Abstract:
A differential antenna and associated control system applied to a digital television (TV). The control system includes a differential antenna receiving a broadcasting signal and generating a differential radio frequency (RF) signal including a positive signal and a negative signal; a control unit generating a selecting signal according to a selected channel; a switch circuit including a switch control circuit and a plurality of matching circuits. Each of the matching circuits optimizes part of the digital TV bandwidth, and the switch control circuit receives the selecting signal to control that an optimized positive signal is generated to the control unit after the positive signal passes through a first matching circuit of the matching circuits, and an optimized negative signal is generated to the control unit after the negative signal passes through a second matching circuit of the matching circuits. The first matching circuit and the second matching circuit optimize the same frequency band, and a differential optimizing signal includes the optimized positive signal and the optimized negative signal. Thus, the control unit generates a selected channel signal accordingly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 099107482 filed in Taiwan, Republic of China on Mar. 15, 2010, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The invention relates to an antenna and a control system thereof and, more particularly, to a differential antenna and a control system applied to a digital television (TV). 
         [0004]    2. Related Art 
         [0005]      FIG. 1A  is a schematic diagram showing a conventional digital TV receiver applied to a host. Generally, when a user watches a digital TV by using a host  100 , he or she needs to connect a digital TV receiver  120  via a connector (such as a universal serial bus connecter (USB) of the host  100 , and application programs of the digital TV is installed in the host  100 . The digital TV receiver  120  includes an antenna  125  and a control unit  128 . The antenna  125  is used for receiving a broadcasting signal of the digital TV, and the control unit  128  further converts the broadcasting signal to a selected channel signal. The selected channel signal is the channel selected by the user and transmitted to the host  100 . 
         [0006]      FIG. 1B  is a block diagram showing a conventional digital TV receiver. The control unit  128  of the digital TV receiver  120  includes a low noise amplifier (LNA)  150 , a band-pass filter (BPF)  160  and a digital video broadcast (DVB) module  170 . The DVB module  170  further includes a radio frequency (RF) tuner  172  and a demodulator  174 . 
         [0007]    First, after the LNA  150  amplifies the RF signal, the BPF  160  keeps the digital TV bandwidth (about 450 MHz to 870 MHz) of the RF signal and transmits it to the DVB module  170 . Second, the DVB module  170  retrieves the selected channel according to the user&#39;s instruction and outputs the selected channel signal to the host  100 , and the host  100  shows the content of the channel on the screen. The RF tuner  172  processes the bandwidth of the selected channel to a base band signal according to the user&#39;s instruction, and the demodulator  174  demodulates the base band signal and converts it to the selected channel signal. 
         [0008]    It is well-known that the digital TV system includes the DVB-T system in Europe, the advanced television system committee (ATSC) system in America, the integrated service digital broadcasting (ISDB-T) system in Japan and south America, the digital media broadcasting (DMB-T/H) system in China and so on. The digital TV bandwidth of the systems above is approximately between 460 MHz to 870 MHz. The bandwidth of the DVB-S satellite digital TV system is approximately between 950 MHz to 2150 MHz, and the bandwidth of the ISDB-S satellite digital TV system is approximately between 11.7 GHz to 12.2 GHz. 
         [0009]    The length of the antenna is in inverse proportion to a receiving frequency, since the size of the antenna applied to the digital TV in market is large, the antenna cannot be disposed inside the host  100  which is relatively small. 
         [0010]    Further, the electromagnetic interference (EMI) occurs in the host  100  in a conventional use, therefore, if the conventional antenna of the digital TV is disposed inside the host  100 , the bad receiving signal due to EMI impact would lead to malfunction of the digital TV and cannot be watched. 
       SUMMARY OF THE INVENTION 
       [0011]    A differential antenna of a digital TV and a control system thereof is provided. Different matching circuits are used to optimize different signals of the bandwidth, and thus the size of an antenna can be reduced greatly. The antenna may be disposed inside any electrical device of the digital TV, and a differential RF signal generated by the antenna resists the EMI of the electrical device effectively. 
