Patent Publication Number: US-7595706-B2

Title: High-frequency distribution circuit for distributing high-frequency signal

Description:
This nonprovisional application is based on Japanese Patent Applications Nos. 2004-263990, 2005-039408, 2005-138352, and 2005-180657 filed with the Japan Patent Office on Sep. 10, 2004, Feb. 16, 2005, May 11, 2005, and Jun. 21, 2005, respectively, the entire contents of which are hereby incorporated by reference. Japanese Application 2005-180657 was published as Japanese Patent Laid-Open No. 2006-345464 on Dec. 21, 2006. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to high-frequency distribution circuits and particularly to high-frequency distribution circuits distributing to a plurality of output terminals a frequency signal received at an input terminal. 
   2. Description of the Background Art 
     FIG. 19  is a block diagram showing a configuration of a receiving unit of a satellite broadcast system as conventional. In  FIG. 19  the satellite broadcast system has the receiving unit including an antenna  103  having a reflector  101  and a low noise block down converter (LNB)  102 , receivers (or load circuits)  104  and  105 , and television sets  106  and  107 . 
   A satellite emits an electric wave α which is received via reflector  101  by LNB  102 . LNB  102  extracts video signals of a plurality of channels from the received electric wave α and also amplifies the video signals with minimized noise, and provides receivers  104  and  105  with video signals (or high-frequency signals) of the channels selected by receivers  104  and  105 , respectively. LNB  102  outputs a signal which is in turn FM-demodulated in each of receivers  104  and  105  and furthermore converted to video and audio signals and provided to television sets  106  and  107 . Television sets  106  and  107  display on their screens the video images of the channels selected by the tuners of receivers  104  and  105 , respectively (see for example Japanese Patent Laying-Open No. 2002-218329). 
   Such LNB  102  is provided therein with a high-frequency distribution circuit distributing a video signal to the two receivers  104  and  105 , and its power supply voltage is supplied from receivers  104  and  105 . 
   The characteristic impedance of a coaxial cable connected to each output terminal of such LNB  102 , switch (SW)-BOX and the like, and the input impedance of receivers  104  and  105  are typically 75Ω. As such, if in LNB  102  or the like a received signal is monitored at one output terminal and the other output terminal has nothing connected thereto, then at the other output terminal the received signal is totally reflected. Thus whether the other output terminal is connected or not provides a difference in level of a received signal monitored at one output terminal, poor isolation, and other similar disadvantages. 
   If any unused output terminal is terminated by a termination of 75Ω, the variation in impedance attributed to whether the output terminal is used or not can be eliminated. Providing the LNB, the SW-BOX or other similar products with a termination as an accessory, however, is significantly costly. 
   A final-stage amplifier or the like may have an attenuator inserted therein to attenuate in level a signal reflected from an output terminal. This, however, requires that the amplifier be increased in gain, which can result in increased current consumption, poor phase noise, and/or similar detriments. 
   SUMMARY OF THE INVENTION 
   Accordingly the present invention mainly contemplates an inexpensive high-frequency distribution circuit capable of preventing variation in level of a received signal, poor isolation and the like attributed to whether an output terminal is used or not. 
   The present high-frequency distribution circuit is a high-frequency distribution circuit that distributes to a plurality of output terminals a high-frequency signal received at an input terminal, and includes: a plurality of high-frequency lines associated with the plurality of output terminals, respectively, and each having one end connected to the input terminal; a terminator resistor associated with each high-frequency line; and a switch circuit associated with each high-frequency line, and passing a high-frequency signal from the other end of an associated high-frequency line to an associated output terminal if a load circuit is connected to the associated output terminal, and grounding the other end of the associated high-frequency line via an associated terminator resistor if the load circuit is not connected to the associated output terminal. 
   The present invention provides another high-frequency distribution circuit that has a plurality of input terminals and a plurality of output terminals and selects a high-frequency signal of a plurality of high-frequency signals, which are provided to the plurality of input terminals, for each output terminal to provide the selected high-frequency signal to the output terminal, and includes: a plurality of high-frequency lines associated with the plurality of output terminals, respectively; a select circuit selecting a high-frequency signal of a plurality of high-frequency signals, which are provided to the plurality of input terminals, for each high-frequency line to provide the selected high-frequency signal to one end of the high-frequency line; a terminator resistor associated with each high-frequency line; and a switch circuit associated with each high-frequency line, and passing a high-frequency signal from the other end of an associated high-frequency line to an associated output terminal if a load circuit is connected to the associated output terminal, and grounding the other end of the associated high-frequency line via an associated terminator resistor if the load circuit is not connected to the associated output terminal. 
   Preferably the high-frequency distribution circuit further includes a control circuit associated with each output terminal, and outputting a first signal if a load circuit is connected to an associated output terminal, and outputting a second signal if the load circuit is not connected to the associated output terminal, wherein the switch circuit passes a high-frequency signal from the other end of the associated high-frequency line to the associated output terminal if an associated control circuit outputs the first signal, and the switch circuit grounds the other end of the associated high-frequency line via the associated terminator resistor if the associated control circuit outputs the second signal. 
