Abstract:
A television signal receiving tuner that is capable of receiving at least terrestrial television broadcast signals includes a trap circuit for attenuating frequency-modulation broadcast-band signals. The trap circuit attenuates frequency-modulation broadcast-band signals that are in proximity to the desired terrestrial television broadcast signals or are broadcast within a band that are received at one half of the frequency of the desired terrestrial television broadcast signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to television signal receiving tuners, and more particularly relates to a television signal receiving tuner that is capable of receiving terrestrial television broadcasts and that has a trap circuit for attenuating frequency-modulation broadcast-band signals. 
     2. Description of the Related Art 
     A conventional television signal receiving tuner (hereinafter referred to as simply a tuner) is shown in FIG.  3 . In the tuner shown in FIG. 3, one terminal  31  a of a filter  31  is connected to an antenna for receiving a terrestrial television broadcast or a cable for receiving cable television (CATV) signals (neither are shown). Another end  31   b  of the filter  31  is connected to an input terminal of a VHF (very high frequency) unit  32  and that of a UHF (ultrahigh frequency) unit  33  which are connected in parallel with each other. An output terminal of the VHF unit  32  and an output terminal of the UHF unit  33  are each connected to an integrated circuit  34 .The integrated circuit  34  has a mixer, an oscillator, and a PLL (phase locked loop) incorporated therein. The mixer frequency-converts a high frequency signal in the VHF band or in the UHF band into an intermediate frequency signal IF and outputs the intermediate frequency signal IF. 
     The VHF unit  32  is constructed by sequentially connecting in series a trap circuit  35 , a VHF antenna tuning circuit  36 , a VHF high frequency amplifier  37  and a VHF interstage tuning circuit  38 . Likewise, the UHF unit  33  is constructed by sequentially connecting in series a UHF antenna tuning circuit  39 , a UHF high frequency amplifier  40 , and a UHF interstage tuning circuit  41 . 
     The trap circuit  35  attenuates FM broadcast signals which interfere with terrestrial television broadcast signals when terrestrial television broadcast signals are received. A parallel resonant circuit constructed with an inductor  42  and a capacitor  43  is connected, via a direct current blocking capacitor  44 , in parallel with a switch diode  45 , the cathode of which is connected to an input terminal  35   a  of the trap circuit  35  and the anode is connected to an output terminal  35   b  thereof. The resonant frequency of the parallel resonant circuit formed by the inductor  42  and the capacitor  43  is set within the FM broadcast-band. The cathode of the switch diode  45  is grounded via a resistor  46  which is provided between the input terminal  35   a  ground. A trap circuit control voltage V f  is applied via a feeding resistor  47  from the integrated circuit  34  to a node between the switch diode  45  and the resistor  46 . 
     In the VHF unit  32 , the VHF antenna tuning circuit  36  outputs a desired VHF band television signal to the VHF high frequency amplifier  37 . The VHF high frequency amplifier  37  amplifies the input television signal, and the VHF interstage tuning circuit  38  outputs the amplified television signal to the integrated circuit  34 . Operations of the circuits that comprise the UHF unit  33  are the same as those circuits described in the VHF unit  32 . Therefore, descriptions of the UHF circuits are omitted. 
     The integrated circuit  34  outputs a band selection voltage or a tuning voltage in accordance with channel selection data DA input from a television receiver (not shown). The band selection voltage consists of a voltage V lo  for selecting a low band in the VHF band, a voltage V hi  for selecting a high band in the VHF band, and a voltage V u  for selecting the UHF band. When each band is selected, the integrated circuit  34  outputs a voltage of 5 V. The band selection voltages V lo  and V hi  for the VHF band are applied to the VHF antenna tuning circuit  36 , the VHF high frequency amplifier  37 , and the VHF interstage tuning circuit  38 .The band selection voltage V u  for the UHF band is applied to the UHF high frequency amplifier  40 . A tuning voltage V t  is applied to the VHF antenna tuning circuit  36 , the VHF interstage tuning circuit  38 , the UHF antenna tuning circuit  39 , and the UHF interstage tuning circuit  41 . 
     For example, in the United States frequency assignments for VHF and UHF band television channels and the FM broadcast-band are shown in Table 1. It should be noticed that the frequency band assigned to channel six is adjacent to that of the FM broadcasting range. 
     Furthermore, some CATV channels are assigned to the FM broadcast-band (for example, in the United States, between 88 MHz and 108 MHz). Accordingly, the trap circuit  35  is activated when signals for channel six or for particular CATV channels are received; otherwise the trap circuit  35  is not activated. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Freq. 
                   
