JP4297117B2 - Tuning circuit - Google Patents

Description

  The present invention relates to a tuning circuit, and more particularly to a tuning circuit that varies a tuning frequency by changing a voltage applied to a variable capacitance diode.

  In recent years, the broadband of wireless devices has been attempted. In particular, an in-vehicle wireless device may be used in an environment in which adjacent interference is severe, and a tuning circuit having a high Q value is desired.

  FIG. 1 is a circuit diagram showing an example of a conventional tuning circuit. The antenna 1 is connected to the primary coil L1 of the transformer 2 and one end of the capacitor C1. The other end of the primary coil L1 is grounded, and the other end of the capacitor C1 is grounded via a variable capacitance diode Vc. A control voltage is applied from the tuning control circuit 3 to the connection point between the capacitor C1 and the variable capacitance diode Vc via the resistor R1. The tuning control circuit 3 reads control voltage data corresponding to the desired reception frequency from the memory 4 and generates a control voltage.

Here, a signal centered on the frequency fr (= 1 / [2π (L1 × Cto) ^1/2 ]) is selected when the combined capacitance of the variable capacitance diode Vc and the capacitor C1 according to the control voltage is Cto. And output from the secondary coil L2 of the transformer 2 to the demodulator at the next stage.

  In Patent Document 1, in a tuning circuit in which a fixed capacitor connected in series, a variable capacitance diode, and a coil are connected in parallel and a tuning voltage is supplied to the variable capacitance diode via a resistor, the fixed capacitor and the coil are connected to each other. It is described that the midpoint of connection is connected to a reference potential.

Patent Document 2 describes a tuning circuit including a tuning coil in which an input coil, an output coil, and a capacitive element or a variable capacitive element are connected in parallel. Patent Document 3 describes a tuning circuit in which two variable capacitance diodes are connected in parallel so as to have opposite polarities on the primary side of a tuning transformer.
Japanese Utility Model Publication No. 63-26125 Japanese Patent Laid-Open No. 9-260988 JP-A-7-283692

  In the conventional tuning circuit, control voltage data corresponding to the desired reception frequency is written in the memory 4 at the time of product shipment, so that the circuit element constants such as the coil L1, capacitor C1, variable capacitance diode Vc, etc. change with time, environmental temperature change, etc. There is a problem that the tuning frequency fluctuates due to the above.

  In a vehicle wireless device, the environmental temperature changes within a range of −30 ° C. to + 85 ° C., for example. Generally, a coil has a positive temperature characteristic, and the inductance of the coil at a high temperature is increased as compared with the inductance of the coil at a normal temperature. Therefore, even if the tuning frequency characteristic at normal temperature is set to cover the desired reception frequency band as shown by the broken line in FIG. 2, the tuning frequency characteristic at high temperature changes as shown by the solid line in FIG. As a result, the desired reception frequency band cannot be covered. In order to reduce the influence of such a change in the tuning frequency, the Q value of the tuning circuit has to be set low, resulting in a problem that the reception sensitivity is lowered.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a tuning circuit that prevents fluctuations in tuning frequency due to changes in circuit element constants over time, environmental temperature changes, and the like.

The tuning circuit of the present invention is a tuning circuit in which a first capacitor and a variable capacitance diode connected in series are connected in parallel with a coil, and a tuning frequency is varied by changing an applied voltage of the variable capacitance diode.
A second capacitor disposed between the coil and ground;
A pulse voltage is supplied to a connection point between the coil and the second capacitor to generate a natural damped vibration, and based on the difference between the detected frequency of the natural damped vibration and a target tuning frequency , by frequency has a tuning control circuit for changing the applied voltage to be a tuning frequency of the target, aging of the circuit parameters, the variation of the tuning frequency due to change in environmental temperature or the like can be prevented.

In the tuning circuit,
The coil is a primary coil of a transformer,
The tuning control circuit can detect the frequency of the natural damped oscillation supplied through a third capacitor connected to one end of the secondary coil of the transformer.

In the tuning circuit,
A switch for disconnecting the antenna from the tuning circuit may be provided between the time when the tuning control circuit supplies a pulse voltage and the time when the applied voltage is changed.