         [0012]    A control system of a digital TV is provided to receive a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal including a positive signal and a negative signal; a control unit generating a selecting signal according to a selected channel; and a switch circuit including a plurality of matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, the switch control circuit receives the selecting signal to control that an optimized positive signal is generated to the control unit after the positive signal passes through a first matching circuit of the matching circuits and an optimized negative signal is generated to the control unit after the negative signal passes through a second matching circuit of the matching circuits, the first matching circuit and the second matching circuit optimize the same bandwidth, the optimized positive signal and the optimized negative signal are a differential optimized signal, and the control unit receives the differential optimized signal and converts the differential optimized signal to a selected channel signal. 
         [0013]    An antenna circuit of a digital TV is further provided for receiving a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal. The differential RF signal includes a positive signal and a negative signal, and a switch circuit includes a plurality of matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, the switch control circuit receives the selecting signal to control that an optimized positive signal is generated to the control unit after the positive signal passes through a first matching circuit of the matching circuits and an optimized negative signal is generated to the control unit after the negative signal passes through a second matching circuit of the matching circuits. The first matching circuit and the second matching circuit optimize the same bandwidth. 
         [0014]    A control system of a digital TV is further provided for receiving a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal; a front-end circuit receiving the differential RF signal and converting it to an amplified positive signal and an amplified negative signal; a control unit generating a selecting signal according to a selected channel; and a switch circuit including a plurality of matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, the switch control circuit receives the selecting signal to control that an optimized positive signal is generated to the control unit after the positive signal passes through a first matching circuit of the matching circuits and an optimized negative signal is generated to the control unit after the negative signal passes through a second matching circuit of the matching circuits, the first matching circuit and the second matching circuit optimize the same bandwidth, the optimized positive signal and the optimized negative signal are a differential optimized signal, and the control unit receives the differential optimized signal and converts it to a selected channel signal. 
         [0015]    An antenna circuit applied to a digital TV is further provided for receiving a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal; a front-end circuit receiving the differential RF signal and converting it to an amplified positive signal and an amplified negative signal; a switch circuit including a plurality of matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, the switch control circuit receives a selecting signal to control that an optimized positive signal is generated to the control unit after the positive signal passes through a first matching circuit of the matching circuits and an optimized negative signal is generated to the control unit after the negative signal passes through a second matching circuit of the matching circuits, the optimized positive signal and the optimized negative signal are a differential optimized signal, and the first matching circuit and the second matching circuit optimize the same bandwidth. 
         [0016]    A control system of a digital TV is further provided for receiving a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal; a front-end circuit receiving the differential RF signal and generating a single ended signal; a DVB module generating a selecting signal according to a selected channel; and a switch circuit including a plurality of matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, and the switch control circuit receives the selecting signal to control that a single ended optimized signal is generated to the DVB module after the single ended signal passes through a first matching circuit of the matching circuits. The DVB module converts the single ended optimized signal to a selected channel signal. 
         [0017]    An antenna circuit of the digital TV is further provided for receiving a digital TV bandwidth which includes a differential antenna receiving a broadcasting signal and generating a differential RF signal; a front-end circuit receiving the differential RF signal and converting a single ended signal; and a switch circuit including a plurality of the matching circuits and a switch control circuit. Each of the matching circuits optimizes part of the digital TV bandwidth, and the switch control circuit receives a selecting signal to control that a single ended optimized signal is generated after the single ended signal passes through a first matching circuit of the matching circuits. 
         [0018]    These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1A  is a schematic diagram showing a conventional digital TV receiving system applied to a host; 
           [0020]      FIG. 1B  is a block diagram showing a conventional digital TV receiver; 
           [0021]      FIG. 2  is a schematic diagram showing a digital TV receiving system in an embodiment of the invention; 
           [0022]      FIG. 3  is a block diagram showing a digital TV receiver in a first embodiment of the invention; 
           [0023]      FIG. 4A  to  FIG. 4D  are schematic diagrams showing matching circuits in different embodiments; 
           [0024]      FIG. 5  is a block diagram showing a digital TV receiver in a second embodiment of the invention; and 
           [0025]      FIG. 6  is a block diagram showing a digital TV receiver in a third embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 2  is a schematic diagram showing a digital TV receiving system in an embodiment of the invention. The digital TV receiving system is disposed at an available area at the edge of the screen of a host  200 , which is not limited herein. The digital TV receiving system may be also disposed in any electrical device such as a digital TV set-top box (STB), a satellite digital TV receiver, a car audio and a video receiver, or a TV which can receive the digital TV signal. 