   Still preferably the load circuit applies a power supply voltage to the output terminal in response to the load circuit being connected to the output terminal; and the control circuit outputs the first signal if the power supply voltage is applied to the associated output terminal, and the control circuit outputs the second signal if the power supply voltage is not applied to the associated output terminal Still preferably the switch circuit includes a SPDT including a common terminal connected to the other end of the associated high-frequency line, a first conduction terminal connected to the associated output terminal, a second conduction terminal connected to one end of the terminator resistor, and a control terminal, and if a first voltage is applied to the control terminal, the common terminal and the first conduction terminal are electrically connected, and if a second voltage is applied to the control terminal, the common terminal and the second conduction terminal are electrically connected; the terminator resistor has the other terminal grounded; and the first signal is the first voltage applied to the first control terminal and the second signal is the second voltage applied to the control terminal. 
   Still preferably the high-frequency distribution circuit further includes: a subordinate terminator resistor associated with each SPDT; and a subordinate SPDT associated with each SPDT, and including a subordinate common terminal connected to a first conduction terminal of an associated SPDT, a first subordinate conduction terminal connected to an associated output terminal, a second subordinate conduction terminal connected to one terminal of an associated subordinate terminator resistor, and a subordinate control terminal, the subordinate SPDT having the subordinate common terminal and the first subordinate conduction terminal electrically connected when the first voltage is applied to the subordinate control terminal, the subordinate SPDT having the subordinate common terminal and the second subordinate conduction terminal electrically connected when the second voltage is applied to the subordinate control terminal. The subordinate terminator resistor has the other terminal grounded and equal voltage is applied to the subordinate control terminal of the subordinate SPDT and the control terminal of the associated SPDT. 
   Still preferably the switch circuit includes a switching element connected in series to an associated terminator resistor between the other end of an associated high-frequency line and a line of a ground potential, and not conducting if the control circuit outputs the first signal and conducting if the control circuit outputs the second signal. 
   Still preferably the high-frequency distribution circuit further includes: a subordinate terminator resistor associated with each switching element; and a subordinate switching element connected in series to an associated subordinate terminator resistor between the other end of an associated high-frequency line and a line of ground potential, and not conducting if the control circuit outputs the first signal and conducting if the control circuit outputs the second signal. 
   Still preferably the high-frequency distribution circuit further includes an amplifier associated with each high-frequency line, and receiving a high-frequency signal from the other end of an associated high-frequency line to amplify the high-frequency signal and provides an associated output terminal with the high-frequency signal amplified, wherein the control circuit activates an associated amplifier if the load circuit is connected to the associated output terminal, and the control circuit inactivates the amplifier if the load circuit is disconnected from the associated output terminal. 
   Still preferably the high-frequency distribution circuit further includes: a subordinate terminator resistor associated with each switch circuit; and a subordinate switch circuit associated with each switch circuit and disposed between an associated switch circuit and an associated output terminal, and passing a high-frequency signal having passed through the associated switch circuit to the associated output terminal if the load circuit is connected to the associated output terminal, and guiding a high-frequency signal, which has leaked from the associated switch circuit, via an associated subordinate terminator resistor to a line of ground potential if the load circuit is disconnected from the associated output terminal. 
   Still preferably the high-frequency distribution circuit is configured as a discrete circuit. 
   Still preferably the high-frequency distribution circuit is configured as an integrated circuit. 
   The present high-frequency distribution circuit is provided with a switch circuit which passes a high-frequency signal form the other end of a high-frequency line to an output terminal if a load circuit is connected to the output terminal and which grounds the other end of the high-frequency line via a terminator resistor if the load circuit is not connected to the output terminal. Variation in level of a received signal, poor isolation and the like attributed to whether the output terminal is used or not can be prevented. Furthermore, lower price can be achieved than when a termination is used. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1-4  are circuit diagrams showing configurations of the present high-frequency distribution circuit in first to fourth embodiments, respectively. 
       FIG. 5  is a circuit diagram showing an exemplary variation of the fourth embodiment. 
       FIG. 6  is a circuit diagram showing a configuration of the present high-frequency distribution in a fifth embodiment. 
       FIG. 7  is a circuit diagram showing an exemplary variation of the fifth embodiment. 
       FIG. 8  is a circuit diagram showing a configuration of the present high-frequency distribution in a sixth embodiment. 
       FIG. 9  is a circuit diagram showing an exemplary variation of the sixth embodiment. 
       FIG. 10  is a circuit diagram showing another exemplary variation of the sixth embodiment. 
       FIGS. 11-14  are circuit diagrams showing still other exemplary variations of the sixth embodiment. 
       FIG. 15  is a circuit diagram showing a configuration of the present high-frequency distribution in a seventh embodiment. 
       FIG. 16  is a circuit diagram showing an exemplary variation of the seventh embodiment. 
       FIG. 17  is a circuit diagram showing another exemplary variation of the seventh embodiment. 
       FIG. 18  is a circuit diagram showing still another exemplary variation of the seventh embodiment. 
       FIG. 19  is a block diagram showing a configuration of a receiving unit of a satellite broadcast system as conventional. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a first embodiment. In  FIG. 1  the high-frequency distribution circuit is provided in a LNB, a SW-BOX or the like and includes an input terminal  1 , high-frequency lines  2  and  3 , a resistor  4 , switch circuits  5  and  6 , terminator resistors  7  and  8 , capacitors  9 ,  10 ,  13  and  14 , amplifiers  11  and  12 , and output terminals  15  and  16 . 
