JPH07240667A - High frequency synchronizing circuit - Google Patents

Abstract

PURPOSE:To prevent the lowering of the characteristic of a high frequency synchronizing circuit to be used many times when a variable capacitance diode for plural circuits is used when the variable capacitance diode is integrally formed within an integrated circuit. CONSTITUTION:The connection point of cathodes with each other is formed; the circuit is composed of variable capacity diodes D 1 and D 2 to which synchronizing voltage VT is added to the connection point and which are made a pair and a synchronizing coil L 1 forming a parallel resonance circuit with DC capacity of the variable capacity diodes D 1 and D 2', the connection point is alternately grounded via a capacitor C 1; and the anode side of the diodes D 1 and D 2 which is the side opposite to the connection point is grounded via a bias resistance R 2. Therefore, because the leakage current from the synchronizing coil L 1 to the connection point is not generated, the lowering of a characteristic can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency tuning circuit using variable capacitance diodes in AM receivers, FM receivers and the like.

[0002]

2. Description of the Related Art A conventional high frequency tuning circuit using a variable capacitance diode is shown in the circuit diagrams of FIGS. In FIG. 4, both ends of the tuning coil L1 are connected to the anodes of the variable capacitance diode D1 and the variable capacitance diode D2 which have substantially the same characteristics with the cathodes connected to each other, and the anode of the variable capacitance diode D2 is grounded. The tuning voltage V is applied from the terminal 1 to the connection point between the cathodes via the resistor R1.
_T is added. When there is only one variable capacitance diode, when a current due to an input signal with a strong electric field strength flows through the variable capacitance diode, the rectification action distorts the waveform to generate harmonics and the capacitance depending on the amplitude of the input signal. Although the inconvenience such as the mixing of the signal of the frequency near the desired input signal due to the fluctuation of the value occurs, the pair of variable capacitance diodes D1 and D2 in which the cathodes are connected are paired.
By using, it is possible to prevent such an inconvenience.

In FIG. 5, variable capacitance diodes D1 and D2 are shown.
Is connected to the ground via the capacitor C1, and the connection point is grounded AC. And the tuning coil L
The center tap of 1 is directly grounded, and the inductance on both sides of the tap is made equal. The configuration shown in FIG. 5 is often used when a plurality of high-frequency tuning circuits used in the same receiver, such as the first and second stages, and variable capacitance diodes for the local oscillation circuit are integrally formed in the integrated circuit. Used. In such a case, since the respective high frequency tuning circuits are closely coupled via the variable capacitance diode in the integrated circuit, the capacitor C1 is required to prevent the high frequency tuning circuits from being coupled. FIG. 6 is a circuit diagram in the case where two high frequency tuning circuits are formed by using four variable capacitance diodes integrally formed in the same integrated circuit surrounded by a dotted line. A capacitor corresponding to the capacitor C1 is represented by one common capacitor C11. Since the cathode of the varactor diode serves as a common integrated circuit substrate, the need for a capacitor thus preventing the coupling of multiple RF tuning circuits can be seen. However, it is technically extremely difficult to completely center the tap of the tuning coil L1 and make the inductances on both sides the same because of the nature of the coil as a circuit element. Therefore, the inductance C of the tuning coil L1 becomes unbalanced, so that the capacitor C is removed from this tap.
There is a drawback that the input signal via 1 easily flows as a leakage current to the connection point, leading to deterioration of the characteristics of the high frequency tuning circuit.

[0004]

It is an object of the present invention to be frequently used in using variable capacitance diodes for a plurality of circuits when they are integrally formed in an integrated circuit. It is to solve the drawbacks that occur in the high frequency tuning circuit of.

[0005]

In the high frequency tuning circuit of the present invention, a connection point of anodes or cathodes is formed, and a pair of variable capacitance diodes, to which a tuning voltage is applied, is connected to the connection point. And a tuning coil forming a parallel resonance circuit with the series capacitance of the variable capacitance diode,
The connection point is AC-grounded via a capacitor, and the end of the variable capacitance diode opposite to the connection point is DC-grounded via a bias resistor. Multiple pairs of variable capacitance diodes may be connected in parallel.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a high frequency tuning circuit of the present invention will be described below with reference to FIG. 1 which is a circuit diagram. The same parts as those in FIGS. 4 and 5 are designated by the same reference numerals. In FIG. 1, a pair of variable capacitance diodes D1 and D2 are connected to each other at their cathodes, and their anodes are connected to a tuning coil L1. The tuning coil L1 forms a parallel resonance circuit with the series capacitance of the variable capacitance diodes D1 and D2.
The tap of the tuning coil L1 is grounded via the bias resistor R2. Note that this tap may be located at a position displaced from the center of the tuning coil L1. Bias resistor R2
Is a resistance of several KΩ to several hundred KΩ. The connection points between the cathodes of the variable capacitance diodes D1 and D2 are connected to the terminal 1 via a resistor R1 of several KΩ for overcurrent protection, and are also grounded via a capacitor C1.
The tuning voltage V _T is applied to the terminal 1.