                 Att. 
               
               
                   
                   
                   
                 Band 
                   
                 Freq. 
               
               
                   
                 Band 
                 Ch. No. 
                 (MHz) 
                 V f   
                 (MHz) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Terrestrial 
                 VHF Low Band 
                 2 to 5 
                 54 to 82 
                 OFF 
                 — 
               
               
                 Television 
                   
                 6 
                 82 to 88 
                 ON 
                 91.25 
               
               
                 Broadcasting 
                 VHF High Band 
                  7 to 13 
                 174 to 216 
                 OFF 
                 — 
               
               
                   
                 UHF 
                 14 to 83 
                 470 to 890 
                 OFF 
                 — 
               
               
                 Cable 
                 VHF Low Band 
                 2 to 6 
                 54 to 88 
                 OFF 
               
               
                 TV 
                 Midband 
                 A-5 to 1  
                  90 to 174 
                 OFF 
                 — 
               
               
                   
                 VHF High Band 
                  7 to 13 
                 174 to 216 
                 OFF 
               
               
                   
                 Superband 
                   J to W 
                 216 to 300 
                 OFF 
                 — 
               
               
                   
                 Hyperband 
                 W + 1 to 
                 300 to 648 
                 OFF 
                 — 
               
               
                   
                   
                 W + 58 
               
               
                   