   High-frequency lines  2  and  3  each have one end connected to input terminal  1 , and have their respective other ends connected to switch circuits  5  and  6  at common terminals  5   c  and  6   c , respectively. Resistor  4  has a sufficiently larger value in resistance than terminator resistors  7  and  8  and is connected between the other ends of high-frequency lines  2  and  3 , respectively. High-frequency lines  2  and  3  is equal in dimension and characteristic impedance (e.g., 75Ω). 
   Switch circuit  5  has a first conduction terminal  5   a  connected via capacitor  9  to an input node of amplifier  11 , and amplifier  11  has an output node connected via capacitor  13  to output terminal  15 . Terminator resistor  7  is connected between a second conduction terminal  5   b  of switch circuit  5  and a ground potential GND line. Terminator resistor  7  has a value in resistance (of 75Ω) equal in value to the characteristic impedance of high-frequency line  2 . 
   If output terminal  15  is connected via a coaxial cable to receiver  104 , switch circuit  5  conducts via common terminal  5   c  and the first conduction terminal  5   a . Note that the coaxial cable connected to output terminal  15  has a characteristic impedance of 75Ω and receiver  104  has an input resistance value of 75Ω for the sake of illustration. High-frequency line  2  passes a high-frequency signal which is in turn transmitted via switch circuit  5 , capacitor  9 , amplifier  11 , capacitor  13 , output terminal  15  and the coaxial cable to receiver  104 . 
   If output terminal  15  is not connected to receiver  104 , switch circuit  5  conducts via common terminal  5   c  and the second conduction terminal  5   b  and high-frequency line  2  has the other end grounded via terminator resistor  7 . Whether output terminal  15  may be connected via the coaxial line to receiver  104  or may not be connected to receiver  104 , an impedance of 75Ω is provided, as seen at input terminal  1  toward output terminal  15 , and does not vary. Note that switch circuit  5  may manually be switched, or may be switched by a control circuit, as described later. 
   Switch circuit  6  has a first conduction terminal  6   a  connected via capacitor  10  to an input node of amplifier  12 , and amplifier  12  has an output node connected via capacitor  14  to output terminal  16 . Terminator resistor  8  is connected between a second conduction terminal  6   b  of switch circuit  6  and a ground potential GND line. Terminator resistor  8  has a value in resistance (of 75Ω) equal in value to the characteristic impedance of high-frequency line  3 . 
   If output terminal  16  is connected via a coaxial cable to receiver  105 , switch circuit  6  conducts via common terminal  6   c  and the first conduction terminal  6   a . Note that the coaxial cable connected to output terminal  16  has a characteristic impedance of 75Ω and receiver  105  has an input resistance value of 75Ω for the sake of illustration. High-frequency line  3  passes a high-frequency signal which is in turn transmitted via switch circuit  6 , capacitor  10 , amplifier  12 , capacitor  14 , output terminal  16  and the coaxial cable to receiver  105 . 
   If output terminal  16  is not connected to receiver  105 , switch circuit  6  conducts via common terminal  6   c  and the second conduction terminal  6   b  and high-frequency line  3  has the other end grounded via terminator resistor  8 . Whether output terminal  16  may be connected via the coaxial line to receiver  105  or may not be connected to receiver  105 , an impedance of 75Ω is provided, as seen at input terminal  1  toward output terminal  16 , and does not vary. Note that switch circuit  6  may manually be switched, or may be switched by a control circuit, as described later. 
   The high-frequency distribution circuit operates as described hereinafter. If output terminals  15  and  16  are connected to receivers  104  and  105 , respectively, switch circuit  5  conducts via common terminal  5   c  and the first conduction terminal  5   a  and switch circuit  6  conducts via common terminal  6   c  and the first conduction terminal  6   a . In that case, the value in resistance as seen at input terminal  1  toward output terminal  15  and that as seen at input terminal  1  towards output terminal  16  are both 75Ω, and a high-frequency signal received at input terminal  1  is distributed to the two output terminals  15  and  16  equally. 
   If output terminal  15  is connected to receiver  104  and output terminal  16  is not connected to receiver  105 , switch circuit  5  conducts via common terminal  5   c  and the first conduction terminal  5   a  and switch circuit  6  conducts via common terminal  6   c  and the second conduction terminal  6   b . In that case the value in resistance as seen at input terminal toward output terminal  15  and that as seen at input terminal  1  toward output terminal  16  are also both 75Ω and a high-frequency signal received at input terminal  1  is distributed toward output terminals  15  and  16  equally. This also applies if output terminal  16  is connected to receiver  15  and output terminal  15  is not connected to receiver  104 . Whether one of output terminals  15  and  16  may be connected to a receiver or not, a high-frequency signal is reliably distributed. 
   In the first embodiment if output terminals  15  and  16  are connected to receivers  104  and  105 , a high-frequency signal is passed from the other ends of high-frequency lines  2  and  3 , respectively, to output terminals  15  and  16 , and if output terminals  15  and  16  are not connected to receiver  104  and  105  then high-frequency lines  2  and  3  have their respective other ends grounded via terminator resistors  7  and  8 . This can prevent variation in level of a received signal, poor isolation and the like attributed to whether output terminals  15  and  16  are connected to receivers  104  and  105 . Furthermore, better operability can be provided than when a termination is used. An externally attached component can be dispensed with, and improved workability and lower price can be achieved. 
