JPH0273710A - Dual tuning circuit - Google Patents

Abstract

PURPOSE:To improve image disturbing characteristic over a wide band by constituting a 3rd inductive impedance by the series connection of a 1st inductance circuit and a variable capacitance diode. CONSTITUTION:The inductive impedance for coupling a dual tuning circuit consists of the series connection with inductors 13-18 and varactor diodes 6-10. The coupling inductive impedance comprising the series connection between the inductors 13-18 and the varactor diodes 6-10 is low at a low band frequency and high at a high frequency band. Thus, the inductors 17, 18 and the varactor diode 10, e.g., are selected properly to match a trap point with an image frequency band over a high band from a low band. Thus, the image disturbing signal is sufficiently attenuated and the image disturbing characteristic is improved over a wide reception band.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable tuning circuit used in television tuners and the like, and particularly to a double tuning circuit suitable for wideband reception.

[Conventional technology]

A conventional double-tuned circuit has a configuration in which the stages of the tuned circuit are connected by a coupling capacitor or a variable capacitance diode for coupling, as described in 1% Japanese Patent Publication No. 1982-201316.

[The problem that the invention attempts to solve]

The above conventional technology uses a variable capacitance diode with a wide variation range to receive signals in a wider band.
'& In cases where inductor switching is performed using a switching diode and reception is performed in two bands.

2. It is not possible to cause the trap point, which occurs at a frequency higher than the tuning point due to inductive coupling between the coupling capacitor or the variable capacitance diode for coupling and the inductor on the primary and secondary sides, to follow the image frequency over a wide band. There were problems such as the fact that the optimum value of the coupling capacitor differed between the two bands, making it impossible to sufficiently attenuate the image interference signal.

The object of the present invention is to improve the above-mentioned drawbacks of the prior art.

The object of the present invention is to provide a double-tuned circuit with good image disturbance characteristics over a wide band.

[Means to solve the problem]

The above objective is achieved by optimizing the coupling by configuring the inductive impedance for coupling of the double-tuned circuit by connecting an inductor and a variable capacitance diode in series so that the trap point occurs in the image band over a wide band. .

[Effect]

The inductive impedance for coupling, which is composed of a series connection of an inductor and a variable capacitance diode, is lower at low frequencies and higher at higher frequencies than in the past.

As a result, the trap points are compared with the conventional one.

At a point far from the tuning point at the low frequency, the same U is applied at the mixed frequency.
The trap point can be generated at a point closer to the image frequency band over a wide band, and the image frequency? It depends on whether it is sufficiently attenuated.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. 1st
In the figure, 1 is an input terminal and 2 is an output terminal.

3.4.5 is a voltage supply terminal, 6 to 10 are variable capacitance diodes, 11 and 12 are switching diodes, and 13 to 10 are variable capacitance diodes.
18 is an inductor, 19-25 are capacitors, 26-3
2 is resistance. Figure 1 shows a double tuning circuit used in television gene chainers, etc., in which the switching diode 11
, 12, the tuning inductors 13 , 14 , 15
, 16 to change the inductance, it is possible to receive two bands, low band and high band. Furthermore, the coupling inductors 17 and 18 are also configured to be switched. During low band reception, a voltage is applied to the terminal 3 to bring the switching diodes 11 and 12' into a non-conducting state. In this case, the inductors 13°14, 15
, 16 act as tuning inductors, and inductor 1
The inductive impedance caused by 7, 18 and the variable capacitance diode acts as a coupling inductance, and by applying a tuning voltage to the terminal 5, the capacitance value of the variable capacitance diode is changed to change the tuning point.

Further, during high band reception, a voltage is applied to the terminal 4 to bring the switching diodes 11 and 12' into conduction. In this case, the inductors 13 and 14 act as tuning inductors, the inductive impedance of the inductor 18 and the variable capacitance diode acts as a coupling piece inductance, and a tuning voltage is applied to the terminal 5 to change the tuning point. The coupling inductor and trap frequency of the double-tuned circuit will be explained with reference to FIG. FIG. 2 (α) shows the equivalent circuit of the embodiment shown in FIG. 34 is an equivalent inductive impedance formed by connecting the variable capacitance diode 8 and inductors 14 and 16 in series, and the inductor 35 is the variable capacitance diode 10.
This is the equivalent inductive impedance due to the series connection of the inductors 17 and 18. Figure 2 (b) shows the inductor 33, 34, 35 of (α) converted by YY-Δ, and the inductance L of the inductor 38 is the same as that of the inductor 33, 34.
, 35 inductance values as LL , L2 , respectively.
When L3 is assumed, the following equation is obtained.