         [0027]    In  FIG. 2 , the digital TV receiving system includes a differential antenna  210 , a switch circuit  220 , and a control unit  250 . The differential antenna  210  is used for receiving a broadcasting signal and generating a differential RF signal, and the switch circuit  220  is connected between the differential antenna  210  and the control unit  250 . The switch circuit  220  optimizes a specific bandwidth corresponding to a selected channel of the differential RF signal. Afterwards, a differential optimized signal is outputted. The control unit  250  converts the differential optimized signal to a selected channel signal selected by the user and transmits the selected channel signal to an internal circuit of the host  200 . Thus, the host  200  displays the content of the selected channel on the screen. 
         [0028]    Generally speaking, the common mode noise of the EMI in the host  200  or other electrical devices is strong. The differential antenna  210  is used to restrain the common mode noise effectively according to an embodiment of the invention. Furthermore, to reduce the size of the differential antenna  210 , the switch circuit  220  is used to optimize the bandwidth corresponding to the channel selected by the user and output the differential optimized signal to the control unit  250 . Consequently, the size of the differential antenna  210  can be reduced to be smaller than 10 cm, and it is easy to dispose the differential antenna  210  at the available area at the edge of the screen. 
         [0029]      FIG. 3  is a block diagram showing a digital TV receiver in a first embodiment of the invention. The differential antenna  210  uses a dipole antenna. Except for the dipole antenna, a loop antenna may also be used as the differential antenna  210 . The differential RF signal outputted by the differential antenna  210  includes a positive signal S+ and a negative signal S−. 
         [0030]    The switch circuit  220  includes a switch control circuit including multiple switches Sw 1  to Sw 4  and a plurality of matching circuits  221  to  228 , and the control unit  250  may output a 2-bit selecting signal to the switches Sw 1  to Sw 4  of the switch circuit  220 . The matching circuits  221  to  228  optimize the different bandwidths of the digital TV bandwidth, and improve the receiving efficiency of the bandwidth. 
         [0031]    For example, when the selecting signal is “00”, the first switch Sw 1  inputs the positive signal S+ to the 450-550 MHz matching circuit  221  which optimizes the signal with the 450 MHz to 550 MHz bandwidth in the positive signal S+, and the optimized positive signal So+ is outputted by the second switch Sw 2 ; when the selecting signal is “01”, the first switch Sw 1  inputs the positive signal S+ to the 550-650 MHz matching circuit  223  which optimizes the signal with the 550 MHz to 650 MHz bandwidth in the positive signal S+, and the optimized positive signal So+ is outputted by the second switch Sw 2 ; when the selecting signal is “10”, the first switch Sw 1  inputs the positive signal S+ to the 650-750 MHz matching circuit  225  which optimizes the signal with the 650 MHz to 750 MHz bandwidth in the positive signal S+, and the optimized positive signal So+ is outputted by the second switch Sw 2 ; when the selecting signal is “11”, the first switch Sw 1  inputs the positive signal S+ to the 750-870 MHz matching circuit  227  which optimizes the signal with the 750 MHz to 870 MHz bandwidth in the positive signal S+, and the optimized positive signal So+ is outputted by the second switch Sw 2 . The principle of the actions to the negative signal S− by the third switch Sw 3 , the fourth switch Sw 4 , and the matching circuits  222 ,  224 ,  226 ,  228  are the same as the above principle, which is omitted herein. The optimized positive signal So+ and the optimized negative signal so− are the differential optimized signal. 
         [0032]    The control unit  250  includes a prepositive circuit  251  and a DVB module  256 . The prepositive circuit  251  includes a differential LNA  252  and a balun  254 . The DVB module  256  further includes a RF tuner  257  and a demodulator  258 . 