   Note that while in the first embodiment the characteristic impedance of high-frequency lines  2  and  3  and the value in resistance of terminator resistors  7  and  8  are equal to that of receivers  104  and  105 , or 75Ω, the former may have a value different from the latter, (e.g. 50Ω) and an impedance converter converting 50Ω to 75Ω may be provided between amplifiers  11  and  12  and output terminals  15  and  16 . 
   Second Embodiment 
     FIG. 2  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a second embodiment. The high-frequency distribution circuit of  FIG. 2  corresponds to that of  FIG. 1  with switch circuits  5  and  6  of  FIG. 1  implemented by single pole double throws (SPDTs)  20  and  21 , respectively. 
   SPDT  20  includes a common terminal  20   c , first and second conduction terminals  20   a  and  20   b , and first and second control terminals  20   d  and  20   e . Common terminal  20   c  is connected to the other end of high-frequency line  2 . The first conduction terminal  20   a  is connected via capacitor  9  to an input node of amplifier  11 . The second conduction terminal  20   b  is connected via terminator resistor  7  and a capacitor  22  to a ground potential GND line. Capacitor  22  is provided to prevent a direct current (dc) current from flowing from the second conduction terminal  20   b  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. 
   If output terminal  15  is connected via a coaxial cable to receiver  104 , SPDT  20  receives a high level (3V) and a low level (0V) at the first and second control terminals  20   d  and  20   e , respectively, and conducts via common terminal  20   c  and the first conduction terminal  20   a.    
   If output terminal  15  is not connected to receiver  104 , SPDT  20  receives the low and high levels at the first and second control terminals  20   d  and  20   e , respectively, and conducts via common terminal  20   c  and the second conduction terminal  20   b , and high-frequency line  2  has the other end grounded via terminator resistor  7 . 
   SPDT  21  includes a common terminal  21   c  , first and second conduction terminals  21   a  and  21   b , and first and second control terminals  21   d  and  21   e . Common terminal  21   c  is connected to the other end of high-frequency line  3 . The first conduction terminal  21   a  is connected via capacitor  10  to an input node of amplifier  12 . The second conduction terminal  21   b  is connected via terminator resistor  8  and a capacitor  23  to a ground potential GND line. Capacitor  23  is provided to prevent a direct current (dc) current from flowing from the second conduction terminal  21   b  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. 
   If output terminal  16  is connected via a coaxial cable to receiver  105 , SPDT  21  receives the high and low levels at the first and second control terminals  21   d  and  21   e , respectively, and conducts via common terminal  21   c  and the first conduction terminal  21   a.    
   If output terminal  16  is not connected to receiver  105 , SPDT  21  receives the low and high levels at the first and second control terminals  21   d  and  21   e , respectively, and conducts via common terminal  21   c  and the second conduction terminal  21   b , and high-frequency line  3  has the other end grounded via terminator resistor  8 . The remainder in configuration and operation is identical to that described in the first embodiment. Accordingly it will not be described repeatedly. 
   The second embodiment can provide the same effect as the first embodiment. Note that the use of the SPDT contributes to increased current consumption, which, however, is as small as negligible. 
   Third Embodiment 
     FIG. 3  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a third embodiment. The high-frequency distribution circuit of  FIG. 3  corresponds to that of  FIG. 1  with switch circuit  5  of  FIG. 1  configured of PIN diodes  31  and  32 , capacitors  33  and  34 , a resistor  35  and first and second control terminals  36  and  37 , and switch circuit  6  configured of PIN diodes  41  and  42 , capacitors  43  and  44 , a resistor  45  and first and second control terminals  46  and  47 . 
   Capacitor  33  is connected between the other end of high-frequency line  2  and capacitor  9 . Diode  31  has an anode connected to one terminal of terminator resistor  7  and has a cathode connected to a node located between capacitors  9  and  33 . Diode  31  has resistance set to have a sufficiently small value when it conducts. Terminator resistor  7  has the other terminal connected via the first control terminal  36  and capacitor  34  to a ground potential GND line. Capacitor  34  is provided to prevent a dc current from flowing from first control terminal  36  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. Diode  32  has an anode connected to the second terminal  37  and a cathode connected to that of diode  31 . Diode  32  has resistance set to have a sufficiently large value when it conducts. Resistor  35  has a value in resistance sufficiently larger than terminator resistors  7  and  8  and is connected between the cathodes of diodes  31  and  32  and the ground potential GND line. 
   If output terminal  15  is connected via a coaxial cable to receiver  104 , the first control terminal  36  receives a first voltage V 1  and the second control terminal  37  receives a second voltage V 2  higher than the first voltage V 1 , and diode  32  conducts and diode  31  does not conduct. This allows a dc current to flow from the second control terminal  37  via diode  32  and resistor  35  to the ground potential GND line. Furthermore, as diode  32  and resistor  35  are sufficiently high in resistance, a high-frequency signal passing through high-frequency line  2  is output via capacitors  33  and  9 , amplifier  11  and capacitor  13  to output terminal  15 . 