L=LL+L2+Upper P animals...
・(1) Actually, the inductive coupling component M between the primary and secondary inductors is also added, and IXL2 L = L 1 + L 2 + −-M ・・
・Bright...(2).

A trap occurs due to the parallel connection of the inductor 38 and the capacitor 25, but if the inductor 35 (L3) is connected in parallel (to strengthen the coupling between the primary and secondary tuned circuits) (2
) equation becomes small, and the trap frequency becomes high. Conversely, if the inductor 35 is made smaller to weaken the coupling, the trap frequency becomes low. In the conventional circuit, the trap point is set to the image frequency at a high frequency. If the trap point approaches the tuning point at low frequencies, the image disturbance characteristics will not only deteriorate (the loss of the tuning circuit will also increase, but if the trap point is tuned to the image frequency at low frequencies, the image disturbance characteristics will deteriorate). However, the trap point becomes a frequency higher than the image frequency at the frequency range, and the image disturbance characteristics deteriorate.

In this embodiment, it is constructed by connecting a coupled inductive impedance t inductor and a variable capacitance diode in series, and the equivalent inductance at low frequencies can be made much larger than the equivalent inductance at high frequencies.

The coupling between the primary and secondary sides is weak at low frequencies.

It can be strong at high frequencies, and the inductor 17°18 in Figure 1
By appropriately selecting the variable capacitance diode 10, the trap point can be matched to the image frequency band from the low band to the noi band, resulting in a double-tuned circuit that can sufficiently attenuate image interference signals. In addition,
Since the coupling capacitor 25 in FIG. 1 has a small capacitance of several PF or less, it can be constructed from a pattern capacitance such as a printed circuit board, and it is also possible to construct an inductor from a pattern for the inductor 17 and IIHC. . A second embodiment is shown in FIG. The embodiment shown in FIG. 3 is a simplified version of the embodiment shown in FIG. 1, in which switching diodes 11 and 12 are grounded through bypass capacitors 40 and 41, and only inductor 39 is provided as a coupling inductor. In the embodiment shown in FIG. 3, a high band trap point is determined by a coupling capacitor 25.

The configuration is such that the low band trap point is optimized by variable inductive impedance created by the series connection of the inductor 39 and the variable capacitance diode 10. FIG. 4 shows a third embodiment. 4 differs from the embodiment shown in FIG. 1 in that the variable capacitor is not provided with a diode 10, and the configuration is such that an optimal coupled inductor can be selected for each of the low band and high band. FIG. 5 shows a fourth embodiment. In the embodiment shown in FIG. 5, the system of the tuning circuit is different from that in the first to third embodiments, and there is no variable capacitance diode 7.8 connected to the inductor 13 or It has a configuration in which diodes 41 and 42 are provided.Even if the tuning circuits are different as in this case, if the combination of the double tuning circuits is set to an appropriate state, sufficient attenuation of the image frequency can be achieved over a wide band as in the previous embodiment. can be obtained over a period of time.

FIG. 6 shows a fifth embodiment. The difference between the embodiment shown in FIG. 6 and that shown in FIG. 1 is that the voltage applied to terminal 3 or terminal 4 is divided and applied as the voltage applied to the variable capacitance diode 10, and the voltage applied to the variable capacitance diode 10 is divided depending on the band.
Voltage to add to 0? The configuration is such that the capacitance value is changed by switching, and the coupling inductors 17 and 1 shown in Fig.
Compared to the configuration in which switching is made to B', the optimum value of the coupling impedance can be set.

〔Effect of the invention〕

According to the present invention, since the trap point of the double-tuned circuit can be generated in the image band over a wide reception band, it is possible to provide a receiver ya1' such as a television set with good image interference characteristics.

[Brief explanation of the drawing]

Fig. 1 is a double tuning circuit diagram showing the first embodiment of the present invention, Fig. 2 is an equivalent circuit diagram of the first embodiment, and Figs. ] is a double tuning circuit diagram shown in FIG. 6, 7, 8, 9, 10, 41, 42...
Variable capacitance diode. 13 , 14 , 15 , 16 , 17 , 18 , 3
9... Inductor, 11, 12... Switching diode. Second force (ill,) (b)

Claims (1)

[Claims] First and second variable capacitance diode circuits each including one or more variable capacitance diodes are connected by a series connection of first and second inductive impedances, and
In a double-tuned circuit in which the connection point of the second inductive impedance is grounded by the third inductive impedance,
A double-tuned circuit characterized in that the third inductive impedance consists of a first inductor circuit and a variable capacitance diode connected in series.