “|b−e|≦threshold value D” and “|f−c|≦threshold value D” is detected, the inversion from a logic 1 value to a logic 0 value, or the inversion from a logic 0 value to a logic 1 value can be exactly determined in each of two consecutive symbols. Therefore, the preamble pattern can be exactly detected. 
         [0042]    In addition, according to the present invention, only one of the two conditions of “|b−e|≦threshold value D” and “|f−c|≦threshold value D” may be satisfied, and the inversion of logical values can be detected before and after one symbol even if only one condition is satisfied. For example, when comparing with the inflection point detector of  FIG. 3  according to the related art, a comparator to compare the output signal S 3  of the absolute value circuit C 5  with the threshold value D may be further installed, and an output signal of the comparator may be delayed in the delay circuit by 8 clock pulses, so that the output signal of the comparator may be supplied to the AND circuit C 9  together with the outputs of the comparators C 7  and C 8 . 
         [0043]      FIG. 7  is a block diagram showing the structure of an inflection point detector applied to an FSK demodulator according to a second embodiment of the present invention. The inflection point detector includes the 32-stage shift register  51 , the inflection point extraction circuit  52 , the amplitude monitoring circuit  53 , the preamble detectors  54  and  55 , the AND gate  56 , the edge detector C 10 , the pre-frequency offset generator C 11 , and the delay circuit C 16  similarly to the structure of the inflection point detector of  FIG. 5 , and further includes a consecutive inflection point generation detector C 17 . 
         [0044]    The consecutive inflection point generation detector C 17  is interposed between the edge detector C 10  and the pre-frequency offset generator C 11 . 
         [0045]    As shown in  FIG. 8 , the consecutive inflection point generation detector C 17  includes an inflection point detection window generator C 18 , an AND gate C 19 , a detected inflection point retaining circuit C 20 , and an AND circuit C 21 . 
         [0046]    The inflection point detection window generator C 18  receives the inflection point timing signal S 4  from the edge detector C 10  and generates an inflection point detection window at each symbol rate interval. The AND gate C 19  performs an AND operation with respect to the inflection point timing signal S 4  and a detection window signal S 6 , which is an output of the inflection point detection window generator C 18 , to generate an inflection point timing signal S 7  after the pass of inflection point detection window. 
         [0047]    The detected inflection point retaining circuit C 20  receives the inflection point timing signal S 7  after the pass of inflection point detection window which is an output of the AND gate C 19 , and the detection window signal S 6  of the inflection point detection window generator C 18  and retains the result about the detection state of the inflection point timing signal S 4  for the H level duration of a prior detection window signal (detection window signal before one symbol). The AND circuit C 21  performs an AND operation with respect to the output signal S 7  of the AND gate C 19  and a prior inflection point detection result retaining signal S 8  which is an output of the detected inflection point retaining circuit C 20  to generate an inflection point timing signal S 5  after the detection of twice consecutive inflection point occurrences. 
         [0048]    Other components of the second embodiment are the same as those of  FIG. 5  according to the first embodiment. Accordingly, the operation of the inflection point detector until the output of the edge detector C 10  and the operation of the inflection point detector after the pre-frequency offset generator are the same as the operations of the first embodiment. 
         [0049]    Thereafter, the operation of the consecutive inflection point generation detector C 17  will be described with reference to the timing chart shown in  FIG. 9 . 
         [0050]    The inflection point detection window generator C 18  generates the detection window signal S 6  from the inflection point timing signal S 4  which is an output of the edge detector C 10 . In this case, the detection window signal S 6  has an H level with a predetermined period which corresponds to a symbol rate interval. The detection window signal S 6  is regulated corresponding to the inflection point timing signal S 4  in such a manner that the timing of the inflection point timing signal S 4  at the H level occurs at the center of the H-level duration of the detection window signal S 6 . The inflection point timing signal S 7  after the pass of inflection point detection window, which is output from the AND gate C 19 , is a signal obtained by AND-gating the inflection point timing signal S 4  by the detection window signal S 6 . 
         [0051]    At a falling edge timing of the detection window signal S 6 , the detected inflection point retaining circuit C 20  determines if the inflection point timing signal S 7  after the pass of inflection point detection window has been at the H level for the H-level duration of the detection window signal S 6  right before the falling edge timing. If the inflection point timing signal S 7  after the pass of inflection point detection window has been at the H level, the detected inflection point retaining circuit C 20  outputs an H level signal. If the inflection point timing signal S 7  after the pass of inflection point detection window has been at a low (L) level, the detected inflection point retaining circuit C 20  outputs an L level signal. An output signal according to the determination result is supplied as the prior inflection point detection result retaining signal S 8  to the AND circuit C 21 . 
         [0052]    The AND circuit C 21  generates the inflection point timing signal S 5  after the detection of twice consecutive inflection point occurrences by gating the inflection point timing signal S 7  after the pass of inflection point detection window by the prior inflection point detection result retaining signal S 8 . The inflection point timing signal S 5  after the detection of twice consecutive inflection point occurrences is supplied to the pre-frequency offset generator C 11  while serving as the consecutive inflection point generation detector C 17 . 
         [0053]    The pre-frequency offset generator C 11  extracts frequency detection values corresponding to inflation points from the inflection point timing signal S 5  after twice consecutive inflection point occurrences detection and the frequency detection signal S 0 , and outputs the frequency detection values serving as the pre-frequency offset signal to the averaging circuit  22  provided at the next stage. 
         [0054]    As described above, according to the second embodiment, since an inflection point timing signal is supplied to a pre-frequency offset generator only if an inflection point is detected in each of two consecutive symbols at a symbol rate, the possibility of detecting the inflection point in the middle of receiving an expected signal is high. In addition, as compared with the first embodiment, the erroneous detection of the inflection point caused by noise can be more reduced, the stability of the frequency offset value can be improved, and the receive characteristic can be improved. 
         [0055]    Although the first and second embodiments have been described in that inflection points are detected by using the 32-stage shift register  51  to store frequency detection signals corresponding to two symbols, the length of a preamble pattern allowing pattern monitoring can be lengthened if the number of the stages of the shift register is increased. Accordingly, the erroneous detection of the inflection points can be reduced. 
         [0056]    In addition, although the second embodiment has been described in that the consecutive inflection point generation detector consecutively detects inflection points twice, if the consecutive inflection point generation detector consecutively detects inflection points three times, erroneous detection can be more reduced. 
         [0057]    In addition, although the hardware configuration of the inflection point detector is realized according to the above embodiments, the inflection points may be detected by performing an inflection point extracting step, an amplitude determining step, a preamble determining step, and an AND gating step through computer processing. 
         [0058]    This application is based on Japanese Patent Application No. 2011-033875 which is incorporated herein by reference.