  According to the present invention, it is possible to prevent fluctuations in the tuning frequency due to changes in circuit element constants over time, environmental temperature changes, and the like.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 3 shows a circuit configuration diagram of an embodiment of the tuning circuit of the present invention. In the figure, an antenna 10 is connected to one end of a primary coil L1 of a transformer 12 and a capacitor C1 through a switch 11. The other end of the capacitor C1 is grounded via the variable capacitance diode Vc and grounded via the capacitor C2. The other end of the primary coil L1 is grounded via a capacitor C3.

  A control voltage is applied from the tuning control circuit 13 to the connection point between the capacitor C1, the variable capacitance diode Vc, and the capacitor C2 via the resistor R1. A pulse voltage is supplied from the tuning control circuit 13 to the connection point between the primary coil L1 and the capacitor C3 via the resistor R2. The switch 11 is supplied with a switching signal from the tuning control circuit 13. One end of the secondary coil L2 of the transformer 12 is grounded, and the other end is connected to the demodulator at the next stage, and is connected to a pulse counter built in the tuning control circuit 13 via a capacitor C4.

  When changing the tuning frequency, the tuning control circuit 13 turns off the switch 11 with a switching signal, applies a control voltage corresponding to the tuning frequency to the variable capacitance diode Vc, applies a pulse voltage to the capacitor C3, A pulse counter that generates a natural damping vibration (that is, a resonance frequency of the resonance circuit) in a resonance circuit composed of C1, C2, variable capacitance diode Vc, and primary coil L1, and incorporates a natural damping vibration supplied through a capacitor C4. By counting only the unit time, the frequency of the natural damped vibration is detected, and the control voltage is varied so that the frequency of the natural damped vibration matches the tuning frequency.

  Here, the capacitor C1 and the variable capacitance diode Vc are for providing a main capacitance for determining the tuning frequency. The capacitor C2 is connected in parallel with the variable capacitance diode Vc to offset (increase) the minimum capacitance value of the variable capacitance diode Vc as shown in FIG. 4, and is not necessarily provided. FIG. 4 shows the case where the voltage-capacitance characteristic is only the variable capacitance diode Vc (solid line) and the case where the capacitor C2 is connected in parallel (broken line).

  The capacitor C3 is for accumulating energy of a pulse voltage (for example, the low level is 0V, the high level is 10V, and the pulse width is 10 msec) supplied from the tuning control circuit 13. Therefore, the larger the capacitance value of the capacitor C3, the more energy is stored, and a natural damping vibration having a long decay time as shown in FIG. 5 is obtained. In addition, this inherently damped oscillation is easily generated by applying a pulse voltage. The higher the Q value of the resonance circuit composed of the capacitors C1 and C2, the variable capacitance diode Vc, and the primary coil L1, the longer the decay time. This makes it easy to detect the frequency of the natural damped vibration.

  Also, assuming that the desired reception frequency is 100 MHz and the capacitance value of the capacitor C3 is 0.1 μF, the impedance of the capacitor C3 at a frequency of 100 MHz is extremely low at 0.0159Ω, and the other end of the primary coil L1 is substantially grounded. Thus, it can be seen that the capacitor C3 does not affect the tuning frequency.

  The capacitor C4 is a coupling capacitor that supplies a natural damping vibration generated by applying a pulse voltage to the capacitor C3 to the tuning control circuit 13. When the capacitance value of the capacitor C4 is 5 pF, the impedance of the capacitor C4 with respect to a frequency of 100 MHz is 318Ω. When the impedance of the demodulator connected to the secondary coil L2 is 50Ω, about 1/7 of the signal component having a frequency of 100 MHz is supplied to the tuning control circuit 13.

  In the pulse counter of the tuning control circuit 13, by comparing the waveform of the natural damped oscillation shown in FIG. 6A supplied through the capacitor C4 with the threshold value Vth, it is binarized as shown in FIG. By counting the number of pulses, the frequency of the natural damped oscillation, that is, the resonance frequency of the resonance circuit is detected.