                 FM 
                 — 
                  88 to 108 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     When signals having frequencies within the terrestrial television broadcasting channel six are received, the trap circuit control voltage V f , (e.g., a 5 V voltage), is applied via the feeding resistor  47  from the integrated circuit  34  to the trap circuit  35 . Since the switch diode  45  is reverse-biased, the switch diode  45  is switched off. With the switched diode  45  switched off, the trap circuit  35  attenuates a signal having a particular frequency, such as 91.25 MHz within the FM broadcast-band, which is in the proximity of the frequency band of channel six. 
     When the terrestrial television broadcast VHF low band, apart from the band of channel six, or VHF high band is received, the trap circuit control voltage V f  is not applied to the trap circuit  35  and the band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  36 . Since the switch diode  45  is forward-biased, the switch diode  45  is switched on. With the switch diode  45  switched on, the input terminal  35   a  and the output terminal  35   b  of the trap circuit  35  are short-circuited, allowing high frequency signals to be received by the VHF antenna tuning circuit  36  without being attenuated by the trap circuit  35 . 
     When CATV signals are received, the trap circuit control voltage V f  is not applied to the trap circuit  35  and the band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  36 . Since the switch diode  45  is forward-biased, the switch diode  45  is switched on. With the switch diode  45  switched on, the input terminal  35   a  and the output terminal  35   b  of the trap circuit  35  are short-circuited, and the CATV signals within the entire VHF band are received by the VHF antenna tuning circuit  36  without being attenuated by the trap circuit  35 . 
     When the above-described conventional tuner receives VHF television signals within the terrestrial television broadcast-band, the integrity of the VHF television signals can be impaired. The frequencies of some of these received signals correspond to twice the frequencies of the FM broadcast-band (between 88 MHz and 108 MHz). It is the second harmonic of these FM broadcast signals that can also interfere with the television signals received by the high frequency amplifier  37  and other components. The quality of the received image is thus degraded if the second harmonic of these FM broadcast signals is not removed. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, a television signal tuner capable of receiving at least terrestrial television broadcast signals is provided. The television signal receiving tuner includes a trap circuit for attenuating frequency-modulation (FM) broadcast-band signals that interfere with the terrestrial television broadcast signals. The trap circuit passes select terrestrial television signals while attenuating (or dampening) FM broadcast-band signals that are in proximity to the selected terrestrial television signals, or have frequencies that when doubled coincide with the selected terrestrial television signals. By suppressing interference from adjacent FM broadcast signals and from second harmonics generated from some other FM broadcast signals, degradation in the quality of received images can be minimized. 
     In a tuner, the attenuated frequency or resonate frequency of the trap circuit changes when with the desired television channel changes. When the desired channel has a frequency that is double the frequency of a FM broadcast-band, second harmonic interference can effect signal quality. Under this condition, suppression of the interference caused by second harmonics of FM broadcast signals can be ensured, which prevents degradation in the quality of the received television images. 
     The tuner may further include a varactor diode. In one preferred embodiment, the resonant frequency or attenuated frequency of the trap filter may be varied by applying a tuning voltage to the varactor diode that corresponds to the desired channel of frequencies to be received. By changing the reception channel, the resonant frequency or attenuated frequency of the trap filter is changed and the range of interference can be minimized. 
     In one preferred embodiment, the trap circuit may include a parallel resonant circuit, a switch diode, and a varactor diode in which the parallel resonant circuit, the switch diode, and the varactor diode are connected in parallel with one another. When the desired television signals are in proximity to the FM broadcast-band or the signals are approximately double the frequencies of a FM broadcast-band signal, the switch diode is reversed biased or switched off, and when other television signals are received, the switch diode is forward biased or switched on. 
     When CATV signals are received, the CATV signals are received without being attenuated by the trap circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a television signal receiving tuner according to the present invention; 
     FIG. 2 is a block diagram of another television signal receiving tuner according to the present invention; and 
     FIG. 3 is a block diagram of a conventional television signal receiving tuner. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a block diagram of the television signal receiving tuner according to a preferred embodiment of the present invention. In the tuner of FIG. 1, one terminal  1   a  of a filter  1  is connected to an antenna for receiving terrestrial television broadcasts or a cable for receiving CATV signals (neither are shown). Another end  1   b  of the filter  1  is connected to the input terminal of a VHF unit  2  and that of a UHF unit  3  which are connected in parallel with each other. An output terminal of the VHF unit  2  and an output terminal of the UHF unit  3  are each connected to an integrated circuit  4 . The integrated circuit  4  has a mixer, an oscillator, and a PLL incorporated therein. The mixer frequency-converts a high frequency signal in the VHF band or an ultra-high frequency in the UHF band into an intermediate frequency signal IF and outputs the intermediate frequency signal IF. 
     The VHF unit  2  is constructed by sequentially connecting in series a trap circuit  5 , a VHF antenna tuning circuit  6 , a VHF high frequency amplifier  7 , and a VHF interstage tuning circuit  8 . Likewise, the UHF unit  3  is constructed by sequentially connecting in series a UHF antenna tuning circuit  9 , a UHF high frequency amplifier  10 , and a UHF interstage tuning circuit  11 . 
     The trap circuit  5  attenuates FM broadcast signals which interfere with terrestrial television broadcast signals when terrestrial television broadcast signals are received. A parallel resonant circuit formed by an inductor  12  and a first capacitor  13  is coupled, via a direct current blocking capacitor  14 , in parallel with a switch diode  15 , the cathode of which is connected to an input terminal  5   a  of the trap circuit  5  and the anode is connected to an output terminal  5   b  thereof. The resonant frequency of the parallel circuit formed by the inductor  12  and the first capacitor  13  is set within the FM broadcast-band. The cathode of the switch diode  15  is grounded via a resistor  16  provided between the input terminal  5   a  and ground. A trap circuit control voltage V f  is applied via a first feeding resistor  17  from the integrated circuit  4  to a node between the switch diode  15  and the resistor  16 . 
     In the VHF unit  2 , the VHF antenna tuning circuit  6  outputs a desired VHF band television signal to be received by the VHF high frequency amplifier  7 , the VHF high frequency amplifier  7  amplifies the input television signal, and the VHF interstage tuning circuit  8  outputs the amplified television signal to the integrated circuit  4 . Operations of the circuits of the UHF unit  3  are similar to those described in the VHF unit  2 . Therefore, descriptions of the circuits that comprise UHF Unit  3  are omitted. 
     The integrated circuit  4  outputs a band selection voltage and a tuning voltage in accordance with channel selection data DA input from a television receiver (not shown). The band selection voltage consists of a voltage V lo  for selecting a low band in the VHF band, a voltage V hi  for selecting a high band in the VHF band, and a voltage V u  for selecting the UHF band. When a band is selected, the integrated circuit  4  outputs a voltage, for example, a 5 V voltage. The VHF band selection voltage V lo  or V hi  is applied to the VHF antenna tuning circuit  6 , the VHF high frequency amplifier  7 , and the VHF interstage tuning circuit  8 . The UHF band selection voltage V u  is applied to the UHF high frequency amplifier  10 . A tuning voltage V t  is applied to the VHF antenna tuning circuit  6 , the VHF interstage tuning circuit  8 , the UHF antenna tuning circuit  9 , and the UHF interstage tuning circuit  11 . 
     For example, in the United States frequency assignments of VHF and UHF band television channels and the FM broadcast-band are shown in Table 2. It should be noted that the frequency band of channel six (between 82 MHz and 88 MHz) is adjacent to the FM broadcasting frequency band (between 88 MHz and 108 MHz). Furthermore, some CATV channels are assigned within the FM broadcast-band (for example, in the United States, between 88 MHz and 108 MHz). 
     Accordingly, FM broadcast signals are attenuated by the trap circuit  5  when signals for channel six or for particular CATV channels are received; otherwise the trap circuit  5  is not activated. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Freq. 
                   
