Television – Receiver circuitry – Tuning
Reexamination Certificate
2000-03-15
2003-06-17
Kostak, Victor R. (Department: 2711)
Television
Receiver circuitry
Tuning
C348S735000, C334S006000
Reexamination Certificate
active
06580468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a television tuner.
2. Description of the Related Art
A conventional television tuner will be described by referring to
FIGS. 5 and 6
. A television tuner comprises: a frequency converter
31
for frequency converting a received television signal to an intermediate frequency signal (hereinbelow, abbreviated as an IF signal) and outputting the IF signal; an intermediate frequency tuning circuit (hereinbelow, abbreviated as an IF tuning circuit)
32
for passing the IF signal outputted from the frequency converter
31
; and an intermediate frequency amplifier
33
for amplifying the IF signal and outputting the amplified IF signal.
First, a television signal received by an antenna (not shown) is amplified by an amplifier (not shown) and the amplified signal is supplied to the frequency converter
31
. The frequency converter
31
has first and second output terminals
31
a
and
31
b
and, although not shown, a voltage controlled oscillator and a mixer. In the mixer, an oscillation signal outputted from the voltage controlled oscillator is mixed with the television signal, thereby performing the frequency conversion and obtaining an IF signal. The IF signals are outputted from the first and second output terminals
31
a
and
31
b
in a balanced state. For example, in a television tuner complied with a domestic specification, a sound intermediate frequency SIF is 54.25 MHz, a color intermediate frequency CIF is 55.17 MHz and a picture intermediate frequency PIF is 58.75 MHz.
The IF tuning circuit
32
has a capacitor
32
a
and a tuning coil
32
b
. The capacitor
32
a
and the tuning coil
32
b
are connected in parallel. One end of the tuning coil
32
b
is connected to the first output terminal
31
a
of the frequency converter
31
and the other end of the tuning coil
32
b
is connected to the second output terminal
31
b
of the frequency converter
31
. A tuning frequency f
0
of the IF tuning circuit
32
is set so as to be almost a center frequency (57 MHz in the domestic specification) between the picture intermediate frequency PIF and the color intermediate frequency CIF.
The intermediate frequency amplifier
33
is connected to both ends of the tuning coil
32
b
via coupling capacitors
34
and
35
. The IF signals outputted from the frequency converter
31
pass through the IF tuning circuit
32
and are supplied to the intermediate frequency amplifier
33
in a balanced state. The IF signals are amplified and outputted.
As shown by a solid line in
FIG. 6
, the level of the IF signal outputted from the IF tuning circuit
32
becomes the maximum value (point A) at the tuning frequency f
0
. An output characteristic drawing a curve of the level of the IF signal, which is gradually lowered as the frequency becomes higher or lower than the tuning frequency f
0
is obtained. The level of the picture intermediate frequency PIF is lower than that at the tuning frequency f
0
(point B). Similarly, the level of each of the color intermediate frequency CIF and the sound intermediate frequency SIF is lower than that at the tuning frequency f
0
(points C and D).
For example, transistors (not shown) are used for the mixer of the frequency converter
31
and an amplifier (not shown) provided at the front stage of the frequency converter
31
. Generally, the gain of the transistor decreases when the ambient temperature rises. In the output characteristic of the IF signal of the frequency converter
31
, as shown by a broken line in
FIG. 6
, the gain decreases as a whole. Consequently, as shown by broken lines in
FIG. 6
, the levels of the sound intermediate frequency signal SIF and the picture intermediate frequency signal PIF decrease from the points D and B to points D′ and B′, respectively. Especially, the decrease in the level of the picture intermediate frequency signal PIF causes a problem such that the S/N ratio of an image displayed on a display of a television deteriorates.
SUMMARY OF THE INVENTION
The invention intends to solve the problem and its object is to suppress attenuation of the level of a picture intermediate frequency PIF when the ambient temperature rises.
In order to solve the problem, according to the invention, there is provided a television tuner comprising: a frequency converter for frequency converting a received television signal into an intermediate frequency signal and outputting the intermediate frequency signal; and an intermediate frequency tuning circuit for passing the intermediate frequency signal, wherein a tuning frequency of the intermediate frequency tuning circuit is changed to a higher value in association with a rise in the ambient temperature.
According to the television tuner of the invention, the intermediate frequency tuning circuit has at least a capacitor and a coil, and a capacitor having a negative temperature characteristic is used as the capacitor.
According to the television tuner of the invention, when the tuning frequency of the intermediate frequency tuning circuit is set to an almost center frequency between a picture intermediate frequency and a color intermediate frequency of the television signal at a specific temperature in an ordinary temperature range, the tuning frequency when the ambient temperature rises by 25° C. is set to be higher than the frequency which is the almost center between the picture intermediate frequency and the color intermediate frequency of the television signal and lower than the picture intermediate frequency.
REFERENCES:
patent: 3619803 (1971-11-01), Klein
patent: 3624512 (1971-11-01), Evans
patent: 4160213 (1979-07-01), Carter
patent: 4260956 (1981-04-01), Harford
patent: 4271433 (1981-06-01), Theriault
patent: 4326296 (1982-04-01), Rafter et al.
patent: 4491976 (1985-01-01), Saitoh
patent: 4891708 (1990-01-01), Baba
patent: 7-22974 (1995-01-01), None
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