JP2003289261A - Television tuner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve gain reduction and NF degradation in a lower zone of a UHF band while maintaining a wide variable range of tuning frequency for an input tuning circuit, and remove second harmonic interference from television signals in the band. <P>SOLUTION: The television tuner comprises the input tuning circuit 1 for tuning to the television signals in the UHF band, and an amplifier element 2 connected with the input tuning circuit 1 through a coupling capacitor 3. The input tuning circuit 1 is connected in series both with a varactor diode 1c and a capacitor 1d serially connected to the varactor diode 1c. The input tuning circuit 1 also comprises first and second inductance elements 1a and 1b connected in parallel with a series circuit of the varactor diode 1c and the capacitor 1d. A third inductance element 1e is provided, which is connected in parallel with the varactor diode 1c. A lowest parallel resonant frequency of the varactor diode 1c and the third inductance element 1e is set to be lower than the lowest frequency of the UHF band. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television tuner, and more particularly to a television tuner which widens a variable range of a tuning frequency in a UHF band and prevents a gain reduction in a low frequency band in the UHF band.

[0002]

2. Description of the Related Art The circuit configuration of a conventional television tuner is shown in FIG. The input tuning circuit 31 has a first inductance 31a whose one end is grounded, a second inductance element 31b connected in series to it, a varactor diode 31c, and a capacitor 31d connected in series to the varactor diode 31c. The cathode of is connected to the ground by the capacitor 31d, and the anode is connected to the second inductance element 31b.

The capacitance value of the capacitor 31d is equal to or greater than that of the varactor diode, and the cathode of the varactor diode 31c is galvanically isolated from the ground by this capacitor 31d. A parallel tuning circuit is configured by the above connection, and the television signal in the UHF band is input to the connection point between the first inductance element 31a and the second inductance element 31b. The tuning voltage V is applied to the cathode of the varactor diode 31c.
t is applied.

An amplifying element 32 such as an FET (field effect transistor) is provided at the next stage of the input tuning circuit 31. Then, the anode of the varactor diode 31c is coupled to the input terminal (first gate) of the amplification element 32 via the coupling capacitor 33. The capacitance value of the coupling capacitor 33 is a small value of 1 to 2 pF (picofarad) in order to prevent the input capacitance component of the amplification element 32 from being added to the input tuning circuit 31 as it is and narrowing the variable range of the tuning frequency. There is.

A bias voltage Vb is applied to the input terminal of the amplifying element 32 via the third inductance element 34. As for the gain of the amplifier element 32, since the capacitance value of the coupling capacitor 33 is small, as shown by the dotted line in FIG. 5, the UHF band (f
It decreases in the low range (f1) of 1 to f2). Therefore, the third inductance element 34 constitutes a resonance circuit (so-called peaking circuit) together with the input capacitance component of the amplification element 32, and its resonance frequency is set to be near the lowest frequency in the UHF band. As a result, the decrease in gain in the low frequency range of the UHF band of the amplifying element 32 due to the small-capacity coupling capacitor 33 is corrected in the direction of raising it as shown by the solid line in FIG.

[0006]

The gain correction by the peaking circuit is in a limited frequency range, and the gain decreases and the NF deteriorates in the frequencies before and after that. In addition, low range (f
When receiving a television signal having a frequency twice as high as that in 1), the television signal in the low frequency band is input to the amplifying element at a relatively high level, so that interference due to the second harmonic also occurs.

Therefore, in the present invention, the gain reduction and NF deterioration in the low frequency range of the UHF band are improved while the variable range of the tuning frequency in the input tuning circuit is widened, and the second harmonic interference due to the television signal in the frequency band is improved. Intended to be removed.

[0008]

In order to solve the above problems, according to the present invention, an input tuning circuit for tuning a television signal in the UHF band and an amplifier coupled to the input tuning circuit via a coupling capacitor are provided. A first varactor diode, a varactor diode, a capacitor connected in series with the varactor diode, and a series circuit including the varactor diode and the capacitor, And a second inductance element, and a third inductance element connected in parallel to the varactor diode is provided, and the lowest parallel resonance frequency of the varactor diode and the third inductance element is the lowest frequency in the UHF band. Below.

Further, the cathode of the varactor diode is grounded by the capacitor, and the anode of the varactor diode is grounded via the first and second inductance elements.

Further, the television signal in the UHF band is input to the connection point between the first inductance element and the second inductance element, and the anode of the varactor diode is connected to the amplification element via the coupling capacitor. Combined

The series resonance frequency of the varactor diode, the capacitor, and the third inductance element is set within the VHF band.

[0012]

FIG. 1 shows the circuit configuration of a television tuner of the present invention. The input tuning circuit 1 has a first inductance 1a whose one end is grounded, a second inductance element 1b connected in series with it, a varactor diode 1c, a capacitor 1d connected in series with it, and a varactor diode 1c connected in parallel. The cathode of the varactor diode 1c is grounded by the capacitor 1d, and the anode of the varactor diode 1c is connected to the second inductance element 1b. The tuning voltage Vt is applied to the cathode of the varactor diode 1c.

The third inductance element 1e is connected to the varactor diode 1c via the DC cut capacitor 1f, and the minimum parallel resonance frequency f∞ of the third inductance element 1e and the varactor diode 1c is U.
It is set to be equal to or lower than the lowest frequency of the HF band.

Further, the lowest series resonance frequency f0 of the third inductance element 1e, the varactor diode 1c and the capacitor 1d is set to be in the VHF band although it is further lowered.