The high-frequency tuning circuit configured in this way is
The tuning voltage V _{T is} applied to the connection point of the variable capacitance diodes D1 and D2.
Is added, the potential of the anode on the side opposite to the connection point of the variable capacitance diodes D1 and D2 and the potential of the tuning coil L1 are ground potential. Therefore, the variable capacitance diode D
The capacitances 1 and D2 can be adjusted by the tuning voltage V _T. Moreover, the tap of the tuning coil L1 is bias resistor R
Since it is grounded via 2, the tuning coil L1 is grounded in terms of direct current, but is floating from the ground in terms of alternating current. As a result, even if the position of the tap of the tuning coil L1 deviates from the center to cause a difference in the inductance on both sides of the tap, the variable capacitance diode D moves from the tap.
No current flows at the connection point of 1 and D2.

FIG. 2 is a circuit diagram showing another embodiment of the high frequency tuning circuit of the present invention. A bias resistor R2 whose one end is grounded is connected to a connection point between the anode of the variable capacitance diode D2 and the tuning coil L1. 1 is different from FIG. As described in FIG. 1, a pair of variable capacitance diodes D
The anodes of 1 and D2 are grounded in a DC manner including the tuning coil L1, but are in a floating state in an AC manner. Therefore, the bias resistor R2 may be connected to the outside of the tuning coil L1 in this way. .

FIG. 3 is a circuit diagram showing still another embodiment of the high frequency tuning circuit of the present invention. The variable capacitance diode paired with the cathodes connected to each other has two sets of variable capacitance diodes D3 and D4 and variable capacitance diodes D5 and D6, and the respective series capacitances are connected in parallel. And
The capacitors connected in parallel form a parallel resonance circuit with the tuning coil L1. The tuning voltage V _T is applied from the terminal 1 to the connection point between the cathodes of the pair of variable capacitance diodes via the resistor R1. Further, the connection point is a capacitor C
It is grounded through 1. Thus, the capacitance value required for the parallel resonant circuit can be adjusted by increasing the number of pairs of variable capacitance diodes in pairs. In the embodiments, the pair of variable capacitance diodes are connected to the cathodes, but may be connected to the anodes.
In addition, a fixed capacitor may be connected in parallel to the series capacitance of the variable capacitance diode as needed. It goes without saying that the high frequency tuning circuit includes an antenna tuning circuit.

[0010]

As described above, in the high frequency tuning circuit of the present invention, the parallel resonance circuit is formed by the series capacitance of the variable capacitance diode paired with the connection point of the anode and the cathode and the tuning coil. , Its connection point is grounded via a capacitor with a tuning voltage applied. The opposite side of the connection point is grounded via a bias resistor. Therefore, the opposite side of the connection point is grounded in terms of DC including the entire tuning coil, but is in a state of floating above the ground in terms of AC. As a result, leakage current does not flow to the connection point side even if the position for direct current grounding for applying a bias voltage to the variable capacitance diode is not exactly limited to the center of the tuning coil. The characteristic of does not deteriorate. This is
A variable capacitance diode used for another circuit is integrally formed in the integrated circuit, and when the variable capacitance diode is used to form a high frequency tuning circuit, this is a particularly excellent advantage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a high frequency tuning circuit of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the high frequency tuning circuit of the present invention.

FIG. 3 is a circuit diagram showing still another embodiment of the high frequency tuning circuit of the present invention.

FIG. 4 is a circuit diagram showing a conventional high frequency tuning circuit.

FIG. 5 is a circuit diagram showing another conventional high frequency tuning circuit.

FIG. 6 is a circuit diagram in the case of configuring a plurality of high frequency tuning circuits by using variable capacitance diodes formed integrally in an integrated circuit.

[Explanation of symbols]

 D1, D2 Variable capacitance diode L1 Tuning coil R2 Bias resistance

Claims (2)

[Claims] 1. A pair of variable capacitance diodes forming a connection point of anodes or cathodes, and a tuning voltage is applied to the connection point, series capacitance of the pair of variable capacitance diodes and parallel resonance. It is composed of a tuning coil forming a circuit, the connection point is AC-grounded via a capacitor, and the end of the variable capacitance diode opposite to the connection point is DC-grounded via a bias resistor. High frequency tuning circuit. 2. A plurality of pairs of variable capacitance diodes forming a connection point between anodes or cathodes, and a tuning voltage is applied to the connection point, and series capacitances of the paired variable capacitance diodes. Are connected in parallel, and are composed of a tuning coil that forms a parallel resonant circuit with the capacitor connected in parallel, the connection point is grounded AC through a capacitor, and the end of the variable capacitance diode on the opposite side of the connection point is biased. A high-frequency tuning circuit characterized by being grounded in a direct current manner through a resistor.