Abstract:
The invention relates to a filter device particularly for receiving television signals, which receives input signals and generates output signals, wherein the filter device forms a Nyquist slope by means of a passive polyphase filter ( 10 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a filter device particularly for devices for receiving television signals as well as a device for receiving television signals including a filter device. 
         [0002]    Filter devices for devices for receiving television signals are known both for receiving analog television signals and for receiving digital television signals. With analog television signals, the transmission is achieved with a vestigial sideband modulation. This means that the sidebands are asymmetrically designed. In order to determine the baseband signal here, a Nyquist slope is therefore necessary, which brings the difference in the amplitude of the double-sideband range and of the single-sideband range into line with each other. 
       BACKGROUND OF THE INVENTION 
       [0003]    In known receiving devices the Nyquist slope is realized in the IF frequency range by external, what are referred to as SAW filters, which are also called Surface Acoustic Wave filters. These SAW filters produce channel selectivity and form a Nyquist slope. Due to the multiplicity of television standards worldwide, SAW filters forming different Nyquist slopes are necessary. These SAW filters forming different Nyquist slopes differ in the bandwidth, intermediate frequency and the width of the double-sideband range. 
         [0004]    In more recent receiving devices, according to newer concepts for receiving multi-standard television signals, a window-SAW-filter is used instead of a multiplicity of SAW filters. The external SAW filter can also be avoided by an internal selectivity. In both cases, an integration of a Nyquist slope must be realized in order to be able to realize an analog television signal reception. 
         [0005]    Typically, a window-SAW-filter is used for receiving digital television signals. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to design a filter device for receiving a vestigial sideband modulated signal particularly for receiving television signals with a Nyquist slope, which device needs reduced expenditure regarding the integration of hybrid television reception and has a common filter for analog and digital television signal reception. The filter device should bring the amplitude of the double-sideband range into line with the amplitude of the single-sideband range at least substantially. Further, it is an object to provide a device for receiving television signals including a filter device. 
         [0007]    The object, according to the invention, is achieved in accordance with the characteristics of claim  1  and claim  9 . Advantageous further embodiments are described in the dependent claims. 
         [0008]    Advantageously, the invention provides a filter device particularly intended for receiving television signals, which receives input signals and generates output signals, wherein the filter device forms a Nyquist slope by means of a passive polyphase filter. Thus, the device can be used with a window-SAW-filter, which is also used for receiving digital television signals. 
         [0009]    It is particularly advantageous, if the passive polyphase filter has at least one or a plurality of simple passive polyphase filter stages. 
         [0010]    For using the filter device for different television standards it is expedient for the device to have a plurality of simple passive polyphase filter stages. 
         [0011]    Particularly advantageous is the arrangement comprising a first simple passive polyphase filter stage, which is designed in such a way that each input phase (φ) is connected to the output of the same phase (φ) by means of a resistor R 2 , furthermore, a capacitor C connects the input of a phase (φ) to the output of a subsequent phase (φ+90°) respectively and resistor R 1  connects an input of a phase (φ) to the inverted output (φ+180°) respectively. 
         [0012]    Furthermore, it is expedient if a second simple passive polyphase filter stage is designed in such a way that each input phase (φ) is connected to the output of the same phase (φ) by means of a resistor R and furthermore, a capacitor C connects the input of a phase (φ) to the output of a subsequent phase (φ+90°) respectively. 
         [0013]    It is particularly advantageous, if at least one first simple polyphase filter stage is used. In addition, it may be expedient if before or after the first simple polyphase filter stage further and different polyphase filter stages are also provided if necessary. Herein, the sequence of the individual passive polyphase filter stages may be interchanged. 
         [0014]    It is particularly advantageous that a good linearity is achieved, because no active components are needed. The Nyquist filter can also be extended to a filter for suppressing the image frequencies, in order to reduce or prevent interference from the adjacent channels. 
         [0015]    These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings: 
           [0017]      FIG. 1  gives a representation of an external SAW filter with a Nyquist slope for receiving analog television signals according to the state of art; 
           [0018]      FIG. 2  gives a representation of a window SAW filter with a Nyquist slope for receiving digital television signals, 
           [0019]      FIG. 3  shows a diagram with a Nyquist slope in the baseband, 
           [0020]      FIG. 4  gives a schematic representation of a filter device according to the invention, 
           [0021]      FIG. 5   a  gives a schematic representation of a simple passive polyphase filter stage, 
           [0022]      FIG. 5   b  shows a diagram of an amplitude response as a function of frequency, 
           [0023]      FIG. 6   a  gives a schematic representation of a simple passive polyphase filter stage, 
           [0024]      FIG. 6   b  shows a diagram of an amplitude response as a function of frequency; and 
           [0025]      FIG. 7  shows a diagram of an amplitude response as a function of frequency. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]      FIG. 1  shows an external SAW filter with a Nyquist slope for receiving analog television signals (ATV) and  FIG. 2  shows a window SAW filter for receiving digital television signals (DTV). Here, the desired channel  1 ,  2  below the curve  3 ,  4  respectively of the respective filter is selected corresponding to the transfer function of the filter. For this purpose,  FIG. 3  shows a diagram with a Nyquist slope in the baseband and the demodulated television signal. In the baseband the complex demodulated television signal comprises a double-sideband range (DSB), which is symmetrical around 0 Hz and a single-sideband range (SSB), which comprises the higher video frequencies. The picture carrier is at 0 Hz after the demodulation. The Nyquist slope in the baseband functions around 0 Hz and brings the DSB signal into line with the level of the SSB signal. 
         [0027]    The invention proposes to carry out a filtering with a Nyquist slope by means of a passive polyphase filter  10 , as represented in  FIG. 4 . The complex demodulation produces a 4-phase signal at 0°, 90°, 180° and 270°. These four phases are supplied to the passive polyphase filter  10 , which comprises one or a plurality of simple passive polyphase filter stages  11 ,  12 ,  13 . Here, the stages  11 ,  12 ,  13  may comprise, for example, one of the filter stages represented in the  FIG. 5   a  or  6   a.    
         [0028]    The simple passive polyphase filter stage according to  FIG. 5   a  is formed in such a way that each input phase is connected to the output of the same phase by means of a resistor R 2 . At the same time, a capacitor C connects the input of a phase φ to the output of a subsequent phase (φ+90°). The resistors R 1  connect the input of a phase φ to the inverted output (p+180°) respectively. 
         [0029]    In  FIG. 5   a  one recognizes a total of four inputs with a 0°, 90°, 180° and 270° phase shift on the left side of the representation of the filter stage, wherein each input phase (φ) is connected to the output of the same phase (φ) represented on the right by means of a resistor R 2 ,  21 ,  22 ,  23 ,  24 . At the same time a capacitor C connects the input of a phase (φ) to the output of a subsequent phase (φ+90°) respectively, thus the input of the phase at 0° to the output of phase at 90° etcetera. Accordingly, it is effected that the resistors R 1 ,  25 ,  26 ,  27 ,  28  connect the input of a phase φ to the inverted output (p+180°), such as, for example, the input of the phase at 0° to the output of the phase at 180°. 
         [0030]    The simple passive polyphase filter stage according to  FIG. 5   a  has the following transfer function H 1  (p): 
         [0000]    
       