                 Att. 
               
               
                   
                   
                   
                 Band 
                   
                 Freq. 
               
               
                   
                 Band 
                 Ch. No. 
                 (MHZ) 
                 V f   
                 (MHz) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Terrestrial 
                 VHF Low Band 
                 2 to 5 
                 54 to 82 
                 OFF 
                 — 
               
               
                 Television 
                   
                 6 
                 82 to 88 
                 ON 
                 91.25 
               
               
                 Broadcasting 
                 VHF High Band 
                  7 to 13 
                 174 to 216 
                 ON 
                 88.5 to 
               
               
                   
                   
                   
                   
                   
                 106.5 
               
               
                   
                 UHF 
                 14 to 83 
                 470 to 890 
                 OFF 
                 — 
               
               
                 Cable 
                 VHF Low Band 
                 2 to 6 
                 54 to 88 
                 OFF 
               
               
                 TV 
                 CATV Midband 
                 A-5 to 1  
                  90 to 174 
                 OFF 
                 — 
               
               
                   
                 VHF High Band 
                  7 to 13 
                 174 to 216 
                 OFF 
               
               
                   
                 Superband 
                   J to W 
                 216 to 300 
                 OFF 
                 — 
               
               
                   
                 Hyperband 
                 W + 1 to 
                 300 to 648 
                 OFF 
                 — 
               
               
                   
                   
                 W + 58 
               
               
                   
