JPS6233383Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a tuning circuit used in the front end of a receiver.

Conventional tuning circuits using M-coupling have the disadvantage that when the tuning frequency is changed, the degree of coupling changes, and as a result, the bandwidth changes, deteriorating the electrical performance such as the image interference ratio of the receiver.

This invention was made to improve the above-mentioned drawbacks, and provides a tuning circuit that can maintain a constant bandwidth even if the tuning frequency is changed by inserting a coil in series with the coupling pickup coil. It is an object.

An embodiment of this invention will be described below with reference to the drawings. Figure 1 shows an embodiment of a tuning circuit according to this invention, in which a coil L ₃ and a variable capacitor connected in series to this coil L ₃ and whose capacitance changes in conjunction with a tuning knob (not shown). a pickup coil L ₂ for supplying an input signal _{to the resonance circuit 1; a pickup coil L 4 for} taking out the output signal of the resonance circuit ₁ ; and coupling degree correction coils L ₁ and L ₅ connected between the input terminal Ti and the pickup coil L ₄ and the output terminal To, respectively.

In a receiver using a tuning circuit configured in this manner, when a tuning knob (not shown) is turned, the capacitance of the variable capacitor _C1 changes in conjunction with this. Now, when you turn the tuning knob to receive a high frequency broadcast wave, as the resonance frequency of resonance circuit 1 changes from low frequency to high frequency, coil L ₃
The degree of coupling between the pick-up coils L ₂ and L ₄ tends to become tighter. Therefore, the resonance characteristic becomes a gentle curve and the bandwidth tends to increase. However, in the tuned circuit according to this embodiment, the pickup coil
A coupling degree correction coil _L1 is connected to _L2 in series with this pick-up coil _L2 , and a coupling degree correction coil _L5 is connected to the pick-up coil _L4 in series with this pick-up coil _L4 . Therefore, the change in the coupling degree of the coil L ₃ and the pickup coils L ₂ and L ₄ due to the change in the resonance frequency is canceled out by the change in the inductance of the coils L ₁ and L ₅ . If the inductance values of the coils L ₁ and L ₅ are selected so that the impedance is small at the lowest resonant frequency of the resonant circuit 1, and the impedance is large at the highest resonant frequency, the resonant frequency can be increased. As the frequency changes from low to high, the pick-up coil
The current flowing through L ₂ and L ₄ decreases. Therefore, the degree of coupling between the coil L ₃ and the pickup coils L ₂ and L ₄ is maintained constant, the resonance characteristics are constant, and the bandwidth does not increase.

FIG. 2 shows an embodiment in which this invention is applied to a double-tuned circuit, and the same parts as in FIG. 1 are given the same reference numerals. In the case of this figure 2, the coil
L ₃ and variable capacitor C ₁ constitute a first resonant circuit 1 a, and variable capacitor C ₂ connected in series to coil L ₆ and linked to variable capacitor C ₁ constitutes a second resonant circuit 1 b. It consists of

In a receiver using a double tuning circuit configured in this manner, when a tuning knob (not shown) is turned, the capacitances of the variable capacitors C ₁ and C ₂ change in conjunction with this. Now, when you turn the tuning knob to receive high frequency broadcast waves, the resonance circuits 1a and 1
As the resonant frequency of b changes from low frequency to high frequency, the degree of coupling between coil L ₃ and pickup coil L ₂ and between coil L ₆ and pickup coil L ₄ increases, but this is the same as explained in the previous embodiment. This is canceled out by the change in the inductance of the coupling correction coils _L1 and _L5 . Therefore, coil _L3 and pick-up coil _L2 , coil _L6 and pick-up coil
The degree of coupling with L ₄ is maintained constant, and the coils L ₃ , L ₆
Although the degree of coupling changes with the resonant frequency, the overall resonance characteristics of the double-tuned circuit are significantly improved. Also, the mutual inductance between the pick-up coil _L2 and the coil _L3 of the resonant circuit is _M1 , the mutual inductance between the pick-up coil _L4 and the coil _L3 is _M2 , and the input terminal Ti and the output terminal The voltage at To is V ₁ ,
Assuming that Vo is the voltage loss rate between the input and output terminals (Vo/V ₁ ), it can generally be determined as follows using a simple equivalent circuit.

Vo/V ₁ ≈M ₂ /M ₁ As is clear from the above equation, the voltage loss rate depends on the values of the mutual inductances M ₁ and M ₂ and does not depend on the frequency. In this way, a constant voltage loss factor can be obtained regardless of the frequency of the tuned circuit, so there is no need to adjust the amplification factor of the amplifier connected after the tuned circuit in response to frequency changes.

As explained above, according to this invention, it is possible to obtain a constant bandwidth and a constant voltage loss rate regardless of the tuning frequency of the tuning circuit, so that constant selectivity and separation can be obtained regardless of the receiving frequency of the receiver. In particular, characteristics such as image interference ratio and spurious interference ratio, which deteriorate as the frequency increases, can be significantly improved. Furthermore, since the degree of coupling can be maintained constant regardless of the tuning frequency, the insertion loss of the tuning circuit can be kept constant, and therefore a constant receiving sensitivity can be obtained regardless of the receiving frequency of the receiver.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of this invention, and FIG. 2 is a circuit diagram showing an example in which this invention is applied to a double-tuned circuit. 1...Resonance circuit, _C1 , _C2 ...Variable capacitor, _L1 , _L5 ...Coupling correction coil, _L2 , _L4
...Pick-up coil, L ₃ , L ₆ ... Coil.

Claims (1)

[Scope of utility model registration request] a resonant circuit comprising a variable capacitor and a coil and whose resonant frequency is varied by adjusting the capacitance of the variable capacitor; a first pickup coil for supplying an input signal to the resonant circuit via the coil; It is connected between the pick-up coil and the signal input terminal, and its impedance value increases as the resonant frequency rises, increasing the degree of coupling between the coil of the resonant circuit and the first pick-up coil over the entire variable frequency range of the resonant frequency. a first coupling correction coil for maintaining a substantially constant value; a second pickup coil for extracting a signal from the resonant circuit via the coil; and a connection between the second pickup coil and the signal output terminal. The impedance value increases as the resonant frequency increases, and the degree of coupling between the coil of the resonant circuit and the second pickup coil is maintained at a substantially constant value over the entire variable frequency range of the resonant frequency. A tuning circuit comprising a second coupling correction coil.