   If output terminal  15  is not connected to receiver  104 , the first control terminal  36  receives the first voltage V 1  and the second control terminal  37  receives a third voltage V 3  lower than the first voltage V 1 , and diode  31  conducts and diode  32  does not conduct. This allows a dc current to flow from the first control terminal  36  via terminator resistor  7 , diode  31  and resistor  35  to the ground potential GND line. Furthermore, as capacitor  33 , diode  31  and capacitor  34  have an impedance set to have a sufficiently lower value than terminator resistor  7  does, high-frequency line  2  has the other end grounded via capacitor  33 , diode  31 , terminator resistor  7  and capacitor  34  for high frequency. 
   Capacitor  43  is connected between the other end of high-frequency line  3  and capacitor  10 . Diode  41  has an anode connected to one terminal of terminator resistor  8  and has a cathode connected to a node located between capacitors  10  and  43 . Diode  41  has resistance set to have a sufficiently small value when it conducts. Terminator resistor  8  has the other terminal connected via the first control terminal  46  and capacitor  44  to a ground potential GND line. Capacitor  44  is provided to prevent a dc current from flowing from first control terminal  46  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. Diode  42  has an anode connected to the second terminal  47  and a cathode connected to that of diode  41 . Diode  42  has resistance set to have a sufficiently large value when it conducts. Resistor  45  has a value in resistance sufficiently larger than terminator resistors  7  and  8  and is connected between the cathodes of diodes  41  and  42  and the ground potential GND line. 
   If output terminal  16  is connected via a coaxial cable to receiver  105 , the first control terminal  46  receives the first voltage V 1  and the second control terminal  47  receives the second voltage V 2  higher than the first voltage V 1 , and diode  42  conducts and diode  41  does not conduct. This allows a dc current to flow from the second control terminal  47  via diode  42  and resistor  45  to the ground potential GND line. Furthermore, as diode  42  and resistor  45  are sufficiently high in resistance, a high-frequency signal passing through high-frequency line  3  is output via capacitors  43  and  10 , amplifier  12  and capacitor  14  to output terminal  16 . 
   If output terminal  16  is not connected to receiver  105 , the first control terminal  46  receives the first voltage V 1  and the second control terminal  47  receives the third voltage V 3  lower than the first voltage V 1 , and diode  41  conducts and diode  42  does not conduct. This allows a dc current to flow from the first control terminal  46  via terminator resistor  8 , diode  41  and resistor  45  to the ground potential GND line. Furthermore, as capacitor  43 , diode  41  and capacitor  44  have an impedance set to have a sufficiently lower value than terminator resistor  8  does, high-frequency line  3  has the other end grounded via capacitor  43 , diode  41 , terminator resistor  8  and capacitor  44  for high frequency. The remainder in configuration and operation is identical to that described in the first embodiment. Accordingly it will not be described repeatedly. 
   The third embodiment can provide the same effect as the first embodiment. 
   Fourth Embodiment 
     FIG. 4  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a fourth embodiment. The high-frequency distribution circuit of  FIG. 4  corresponds to that of  FIG. 1  plus control circuits  51  and  52 , high-frequency lines  53  and  54 , and capacitors  55  and  56 . 
   High-frequency line  53  and capacitor  55  are connected in series between output terminal  15  and a ground potential GND line and configure a lowpass filter which prevents a high-frequency signal from passing therethrough and passes dc voltage therethrough. Control circuit  51  determines whether dc voltage is applied at a node N 53  located between high-frequency line  53  and capacitor  55 , and controls switch circuit  5  in accordance with the decision. 
   If output terminal  15  is connected via a coaxial cable to receiver  104 , receiver  104  supplies an output terminal of an LNB, an SW-BOX or the like, i.e., output terminal  15  of the high-frequency distribution circuit, via the coaxial cable with dc voltage as a power supply voltage for the LNB, the SW-BOX or the like. Output terminal  15  receives the dc voltage which is in turn transmitted on high-frequency line  53  to node N 53 . As node N 53  receives the dc voltage, control circuit  51  responsively controls switch circuit  5  to conduct via common terminal  5   c  and the first conduction terminal  5   a  to pass a high-frequency signal to output terminal  15 . 
   If output terminal  15  is not connected to receiver  104 , output terminal  15  and hence node N 53  do not receive dc voltage. Responsively, control circuit  51  controls switch circuit  5  to conduct via common terminal  5   c  and the second conduction terminal  5   b  to terminate the other end of high-frequency line  2 . 
   High-frequency line  54  and capacitor  56  are connected in series between output terminal  16  and a ground potential GND line and configure a lowpass filter which prevents a high-frequency signal from passing therethrough and passes dc voltage therethrough. Control circuit  52  determines whether dc voltage is applied at a node N 54  located between high-frequency line  54  and capacitor  56 , and controls switch circuit  6  in accordance with the decision. 
   If output terminal  16  is connected via a coaxial cable to receiver  105 , receiver  105  supplies an output terminal of the LNB, an SW-BOX or the like, i.e., output terminal  16  of the high-frequency distribution circuit, via the coaxial cable with dc voltage as a power supply voltage for the LNB, the SW-BOX or the like. Output terminal  16  receives the dc voltage which is in turn transmitted on high-frequency line  54  to node N 54 . As node N 54  receives the dc voltage, control circuit  52  responsively controls switch circuit  6  to conduct via common terminal  6   c  and the first conduction terminal  6   a  to pass a high-frequency signal to output terminal  16 . 
