Television – Receiver circuitry – Tuning
Reexamination Certificate
2000-11-16
2003-12-16
Lee, Michael H. (Department: 2614)
Television
Receiver circuitry
Tuning
C348S725000, C455S295000, C455S317000
Reexamination Certificate
active
06665022
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a VHF high frequency input circuit of a TV tuner.
A constitution of a VHF high frequency input circuit of a conventional TV tuner will be described with reference to FIG.
3
. One end of an FM trap circuit
32
is connected to an input end S
1
via a DC blocking capacitor
31
.
The FM trap circuit
32
constitutes a parallel resonance circuit composed of a capacitor
32
a
and an inductor
32
b
, and resonates to a frequency of an FM broadcasting signal. An inductor
36
and an inductor
37
are connected in series to the other end of the FM trap circuit
32
via a DC blocking capacitor
33
, and the other end of the inductor
37
is grounded via a DC blocking capacitor
38
.
Further, an anode of a switching diode
40
is connected via a DC blocking capacitor
39
to a connecting point connecting the inductor
36
with the inductor
37
. Further, one end of an inductor
43
is connected to a cathode of the diode
40
via a DC blocking capacitor
42
. A cathode of a tuning varactor diode
44
is connected to the other end of the inductor
43
, and an anode of the diode
44
is grounded. Further, the other end of the inductor
43
is connected to a high frequency amplifier
46
via a DC blocking capacitor
45
.
In addition, a cathode of a switching diode
34
is connected to one end of the FM trap circuit
32
. Further, an anode of a diode
34
is connected to one end of an inductor
41
, and also is connected to a connecting point connecting the inductor
36
with the DC blocking capacitor
33
. In addition, the other end of the inductor
41
is connected to the cathode of the diode
40
.
Further, a terminal Fm, as for application of a bias voltage to the diodes
34
and
40
, is connected to the other end of the FM trap circuit
32
via a resistor, a terminal Lb is connected to the other end of the inductor
37
via a resistor, and a terminal Hb is connected to the anode of the switching diode
40
via a resistor. In addition, a terminal Tu, as for application of a tuning voltage to the varactor diode
44
is connected to the one end of the inductor
43
via a resistor.
Next, an operation of an input tuning circuit of the conventional TV tuner will be described. When receiving a proximity TV signal of a frequency the closest to an FM broadcasting band, the switching diode is brought into a nonconductive state by respectively imparting voltages Vlo and Vfm (Vfm>Vlo) to the terminals Lb and Fm and by applying a reverse bias voltage to the switching diode
34
, and then an FM broadcasting signal is attenuated by making the FM trap circuit
32
operable. Simultaneously, the switching diode
40
is also brought into a nonconductive state by grounding the terminal Hb and by applying a reverse bias voltage to the diode
40
. Further, a voltage Vtu is applied to the varactor diode
44
from the terminal Tu. With such operations, an output signal of the received proximity TV signal is transmitted to a high frequency amplifier
46
by tuning in to a frequency of a received signal by means of an inductance composed of the inductor
36
and the inductor
37
connected to each other in series and of the inductor
41
and the inductor
43
connected to each other in series and by means of a capacitance of the varactor diode
44
.
Further, when receiving a VHF low band except the proximity TV signal of a frequency the closest to the FM broadcasting band, the switching diode
34
is brought into a conductive state as a by-pass route of the FM trap circuit
32
by grounding the terminal Fm and by adding a bias voltage Vlo to the terminal Lb. Simultaneously, the switching diode
40
is brought into a nonconductive state by grounding the terminal Hb and by applying a reverse bias voltage to the switching diode
40
. With such operations, similarly as described above, the output signal of the received VHF low band can be transmitted to the high frequency amplifier
46
by tuning in to a frequency of a received signal by means of an inductance composed of the inductor
36
and the inductor
37
connected to each other in series and of the inductor
41
and the inductor
43
connected to each other in series and by means of a capacitance of the varactor diode
44
.
Further, when receiving a VHF high band, the switching diode
34
is brought into a conductive state as a by-pass route of the FM trap circuit
32
by grounding the terminal Fm and by adding a bias voltage Vhi to the terminal Hb. Simultaneously, the switching diode
40
is brought into a conductive state by grounding the terminal Lb and by applying a bias voltage to the diode
40
. With such operations, the output signal of the received VHF high band is transmitted to the high frequency amplifier
46
by tuning in to a frequency of a received signal by means of an inductance composed of the inductor
37
and the inductor
43
and by means of a capacitance of the varactor diode
44
.
In the conventional TV tuner as described above, the diode
34
, the capacitor
33
, the inductor
36
, and the inductor
41
are connected to one another. However, these connections are conducted by means of a conductor land provided on the printed circuit board. Further, since the size of the conductor land is large, an inductance thereof is not negligible. An equivalent circuit of a portion of the conductor land paid attention to a side of the FM trap circuit
32
when receiving a VHF high band is illustrated in FIG.
4
. That is, an inductor
47
having a very small inductance by means of the conductor land described above is intervened between the anode of the diode
34
and the other end of the capacitor
33
. Therefore, drawbacks as described hereinafter will be generated.
When receiving the VHF high band, as described above, the switching diode
34
is brought into a conductive state without transmitting a received signal to the FM trap circuit
32
. However, at this time, the inductor
47
having a very small inductance is connected to the diode
34
in series. Accordingly, the inductor
47
is connected to the FM trap circuit
32
in parallel. In the mean time, since a capacitance value of the capacitor
33
is remarkably large compared with a capacitance value of the capacitor
32
a
, the capacitor
33
is negligible. Further, since an inductance of the inductor
32
b
is exceedingly large compared with an inductance of the inductor
47
, the inductor
32
b
is negligible.
Accordingly, a parallel resonance circuit composed of the capacitor
32
a
and the inductor
47
is constituted from the result described above. Further, a resonance frequency of a parallel resonance circuit by means of a capacitance of the capacitor
32
a
set for an FM trap frequency and a very small inductance of the inductor
47
resides in a receiving VHF high band frequency band. Therefore, a received VHF high band signal receives attenuation in the vicinity of this resonance frequency, therefore has worsened characteristics such as NF, input sensitivity, and S/N.
SUMMARY OF THE INVENTION
Thus, an object of the present invention is to provide a TV tuner in which when receiving a VHF high band, a received VHF high band signal receives no attenuation at all and no influence is exerted on characteristics such as NF, input sensitivity, and S/N.
An input circuit of a TV tuner according to the present invention includes a parallel resonance circuit composed of a capacitor and a first inductor with one end of the parallel resonance circuit being connected to an input end, an input tuning circuit connected to the other end of the parallel resonance circuit described above and capable of tuning in to frequencies of respective bands by being switched to a low band and a high band of VHF bands, and a varactor diode brought into a nonconductive state when receiving a proximity TV signal of a frequency the closest to an FM broadcasting band and brought into a conductive state when receiving other TV signals. One end of the varactor diode is connected to one end of the parallel resonance circuit described above. The other end of t
Furuta Toshiro
Yamamoto Masaki
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Lee Michael H.
LandOfFree
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