JPH11205699A - Input circuit for television tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To absorb and attenuate unnecessary FM bands when receiving ground waves through a more simplified FM trap circuit and further to receive both the ground wave and cable television(CATV) through ON/OFF operation of a switch. SOLUTION: A first trap circuit 4 connecting in parallel a first coil 4a and a first capacitor 4b and a second trap circuit 5 connecting in parallel a second coil 5a and a second capacitor 5b are connected in series and are provided in series to a signal transmission line, wherein the trap frequency of the first and second trap circuits 4 and 5 is set to the frequency of an FM broadcasting band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input circuit of a television tuner, and more particularly to a so-called terrestrial television signal or cable television (hereinafter C).
The present invention relates to an input circuit of a television tuner suitable for receiving a signal (referred to as ATV).

[0002]

2. Description of the Related Art FIG. 3 shows an input circuit (hereinafter simply referred to as an input circuit) of a conventional television tuner. In FIG. 3, the input circuit includes an IF trap circuit 31 for attenuating a signal in a television intermediate frequency band, an FM trap circuit 32 for attenuating a signal in an FM broadcast band (FM broadcast signal), and an antenna tuning circuit 33. ing. The input terminal 31a of the IF trap circuit 31 is connected to an antenna for receiving terrestrial television or a cable (both not shown) for transmitting a CATV signal, and the output terminal 33a of the antenna tuning circuit 33 has a high frequency. It is connected to an amplifier (not shown). In CATV, channels are also allocated to the FM broadcast band (for example, 88 MHz to 108 MHz in the U.S. specification). The FM trap circuit 32 is a circuit for attenuating an FM broadcast signal that interferes when receiving a terrestrial television signal, and includes a first coil 34a and a first capacitor 3 connected in parallel.
4b, a first trap circuit 34 provided in series on the signal transmission path, and a second coil 3 connected in parallel.
5A, a second capacitor 35b and a third capacitor 35c connected in series to the second capacitor 35b, and a second trap circuit 35 provided between the signal transmission path and the ground. One end of the first trap circuit 34 is connected to the input terminal 32 a of the FM trap circuit 32, and the other end is connected to the output terminal 32 b of the FM trap circuit 32 via the DC blocking capacitor 36.

In a television tuner of the United States specification, the trap frequency of the first trap circuit 34 is set to, for example, 95 MHz, and the trap frequency of the second trap circuit 35 is set to, for example, 105 MHz. The cathode of the first switch diode 37 is connected to the input terminal 32a, and the anode is connected to the output terminal 32a.
b, and the first switch diode 37 and the first trap circuit 34 are connected in parallel. Further, by providing the resistor 38 between the input terminal 32a and the ground, the cathode of the first switch diode 37 is connected to the ground via the resistor 38.

Next, the second trap circuit 35 is connected via a second switch diode 39 to the other end of the first trap circuit 34 provided in series on the signal transmission line, thereby providing a signal. It is connected between the transmission line and the ground. Then, a connection point between the second coil 35a and the second capacitor 35b connected in parallel with the anode of the second switch diode 39 and the FM trap voltage terminal 4
The power supply resistor 41 is connected between the power supply resistor 41 and the power supply resistor 41. Note that FM
The antenna tuning circuit 33 connected to the output terminal 32b of the trap circuit 32 is supplied with a band switching voltage from a high band switching terminal 42 and a low band switching terminal 43, thereby providing a terrestrial television signal. Alternatively, the CATV signal is divided into low-band and high-band television signals for tuning.

In the above configuration, when a terrestrial television signal is received, both the first trap circuit 34 and the second trap circuit 35 in the FM trap circuit 32 function, and the CATV signal is transmitted. When receiving, these first trap circuits 3
The fourth and second trap circuits 35 are prevented from functioning together. Hereinafter, this operation will be described in detail. First,
When receiving a terrestrial television signal, a voltage of, for example, 5 V is applied to the FM trap voltage terminal 40. Then, a voltage is applied to the second switch diode 39 in the forward direction, so that the second switch diode 39 becomes conductive and a voltage drop occurs in the resistor 38. On the other hand, a band switching voltage is supplied to the antenna tuning circuit 33 from the high band switching terminal 42 or the low band switching terminal 43, and the band switching voltage is supplied to the first switching diode via the antenna tuning circuit 33. It is also transmitted to 37 anodes.

Here, the potential applied to the anode of the first switch diode 37 is set to be lower than the potential applied to the cathode of the first switch diode 37. Then, a reverse voltage is applied to the first switch diode 37, so that the first switch diode 37 is turned off. Therefore, the FM trap circuit 32 shown in FIG. 3 is equivalent to that shown in FIG. Circuit. Here, a coil 35d of the second trap circuit 35 in FIG. 4 is a coil having an equivalent inductance at a trap frequency by the second coil 35a and the second capacitor 35b in FIG. As a result, the FM broadcast signal is attenuated by the first trap circuit 34 and the second trap circuit 35,
Interference during reception of terrestrial television signals is eliminated.

