UHF/VHF turner

Television – Receiver circuitry – Tuning

Reexamination Certificate

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Details

C348S732000, C348S733000, C348S505000, C348S555000

Reexamination Certificate

active

06392715

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a UHF/VHF tuner and, more particularly, to a UHF/VHF tuner in which complication in structure and increase in manufacturing costs are suppressed and broadcast receiving performance is improved by reducing variation in the reception sensitivity according to frequencies when a broadcast in the VHF band is received.
Hitherto, in a UHF/VHF tuner for U.S.A., an RF circuit for UHF and an RF circuit for VHF are separately provided and a frequency converting stage of each of the UHF and VHF circuits is constructed by one mixer and one local oscillator.
FIG. 3
is a block diagram showing an example of the construction of main components of a known UHF/VHF tuner.
FIG. 4
is a block diagram in which an example of the construction of frequency converting stages of the UHF/VHF tuner shown in
FIG. 3
is illustrated in a partial circuit diagram. Component elements which are the same as those shown in
FIG. 3
are designated by the same reference numerals.
As shown in
FIG. 3
, the known UHF/VHF tuner comprises a UHF antenna (ANT) tuning circuit
31
, a UHF high frequency amplifier (RFAMP)
32
, a UHF interstage tuning circuit
33
, a UHF mixer (MIX)
34
, a UHF local oscillator (OSC)
35
, a VHF antenna (ANT) tuning circuit
36
, a VHF high frequency amplifier (RFAMP)
37
, a VHF interstage tuning circuit
38
, a VHF mixer (MIX)
39
, a VHF local oscillator (OSC),
40
, an intermediate frequency amplifier (IFAMP)
41
, a reception signal input terminal
42
, and an intermediate frequency signal output terminal
43
. The component elements
31
to
43
are connected as shown in FIG.
3
.
As shown in
FIG. 4
, the UHF local oscillator
35
is constructed by a first circuit part
35
(
1
) including a transistor for oscillation integrated in an integrated circuit
44
and a second circuit part
35
(
2
) externally attached to the integrated circuit
44
. The VHF local oscillator
40
is also constructed by a first circuit part
40
(
1
) including a transistor for oscillation integrated in the integrated circuit
44
and a second circuit part
40
(
2
) externally attached to the integrated circuit
44
.
The second circuit part
35
(
2
) of the UHF local oscillator
35
comprises a varactor diode
35
1
, an inductor
35
2
, capacitors
35
3
to
35
8
, and resistors
35
9
and
35
10
. Those elements
3
51
to
35
10
are connected as shown in FIG.
4
. The second circuit part
40
(
2
) of the VHF local oscillator
40
comprises varactor diodes
40
1
to
40
3
, inductors
40
4
to
40
6
, capacitors
40
7
to
40
16
, a switching diode
40
17
, and resistors
40
18
to
40
23
. Those elements
40
1
to
40
23
are connected as shown in FIG.
4
. Further, a PLL
45
is arranged in the integrated circuit
44
. Band switching terminals BS
2
and BS
3
in the PLL
45
are connected as shown in FIG.
4
and tuning voltage terminals TU (U) and TU (V) in the PLL
45
are connected also as shown in FIG.
4
.
The known UHF/VHF tuner having the above construction operates as follows.
In case of receiving a broadcast in the UHF band, a reception signal is supplied to the reception signal input terminal
42
. From the reception signal, only a reception signal of a desired channel is selected by the UHF antenna tuning circuit
31
and is amplified to a predetermined level by the UHF amplifying circuit
32
. From the amplified reception signal, only a reception signal of the desired channel is further selected by the UHF interstage tuning circuit
33
and is supplied to the UHF mixer
34
. The UHF mixer
34
mixes the frequencies of the reception signal selected by the UHF interstage tuning circuit
33
and a local oscillation signal generated by the UHF local oscillator
35
. The intermediate frequency amplifier
41
selectively amplifies an intermediate frequency signal indicative of the frequency as the difference between the reception signal and the local oscillation signal and leads the resultant signal to the intermediate frequency signal output terminal
43
.
In this instance, in the UHF local oscillator
35
, the oscillation frequency is set by values such as capacitance values of the varactor diode
35
1
and the capacitors
35
3
and
35
4
and the inductance value of the inductor
35
2
. Among them, the capacitance value of the varactor diode
35
1
is set by an output tuning voltage of the tuning voltage terminal TU (U) in the PLL
45
supplied to the varactor diode
35
1
via the resistor
35
10
.
In the event of receiving a broadcast in the VHF band, a reception signal is supplied to the reception signal input terminal
42
. From the reception signal, only a reception signal of a desired channel is selected by the VHF antenna tuning circuit
36
and is amplified by the VHF amplifying circuit
37
to a predetermined level. From the amplified reception signal, only a reception signal of the desired channel is further selected by the VHF interstage tuning circuit
38
and is supplied to the VHF mixer
39
. The VHF mixer
39
mixes the frequencies of the reception signal selected by the VHF interstage tuning circuit
38
and the local oscillation signal generated by the VHF local oscillator
40
. The intermediate frequency amplifier
41
selectively amplifies an intermediate frequency signal indicative of the frequency as the difference between the reception signal and the local oscillation signal and leads the resultant signal to the intermediate frequency signal output terminal
43
.
In this instance, in the VHF local oscillator
40
, when a high-band channel is selected, the oscillation frequency is set by values such as capacitance values of the varactor diode
40
, and the capacitors
40
7
,
40
8
, and
40
13
, and the inductance value of the inductor
40
4
. On the other hand, when a low-band channel is selected, the oscillation frequency is set by values such as capacitance values of the varactor diode
40
1
, and the capacitors
40
7
to
40
10
, and
40
13
, and the inductance values of the inductors
40
4
and
40
5
. When either one of the channels is selected, the capacitance value of the varactor diode
40
1
is set by an output tuning voltage of the tuning voltage terminal TU (V) in the PLL
45
supplied to the varactor diode
40
1
via the resistor
40
18
. When the high band is selected, a positive voltage is applied to the band switching terminal BS
2
in the PLL
45
. The positive voltage is supplied to the anode of the switching diode
40
17
through the resistor
40
20
and the inductor
40
5
to thereby turn on the switching diode
40
17
. Consequently, the inductor
40
5
and the capacitors
40
9
and
40
10
are bypassed from an oscillation frequency determining circuit in the VHF local oscillator
40
. The oscillation frequency determining circuit is constructed by the varactor diode
40
1
, the capacitors
40
7
,
40
8
, and
40
13
and the inductor
40
4
. On the other hand, when the low band is selected, the positive voltage is applied to the band switching terminal BS
3
in the PLL
45
. The positive voltage is supplied to the cathode of the switching diode
40
17
via the resistor
40
22
to thereby turn off the switching diode
40
17
. Consequently, the inductor
40
5
and the capacitors
40
9
and
40
10
construct the oscillation frequency determining circuit of the VHF local oscillator
40
together with the varactor diode
40
1
, the capacitors
40
7
,
40
8
, and
40
13
, and the inductor
40
4
.
According to the known UHF/VHF tuner, however, when the broadcast in the VHF band is received, it is difficult for the first circuit part
40
(
1
) including the transistor for oscillation to generate a sufficient local oscillation signal in a wide frequency range of both of the high and low bands. In the case where a broadcast of the high frequency side of the high band and/or the low frequency side of the low band is selected, the level of the local oscillation signal becomes lower than that of the local oscillation signal when a broadcast of other frequency is selected and there is a problem such that the re

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