   If output terminal  16  is not connected to receiver  105 , output terminal  16  and hence node N 54  do not receive dc voltage. Responsively, control circuit  52  controls switch circuit  6  to conduct via common terminal  6   c  and the second conduction terminal  6   b  to terminate the other end of high-frequency line  3 . The remainder in configuration and operation is identical to that described in the first embodiment. Accordingly it will not be described repeatedly. 
   The fourth embodiment can provide the same effect as the first embodiment. Furthermore, it can also prevent variation in level of a received signal, poor isolation and the like attributed to variation in impedance caused as receivers  104  and  105  connected to output terminals  15  and  16  are powered on/off. 
     FIG. 5  is a circuit diagram showing an exemplary variation of the fourth embodiment. In this exemplary variation if node N 53  receives dc voltage, control circuit  51  controls switch circuit  5  to conduct via common terminal  5   c  and the first conduction terminal  5   a  and also activates amplifier  11 . If node N 53  does not receive dc voltage, control circuit  51  controls switch circuit  5  to conduct via common terminal  5   c  and the second conduction terminal  5   b  and also inactivates amplifier  11 . 
   If node N 54  receives dc voltage, control circuit  52  controls switch circuit  6  to conduct via common terminal  6   c  and the first conduction terminal  6   a  and also activates amplifier  12 . If node N 54  does not receive dc voltage, control circuit  52  controls switch circuit  6  to conduct via common terminal  6   c  and the second conduction terminal  6   b  and also inactivates amplifier  12 . If amplifiers  11  and  12  are not required they can be inactivated. Reduced power consumption can thus be achieved. 
   Fifth Embodiment 
     FIG. 6  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a fifth embodiment. The high-frequency distribution circuit of  FIG. 6  corresponds to that of  FIG. 1  plus control circuits  61  and  62 . 
   Control circuit  61  operates in response to a switch signal φ 1  to control switch circuit  5 . Switch signal φ 1  may be applied from receiver  104 , generated in the high-frequency distribution circuit in response to detecting that a coaxial cable is connected to output terminal  15 , or generated in response to a user instruction. 
   If output terminal  15  is connected via the coaxial cable to receiver  104 , switch signal φ 1  is set high. In response to signal φ 1  set high, control circuit  61  applies a first control signal to switch circuit  5  to control switch circuit  5  to conduct via common terminal  5   c  and the first conduction terminal  5   a  to pass a high-frequency signal to output terminal  15 . 
   If output terminal  15  is not connected to receiver  104 , switch signal φ 1  is set low. In response to signal φ 1  set low, control circuit  61  applies a second control signal to switch circuit  5  to control switch circuit  5  to conduct via common terminal  5   c  and the second conduction terminal  5   b  to pass a high-frequency signal to terminate the other end of high-frequency line  2 . 
   Control circuit  62  operates in response to switch signal φ 2  to control switch circuit  6 . Switch signal φ 2  is generated in the same method as switch signal φ 1 . 
   If output terminal  16  is connected via the coaxial cable to receiver  105 , switch signal φ 2  is set high. In response to signal φ 2  set high, control circuit  62  applies the first control signal to switch circuit  6  to control switch circuit  6  to conduct via common terminal  6   c  and the first conduction terminal  6   a  to pass a high-frequency signal to output terminal  16 . 
   If output terminal  16  is not connected to receiver  105 , switch signal φ 2  is set low. In response to signal φ 2  set low, control circuit  62  applies the second control signal to switch circuit  6  to control switch circuit  6  to conduct via common terminal  6   c  and the second conduction terminal  6   b  to pass a high-frequency signal to terminate the other end of high-frequency line  3 . The remainder in configuration and operation is identical to that described in the first embodiment. Accordingly it will not be described repeatedly. 
   The fifth embodiment can provide the same effect as the first embodiment. 
     FIG. 7  is a circuit diagram showing an exemplary variation of the fifth embodiment. In this exemplary variation control circuit  61  operates in response to switch signal φ 1  having the high level to control switch circuit  5  to conduct via common terminal  5   c  and the first conduction terminal  5   a , and also to activate amplifier  11 . Furthermore control circuit  61  operates in response to switch signal φ 1  having the low level to control switch circuit  5  to conduct via common terminal  5   c  and the second conduction terminal  5   b , and also to inactivate amplifier  11 . 
   Control circuit  62  operates in response to switch signal φ 2  having the high level to control switch circuit  6  to conduct via common terminal  6   c  and the first conduction terminal  6   a , and also to activate amplifier  12 . Furthermore control circuit  62  operates in response to switch signal φ 2  having the low level to control switch circuit  6  to conduct via common terminal  6   c  and the second conduction terminal  6   b , and also to inactivate amplifier  12 . If amplifiers  11  and  12  are not required they can be inactivated. Reduced power consumption can thus be achieved. 
