JP3400165B2 - Oscillator and filter - Google Patents

Description

Detailed Description of the Invention

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oscillator and a filter using a resonance circuit, and more particularly to an oscillator, a filter and a filter which can be adjusted to reduce the capacitance connected to a shunt between a piezoelectric vibrator and ground. The present invention relates to a voltage controlled oscillator.

[0003]

2. Description of the Related Art Piezoelectric vibrators that utilize the piezoelectric effect of quartz and the like have a higher Q, better temperature characteristics, and a smaller shape than conventional LC resonators, etc. -Widely used due to miniaturization.

FIG. 8 shows an example of a Colpitts oscillator composed of a crystal unit and a CMOS inverter. Capacitors C1 and C2 are connected to a shunt between both ends of the crystal unit and ground. These capacitors C1 and C2
Is for determining the negative resistance of the oscillator, and it is necessary to set each capacitance value to an optimum value.

However, as shown by the dotted line in FIG. 9,
When this oscillator is actually formed on a printed circuit board, as is well known, stray capacitance is inevitably developed between the wiring pattern on the printed circuit board surface and the lead wires of each component. Of course, this stray capacitance may appear between the crystal unit and the ground, and may become larger than the originally required capacitance value of the capacitors C1 and C2. At this time, the effective resistance of the oscillator increases. Therefore, the oscillation becomes unstable, and in the worst case, there may be a problem that the oscillation stops. In order to solve this, the layout of each component or the position of the wiring pattern is changed, or an inductance that behaves in the opposite manner to the capacitance in order to reduce the stray capacitance, that is, a method of connecting the coil in parallel with the stray capacitance Was taken.

However, in the former case, the layout must be changed by repeating trial and error until the stray capacitance becomes a small value, which leads to an increase in cost, and the stray capacitance can always be reduced to a desired capacitance value. There was a drawback that it was not always. On the other hand, also in the latter case, the inserted coil constitutes the LC parallel resonance circuit together with the stray capacitance, and not only the desired oscillation frequency, that is, the frequency determined based on the resonance of the crystal oscillator, but also the LC parallel resonance circuit is used. Since spurious is generated, there is a defect that an extremely complicated structure must be adopted, such as inserting a resistor at a necessary place and suppressing it.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an oscillator and a filter that can be adjusted in a direction to reduce the capacitance value of a capacitor connected to a shunt between a resonance circuit and ground.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an oscillator and a filter using a resonance circuit (including a piezoelectric vibrator made of a piezoelectric crystal such as quartz or a piezoelectric ceramic), and A negative value capacitor means for giving a negative capacitance value is connected in parallel to a capacitor Cs connected to a shunt between the ground and the negative value capacitor means, and in particular, the negative value capacitor means is an operational amplifier and an operational amplifier of the operational amplifier. Capacitor C connected between the negative input terminal and ground
f, a resistor Rf connected between the negative input terminal and the output terminal of the operational amplifier, and a resistor Rs connected between the positive input terminal and the output terminal of the operational amplifier, the positive input of the operational amplifier Connect the terminal to the capacitor Cs
It is configured to be connected to the terminal on the resonance circuit side.

[0009]

EXAMPLES The present invention will now be described in detail based on examples. As mentioned above, there is a defect that spurious occurs when a coil is inserted in parallel with the capacitor in order to reduce the capacitance value of the capacitor that is connected to the shunt or develops between the piezoelectric vibrator and the ground. However, it has been found that, for example, as shown in FIG. 1, this defect can be solved by connecting a negative value capacitor [-Cv] having a negative capacitance value in parallel instead of the coil.

FIG. 2 is a circuit diagram for explaining the principle of the negative value capacitor means for obtaining a negative capacitance value equivalently. First, the circuit of FIG. 1A includes an operational amplifier A, a resistor Rs connected between the negative input terminal of the operational amplifier A and ground,
A resistor Rf connected between the negative input terminal and the output terminal of the operational amplifier A, a capacitor Cf connected between the positive input terminal and the output terminal of the operational amplifier A, and a capacitor connected between the positive input terminal of the operational amplifier A and the ground. It is composed of Cs, and the positive input terminal of the operational amplifier A is input and the output terminal is output.

Assuming that the operational amplifier A is an ideal operational amplifier, the input voltage is e1 and the output voltage is e0.
And when

[Equation 1] And the current flowing from the input is i1,

[Equation 2] And when

[Equation 3]

[Equation 4] Then, by substituting equation (1) into equation (4) and erasing e0,

[Equation 5] Furthermore, substituting equations (3) and (5) into equation (2),

[Equation 6]

[Equation 7] And this equation (7) gives the input admittance of this circuit. Since this equation can be thought of as representing pure capacitance, the capacitance value C seen from the input of this circuit is

[Equation 8] Becomes Here, considering the capacitor Cs as a capacitor connected to a shunt between the crystal unit and the ground, the last term on the right side of the equation (8) [−Cf · Rf / Rs] is equivalent to the capacitor Cs. Acts as a negative capacitance,
It will be appreciated that the capacitance value of the capacitor Cs can be reduced.

