Abstract:
The present invention provides an OFDM receiver having a level control section comprising comparators which respectively compare a first signal outputted from an ADC with threshold values, counters which respectively count the frequencies with which the level of the first signal exceeds the threshold values, based on second and third signals corresponding to the results of comparison by the comparators, a moving average unit which calculates an average value of the level of the first signal lying in a predetermined period, based on fourth and fifth signals corresponding to the frequencies counted by the these counters, and a DAC which generates a gain control signal for controlling an AMP in such a manner that the average level of the first signal outputted from the ADC becomes a predetermined value, according to a sixth signal calculated by the moving average unit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an OFDM (orthogonal Frequency Division Multiplex) receiver such as a digital terrestrial broadcasing receiver or the like, and particularly to level control on a received signal converted into an intermediate frequency by a tuner. 
         [0002]      FIG. 2  is a block diagram showing the outline of a conventional digital terrestrial broadcasting receiver. 
         [0003]    The digital terrestrial broadcasting receiver has an antenna  1  which receives an OFDM-modulated wireless or radio signal RF lying in an UHF (Ultra High Frequency) band, and a tuner  3  which frequency-converts the radio signal RF received by the antenna  1  in accordance with a local oscillation signal LO of a local oscillator  2  to thereby generate an intermediate frequency signal IF for a desired reception channel. The intermediate frequency signal IF is amplified in such a manner that average power becomes a constant value by a variable gain amplifier (hereinafter called “AMP”)  4  whose amplification is controlled by a gain control signal AGC, after which it is supplied to an analog-digital converter (hereinafter called “ADC”)  5 . A fast Fourier transformer (hereinafter called “FFT”)  6  and a power calculator  7  are connected to the output side of the ADC  5 . 
         [0004]    The FFT  6  converts a time-domain signal converted to a digital value by the ADC  5  to a frequency-domain signal corresponding to a plurality of carriers constituting OFDM. Although not shown in the drawing, an equalizing unit which generates receive data in sync between the plurality of carriers, an error correction unit, a video/audio reproducing unit, etc. are connected to the output side of the FFT  6 . 
         [0005]    On the other hand, the power calculator  7  calculates a value corresponding to the average power of the output signal of the AMP  4 , based on the time-domain signal converted to the digital value by the ADC  5 . The output signal of the power calculator  7  is supplied to a digital-analog converter (hereinafter called “DAC”)  8 , where it is converted into an analog signal, followed by being supplied to the AMP  4  as a gain control signal AGC. 
         [0006]    In the digital terrestrial broadcasting receiver, a wireless or radio signal RF lying in a range of 450 MHz to 700 MHz, which has been received by the antenna  1 , is converted into an intermediate frequency signal IF whose band is about 450 kHz with its center frequency as about 500 kHz by the tuner  3 . For example, 108 carriers are multiplexed into the intermediate frequency signal IF. Each modulated wave has been quadrature-modulated based on data constituting broadcast contents and a control signal. The intermediate frequency signal IF outputted from the tuner  3  is amplified to a predetermined level by the AMP  4 , followed by being supplied to the ADC  5 , where it is converted into a digital value in accordance with a sampling clock of 2 MHz, for example. The received signal converted into digital form by the ADC  5  is supplied to the FFT  6 , where it is separated into signals set every carrier, thereby generating receive data, after which they are reproduced as images and sound. 
         [0007]    Further, the received signal converted into the digital value by the ADC  5  is supplied to the power calculator  7 , where average power set for a predetermined period (e.g., 1 symbol period corresponding to modulation unit of carrier=about 1 ms) is calculated. The value of the average power calculated by the power calculator  7  is converted into an analog gain control signal ACC by the DAC  8 , followed by being supplied to the AMP  4 . In the AMP  4 , its amplification is reduced when the gain control signal AGC becomes larger, whereas when the gain control signal AGC becomes smaller, its amplification increases. Thus, the average power of the output signal of the AMP  4  converges to a predetermined value. 
         [0008]    Accordingly, the received signal suitably converted into the digital value is obtained from the ADC  5  by setting the average power outputted from the AMP  4  so as to reach the optimum input level of the ADC  5 . 
         [0009]    The above related art refers to a patent document 1 (Japanese Unexamined Patent Publication No. 2004-153811). 
         [0010]    However, the following problems arise upon calculation of the average power by the power calculator  7 . 
         [0011]    That is, there is a need to calculate instantaneous power and calculate its average value upon calculation of average power. Since the instantaneous power is proportional to the square of the voltage, it is necessary to perform a square calculation of the digital value outputted from the ADC  5  in accordance with a sampling clock. Since the range of the digital value outputted from the ADC  5  is of a relatively wide-ranging value, the number of digits for the squared result becomes large. Further, there is a need to accumulatively add the values of respective instantaneous power for one symbol period (about 1 ms) for the purpose of determining the average power. Therefore, the problems arose in that there was a need to perform a high-capacity calculation at high speed, and the scale of the power calculator  7  would increase. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention has been made in view of the foregoing. It is therefore an object of the present invention to provide an OFDM receiver having a level control function, which is capable of holding an intermediate frequency signal supplied to an ADC at a predetermined level. 