As a result, the impedance characteristics of the varactor diode 1c, the capacitor 1d and the third inductance element 1e are parallel resonance frequency f as shown in FIG.
Above ∞, it becomes capacitive, and further, the variation range ΔC (capacity ratio) of the equalization capacity in the frequency range (f1 to f2) of the UHF band expands. Therefore, the tuning frequency range of the input tuning circuit 1 is also expanded.

The capacitance value of the capacitor 1d is equal to or less than that of the varactor diode, and the cathode of the varactor diode 1c is galvanically isolated from the ground by this capacitor 1d. A parallel tuning circuit is constituted by the above connection, and the tuning frequency is set by the tuning voltage. The television signal in the UHF band is used for the first inductance element 1a and the second inductance element 1
It is input to the connection point with b. Therefore, the impedance on the front stage side is stepped up by the two inductance elements 1a and 1b, and the Q of the input tuning circuit 1 becomes high.

At the next stage of the input tuning circuit 1, an amplification element 2 such as FET (field effect transistor) is provided. The anode of the varactor diode 1c is coupled to the input terminal (first gate) of the amplification element 2 via the coupling capacitor 3. Since the tuning frequency range of the input tuning circuit 1 is expanded, the capacitance value of the coupling capacitor 3 is selected to be about 10 pF (picofarad), which is a larger value than before. The input capacitance component of the amplification element 2 is added to the input tuning circuit 1 via the coupling capacitor 3, but does not narrow the variable range of the tuning frequency.

A bias voltage Vb is applied to the input terminal of the amplifying element 2 via the resistor 4. The resistor 4 is grounded by the DC cut capacitor 5. Also, the AGC voltage is supplied to the AGC terminal (second gate). Since the capacitance value of the coupling capacitor 3 is large, the gain of the amplifying element 2 does not decrease in the low range (f1) of the UHF band (f1 to f2) as shown in FIG. Therefore, the decrease in gain in the low frequency range and the deterioration of the NF are improved, and the gain characteristic having a peak in the low frequency range does not occur, so that the interference due to the second harmonic becomes less likely to occur.

[0019]

As described above, according to the present invention, the input tuning circuit and the amplifying element coupled to the input tuning circuit via the coupling capacitor are provided, and the input tuning circuit includes the varactor diode and the varactor diode in series. A third inductance element connected in parallel with the varactor diode, comprising a connected capacitor and first and second inductance elements connected in series with each other and in parallel with the series circuit of the varactor diode and the capacitor. Since the minimum parallel resonance frequency due to the varactor diode and the third inductance element is set to be equal to or lower than the minimum frequency in the UHF band, the variable range of the tuning frequency for tuning in the UHF band is widened, so that the capacitance value of the coupling capacitor is increased. it can. As a result, the gain of the amplifying element is increased up to the low range of the UHF band, and the NF is improved. Further, the peaking circuit for raising the gain in the low frequency band is unnecessary. further,
The elimination of the peaking circuit also eliminates double harmonic interference due to low frequency television signals.

Further, since the cathode of the varactor diode is grounded by the capacitor and the anode of the varactor diode is grounded via the first and second inductance elements, a tuning voltage for changing the capacitance of the varactor diode is applied to the cathode. Just enough.

Since a UHF band television signal is input to the connection point between the first inductance element and the second inductance element and the anode of the varactor diode is coupled to the amplification element via the coupling capacitor, input tuning is performed. It is possible to obtain the matching between the impedance on the front side of the circuit and the input impedance of the amplification element, and to increase the Q of the input tuning circuit.

Further, the series resonance frequency of the varactor diode, the capacitor, and the third inductance element is V
Since it is set within the HF band, interference by a television signal in the VHF band can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a television tuner of the present invention.

FIG. 2 is an impedance characteristic diagram in the input tuning circuit of the television tuner of the present invention.

FIG. 3 is a gain characteristic diagram of the television tuner of the present invention.

FIG. 4 is a circuit diagram of a conventional television tuner.

FIG. 5 is a gain characteristic diagram of a conventional television tuner.

[Explanation of symbols]

1-input tuning circuit 1a First inductance element 1b Second inductance element 1c Varactor diode 1d capacitor 1e Third inductance element 1f DC cut capacitor 2 amplification element 3 coupling capacitors 4 resistance 5 DC cut capacitor

Claims (4)

[Claims] 1. An input tuning circuit for tuning a television signal in the UHF band, and an amplifier element coupled to the input tuning circuit via a coupling capacitor, the input tuning circuit comprising: a varactor diode; A capacitor connected in series with a varactor diode, and first and second inductance elements connected in series with each other in parallel with a series circuit of the varactor diode and the capacitor, and connected in parallel with the varactor diode. A television tuner characterized in that a minimum parallel resonance frequency of the varactor diode and the third inductance element is set to be equal to or lower than the minimum frequency of the UHF band. 2. The television according to claim 1, wherein a cathode of the varactor diode is grounded by the capacitor, and an anode of the varactor diode is grounded through the first and second inductance elements. Tuner. 3. A television signal in the UHF band is input to a connection point between the first inductance element and the second inductance element, and an anode of the varactor diode is connected to the amplification element via the coupling capacitor. The television tuner according to claim 2, wherein the television tuner is combined. 4. The television tuner according to claim 2, wherein a series resonance frequency of the varactor diode, the capacitor, and the third inductance element is set within a VHF band.