Television tuner intermediate-frequency circuit that hardly suffers disturbance of adjacent channel

1. The object of the invention is to configure so that a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in an adjacent channel can be damped without having an effect upon a tuning characteristic of an intermediate frequency tuning circuit.2. To achieve the object, an integrated circuit including a balanced type mixer provided with a first output terminal and a second output terminal as well as a balanced type intermediate-frequency amplifier provided with a first input terminal and a second input terminal is provided, the first output terminal and the first input terminal as well as the second output terminal and the second input terminal are respectively connected via a first capacitative element formed in the integrated circuit, a first external terminal connected to the first output terminal, a second external terminal connected to the second output terminal and a third external terminal connected to the first input terminal are provided to the integrated circuit, an intermediate frequency tuning circuit is connected between the first external terminal and the second external terminal, and the first external terminal and the third external terminal are connected via an inductance element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an intermediate-frequency circuit of a television tuner, and more specifically relates to an intermediate-frequency circuit formed of a integrated circuit of a television tuner.

2. Description of the Related Art

Referring toFIG. 6, an intermediate-frequency circuit of a conventional type television tuner will be described below. An integrated circuit21includes a mixer22, an intermediate-frequency amplifier23and in addition, a local oscillator (not shown). The mixer22is formed as a balanced type mixer provided with balanced output terminals, a television signal RF of a receive channel selected by an external double-tuned circuit is input to the mixer22and a signal oscillated from the local oscillator is input to the mixer22. An intermediate-frequency signal after frequency conversion is output from the mixer22.

The intermediate-frequency amplifier23is formed as a balanced type amplifier provided with balanced input terminals. The balanced output terminal of the mixer22and the balanced input terminal of the intermediate-frequency amplifier23are connected via each connecting capacitative element24in the integrated circuit21and an intermediate-frequency signal is input to the intermediate-frequency amplifier23. In the integrated circuit21, two external terminals21aand21brespectively connected to the balanced output terminals of the mixer22are also provided and an intermediate frequency tuning circuit25is connected between the external terminals21aand21b. The intermediate frequency tuning circuit25is provided with a tuning characteristic that the intermediate frequency turning circuit tunes to a substantially intermediate frequency between a video intermediate carrier Fp and a chrominance subcarrier Fc as shown in FIG.7.

As described above, a television signal of an adjacent channel is removed by providing the intermediate frequency tuning circuit25connected between the external terminals21aand21bbetween the mixer22and the intermediate-frequency amplifier23, only a television signal of a receive channel is input to the intermediate-frequency amplifier23and an intermediate-frequency signal IF output from the intermediate-frequency amplifier23is input to an image detector (not shown) via a SAW filter (not shown) provided outside the integrated circuit21(as well as other components).

However, in this configuration, a television signal of an adjacent channel cannot be sufficiently damped and a television signal of a receive channel is disturbed by a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in the adjacent channel. For a measure against this, a trap circuit for damping a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in an adjacent channel has only to be provided in the tuning circuit25, however, then, there occurs a problem that the intermediate frequency tuning circuit25and the trap circuit have an effect upon each other and it becomes difficult to set a tuning frequency and a trapping frequency.

SUMMARY OF THE INVENTION

One object of the invention is to provide an arrangement in which a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in an adjacent channel can be damped without having an effect upon a tuning characteristic of an intermediate frequency tuning circuit.

To achieve the above and other objects, the invention is provided with an integrated circuit including a balanced type mixer provided with a first output terminal and a second output terminal for converting a frequency as well as a balanced type intermediate-frequency amplifier provided with a first input terminal and a second input terminal, the first output terminal and the first input terminal as well as the second output terminal and the second input terminal are respectively connected via each first capacitative element formed in the integrated circuit, a first external terminal connected to the first output terminal, a second external terminal connected to the second output terminal and a third external terminal connected to the first input terminal are provided to the integrated circuit, an intermediate frequency tuning circuit that tunes to a substantially central frequency in an intermediate frequency band is connected between the first external terminal and the second external terminal, and the first external terminal and the third external terminal are connected via an inductance element.

Also, a second capacitative element is connected to the inductance element in series, a series-parallel resonance circuit is formed by the first capacitative element, the inductance element and the second capacitative element, a series resonance frequency of the series-parallel resonance circuit is substantially equalized to the central frequency and a parallel resonance frequency is equalized to a frequency of a voice intermediate carrier of an adjacent channel on a downside.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, an intermediate-frequency circuit of a television tuner according to the invention will be described below. As shown inFIG. 1, an integrated circuit1includes a mixer2, an intermediate-frequency amplifier3and in addition, a local oscillator (not shown). The mixer2is formed as a balanced type mixer provided with a first output terminal2aand a second output terminal2b, a television signal RF of a receive channel selected by a double-tuned circuit (not shown) provided outside the integrated circuit1is input to two balanced input terminals2cand2dof the mixer2and a signal oscillated from the local oscillator in the integrated circuit1is input to the mixer. An intermediate-frequency signal after frequency conversion is output from the mixer2.

The intermediate-frequency amplifier3is formed as a balanced type amplifier provided with a first input terminal3aand a second input terminal3b. The first output terminal2aof the mixer2and the first input terminal3aof the intermediate-frequency amplifier3as well as the second output terminal2bof the mixer2and the second input terminal3bof the intermediate-frequency amplifier3are respectively connected via first capacitative elements4,4formed in the integrated circuit1. Hereby, an intermediate-frequency signal is input to the intermediate-frequency amplifier3in a balanced state.

