Television – Receiver circuitry – Television receiver adapted to receive radio broadcast or in...
Reexamination Certificate
2001-08-23
2004-09-14
Lee, Michael H. (Department: 2614)
Television
Receiver circuitry
Television receiver adapted to receive radio broadcast or in...
C348S731000, C348S706000
Reexamination Certificate
active
06791626
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an input switching circuit for a television tuner, which switches input between a television signal and an FM broadcasting signal.
2. Description of the Related Art
In a television tuner which allows reception of FM broadcasting signals, input to a tuning circuit is switched between a television signal and an FM broadcasting signal.
FIG. 3
shows the configuration of a conventional input switching circuit for such a television tuner. Referring to
FIG. 3
, between a first input terminal
41
to which a television signal is input and an output terminal
42
, a high-pass filter
43
, a trap circuit
44
, and a first switching circuit
45
are provided, in that order. The high-pass filter passes the lowest frequency of the television signal (approximately 50 MHz) and higher, and the trap circuit
44
attenuates frequencies around 27 MHz which is used as the citizens' band.
The first switching circuit
45
includes two switching diodes
45
a
and
45
b
connected in series between the first input terminal
41
and the output terminal
42
, the anodes thereof being connected to each other, and a switching diode
45
c
, the cathode thereof being connected to the anodes of the switching diodes
45
a
and
45
b
and the anode thereof being grounded at high frequencies.
The node between the anodes of the switching diodes
45
a
and
45
b
and the cathode of the switching diode
45
c
is connected to a power supply terminal
46
, and the cathodes of the switching diodes
45
a
and
45
b
and the anode of the switching diode
45
c
are connected to a switching terminal
47
.
Meanwhile, between a second input terminal
48
to which an FM broadcasting signal is input and the output terminal
42
, a high-pass filter
49
which passes the FM broadcasting signal, a second switching circuit
50
, and a tuned circuit
51
which is tuned to the frequencies (88 to 108 MHz) of the FM broadcasting signal are provided, in that order. The second switching circuit
50
includes two switching diodes
50
a
and
50
b
connected in series between the second input terminal
48
and the output terminal
42
, the anodes thereof being connected to each other, and a switching diode
50
c
, the cathode thereof being connected to the anodes of the switching diodes
50
a
and
50
b
and the anode thereof being grounded at high frequencies.
The node between the anodes of the switching diodes
50
a
and
50
b
and the cathode of the switching diode
50
c
is connected to the switching terminal
47
, and the cathodes of the switching diodes
50
a
and
50
b
and the anode of the switching diode
50
c
are connected to the collector of a switching transistor
52
.
The collector of the switching transistor
52
is connected to the power supply terminal
46
, the base thereof is connected to the switching terminal
47
, and the emitter thereof is grounded.
One end of the tuned circuit
51
is connected to the cathode of the switching diode
50
b
, and the other end thereof is grounded.
Furthermore, although not shown, an input tuning circuit which can be tuned to different frequencies of the television signal and the FM broadcasting signal is provided subsequent to the output terminal
42
.
In the construction as above, when receiving a television signal, a low level switching voltage (e.g., 0 volts) is applied to the switching terminal
47
. Then, in the first switching circuit
45
, the switching diodes
45
a
and
45
b
are turned on and the switching diode
45
c
is turned off. Thus, the television signal input to the first input terminal
41
is passed through the first switching circuit
45
and output to the output terminal
42
. Referring to
FIG. 4
, A indicates the transmission characteristics between the first input terminal
41
and the output terminal
42
at this time, in which frequencies around 27 MHz are attenuated and frequencies at and above approximately 50 MHz are transmitted.
At this time, the switching transistor
52
is turned off; thus, in the second switching circuit
50
, the switching diodes
50
a
and
50
b
are turned off and the switching diode
50
c
is turned on. Thus, the FM broadcasting signal input to the second input terminal
48
is blocked by the second switching circuit
50
, not being output to the output terminal
42
.
Meanwhile, when receiving an FM broadcasting signal, a high level switching voltage (e.g., a voltage equivalent to the voltage applied to the power supply terminal
46
) is applied to the switching terminal
47
. Then, in the first switching circuit
45
, the switching diodes
45
a
and
45
b
are turned off and the switching diode
45
c
is turned on. Thus, the television signal input to the first input terminal
41
is blocked, not being output to the output terminal
42
.
At this time, the switching transistor
52
is turned on; thus, in the second switching circuit
50
, the switching diodes
50
a
and
50
b
are turned on and the switching diode
50
c
is turned off. Thus, the FM broadcasting signal input to the second input terminal
48
is passed through the second switching circuit
50
and output to the output terminal
42
.
As described above, in the second switching circuit
50
, the switching diodes
50
a
and
50
b
are turned off and the switching diode
50
c
is turned on when receiving the television signal. However, at this time, the switching diodes
50
a
and
50
b
have a minute capacitance on the order of 0.5 pF across the terminals thereof, the terminals of the switching diodes
50
c
are not short-circuited completely, and the tuned circuit
51
is provided between the second switching circuit
50
and the output terminal
42
(i.e., externally to the tuned circuit
50
); thus, the FM broadcasting signal input to the second input terminal
48
leaks from the second switching circuit
50
, is tuned to by the tuned circuit
51
, and appears at the output terminal
42
.
Referring to
FIG. 4
, B indicates the transmission characteristics between the second input terminal
48
and the output terminal
42
at this time, in which a peak indicated by b is observed between 88 MHz and 108 MHz. Thus, interference due to the FM broadcasting signal occurs in receiving the television signal.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an input switching circuit for a television tuner, in which leakage of an FM broadcasting signal is reduced when receiving a television signal, preventing interference due to the FM broadcasting signal.
To this end, the present invention provides an input switching circuit for a television tuner. The television tuner includes a first input terminal, to which a television signal is input; a second input terminal, to which an FM broadcasting signal is input; an output terminal; a first switching circuit provided between the first input terminal and the output terminal, which, between the first input terminal and the output terminal, passes the television signal when receiving the television signal while blocking the television signal when receiving the FM broadcasting signal; and a second switching circuit provided between the second input terminal and the output terminal, which, between the second in put terminal and the output terminal, passes the FM broadcasting signal when receiving the FM broadcasting signal while blocking the FM broadcasting signal when receiving the television signal. The second switching circuit includes first two diodes connected in series with each other between the second input terminal and the output terminal, which are turned off when receiving the television signal and turned on when receiving the FM broadcasting signal; and a second diode connected between the node between the first two diodes and the ground, which is turned on when receiving the television signal and turned off when receiving the FM broadcasting signal. Furthermore, a parallel-tuned circuit which is tuned to the frequencies of the FM broadcasting signal is connected in parallel to the seco
Furuta Toshiro
Yamamoto Masaki
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Lee Michael H.
Tran Trang U.
LandOfFree
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