         [0013]    According to one aspect of the present invention, for attaining the above object, there is provided an OFDM receiver which receives an OFDM-modulated wireless signal and converts the same into an intermediate frequency signal and which amplifies the intermediate frequency signal by an AMP and thereafter processes the same so as to be converted into a digital signal by an ADC, comprising a level control section including: 
         [0014]    comparators which respectively compare a signal level outputted from the ADC with threshold values; 
         [0015]    counters which respectively count the frequencies with which the signal level exceeds the threshold values, based on the results of comparison by the comparators; 
         [0016]    a moving average unit which calculates an average value of the signal level lying within a predetermined period on the basis of the frequencies counted by the counters; and 
         [0017]    a DAC which generates a gain control signal for controlling the AMP in such a manner that an average level of the digital signal outputted from the ADC reaches a predetermined value, according to the average value calculated by the moving average unit. 
         [0018]    In the present invention, there is provided a level control section which generates a gain control signal for controlling an AMP by comparators, counters, a moving average unit and a DAC Thus, an advantageous effect is brought about in that an intermediate frequency signal supplied to an ADC can be held at a predetermined level with a simple circuit configuration as compared with the conventional level control section constituted of the power calculator in which the multiplier and adder large in the number of digits are combined together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
           [0020]      FIG. 1  is a configuration diagram of a level control section of a digital terrestrial broadcasting receiver showing an embodiment of the present invention; 
           [0021]      FIG. 2  is a block diagram showing the outline of a conventional digital terrestrial broadcasting receiver; and 
           [0022]      FIG. 3  is an explanatory diagram illustrating the principle of operation of the level control section  10  shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The above and other objects and novel features of the present invention will become more completely apparent from the following descriptions of preferred embodiment when the same is read with reference to the accompanying drawings. The drawings, however, are for the purpose of illustration only and by no means limitative of the scope of the invention. 
       Preferred Embodiment 
       [0024]      FIG. 1  is a configuration diagram of a level control section of a digital terrestrial broadcasting receiver showing an embodiment of the present invention. 
         [0025]    The level control section  10  is provided in place of the power calculator  7  and the DAC  8  employed in the digital terrestrial broadcasting receiver shown in  FIG. 2 . Other configurations are similar to those shown in  FIG. 2 . 
         [0026]    The level control section  10  has comparators (CMP)  11  and  12  which respectively compare a signal S 1  converted to a digital value by an ADC  5  with threshold values th 1  and th 2  (where th 1 &gt;th 2 ). The comparator  11  outputs a signal S 2  brought to “1” when S 1 &gt;th 1  and brought to “0” when S 1 ≦th 1 . Further, the comparator  12  outputs a signal S 3  brought to “1” when S 1 &gt;th 2  and brought to “0” when S 1 ≦th 2 . Counters (CNT)  13  and  14  are respectively connected to the output sides of the comparators  11  and  12 . 
         [0027]    The counter  13  counts a sampling clock CLK of the ADC  5  when the signal S 2  supplied thereto from the comparator  11  is “1”. The counter  14  counts the sampling clock CLK of the ADC  5  when the signal S 3  supplied thereto from the comparator  12  is “1”. These counters  13  and  14  respectively output signals S 4  and S 5  corresponding to count results every  1  symbol period. The output side of the counter  13  is connected to a comparator  15  which compares the signal S 4  with a threshold value th 3 , whereas the output side of the counter  14  is connected to a comparator  16  which compares the signal S 5  with a threshold value th 4 . 
         [0028]    The comparator  15  outputs a signal S 6  brought to “−1” when S 4 &gt;th 3  and brought to “+1” when S 1 ≦th 1 . The comparator  16  outputs a signal S 7  brought to “−1” when S 5 &gt;th 4  and brought to “+1” when S 5 &lt;th 4 . The output sides of the comparators  15  and  16  are connected to a moving average unit  17 . 
         [0029]    The moving average unit  17  accumulatively adds the average values of the signals S 6  and S 7  in accordance with a symbol timing signal TIM for a predetermined period and outputs the so-added value as a signal S 8 . The signal S 8  is supplied to a DAC  18 . 
         [0030]    The DAC  18  converts the signal S 8  outputted from the moving average unit  17  into an analog signal and supplies the same to an AMP  4  as a gain control signal AGC. 
         [0031]      FIG. 3  is an explanatory diagram showing the principle of operation of the level control section  10  shown in  FIG. 1 . 
         [0032]    Since an intermediate frequency signal IF inputted to the AMP  4  is of an OFDM-modulated signal, it is constituted of one obtained by adding a plurality of orthogonal sinusoidal waves together, whose amplitude can be approximated by a normal distribution. 
         [0033]    Therefore, assuming that the instantaneous value of the intermediate frequency signal IF is expressed in complex number as z(t)=x(t)+jy(t), normal distributions p(x) and p(y) about the amplitudes of orthogonal components x(t) and y(t) are respectively expressed in the following independent equations: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       p 
                        
