JPH0454014A - Double tuning circuit - Google Patents

Abstract

PURPOSE:To suppress the frequency dependency of the selectivity by connecting other terminals of each capacitor of each parallel resonance circuit, connecting the connecting point to ground via other capacitor, and connecting each inductor to ground. CONSTITUTION:A high frequency signal passed through a tuning capacitor C4 flows to ground via a capacitor C6 and part of the signal flows to a tuning capacitor C5 coupled with a tuning capacitor C4. The impedance of the capacitor C6 becomes larger as the frequency becomes lower (inversely proportional to the frequency) and the flowing quantity of the signal from the capacitor C4 to the capacitor C5 is increased. As a result, the frequency band of an output signal VOUT is spread. As a result, as the frequency becomes lower, the frequency band of the signal VOUT becomes wider and the sharpness of the selectivity Q is unsharpened. However, in the case of the dual tuning circuit having a characteristic in which the selectivity Q is unsharpened as the frequency becomes higher, the fluctuation of the selectivity Q depending on the frequency is suppressed and a stable characteristic is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a tuning circuit such as an FM tuner, and particularly to a double tuning circuit suitable for an input section of a front end.

[Conventional technology]

In an FM tuner or the like, a tuning circuit is provided at the input section of the front end in order to tune to a desired reception frequency. A parallel resonant circuit consisting of a variable capacitor and an inductance coil is used as the tuning circuit. A single parallel resonant circuit is called a singly tuned circuit.
A circuit with two or more is called a double-tuned circuit.

A double-tuned circuit is used for the purpose of improving the selectivity of reception frequencies, and is generally constructed by capacitively coupling a plurality of parallel resonant circuits having the same configuration. FIG. 3 shows an example of a conventional double-tuned circuit.

As shown in FIG. 3, an input signal vIN from an input signal source S such as an antenna is coupled via a coupling capacitor C to a selected predetermined frequency signal in a resonant circuit consisting of a tuning capacitor 01 and a tuning coil L. Capacitor C
2 is further selected by a resonant circuit consisting of a tuning capacitor C1 and a tuning coil L2.
OUT - Supplied to Dance ZL.

[Problem to be solved by the invention]

A problem with the conventional double-tuned circuit described above is that the selectivity Q varies depending on the receiving frequency.

That is, as shown in FIG. 4, for example, the values of the tuning capacitor C%C are 20 (pF) and 25 (pF), respectively.
, 30 (pF), and 35 (pF), the lower the reception frequency, the sharper the selectivity Q and the narrower the frequency bandwidth, and the higher the reception frequency, the sweeter the selectivity Q. This causes the phenomenon that the bandwidth becomes wider. In this way, the selectivity Q of the tuned circuit is
It is undesirable for the characteristics of the receiver to fluctuate in the receiving frequency.

An object of the present invention is to provide a double-tuned circuit that can suppress frequency dependence of selectivity.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention provides a double-tuned circuit in which at least two parallel resonant circuits each including a capacitor and an inductance are capacitively coupled at one end thereof, in which the other ends of each capacitor of each of the parallel resonant circuits are connected to each other. The connection point is grounded via another capacitor, and each inductance is grounded.

[Effect]

According to the present invention, capacitors in a plurality of parallel resonant circuits that are capacitively coupled to each other are coupled to each other, and a part of the high frequency signal from the capacitor in the previous stage goes around to the capacitor in the latter stage, and the resonant circuit The degree of the coupling increases as the frequency becomes lower and the impedance of the other added capacitor increases. As a result, as the frequency becomes lower, the frequency bandwidth of selectivity Q becomes wider, suppressing the variation in selectivity due to differences in frequency,
It becomes possible to obtain stable selection performance.

〔Example〕

Next, embodiments of the present invention will be described based on the drawings.

FIG. 1 shows an embodiment of the present invention. Input signal from input signal source S such as antenna ■IN is coupling capacitor C
1 is input. The other end of the coupling capacitor C1 has one end of a first tuning circuit consisting of a variable capacitance tuning capacitor C1 and a tuning coil L, and a coupling capacitor C1.
One end of 2 is connected. Coupling capacitor C
One end of a second tuning circuit consisting of a variable capacitance tuning capacitor C1 and a tuning coil L2 and one end of a coupling capacitor C3 are connected to the other end of the coupling capacitor C3. The other end of the coupling capacitor C3 is connected to the load impedance ZL, and the output signal ■ is here. 0. is output.

The first tuning circuit (C,L,) and the second tuning path (C,L,)
) are capacitively coupled by a coupling capacitor C2. The other end of tuning capacitor c4 and tuning capacitor C5
It is commonly connected to the other end, and the capacitor C is connected from that connection point.
It is grounded via 6. Tuning coil L, L
The other ends of both are directly grounded.

In the above configuration, the input signal vIN is applied to the tuning capacitor C4 and the tuning coil Ll via the coupling capacitor CI, but the high frequency signal passing through the tuning capacitor C flows to the ground via the capacitor C6, and
A portion of it flows into tuning capacitor C5, which is in coupling relationship with tuning capacitor C. The impedance of the capacitor C6 is inversely proportional to the frequency, and increases as the frequency decreases, and the amount of signal flowing from the tuning capacitor C to the tuning capacitor C5 increases. As a result, the frequency band of the output signal (2) becomes wider. As a result, as the 11T frequency becomes lower, the frequency band tlT of the output signal V becomes wider, and the selectivity Q becomes less sharp. However, in the case of a double-tuned circuit which originally has the characteristic that the selectivity Q becomes weaker as the frequency becomes higher, from the viewpoint of characteristics, fluctuations in the selectivity Q depending on the frequency are suppressed and stable characteristics are obtained. An example of this is shown in FIG. As can be seen from a comparison with the conventional example in FIG. 4, the selectivity is unified.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to suppress the selectivity from varying depending on the receiving frequency, and stable tuning operation is possible.

S...Input signal source

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a selection characteristic diagram using a tuning capacitor as a parameter, Fig. 3 is a circuit diagram showing a conventional double-tuned circuit, and Fig. 4 is a diagram showing a tuning capacitor as a parameter. FIG. 3 is a conventional selection characteristic diagram. vIN...Input signal■...Output signal UT C1...Coupling capacitor C2...Coupling capacitor C3...Coupling capacitor C4...Tuning capacitor C5...Tuning capacitor C6...・Capacitor L, ... Tuning coil L2 ... Tuning coil Z, ... Load impedance

Claims (1)

[Claims] In a double-tuned circuit in which at least two or more parallel resonant circuits each consisting of a capacitor and an inductance are capacitively coupled at one end thereof, the other ends of each capacitor of each of the parallel resonant circuits are coupled to each other, and the coupling A double-tuned circuit characterized in that a point is grounded through another capacitor, and each inductance is grounded.