         [0033]    The differential LNA  252  of the prepositive circuit  251  receives and amplifies the differential optimized signal. Afterwards, the balun  254  converts the differential optimized signal to a single ended optimized signal and inputs the single ended optimized signal to the DVB module  256 . The DVB module  256  outputs the selecting signal according to the user&#39;s instruction, retrieves the selected channel from the single ended optimized signal and converts it to the selected channel signal to output to the host (or the electrical device). The host (or the electrical device) displays video on the corresponding screen. Furthermore, the RF tuner  257  converts the single ended optimized signal to the base band signal according to the user&#39;s instruction. The demodulator  258  outputs the selecting signal according to the user&#39;s instruction, demodulates the base band signal and converts it to the selected channel signal. 
         [0034]    After the user selects a channel in the host (or the electrical device), the selected channel is transmitted to the DVB module  256 . The demodulator  258  of the DVB module  256  outputs the selecting signal to the switch circuit  220 , and the DVB module  256  converts the received single ended optimized signal to the selected channel signal. For example, if the selected channel is between 650 MHz and 750 MHz, the demodulator  258  outputs the selecting signal “10” to the switches Sw 1  to Sw 4 . Similarly, the optimized positive signal So+ is generated after the positive signal S+passes through the 650-750 MHz matching circuit  225 , and the optimized negative signal so− is also generated after the negative signal S− passes through the 650-750 MHz matching circuit  226 . After the control unit  250  receives the differential optimized signal, the single ended optimized signal is inputted into the DVB module  256  after it passes through the differential LNA  252  and the balun  254 . The DVB module  256  outputs the selected channel signal to the host (or the electrical device), and the video is displayed on the corresponding screen. 
         [0035]    As shown in  FIG. 4A  to  FIG. 4D , the matching circuits may be a T-shaped circuit or a π-shaped circuit. In  FIG. 4A , the T-shaped circuit includes two inductors Lt 1  and Lt 2  connected in series between the input end S and the output end So. A capacitor Ct is connected between a connector point of the two inductors Lt 1 , Lt 2  and a ground. In  FIG. 4B , the T-shaped circuit includes two capacitors Ct 1  and Ct 2  connected in series between the input end S and the output end So. An inductor Lt is connected between the connector point of the two capacitors Ct 1 , Ct 2  and the ground. In  FIG. 4C , the π-shaped circuit includes an inductor Lp connected between the input end S and the output end So, a capacitor Cp 1  connected between the input end S and the ground, and a capacitor Cp 2  connected between the output end So and the ground. In  FIG. 4D , the π-shaped circuit includes a capacitor Cp connected between the input end S and the output end so, an inductor Lp 1  connected between the input end S and the ground, and an inductor Lp 2  connected between the output end so and the ground. The matching circuits with different bandwidths can be obtained by adjusting the value of the inductor and the capacitor of the T-shaped circuit or the π-shaped circuit. 
         [0036]    As stated above, the switch circuit is used to optimize the signal with different bandwidths to reduce the size of the differential antenna. Persons having ordinary skill in the art may make various modifications and changes according to the switch circuit in the embodiment. For example, in  FIG. 3 , a common mode filter may be added between the input end of the switch circuit  220  and the output end of the differential LNA  252  to filter the common mode noise completely. 
         [0037]      FIG. 5  is a block diagram showing a digital TV receiver in a second embodiment of the invention. The digital TV receiver includes a differential antenna  310 , a front-end circuit  320 , a switch circuit  330 , and a control unit  350 . The front-end circuit  320  is a differential LNA  322 . 
         [0038]    According to the second embodiment of the invention, the differential antenna  310  receives the broadcasting signal and outputs the differential RF signal which includes a positive signal S+ and a negative signal S−. The differential LNA  322  receives the differential RF signal and converts it to an amplified differential RF signal which includes an amplified positive signal Si+ and an amplified negative signal Si−. 
         [0039]    The switch circuit  330  includes the switch control circuit including multiple switches Sw 1  to Sw 4  and a plurality of the matching circuits  331  to  338 . The control unit  350  outputs a 2-bit selecting signal to the switches Sw 1  to Sw 4  of the switch circuit  330 . The matching circuits  331  to  338  optimize the different bandwidths of the digital TV the bandwidth, and improve the receiving efficiency of the bandwidth. 