                       
                         ( 
                         x 
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             π 
                              
                             
                                 
                             
                              
                             σ 
                           
                         
                       
                        
                       
                          
                         
                           
                             x 
                             2 
                           
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               σ 
                               2 
                             
                           
                         
                       
                     
                   
                   , 
                   
                     
                       p 
                        
                       
                         ( 
                         y 
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             π 
                              
                             
                                 
                             
                              
                             σ 
                           
                         
                       
                        
                       
                          
                         
                           
                             y 
                             2 
                           
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               σ 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0034]    Assuming now that r(t) is taken as the amplitude of the present signal, r(t) is expressed in the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     r 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           x 
                           2 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                       + 
                       
                         
                           y 
                           2 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0035]    A probability density distribution p(r) of the amplitude is expressed in the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     p 
                      
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       r 
                       
                         σ 
                         2 
                       
                     
                      
                     
                        
                       
                         
                           r 
                           2 
                         
                         
                           2 
                            
                           
                               
                           
                            
                           
                             σ 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0036]    A curve Ca in  FIG. 3  shows the relationship between the amplitude r and the probability density distribution p(r) at the time that the horizontal axis is taken as the amplitude r and the vertical axis is taken as the probability density distribution p(r). The area of a portion surrounded by the curve Ca and the horizontal axis corresponds to the number of samples by the ADC  5 . That is, the amplitude r on the horizontal axis of  FIG. 3  corresponds to the value of the signal S 1  corresponding to the output of the ADC  5  in  FIG. 1 . 
         [0037]    The comparators  11  and  12  respectively compare the amplitude r of the signal S 1  subjected to sampling and converted in digital form, with the threshold values th 1  and th 2 . The counters  13  and  14  respectively count the number of times in which the amplitude r has exceeded the threshold values th 1  and th 2  respectively. 
         [0038]    Thus, when the probability density distribution of the signal S 1  is represented in the form of the curve Ca shown in  FIG. 3 , for example, the value of a signal S 4   a  outputted from the counter  13  in accordance with the result of sampling during one symbol period corresponds to the area of a portion surrounded by the curve Ca and the horizontal axis in  FIG. 3  and located on the right side of the vertical line at r=th 1 . The value of a signal S 5   a  outputted from the counter  14  corresponds to the area of a portion surrounded by the curve Ca and the horizontal axis in  FIG. 3  and located on the right side of the vertical line at r=th 2 . 
         [0039]    The values of the signals S 4   a  and S 5   a  are respectively compared with the threshold values th 3  and th 4  by the comparators  15  and  16 . The comparators  15  and  16  respectively output signals S 6   a  and S 7   a  brought to −1 when they are larger than the threshold values and brought to +1 when they are smaller than the threshold values. The signals S 4   a  and S 5   a  are accumulatively added by the moving average unit  17  for a predetermined period in accordance with the symbol timing signal TIM, and the so-added value is outputted therefrom as a signal S 8   a.    
         [0040]    Next, when the average level of the signal S 1  becomes larger, the probability density distribution of the signal S 1  spreads in the horizontal direction as represented by a curve Cb of  FIG. 3 , and its peak value in the vertical axis direction becomes smaller. In this case, the value of a signal S 4   b  outputted from the counter  13  corresponds to the area of a portion surrounded by the curve Cb and the horizontal axis in  FIG. 3  and located on the right side of the vertical line at r=th 1 . The value of a signal S 5   b  outputted from the counter  14  corresponds to the area of a portion surrounded by the curve Cb and the horizontal axis of  FIG. 3  and located on the right side of the vertical line at r=th 2 . Accordingly, S 4   a &lt;S 4   b  and S 5   a &lt;S 5   b.    
         [0041]    When the signals S 4   b  and S 5   b  increase and exceed the threshold values th 3  and th 4  respectively, the signals S 6   a  and S 7   a  outputted from the comparators  15  and  16  become −1, and the value of a signal S 5   b  outputted from the moving average unit  17  becomes smaller than the signal S 8   a  used up to now. 
         [0042]    The signal S 8   a  is converted into an analog signal by the DAC  18 , which in turn is supplied to the AMP  4  as a gain control signal AGC. Therefore, the amplification of the AMP  4  is reduced so that its output level is lowered. Thus, the average level of the signal S 1  outputted from the ADC  5  is reduced. With such feedback action, the average level of the signal S 1  outputted from the ADC  5  is held at predetermined level. 
         [0043]    As described above, the level control section  10  of the present embodiment has the advantage that since it is constituted of the comparators  11  and  12  or the like and the counters  13  and  14  or the like, the intermediate frequency signal IF supplied to the ADC  5  can be held at a predetermined level with a simple circuit configuration as compared with the case in which the power calculator in which the conventional multiplier and adder large in the number of digits are combined together is used. Incidentally, the level control section of the present embodiment can be applied not only to the digital terrestrial broadcasting receiver but also to other OFDM receivers. 
         [0044]    While the preferred form of the present invention has been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention is to be determined solely by the following claims.