  FIG. 7 shows a flowchart of processing executed by the tuning control circuit 13. In the figure, in step S1, an instruction for a tuning frequency (target frequency fa) supplied from an external host device is fetched. In step S2, the switch 11 is turned off by the switching signal, and the antenna 10 is disconnected from the resonance circuit. In step S3, the control voltage Va corresponding to the target frequency fa is applied to the variable capacitance diode Vc. The tuning control circuit 13 has a table in which the control voltage Va corresponding to the target frequency fa is registered in advance.

  Next, a pulse voltage is applied to the capacitor C3 in step S4. In step S5, the intrinsically damped vibration supplied through the capacitor C4 is counted for a unit time by a built-in pulse counter to detect the frequency f 'of the inherently damped vibration.

  Next, in step S6, it is determined whether or not the absolute value of the difference frequency (f′−fa) between the frequency f ′ of the natural damped vibration and the target frequency fa is less than a predetermined value (for example, 1 KHz). If the value is less than the predetermined value, the switch 11 is turned on in step S10 to connect the antenna 10 and the process is terminated.

  On the other hand, if the absolute value of the difference frequency is greater than or equal to a predetermined value, it is determined in step S7 whether or not the sign of the difference frequency (f′−fa) is positive. If it is positive, the capacitance value of the variable capacitance diode is increased. In step S8, the control voltage Va is decreased by a minute voltage ΔV. If negative, the control voltage Va is increased by a minute voltage ΔV in step S9 in order to decrease the capacitance value of the variable capacitance diode, and then the process proceeds to step S4. .

  In this way, the resonance frequency of the resonance circuit composed of the capacitors C1, C2, the variable capacitance diode Vc, and the primary coil L1 is set to the tuning frequency (target frequency fa) regardless of the element constant change with time and the environmental temperature change. Therefore, the Q value of the tuning circuit can be increased and the reception sensitivity can be improved.

  By the way, when detecting the frequency of the natural damped oscillation, the switch 11 is turned off and the antenna 10 is disconnected from the resonance circuit because the received signal received by the antenna 10 is noise as capacitors C1, C2, variable capacitance diode Vc, primary. This is to prevent erroneous detection of the frequency of the inherently damped vibration that is input to the resonance circuit composed of the coil L1.

  Since the antenna 10 itself also has impedance due to radiation resistance and space capacitance, as shown in FIG. 8, when the switch 11 is turned off, it consists of a resistor Rd, a capacitor Cd, and a coil Ld and corresponds to the impedance of the antenna 10. The pseudo load 20 may be connected to a resonance circuit including the capacitors C1 and C2, the variable capacitance diode Vc, and the primary coil L1.

It is a circuit block diagram of an example of the conventional tuning circuit. It is a figure which shows the tuning frequency characteristic of the conventional tuning circuit. It is a circuit block diagram of one Embodiment of the tuning circuit of this invention. It is a figure which shows the voltage-capacitance characteristic of the variable capacity diode Vc. It is a wave form diagram of a natural damping vibration. It is a figure for demonstrating binarization of a natural damping vibration. It is a flowchart of the process which a tuning control circuit performs. It is a figure for demonstrating a pseudo load.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Antenna 11 Switch 12 Transformer 13 Tuning control circuit L1 Primary coil L2 Secondary coil C1-C4 Capacitor Vc Variable capacity diode R1, R2 Resistance

Claims (3)

In a tuning circuit in which a first capacitor and a variable capacitance diode connected in series are connected in parallel with a coil, and a tuning frequency is varied by changing an applied voltage of the variable capacitance diode,
A second capacitor disposed between the coil and ground;
A pulse voltage is supplied to a connection point between the coil and the second capacitor to generate a natural damped vibration, and based on the difference between the detected frequency of the natural damped vibration and a target tuning frequency , tuning circuit, characterized in that the frequency has a tuning control circuit for changing the applied voltage to be a tuning frequency of the target. The tuning circuit of claim 1, wherein
The coil is a primary coil of a transformer,
The tuning control circuit detects a frequency of the natural damped oscillation supplied through a third capacitor connected to one end of a secondary coil of the transformer. The tuning circuit of claim 2 , wherein
A tuning circuit comprising a switch for disconnecting an antenna from the tuning circuit during a period from when the tuning control circuit supplies a pulse voltage to when the applied voltage is changed.

Images