         [0040]    For example, when the selecting signal is “00”, the first switch Sw 1  inputs the amplified positive signal Si+ to the 450-550 MHz matching circuit  331  which optimizes the signal with the 450 MHz to 550 MHz bandwidth in the amplified positive signal Si+, and then the optimized positive signal So+ is generated and outputted by the second switches Sw 2 ; when the selecting signal is “01”, the first switch Sw 1  inputs the amplified positive signal Si+ to the 550-650 MHz matching circuit  333  which optimizes the signal with the 550 MHz to 650 MHz bandwidth in the amplified positive signal Si+, and then the optimized positive signal So+ is generated and outputted by the second switches Sw 2 ; when the selecting signal is “10”, the first switch Sw 1  inputs the amplified positive signal Si+ to the 650-750 MHz matching circuit  335  which optimizes the signal with the 650 MHz to 750 MHz bandwidth in the amplified positive signal Si+, and then the optimized positive signal So+ is generated and outputted by the second switches Sw 2 ; when the selecting signal is “11”, the first switch Sw 1  inputs the amplified positive signal Si+ to the 750-870 MHz matching circuit  337  which optimizes the signal with the 750 MHz to 870 MHz bandwidth in the amplified positive signal Si+, and then the optimized positive signal So+ is generated and outputted by the second switches Sw 2 . The principle of actions to the amplified negative signal Si− by the third switch Sw 3 , the fourth switch Sw 4 , and the matching circuits  332 ,  334 ,  336 ,  338  are the same as the above principle, which is omitted herein. The optimized positive signal So+ and the optimized negative signal so− are the differential optimized signal. 
         [0041]    The control unit  350  includes a prepositive circuit  351  and a DVB module  356 . The prepositive circuit  351  is a balun, and the DVB module  356  includes a RF tuner  357  and a demodulator  358 . 
         [0042]    The balun  352  converts the differential optimized signal to the single ended optimized signal and inputs it to the DVB module  356 . The DVB module  356  outputs the selecting signal according to the user&#39;s instruction, retrieves the selected channel from the single ended optimized signal and converts it to the selected channel signal to the host (or the electrical device). The host (or the electrical device) displays video signal on the corresponding screen. The RF tuner  357  converts the single ended optimized signal to the base band signal according to the user&#39;s instruction. The demodulator  358  outputs the selecting signal according to the user&#39;s instruction, demodulates the base band signal and converts it to the selected channel signal. 
         [0043]    After the user determines a selected channel at the host (or the electrical device), the selected channel is transmitted to the DVB module  356 . The demodulator  358  of the DVB module  356  outputs the selecting signal to the switch circuit  330 , and the DVB module  356  converts the received single ended optimized signal to the selected channel signal. For example, if the selected channel is in 650 MHz to 750 MHz, the demodulator  358  outputs the “10” selecting signal to the switches Sw 1  to Sw 4 . Thus, after the amplified positive signal Si+passes through the 650-750 MHz matching circuit  335 , the optimized positive signal So+ is generated Similarly, after the amplified negative signal Si− passes through the 650-750 MHz matching circuit  336 , the optimized negative signal So− is generated after the control unit  350  receives the differential optimized signal, the single ended optimized signal is inputted to the DVB module  356  after it passes through the balun  352 . Then, the DVB module  356  outputs the selected channel signal to the host (or the electrical device), and the video is displayed on the corresponding screen. 
         [0044]    Persons having ordinary skill in the art may further add a common mode filter to the front-end circuit  320  or the prepositive circuit  351  of the digital TV receiver in the second embodiment to filter the common mode noise completely. 
         [0045]      FIG. 6  is a block diagram showing a digital TV receiver in a third embodiment of the invention. The digital TV receiver includes a differential antenna  410 , a front-end circuit  420 , a switch circuit  430 , and a DVB module  450 . The differential antenna  410  receives the broadcasting signal and converts it to the differential RF signal which includes a positive signal S+ and a negative signal S−. 
         [0046]    The front-end circuit  420  receives the differential RF signal and converts it to a single ended signal. The front-end circuit  420  further includes a differential LNA  422  and a balun  424 . The differential LNA  422  receives and amplifies the differential RF signal. Afterwards, the balun  424  converts the amplified differential RF signal to the single ended signal S. 