                 FM 
                 — 
                  88 to 108 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     When signals having frequencies within the terrestrial television broadcasting channel six or the VHF high band are received, the trap circuit control voltage V f  is applied via the first feeding resistor  17  from the integrated circuit  4  to the trap circuit  5 . Since the switch diode  15  is reverse-biased, the switch diode  15  inhibits current flow from node  5   b  to node  5   a  and thus is switched off. Accordingly, the trap circuit  5  attenuates a signal of a particular frequency when switch diode  15  is switched off, such as 91.25 MHz, within the FM broadcast-band. 
     When signals having frequencies within the terrestrial television broadcasting VHF low band, apart from the band of channel six, are received, the trap circuit control voltage V f  is not applied to the trap circuit  5  and the band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  6 . Since the switch diode  15  is forward-biased, the switch diode permits current flow from node  5   b  to node  5   a  and thus the switch diode  15  is switched on. Therefore, since the input terminal  5   a  and the output terminal  5   b  of the trap circuit  5  are short-circuited, high frequency signals are received by the VHF antenna tuning circuit  6  without being attenuated by the trap circuit  5 . 
     When CATV signals are received, the trap circuit control voltage V f  is not applied to the trap circuit  5  and the band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  6 . Since the switch diode  15  is forward-biased, the switch diode  15  is switched on. Therefore, since the input terminal  5   a  and the output terminal  5   b  of the trap circuit  5  are short-circuited, CATV signals within the entire VHF band are passed to the VHF antenna tuning circuit  6 . 
     FIG. 2 shows a preferred embodiment of the television signal receiving tuner according to the present invention. In this preferred embodiment, a trap circuit  5 ′ can be tuned (also referred to as programmed) by a using a tuning voltage that tracks signals having frequencies to be attenuated. The trap circuit  5 ′ can be tuned to attenuate FM broadcast signals which interfere with terrestrial television broadcast signals that are received by the tuner. The parallel resonant circuit formed by an inductor  12  and a first capacitor  13  is coupled, via a direct current blocking capacitor  14 , in parallel with a switch diode  15 , the cathode of which is connected to an input terminal  5   a ′ of the trap circuit  5  and the anode is connected to an output terminal  5   b ′ thereof. The cathode of the switch diode  15  is grounded via a resistor  16  provided between the input terminal  5   a ′ and ground. A trap circuit control voltage V f  is applied via a first feeding resistor  17  from an integrated circuit  4  to a node between the switch diode  15  and the resistor  16 . Furthermore, the cathode of a varactor diode  18  is coupled with one terminal of a second capacitor  19 , the anode thereof is coupled with the input terminal  5   a ′ of the trap circuit  5 ′, and the other terminal of the second capacitor  19  is coupled with the output terminal  5   b ′ of the trap circuit  5 ′. A tuning voltage V t  is applied via a second feeding resistor  20  from the integrated circuit  4  to a node between the varactor diode  18  and the second capacitor  19 . Since the capacitance of the varactor diode  18  is varied by the tuning voltage V t , the attenuation frequency of the trap circuit  5 ′ is varied. 
     When signals having frequencies within or adjacent to the terrestrial television broadcasting channel, such as channel six, for example, or other VHF high band signals that are received, the trap circuit control voltage V f  is applied to the trap circuit  5 ′ via the first feeding resistor  17  from the integrated circuit  4 . Since the switch diode  15  is reverse-biased, the switch diode  15  is switched off. By applying a tuning voltage V t  to the varactor diode  18  corresponding to a channel to be received, the capacitance of the varactor diode  18  is varied. The capacitance of varactor diode  18  tunes the trap circuit  5 ′ to a predetermined frequency. Signals at this predetermined frequency are attenuated by the trap circuit  5 ′. When signals having frequencies within channel six are received, the trap circuit  5 ′ attenuates signals having a frequency near 91.25 MHz. When signals having frequencies within the VHF high band are received, the trap circuit  5 ′ attenuates signals having a frequency which is half of the center frequency of a channel to be received. The tuning voltage V t  also causes the tuning frequency of a VHF antenna tuning circuit  6  and that of a VHF interstage tuning circuit  8  to change. When VHF high band signals are received, the tuning frequencies of the VHF antenna tuning circuit  6  and those of the VHF interstage tuning circuit  8  range from 174 MHz to 216 MHz, and the trap circuit  5 ′ is set so that the attenuated frequency varies between about 88.5 MHz and about 106.5 MHz. 
     When signals having frequencies within the terrestrial television broadcasting VHF low band, apart from the band of channel six, are received, the trap circuit control voltage V f  is not applied to the trap circuit  5 ′ and a band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  6 . Since the switch diode  15  is forward-biased, the switch diode  15  is switched on. Since the input terminal  5   a ′ and the output terminal  5   b ′ of the trap circuit  5 ′ are short-circuited, high frequency signals are received by the VHF antenna tuning circuit  6  without being attenuated by the trap circuit  5 ′. 
     When CATV signals are received, the trap circuit control voltage V f  is not applied to the switch diode  15  and the band selection voltage V hi  or V lo  is applied to the VHF antenna tuning circuit  6 . Since the switch diode  15  is forward-biased, the switch diode  15  is switched on. Therefore, since the input terminal  5   a ′ and the output terminal  5   b ′ of the trap circuit  5 ′ are short-circuited, CATV signals having frequencies within the entire VHF band are received by the VHF antenna tuning circuit  6 .