         
           
             
               
                 H 
                 1 
               
                
               
                 ( 
                 p 
                 ) 
               
             
             = 
             
               
                 
                   p 
                    
                   
                       
                   
                    
                   
                     C 
                     1 
                   
                 
                 + 
                 
                   j 
                    
                   
                     ( 
                     
                       
                         1 
                         
                           R 
                           2 
                         
                       
                       - 
                       
                         1 
                         
                           R 
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   p 
                    
                   
                       
                   
                    
                   
                     C 
                     1 
                   
                 
                 + 
                 
                   ( 
                   
                     
                       1 
                       
                         R 
                         2 
                       
                     
                     + 
                     
                       1 
                       
                         R 
                         1 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    This transfer function has a zero position with the imaginary frequency f 01 : 
         [0000]    
       
         
           
             
               f 
               
                 0 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 - 
                 j 
               
                
               
                   
               
                
               
                 1 
                 
                   2 
                    
                   π 
                 
               
                
               
                 
                   
                     R 
                     1 
                   
                   - 
                   
                     R 
                     2 
                   
                 
                 
                   
                     R 
                     1 
                   
                    
                   
                     R 
                     2 
                   
                    
                   
                     C 
                     1 
                   
                 
               
             
           
         
       
     
         [0000]    and has a pole with the negative real frequency f x1 : 
         [0000]    
       
         
           
             
               f 
               
                 x 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 - 
                 
                   1 
                   
                     2 
                      
                     π 
                   
                 
               
                
               
                 
                   
                     R 
                     1 
                   
                   + 
                   
                     R 
                     2 
                   
                 
                 
                   
                     R 
                     1 
                   
                    
                   
                     R 
                     2 
                   
                    
                   
                     C 
                     1 
                   
                 
               
             
           
         
       
     
         [0031]    The amplitude response IHI is represented in  FIG. 5   b  as a function of the frequency f. The absolute value of the transfer function with frequency f=0 is: 
         [0000]    
       
         
           
             
                
               
                 
                   H 
                   1 
                 
                  
                 