         [0047]    The switch circuit  430  includes the switch control circuit including multiple switches Sw 1  to Sw 2  and a plurality of the matching circuits  432  to  435 , and the DVB module  450  may output the 2-bit selecting signal to the switches Sw 1  to Sw 2  of the switch circuit  430 . The matching circuits  432  to  435  optimize the digital TV bandwidth with different bandwidths, and improve the receiving efficiency of the bandwidth. 
         [0048]    For example, when the selecting signal is “00”, the first switches Sw 1  inputs the single ended signal S to the 450-550 MHz matching circuit  432  which optimizes the signal with the 450 MHz to 550 MHz bandwidth in the single ended signal S, and the single ended optimized signal So is outputted by the second switches Sw 2 ; when the selecting signal is “01”, the first switches Sw 1  inputs the single ended signal S to the 550-650 MHz matching circuit  433  which optimizes the signal with the 550 MHz to 650 MHz bandwidth in the single ended signal S, and the single ended optimized signal So is outputted by the second switches Sw 2 ; when the selecting signal is “10”, the first switches Sw 1  inputs the single ended signal S to the 650-750 MHz matching circuit  434  which optimizes the signal with the 650 MHz to 750 MHz bandwidth in the single ended signal S, and the single ended optimized signal So is outputted by the second switches Sw 2 ; when the selecting signal is “11”, the first switches Sw 1  inputs the single ended signal S to the 750-870 MHz matching circuit  435  which optimizes the signal with the 750 MHz to 870 MHz bandwidth in the single ended signal S, and the single ended optimized signal So is outputted by the second switches Sw 2 . 
         [0049]    The DVB module  450  outputs the selecting signal according to the user&#39;s instruction, retrieves the selected channel selected by the user from the single ended optimized signal and converts it to the selected channel signal to the host (or the electrical device). The host (or the electrical device) displays the content of the channel signal on the corresponding screen. The DVB module  450  further includes a RF tuner  457  and a demodulator  458 . The RF tuner  457  converts the single ended optimized signal to the base band signal according to the user&#39;s instruction. The demodulator  458  outputs the selecting signal according to the user&#39;s instruction, demodulates the base band signal and converts it to the selected channel signal. 
         [0050]    When the user determines a selected channel at the host (or the electrical device), the selected channel is transmitted to the DVB module  450 . The demodulator  458  of the DVB module  450  outputs the selecting signal to the switch circuit  430 , and the DVB module  450  converts the received single ended optimized signal to the selected channel signal. For example, if the selected channel is between 650 MHz to 750 MHz, the demodulator  458  outputs the “10” selecting signal to the switches Sw 1  to Sw 2 . Thus, the single ended optimized signal So is generated after the single ended signal S passes through the 650-750 MHz matching circuit  434 . The DVB module  450  receives the single ended optimized signal So, and then it can output the selected channel signal to the host (or the electrical device), and the content of the selected channel is displayed on the corresponding screen. 
         [0051]    Besides, the number of the matching circuits is not limited herein. Persons having ordinary skill in the art may design the matching circuits with different bandwidths, and match with the switch control circuit including different switches to achieve the switch circuit in the embodiment. Moreover, although the antenna and the digital TV receiver take the digital TV bandwidth 450 MHz to 870 MHz as an example, they may also be applied to a DVB-S satellite digital TV system or an ISDB-S satellite digital TV system. 
         [0052]    Persons having ordinary skill in the art may change the front-end circuit, for example, they may exchange the positions of the differential LNA  422  and the balun  424  of the front-end circuit  420  in the third embodiment, or convert the differential RF signal to the single ended signal. 
         [0053]    Although the antenna circuit of the digital TV and the receiving system are disposed in the host (or the electrical device), the digital TV receiving system is not limited to be disposed in the host (or the electrical device). Persons having ordinary skill in the art also may dispose the receiving system outside the host (or the electrical device) to achieve the same effect. 
         [0054]    Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, the disclosure is not for limiting the scope of the invention. Persons having ordinary skill in the art may make various modifications and changes without departing from the scope. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments described above.