Next, when a CATV signal is received, no voltage is applied to the FM trap voltage terminal 40,
A band switching voltage is applied to the high band switching terminal 42 or the low band switching terminal 43. Then, a forward voltage is applied to the first switch diode 37, so that the first switch diode 37 is turned on, and a voltage drop occurs in the resistor 38. As a result, a reverse voltage is applied to the second switch diode 39, so that the second switch diode 39 is turned off. Accordingly, the input terminal 32a and the output terminal 32b of the FM trap circuit 32 are short-circuited by the first switch diode 37, and the second trap circuit 35 is separated from the signal transmission path by the second switch diode 39. , The first trap circuit 34 and the second trap circuit 35 become invalid. As a result, the CATV signal allocated to the FM broadcast band can also be received.

[0008]

By the way, like the second trap circuit 35, a trap circuit provided between the signal transmission line and the ground can be originally constituted by a series resonance circuit composed of only a coil and a capacitor. However, in that case, to increase the Q of the trap circuit, it is necessary to increase the inductance of the coil. However, since the size of the coil is increased due to this, the second coil 3
A second capacitor 35b is provided in parallel with 5a. As a result, there is a problem that parts increase. In the case of receiving a CATV signal, a second switch diode 39 must be separately provided in order to disconnect the second trap circuit 35 from the signal transmission line, which also increases the number of components. There was a problem. The present invention solves these problems, and an object of the present invention is to realize an FM trap circuit with a small number of components, a simple circuit, and a high Q.

[0009]

In order to solve the above-mentioned problems, an input circuit of a television tuner according to the present invention comprises: a first trap circuit in which a first coil and a first capacitor are connected in parallel; A second trap circuit in which a second coil and a second capacitor are connected in parallel is provided, and the first trap circuit and the second trap circuit are connected in series and provided in series on a signal transmission path, The trap frequency of the first trap circuit and the trap frequency of the second trap circuit were set to frequencies in the FM broadcast band.

The input circuit of the television tuner of the present invention has a switch diode that is conductive when receiving a terrestrial television signal and is non-conductive when receiving a cable television signal. Was connected in parallel to a first trap circuit and a second trap circuit connected in series at a high frequency.

Further, the input circuit of the television tuner of the present invention includes an antenna tuning circuit for supplying a band switching voltage and a trap voltage terminal for supplying a voltage to one end of a switch diode. To the other end of the switch diode, and by the voltage supplied to the trap voltage terminal,
The switch diode was made conductive or non-conductive.

[00012]

FIG. 1 shows an input circuit (hereinafter simply referred to as an input circuit) of a television tuner of the present invention. In FIG. 1, the input circuit includes an IF trap circuit 1 for attenuating a signal in a television intermediate frequency band, an FM trap circuit 2 for attenuating a signal in an FM broadcast band (FM broadcast signal), and an antenna tuning circuit 3. ing. And I
An input terminal 1a of the F trap circuit 1 is connected to an antenna for receiving a terrestrial television signal or a cable (both not shown) for transmitting a CATV signal, and an output terminal 3a of the antenna tuning circuit 3 is connected to a high frequency amplifier. (Not shown). In CATV, channels are also allocated to the FM broadcast band (for example, 88 MHz to 108 MHz in the U.S. specification). The FM trap circuit 2 is a circuit for attenuating an FM broadcast signal that interferes when receiving a terrestrial television signal, and includes a first coil 4a and a first capacitor 4b connected in parallel. A trap circuit 4 and a second trap circuit 5 including a second coil 5a and a second capacitor 5b connected in parallel are connected in series at one end, and
They are provided in series on the signal transmission line. The other end of the first trap circuit 4 is connected to the input terminal 2a of the FM trap circuit 2.
And the other end of the second trap circuit 5 is connected to the output terminal 2b of the FM trap circuit 2. In a television tuner of the United States specification, the trap frequency of the first trap circuit 4 is set to, for example, 95 MHz, and the trap frequency of the second trap circuit 5 is set to, for example, 105 MHz.

Further, between the input terminal 2a and the output terminal 2b,
A DC blocking capacitor 6 and a switch diode 7 connected in series are provided. Thus, the switch diode 7 is provided in parallel with respect to the first trap circuit 4 and the second trap circuit 5 connected in series at high frequency. Note that the anode of the switch diode 7 is connected to the output terminal 2b. Next, one end of the power supply resistor 8 is connected to a connection point between the DC blocking capacitor 6 and the switch diode 7, and the other end is connected to the FM trap voltage terminal 9. The antenna tuning circuit 3 connected to the output terminal 2b of the FM trap circuit 2 has a high band switching terminal 10 and a low band switching terminal 1
The band switching voltage is supplied from 1
As a result, the television signal or the CATV signal is divided into low-band and high-band television signals for tuning.