   Sixth Embodiment 
     FIG. 8  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in a sixth embodiment. The high-frequency distribution circuit of  FIG. 8  differs from that of  FIG. 1  in that input terminal  1  and resistor  4  are removed and input terminal  65  and  66  and a 2×2 switch circuit  67  are introduced. Input terminals  65  and  66  receive different high-frequency signals, respectively. 2×2 switch circuit  67  selects for one output terminal  15  one of two such high-frequency signals provided to the two input terminals  65  and  66  and provides the selected high-frequency signal to output terminal  15 . Furthermore 2×2 switch circuit  67  selects for the other output terminal  16  one of two such high-frequency signals provided to the two input terminals  65  and  66  and provides the selected high-frequency signal to output terminal  16 . As such, output terminals  15  and  16  may receive identical high-frequency signals, respectively, or may receive different high-frequency signals, respectively. 
   If this high-frequency distribution circuit also has output terminals  15  and  16  with receivers  104  and  105  connected thereto, it passes a high-frequency signal from the other ends of high-frequency lines  2  and  3  to output terminals  15  and  16 , If the high-frequency distribution circuit has output terminals  15  and  16  without receivers  104  and  105  connected thereto, high-frequency lines  2  and  3  have their respective other ends grounded via terminator resistors  7  and  8 , respectively. This can prevent variation in level of a received signal, poor isolation and the like attributed to whether output terminals  15  and  16  are connected to receivers  104  and  105 . Furthermore, better operability can be provided than when a termination is used. An externally attached component can be dispensed with, and improved workability and lower price can be achieved. 
   Note that it is needless to say that as shown in  FIGS. 9-14 , the high-frequency distribution circuits of  FIGS. 2-7  with input terminal  1  and resistor  4  replaced with input terminals  65  and  66  and 2×2 switch circuit  67  are equally effective. 
   Seventh Embodiment 
   If, for the high-frequency distribution circuit of  FIG. 8 , one desires for example that a high-frequency signal provided to input terminal  65  be provided to output terminal  15  alone, a portion of the high-frequency signal would leak via 2×2 switch circuit  67  toward output terminal  16 . The leaked high-frequency signal is terminated at switch circuit  6  and terminator resistor  8 . However, a portion of the leaked high-frequency signal further leaks via switch circuit  6  to output terminal  16 . If output terminal  16  has a varying impedance connected thereto, the impedance&#39;s variation causes the high-frequency signal leaking toward output terminal  16  to vary in amplitude and as a result a high-frequency signal at output terminal  15  would have a varied amplitude. A seventh embodiment addresses such disadvantage. 
     FIG. 15  is a circuit diagram showing a configuration of the present high-frequency distribution circuit in the seventh embodiment in comparison with  FIG. 8 . The high-frequency distribution circuit of  FIG. 15  differs from that of  FIG. 8  in that the former has switch circuits  71  and  72  and terminator resistors  73  and  74  added thereto. 
   Switch circuit  5  has the first conduction terminal  5   a  connected to switch circuit  71  at a common terminal  71   c . Switch circuit  71  has a first conduction terminal  71   a  connected via capacitor  9  to amplifier  11  at an input node. Switch circuit  71  has a second conduction terminal  71   b  connected via terminator resistor  73  to a ground potential GND line. Switch circuits  5  and  71  are similarly switched. When switch circuit  5  conducts via terminals  5   a  and  5   c , switch circuit  71   c  conducts via terminals  71   a  and  71   c . When switch circuit  5  conducts via terminals  5   b  and  5   c , switch circuit  71  conducts via terminals  71   b  and  71   c.    
   Switch circuit  6  has the first conduction terminal  6   a  connected to switch circuit  72  at a common terminal  72   c . Switch circuit  71  has a first conduction terminal  72   a  connected via capacitor  10  to amplifier  12  at an input node. Switch circuit  72  has a second conduction terminal  72   b  connected via terminator resistor  74  to a ground potential GND line. Switch circuits  6  and  72  are similarly switched. When switch circuit  6  conducts via terminals  6   a  and  6   c , switch circuit  72  conducts via terminals  72   a  and  72   c . When switch circuit  6  conducts via terminals  6   b  and  6   c , switch circuit  72  conducts via terminals  72   b  and  72   c.    
   If one desires that a high-frequency signal provided to input terminal  65  be provided to output terminal  15  alone, a portion of the high-frequency signal would leak via 2×2 switch circuit  67  toward output terminal  16 . The leaked high-frequency signal is terminated at switch circuit  6  and terminator resistor  8 . However, a portion of the leaked high-frequency signal further leaks via switch circuit  6  toward switch circuit  72 . The high-frequency signal having leaked from switch  6  is terminated at switch circuit  72  and terminator resistor  74 . As a result, a high-frequency signal leaking to output terminal  16  can significantly be reduced in amplitude, and if a varying impedance is connected to output terminal  16 , the effect that the variation of the impedance has on the amplitude of the high-frequency signal at output terminal  15  can be reduced. 
     FIG. 16  is a circuit diagram showing an exemplary variation of the seventh embodiment in comparison with  FIG. 9 . The high-frequency distribution circuit of  FIG. 16  differs from that of  FIG. 9  in that SPDTs  75  and  76 , terminator resistors  77  and  78 , and capacitors  79  and  80  are additionally introduced. 
   SPDT  75  includes a common terminal  75   c , first and second conduction terminals  75   a  and  75   b , and first and second control terminals  75   d  and  75   e . Common terminal  75   c  is connected to SPDT  20  at the first conduction terminal  20   a . The first conduction terminal  75   a  is connected via capacitor  9  to an input node of amplifier  11 . The second conduction terminal  75   b  is connected via terminator resistor  77  and a capacitor  79  to a ground potential GND line. 
