Television – Receiver circuitry – Tuning
Reexamination Certificate
1998-03-09
2001-10-23
Eisenzopf, Reinhard J. (Department: 2614)
Television
Receiver circuitry
Tuning
C348S725000, C455S249100, C455S251100
Reexamination Certificate
active
06307599
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an analog-digital broadcast shared receiving tuner, and particularly to an analog-digital broadcast shared receiving tuner capable of reducing the interference of a high-level analog broadcast of an adjacent channel upon reception of a low-level digital broadcast assigned to a vacant channel of an analog broadcast.
2. Description of the Related Art
In recent years, a digital broadcast came into service in a television-broadcasting field in addition to an analog broadcast which has been performed up to now.
Described specifically, a low-level digital broadcast is assigned to a vacant channel of an analog broadcast to transmit an analog-digital mixed broadcast.
Meanwhile, a double conversion type analog-digital broadcast shared receiving tuner having two frequency conversion stages so as to be able to receive a digital broadcast in a satisfactory state has been used as an analog-digital broadcast shared receiving tuner used for receiving such an analog-digital mixed broadcast.
FIG. 3
is a diagram for describing an example of a configuration of such an analog-digital mixed broadcast. FIG.
4
is a block diagram showing one example of the already-known double conversion type analog-digital broadcast shared receiving tuner.
As shown in
FIG. 3
, the analog-digital mixed broadcast is one represented in a signal form in which a digital broadcast channel D is assigned to a vacant channel between analog broadcast channels A
1
and A
2
. The level of the digital broadcast is set so as to reach a low level of about 10 dB with respect to the level of the analog broadcast to eliminate interference with the analog broadcast.
Further, as shown in
FIG. 4
, the double conversion type analog-digital broadcast shared receiving tuner comprises an automatic gain control high-frequency amplifier
41
, a first frequency conversion stage
42
comprised of a first frequency mixer
43
and a first local oscillator
44
, a first intermediate frequency signal pass filter
45
, a first intermediate frequency signal amplifier
46
, a second frequency conversion stage
47
comprised of a second frequency mixer
48
and a second local oscillator
49
, a second intermediate frequency signal pass filter
50
, a second intermediate frequency signal amplifier
51
and a second intermediate frequency signal output terminal
52
. A receiving antenna
53
is electrically connected to the input side of the automatic gain control high-frequency amplifier
41
.
The analog-digital broadcast shared receiving tuner constructed as described above is roughly operated as follows:
When an analog-digital broadcast (50 MHz to 800 MHz) is received by the antenna
53
, the automatic gain control high-frequency amplifier
41
effects automatic gain control amplification on the received high-frequency signal in response to a gain control voltage applied across a gain control terminal
41
c
and supplies the amplified signal to the first frequency conversion stage
42
subsequent to the automatic gain control high-frequency amplifier
41
. In the first frequency conversion stage
42
, the first frequency mixer
43
frequency-mixes the high-frequency signal into a first local oscillation signal supplied from the first local oscillator
44
and supplies the frequency-mixed signal to the first intermediate frequency signal pass filter
45
subsequent to the first frequency mixer
43
. The first intermediate frequency signal pass filter
45
extracts only a first intermediate frequency signal (1 GHz as an example) from the frequency-mixed signal and supplies it to the first intermediate frequency signal amplifier
46
subsequent to the first intermediate frequency signal pass filter
45
. The first intermediate frequency signal amplifier
46
amplifies the first intermediate frequency signal to a predetermined level and supplies it to the second frequency conversion stage
47
subsequent to the first intermediate frequency signal amplifier
46
. In the second frequency conversion stage
47
, the second frequency mixer
48
frequency-mixes the first intermediate frequency signal into a second local oscillation signal supplied from the second local oscillator
49
and supplies the frequency-mixed signal to the second intermediate frequency signal pass filter
50
subsequent to the second frequency mixer
48
. The second intermediate frequency signal pass filter
50
extracts only a second intermediate frequency signal (44 MHz as an example) from the frequency-mixed signal and supplies it to the second intermediate frequency signal amplifier
51
subsequent to the second intermediate frequency signal pass filter
50
. The second intermediate frequency signal amplifier
51
amplifies the second intermediate frequency signal to a predetermined level and supplies it to an available circuit (not shown) through the second intermediate frequency signal output terminal
52
.
In this case, the frequency of the first local oscillation signal of the first frequency conversion stage
42
is selectively adjusted in the analog-digital broadcast shared receiving tuner so that a broadcast for one broadcast channel of the analog broadcast channels A
1
and A
2
or the digital broadcast channel D is received.
The already-known analog-digital broadcast shared receiving tuner has an advantage that the analog and digital broadcasts can be selectively received without individually providing an analog broadcast receiving portion and a digital broadcast receiving portion. However, a problem arises in that when it is desired to set the reception sensitivity of the analog broadcast by the already-known analog-digital broadcast shared receiving tuner substantially equal to the receiving sensitivity of this type (single conversion type) analog broadcast receiving tuner having one frequency conversion stage, interference components resultant from the analog broadcast adjacent to the digital broadcast channel D enter into the digital broadcast upon reception of the digital broadcast, so that the received digital broadcast is greatly degraded in quality.
SUMMARY OF THE INVENTION
With the foregoing problem in view, it is therefore an object of the present invention to provide an analog-digital broadcast shared receiving tuner capable of improving the quality of a received digital broadcast without a reduction in receiving sensitivity of an analog broadcast.
In order to achieve the above object, an analog-digital broadcast shared receiving tuner of the present invention has a variable attenuator electrically connected to the input side of a frequency mixer, which is capable of performing switching between zero attenuation and predetermined attenuation, and means for switching the attenuation of the variable attenuator to the zero attenuation upon reception of an analog television broadcast and switching the attenuation thereof to the predetermined attenuation upon reception of a digital television broadcast.
According to the means, since the attenuation of the connected variable attenuator is set so as to reach the zero attenuation upon reception of the analog television broadcast, the variable attenuator becomes equal to a non-connected state so that comparatively high receiving sensitivity can be obtained. Since the attenuation of the connected variable attenuator is set so as to reach the predetermined attenuation upon reception of the digital television broadcast, interference components resultant from the analog television broadcast, which enter into the digital broadcast received signal, are sufficiently attenuated to an interference non-allowable degree by the variable attenuator, so that the digital broadcast received signal good in quality can be obtained. Incidentally, the digital television broadcast received signal is also attenuated by the variable attenuator in the same manner as described above. However, since the digital signal can be reproduced if only binary states (such as on and off states) can be identified, the quality of the received signal is not degraded due to th
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Eisenzopf Reinhard J.
Tran Trang U.
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