A first external terminal laconnected to the first output terminal2aof the mixer2and a second external terminal1bconnected to the second output terminal2bare also provided to the integrated circuit1and an intermediate frequency tuning circuit5is connected between these external terminals1aand1b. The intermediate frequency tuning circuit5is formed by a parallel tuning circuit composed of an inductance element5aand a capacitative element5band is provided with a tuning characteristic that the intermediate frequency tuning circuit tunes to a substantially intermediate frequency between a video intermediate carrier Fp and a chrominance subcarrier Fc respectively shown in FIG.2.

A third external terminal1cconnected to the first input terminal3aof the intermediate-frequency amplifier3is also provided to the integrated circuit1and an inductance element6is connected between the first external terminal1aand the third external terminal1c.As a result, a parallel resonance circuit7is formed by the first capacitative element4and the inductance element6. The parallel resonance circuit7is set so that it resonates with a voice intermediate carrier −Fs of an adjacent channel on the downside adjacent on the downside (i.e. the lower side of the response curve) of a receive channel.

As a result, an equivalent circuit shown inFIG. 3is formed between the mixer2and the intermediate-frequency amplifier3. As the intermediate frequency tuning circuit5tunes to the substantially intermediate frequency between the video intermediate carrier Fp and the chrominance subcarrier. Fc and the parallel resonance circuit7resonates with the voice intermediate carrier −Fs of the adjacent channel on the downside, a transmission characteristic between the mixer2and the intermediate-frequency amplifier3is as shown inFIG. 4, and the voice intermediate carrier −Fs of the adjacent channel on the downside is trapped. Therefore, the intermediate-frequency circuit according to the invention hardly suffers disturbance by a voice intermediate-frequency signal of the adjacent channel on the downside. In this case, as the parallel resonance circuit7is independent from the intermediate frequency tuning circuit5, a resonance frequency or a tuning frequency never has an effect upon each other and the setting of their frequencies becomes easy. An intermediate-frequency signal IF output from the intermediate-frequency amplifier3is input to a video detector (not shown) via an SAW filer (not shown) provided outside the integrated circuit1and others.

In the above description, the parallel resonance circuit7is made to resonate with the voice intermediate carrier −Fs of the adjacent channel on the downside, however, the parallel resonance circuit may also be made to resonate with a video intermediate carrier +Fp of an adjacent channel on the upside adjacent on the upside of the receive channel. A transmission characteristic in this case is as shown by a dotted line in FIG.4and the intermediate-frequency circuit according to the invention hardly suffers disturbance by the video intermediate-frequency signal on the adjacent channel on the upside.

If a second capacitative element8is connected to the inductance element6shown inFIG. 1in series, a sires-parallel resonance circuit9is formed by the inductance element6, the first capacitative element4and the second capacitative element8as shown in FIG.5. In this case, if a series resonance frequency by the inductance element6and the second capacitative element8is set to the substantially intermediate frequency between the video intermediate carrier Fp of the receive channel and the chrominance subcarrier Fc and the whole parallel resonance frequency is set to the voice intermediate carrier −Fs of the adjacent channel on the downside, a transmission characteristic between the mixer2and the intermediate-frequency amplifier3is as shown in FIG.4and the intermediate-frequency circuit according to the invention similarly hardly suffers disturbance by a voice intermediate-frequency signal of the adjacent channel on the downside.

As described above, the parallel resonance circuit7or the series-parallel resonance circuit9having no effect upon the intermediate frequency tuning circuit5can be provided by connecting the inductance element6or a series circuit composed of the inductance element6and the second capacitative element8between the first external terminal and the third external terminal1crespectively provided to the integrated circuit1, and the intermediate-frequency circuit according to the invention can be made so that it hardly suffers disturbance from an adjacent channel by setting these resonance frequencies.

As described above, the intermediate-frequency circuit according to the invention is provided with the integrated circuit including the balanced type mixer provided with the first output terminal and the second output terminal as well as the balanced type intermediate-frequency amplifier provided with the first input terminal and the second input terminal, and then the first output terminal and the first input terminal as well as the second output terminal and the second input terminal are respectively connected via the first capacitative element formed in the integrated circuit, the first external terminal connected to the first output terminal, the second external terminal connected to the second output terminal and the third external terminal connected to the first input terminal are provided to the integrated circuit, and the intermediate frequency tuning circuit is connected between the first external terminal and the second external terminal and the first external terminal and the third external terminal are connected via the inductance element, therefore the parallel resonance circuit is formed by the inductance element and the first capacitative element, and the intermediate-frequency circuit is made to hardly suffer disturbance by a voice intermediate-frequency signal or a video intermediate-frequency signal by resonating the parallel resonance circuit with a frequency of the voice intermediate carrier of the adjacent channel on the downside or a frequency of the video intermediate carrier of the adjacent channel on the upside.

The second capacitative element is connected to the inductance element in series, the series-parallel resonance circuit is formed by the first capacitative element, the inductance element and the second capacitative element, the series resonance frequency of the series-parallel resonance circuit is substantially equalized to the central frequency of the intermediate frequency band and the parallel resonance frequency is equalized to a frequency of the voice intermediate carrier of the adjacent channel on the downside, therefore the tuning characteristic of the intermediate frequency band becomes sharp and further, the intermediate-frequency circuit more hardly suffers disturbance by a voice intermediate-frequency signal of the adjacent channel on the downside.