Television – Receiver circuitry – Television receiver adapted to receive radio broadcast or in...
Reexamination Certificate
1999-06-02
2002-11-19
Miller, John (Department: 2614)
Television
Receiver circuitry
Television receiver adapted to receive radio broadcast or in...
C348S725000, C348S731000, C348S512000, C455S196100, C455S255000
Reexamination Certificate
active
06483552
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a television tuner which controls the oscillation frequency of a local oscillator by using a PLL circuit and can receive both a television signal and an FM broadcasting signal.
2. Related Art
A conventional television tuner comprises, as shown in
FIG. 4
, a VHF tuner
21
, a UHF tuner
22
, a PLL circuit
23
, and a station selection control circuit
24
. A VHF band television signal TV and an FM broadcasting signal FM are supplied to the VHF tuner
21
and a UHF band television signal TU is supplied to the UHF tuner
22
. The VHF tuner
21
has an FM switching circuit
21
a
for performing switching between the VHF band television signal TV and the FM broadcasting signal FM and receiving the switched signal, a tuning circuit
21
b
, a mixer
21
c
, a local oscillator
21
d
, and the like. An intermediate frequency signal IF is outputted from the mixer
21
c.
The tuning circuit
21
b
can be switched between a case of receiving a television signal in the high band of the VHF band and a case of receiving a television signal in the low band of the VHF band. The FM broadcasting signal can be received in a state where the tuning circuit
21
b
is switched to receive the television signal in the low band of the VHF band. Although not illustrated, the UHF tuner
22
has also a tuning circuit, a mixer, and the like and an intermediate frequency signal of the UHF band television signal is outputted via the mixer
21
c
of the VHF tuner
21
.
The PLL circuit
23
comprises a first fixed frequency divider
23
a
, a variable frequency divider
23
b
, a phase comparator
23
c
, a low pass filter
23
d
, a second fixed frequency divider
23
e
, and a reference oscillator
23
f
. The first fixed frequency divider
23
a
, the variable frequency divider
23
b
, the phase comparator
23
c
, and the low pass filter
23
d
construct a closed loop with the local oscillator
21
d
. The first fixed frequency divider
23
a
divides the frequency of a local oscillation signal outputted from the local oscillator
21
d
at a predetermined frequency dividing ratio. The variable frequency divider
23
b
further divides the frequency of the local oscillation signal divided by the fixed frequency divider
23
a
on the basis of frequency dividing ratio data DA outputted from the channel selection control circuit
24
in correspondence with the frequency of a reception channel and outputs the resultant signal to the phase comparator
23
c.
Meanwhile, a reference oscillation signal outputted from the reference oscillator
23
f
is also divided by the second fixed frequency divider
23
e
, thereby obtaining a reference frequency. The reference frequency is supplied to the phase comparator
23
c
. The phase comparator
23
c
compares the phases of two signals and outputs an error signal corresponding to the phase difference to the low pass filter
23
d
. The low pass filter
23
d
smoothes the error signal into a DC voltage. The DC voltage is used as a control voltage and applied to a varactor diode (not shown) in the local oscillator
21
d
, thereby controlling the frequency of the local oscillation signal outputted from the local oscillator
21
d
to be stable.
The station selection control circuit
24
receives a station selection signal S from a station selection operating unit in a television receiver (not shown), outputs the frequency dividing ratio data DA to the variable frequency divider
23
b
and outputs switching voltages BS
1
to BS
4
to the VHF tuner
21
and the UHF tuner
22
. The switching voltage BS
1
is outputted when the low-band television signal is received. The switching voltage BS
2
is outputted when the high-band television signal is received. The switching voltage BS
3
is outputted when the FM broadcasting signal is received. The switching voltage BS
4
is outputted when the UHF band television signal is received. The switching voltages BS
1
and BS
2
are supplied to the tuning circuit
21
b
in the VHF tuner
21
. The switching voltage BS
3
is supplied to the FM switching circuit
21
a
in the VHF tuner
21
. The switching voltage BS
4
is supplied to the UHF tuner
22
.
The tuning circuit
21
b
is switched so as to be tuned to the television signal in the low band of the VHF band and the FM broadcasting signal by the switching voltage BS
1
. The tuning circuit
21
b
is switched so as to be tuned to the television signal in the high band of the VHF band by the switching voltage BS
2
. The FM switching circuit
21
a
outputs the FM broadcasting signal to the tuning circuit
21
b
by the switching voltage BS
3
. Further, the switching voltage BS
4
makes the UHF tuner
22
operative.
Each of the VHF television signal TV and the UHF television signal TU which are supplied to the television tuner has two kinds of television signals; what is called a ground wave television signal and a cable television signal sent via a cable (called a CATV signal). The ground wave television signal is according to what is called a standard frequency arrangement. The frequency of the CATV signal is, however, often slightly deviated from the frequency of the corresponding ground wave television signal. This is generally called an offset. The offset frequency lies within a range from few kHz to ten-odd kHz. The step frequency of the PLL circuit
23
is generally set to not 50 kHz but 31.25 kHz so as to be received even by an offset channel.
The step frequency indicates the minimum frequency interval of local oscillation signals outputted from the local oscillator
21
d
and is determined by the comparison frequency supplied to the phase comparator
23
c
and the frequency dividing ratio of the fixed frequency divider
23
a
. Generally, the oscillation frequency of the reference oscillator
23
f
is set to 4 MHz and the frequency dividing ratio of the second fixed frequency divider
23
e
is set to 1024, so that the reference frequency is 4000/1024=3.90625 kHz. In order to set the step frequency to 31.25 kHz, the frequency dividing ratio of the fixed frequency divider
23
a
is set to 31.25/3.90625=8.
The FM broadcasting signal is outputted at intervals of 100 kHz and, moreover, the frequency minimum unit is 100 kHz. When the step frequency of the PLL circuit
23
is 31.25 kHz, the oscillation frequency of the local oscillator
21
d
is therefore deviated from each of many FM broadcasting frequencies by ten-odd kHz. The sound quality of the FM broadcasting consequently deteriorates.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a television tuner in which the step frequency of the PLL circuit
23
when the FM broadcasting signal is received is made different from that when the television signal is received, thereby realizing just-tuning of the FM broadcasting signal.
In order to solve the problem, there is provided a television tuner capable of receiving both of a television broadcast and an FM radio broadcast, comprising: a mixer for frequency converting a signal of each of the broadcasts into an intermediate frequency signal; a local oscillator for supplying a local oscillation signal to the mixer; and a PLL circuit for controlling the frequency of the local oscillation signal, wherein the PLL circuit varies the frequency of the local oscillation signal in accordance with the case of receiving the television broadcast and the case of receiving the FM radio broadcast by using different step frequencies.
Preferably, according to the television tuner, the PLL circuit comprises: a reference oscillator for outputting an oscillation signal of a predetermined frequency; a reference frequency divider for dividing the frequency of the oscillation signal and outputting the resultant signal as a reference oscillation signal; and a comparison frequency divider for dividing the frequency of the local oscillation signal outputted from the local oscillator and outputting the resultant signal as a comparison signal, the frequency dividing ratio of the reference frequency divid
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Lo Linus H
Miller John
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