                   ( 
                   
                     p 
                     = 
                     0 
                   
                   ) 
                 
               
                
             
             = 
             
               
                 
                   
                     1 
                     
                       R 
                       2 
                     
                   
                   - 
                   
                     1 
                     
                       R 
                       1 
                     
                   
                 
                 
                   
                     1 
                     
                       R 
                       2 
                     
                   
                   + 
                   
                     1 
                     
                       R 
                       1 
                     
                   
                 
               
               = 
               
                 
                   
                     
                       R 
                       1 
                     
                     - 
                     
                       R 
                       2 
                     
                   
                   
                     
                       R 
                       1 
                     
                     + 
                     
                       R 
                       2 
                     
                   
                 
                 = 
                 
                   H 
                   0 
                 
               
             
           
         
       
     
         [0032]      FIG. 6   b  shows a further simple passive polyphase filter stage  30 , which is formed in such a way that each input (φ), in  FIG. 6   a  on the left, is connected to the output of the same phase (φ) by means of a resistor R,  31 ,  32 ,  33 ,  34 , in  FIG. 6   a  on the right. At the same time a capacitor C,  35 ,  36 ,  37 ,  38  connects the input of a phase (φ) to the output of a following phase ((φ)+90°). 
         [0000]    The transfer function results in: 
         [0000]    
       
         
           
             
               
                 H 
                 2 
               
                
               
                 ( 
                 p 
                 ) 
               
             
             = 
             
               
                 
                   1 
                   R 
                 
                 - 
                 
                   j 
                    
                   
                       
                   
                    
                   p 
                    
                   
                       
                   
                    
                   C 
                 
               
               
                 
                   1 
                   R 
                 
                 + 
                 
                   p 
                    
                   
                       
                   
                    
                   C 
                 
               
             
           
         
       
     
         [0000]    The transfer function has a zero position with the imaginary frequency f 02 : 
         [0000]    
       
         
           
             
               f 
               02 
             
             = 
             
               
                 - 
                 j 
               
                
               
                   
               
                
               
                 1 
                 
                   2 
                    
                   π 
                 
               
                
               
                 1 
                 RC 
               
             
           
         
       
     
         [0000]    and has poles with negative real frequency f x2 : 
         [0000]    
       
         
           
             
               f 
               
                 x 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             
               
                 - 
                 
                   1 
                   
                     2 
                      
                     π 
                   
                 
               
                
               
                 1 
                 RC 
               
             
           
         
       
     
         [0033]    The amplitude response IHI is represented in  FIG. 6   b  as a function of the frequency f. The absolute value of the transfer function with frequency f=0 is: 
         [0000]    
       
         
           
             
                
               
                 
                   H 
                   2 
                 
                  
                 
                   ( 
                   
                     p 
                     = 
                     0 
                   
                   ) 
                 
               
                
             
             = 
             
               
                 
                   1 
                   R 
                 
                 
                   1 
                   R 
                 
               
               = 
               
                 1 
                 = 
                 
                   0 
                    
                   
                       
                   
                    
                   dB 
                 
               
             
           
         
       
     
         [0034]    In order to form a Nyquist slope, a simple passive polyphase filter stage according to  FIG. 5   a  is expedient. The zero position of this stage can be set on the negative end of the double-sideband range with f 0 =−1.25 MHz for the television standards I and L and −0.75 MHz for the television standards M, N, B, G, D and K. Furthermore, the absolute value of the transfer function in the case f=0 is set to ½, thus to −6 dB. Hence R 1 =3*R 2  is the result. 
         [0035]    In order to use one Nyquist slope for various television standards and manage a process variation it is suggested according to the invention that the zero position be shifted to a lower frequency particularly nearer to 0 Hz and that the higher negative frequencies of the double-sideband signal be suppressed by additional simple passive polyphase filter stages as represented in  FIG. 3  in the diagram of an amplitude response as a function of the frequency. 
         [0036]    Here, the further simple passive polyphase filter stages may be stages according to  FIG. 5   a  or  FIG. 6   a . With the use of a passive polyphase filter with N stages the absolute value of the cumulative transfer function results in: 
         [0000]      | H ( P= 0)|= H   10   *H   20   * . . . *H   NO =½. 
         [0037]    Using a plurality of different stages allows the image frequency gain over a further frequency range and the optimization of the amplitude waviness of the passband. 
         [0038]      FIG. 7  shows that the Nyquist slope turns at 0 Hz or −6 dB respectively as a result of a process variation or a temperature fluctuation and the zero positions are dependent on the frequency. Thus the picture contents, which are more strongly amplified with negative frequencies, are more present with positive frequencies and vice versa. This means that a process variation and a possible temperature fluctuation do not have any special influence on the characteristic of the output signal.