   The first and second control terminals  75   d  and  75   e  of SPDT  75  receive a signal having the same level as the first and second control terminals  20   d  and  20   e  of SPDT  20 . SPDTs  20  and  75  are similarly switched. If SPDT  20  conducts via terminals  20   a  and  20   c , SPDT  75  conducts via terminals  75   a  and  75   c . If SPDT  20  conducts via terminals  20   b  and  20   c , SPDT  75  conducts via terminals  75   b  and  75   c.    
   SPDT  76  includes a common terminal  76   c , first and second conduction terminals  76   a  and  76   b , and first and second control terminals  76   d  and  76   e . Common terminal  76   c  is connected to SPDT  21  at the first conduction terminal  21   a . The first conduction terminal  76   a  is connected via capacitor  10  to an input node of amplifier  12 . The second conduction terminal  76   b  is connected via terminator resistor  78  and a capacitor  80  to a ground potential GND line. 
   The first and second control terminals  76   d  and  76   e  of SPDT  76  receive a signal having the same level as the first and second control terminals  21   d  and  21   e  of SPDT  21 . SPDTs  21  and  76  are similarly switched. If SPDT  21  conducts via terminals  21   a  and  21   c , SPDT  76  conducts via terminals  76   a  and  76   c . If SPDT  21  conducts via terminals  21   b  and  21   c , SPDT  76  conducts via terminals  76   b  and  76   c.    
   This exemplary variation also has the same effect as the seventh embodiment. 
     FIG. 17  is a circuit diagram showing another exemplary variation of the seventh embodiment in comparison with  FIG. 10 . The high-frequency distribution circuit of  FIG. 17  differs from that of  FIG. 10  in that PIN diodes  81 ,  82 ,  91 ,  92 , capacitors  83 ,  84 ,  93 ,  94 , resistors  85 ,  88 ,  95 ,  98 , first control terminals  86 ,  96 , and second terminals  87 ,  97  are additionally introduced. 
   Capacitor  83  is connected between one terminal of resistor  35  and capacitor  9 . Diode  81  has an anode connected to one terminal of terminator resistor  88  and has a cathode connected to a node located between capacitors  88  and  9 . Diode  81  has resistance set to have a sufficiently small value when it conducts. Terminator resistor  88  has the other terminal connected via the first control terminal  86  and capacitor  84  to a ground potential GND line. Capacitor  84  is provided to prevent a dc current from flowing from first control terminal  86  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. Diode  82  has an anode connected to the second terminal  87  and a cathode connected to that of diode  81 . Diode  82  has resistance set to have a sufficiently large value when it conducts. Resistor  85  has a value in resistance sufficiently larger than terminator resistor  88  and is connected between the cathodes of diodes  81  and  82  and the ground potential GND line. 
   The first and second control terminals  86  and  87  receive the same voltages as the first and second control terminals  36  and  37 , respectively. If diode  32  conducts and diode  31  does not conduct, diode  82  conducts and diode  81  does not conduct. If diode  32  does not conduct and diode  31  conducts, diode  82  does not conduct and diode  81  conducts. 
   Capacitor  93  is connected between one terminal of resistor  45  and capacitor  10 . Diode  91  has an anode connected to one terminal of terminator resistor  98  and has a cathode connected to a node located between capacitors  93  and  10 . Diode  91  has resistance set to have a sufficiently small value when it conducts. Terminator resistor  98  has the other terminal connected via the first control terminal  96  and capacitor  94  to a ground potential GND line. Capacitor  94  is provided to prevent a dc current from flowing from first control terminal  96  to the ground potential GND line and has a sufficiently low impedance for a high-frequency signal. Diode  92  has an anode connected to the second terminal  97  and a cathode connected to that of diode  91 . Diode  92  has resistance set to have a sufficiently large value when it conducts. Resistor  95  has a value in resistance sufficiently larger than terminator resistor  98  and is connected between the cathodes of diodes  91  and  92  and the ground potential GND line. 
   The first and second control terminals  96  and  97  receive the same voltages as the first and second control terminals  46  and  47 , respectively. If diode  42  conducts and diode  41  does not conduct, diode  92  conducts and diode  91  does not conduct. If diode  42  does not conduct and diode  41  conducts, diode  92  does not conduct and diode  91  conducts. 
   This exemplary variation also has the same effect as the seventh embodiment. 
     FIG. 18  is a circuit diagram showing a still another exemplary variation of the seventh embodiment in comparison with  FIG. 11 . The high-frequency distribution circuit of  FIG. 18  differs from that of  FIG. 11  in that switch circuits  71  and  72  and terminator resistors  73  and  74  are additional introduced. Switch circuits  71  and  72  and terminator resistors  73  and  74  are connected and operate, as has been described with reference to  FIG. 15 . 
   This exemplary variation also has the same effect as the seventh embodiment. 
   Note that the above described high-frequency distribution circuit may be configured as an integrated circuit having a transistor, a diode, a resistor, a capacitor and the like provided on a single semiconductor substrate, or may be a discrete circuit having an individual component arranged on a printed circuit board and connected. 
   Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.