In the above configuration, when a terrestrial television signal is received, the first trap circuit 4 and the second trap circuit 5 are operated, and C
When the ATV signal is received, the first trap circuit 4 and the second trap circuit 5 are disabled. Hereinafter, this operation will be described in detail. First, when receiving a terrestrial television signal, a voltage of, for example, 5 V is applied to the cathode of the switch diode 7 from the FM trap voltage terminal 9 via the resistor 8. On the other hand, a band switching voltage is supplied to the antenna tuning circuit 3 from the high band switching terminal 10 or the low band switching terminal 11, and the band switching voltage is supplied to the antenna tuning circuit 3 via the antenna tuning circuit 3. It also comes to join. At this time, the voltage applied to the anode of the switch diode 7 is set in advance so as to be lower than the voltage applied to the cathode of the switch diode 7. Therefore, a voltage is applied to the switch diode 7 in the reverse direction, so that the switch diode 7 is turned off, and the FM trap circuit 2 shown in FIG. 1 becomes an equivalent circuit shown in FIG.
As a result, the FM broadcast signal is attenuated by the first trap circuit 4 and the second trap circuit 5, so that interference at the time of receiving a terrestrial television signal can be eliminated. Moreover,
Since the two trap circuits 4 and 5 are each configured by a parallel resonance circuit, the number of components is reduced, and the inductance values of the first coil 4a and the second coil 5a are reduced.
Therefore, even if the shape of the coil is reduced, the Q can be increased, so that a large attenuation can be obtained.

Next, when a CATV signal is received, no voltage is applied to the FM trap voltage terminal 9 and a band switching voltage is applied to the high band switching terminal 10 or the low band switching terminal 11. Then, a forward voltage is applied to the switch diode 7 to be in a conductive state. Therefore, the input terminal 2a and the output terminal 2b of the FM trap circuit 2 are short-circuited by the switch diode 7, so that F
The M trap circuit 2 becomes invalid. As a result, the CATV signal allocated to the FM broadcast band can be received. When the CATV signal is received, the FM trap circuit 2 is invalidated by using only one switch diode 7, so that the number of parts is reduced. In addition,
In the case of a tuner that receives only a terrestrial television signal, it is only necessary to remove the switch diode 7 and the like and to provide only the first trap circuit 4 and the second trap circuit 5 in the signal transmission path. Is less.

[0016]

As described above, the input circuit of the television tuner according to the present invention comprises a first trap circuit in which a first coil and a first capacitor are connected in parallel, a second coil and a second coil. A second trap circuit in which the first and second capacitors are connected in parallel, the first trap circuit and the second trap circuit are connected in series, and the second trap circuit is provided in series on the signal transmission path, and the trap of the first trap circuit is provided. By setting the frequency and the trap frequency of the second trap circuit to the frequency of the FM broadcast band, a high Q can be obtained with a simple circuit having a smaller number of components.

Further, the input circuit of the television tuner of the present invention has a switch diode that is conductive when receiving a terrestrial television signal and is non-conductive when receiving a cable television signal. By connecting the first trap circuit and the second trap circuit connected in series in parallel at high frequency,
A terrestrial television signal and a CATV signal can be received with a simpler circuit.

The input circuit of the television tuner of the present invention includes an antenna tuning circuit for supplying a band switching voltage and a trap voltage terminal for supplying a voltage to one end of a switch diode. The band switching voltage supplied to the input tuning circuit is used because the switch diode is turned on or off by the voltage supplied to the other end of the switch diode via the trap voltage terminal. Terrestrial television signal reception status and CAT
It can be switched to the V signal receiving state.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an input circuit of a television tuner of the present invention.

FIG. 2 is an equivalent circuit diagram of the input circuit of the television tuner of the present invention when receiving a terrestrial television signal.

FIG. 3 is a circuit diagram of an input circuit of a conventional television tuner.

FIG. 4 shows an input circuit of a conventional television tuner.
FIG. 3 is an equivalent circuit diagram when a terrestrial television signal is received.

[Description of Signs] 1, IF trap circuit 1a, 2a Input terminal 2 FM trap circuit 2b, 3a Output terminal 3 Antenna tuning circuit 4 First trap circuit 4a First coil 4b First capacitor 5 Second FM trap Circuit 5a Second coil 5b Second capacitor 6 DC blocking capacitor 7 Switch diode 8 Power supply resistor 9 FM trap voltage terminal 10 High band switching terminal 11 Low band switching terminal

Claims (3)

[Claims] A first trap circuit in which a first coil and a first capacitor are connected in parallel; and a second trap circuit in which a second coil and a second capacitor are connected in parallel. Connecting the first trap circuit and the second trap circuit in series, and providing them in series in a signal transmission path, and setting the trap frequency of the first trap circuit and the trap frequency of the second trap circuit. Is set to the frequency of the FM broadcast band. 2. A switch diode, which is conductive when receiving a terrestrial television signal and is non-conductive when receiving a cable television signal, wherein the switch diode is connected to the first series-connected diode. 2. The input circuit of a television tuner according to claim 1, wherein said input circuit is connected in parallel to said trap circuit and said second trap circuit in terms of high frequency. 3. An antenna tuning circuit for supplying a band switching voltage and a trap voltage terminal for supplying a voltage to one end of the switch diode, wherein the band switching voltage is supplied to the other of the switch diode via the antenna tuning circuit. 3. The input circuit of a television tuner according to claim 2, wherein the switch diode is turned on or off by a voltage supplied to an end of the switch and the trap voltage terminal.