FIG. 3 shows an embodiment of an oscillator according to the present invention constructed by using the circuit shown in FIG. 2A. In FIG. 3A, A1 is an operational amplifier for driving a crystal unit, A circuit in the dotted line in the figure including the operational amplifier A2 provides a negative capacitance means for giving a negative capacitance value for reducing the capacitance value of the capacitor Cs (C1) connected to the shunt between the crystal unit and the ground. Function as. Further, in FIG. 3B, in order to reduce the capacitance value of the capacitor Cs (C2), the operational amplifier A constituting the negative value capacitor means is used as a driving amplifier of the oscillator to reduce the number of constituent elements. It is a thing. However, in this circuit, since the point P and the point Q are connected to use the operational amplifier A as an amplifier for driving the oscillator, the capacitor C
Since f is connected in parallel with the crystal unit, the result is that the parallel capacitance (capacitance between electrodes) of the crystal unit is substantially increased. As is well known, in order to stabilize the oscillation, it is desirable to set the parallel capacitance of the crystal unit to a value as small as possible. (One of the conditions for a good oscillator is to reduce the ratio of the equivalent series capacitance of the crystal oscillator to the capacitance between electrodes, the so-called capacitance ratio.) Therefore, stable oscillation is obtained in this circuit. Therefore, it is desirable to insert a buffer circuit between the points P and Q so that the capacitor Cf is not substantially connected in parallel with the crystal unit.

FIG. 2B is a modification of FIG. 2A, in which the capacitor Cf is connected between the positive input terminal and the output terminal of the operational amplifier A so as to eliminate the above-mentioned inconvenience. It was found that the positions of and were exchanged, and that the same effect as in the above case can be obtained also by this configuration. That is, in the figure, when the input voltage is e1 and the output voltage is e0,

[Equation 9] Next to

[Equation 10]

[Equation 11] From equations (9) and (11)

[Equation 12]

[Equation 13] From equations (10) and (13)

[Equation 14] Therefore, this equation (14) gives the input admittance of this circuit. This equation is exactly the same as the equation (7), and similarly satisfies the equation (8), so that the same action as in FIG. 2A is exhibited.

FIG. 4 shows a modified embodiment of the oscillator according to the present invention constructed by using the circuit of FIG.
In (a), A1 is an operational amplifier for driving the crystal unit, and the circuit inside the dotted line including the operational amplifier A2 is
It functions as a negative value capacitor means for reducing the capacitance value of the capacitor Cs (C1) connected to the shunt between the crystal unit and the ground and giving a negative capacitance value. Further, FIG. 4B shows that the number of constituent elements is reduced by using the operational amplifier A which constitutes the negative value capacitor means as an amplifier for driving the oscillator. Figure 3 (b)
Unlike the circuit of, in this circuit, the operational amplifier A
Even if the points P and Q are connected to use as a driving amplifier for the oscillator, the resistor Rs is only connected in parallel with the crystal unit and the capacitor Cf is connected in parallel with the crystal unit. Therefore, it has a feature that it is not necessary to insert a buffer circuit.

For example, as shown in FIG. 5, by using the circuits of FIGS. 2A and 2B at both ends of the crystal unit, a capacitor Cs1 (C) connected to a shunt between the crystal unit and the ground is provided.
It is also possible to reduce the capacitance values of both 1) and Cs2 (C2).

Next, by using the circuits of FIGS. 2 (a) and 2 (b), fine adjustment means of the oscillation frequency of the crystal oscillator can be constructed without using a trimmer capacitor.
Also, it will be described that a voltage controlled crystal oscillator (VCXO) can be configured without using a variable capacitance diode (varicap).

In order to finely adjust the oscillation frequency of the crystal oscillator, a trimmer capacitor, for example, a semi-fixed capacitor is connected to the shunt between the crystal oscillator and the ground, and the capacitance value is changed. It was common practice. As described above, from expression (8), [-Cf ·
Rf / Rs] equivalently acts as a negative capacitance with respect to the capacitor Cs and can reduce the capacitance value of the capacitor Cs. Therefore, for example, in FIG. 4B, the capacitance value of the capacitor C2 is slightly increased in advance. Set to Rf
Alternatively, if Rs is a variable resistor and the variable resistor is adjusted to reduce the shunt capacitance between the crystal unit and the ground, the frequency can be changed. Where R
As the f and Rs, a pattern resistor formed by printing a conductive paste on the surface of a printed circuit board on which oscillator parts are mounted is adopted, and trimming is performed with a laser to change the resistance value, thereby reducing the size. It is also possible to plan. Normally, such a trimming resistor has a higher resistance value as it is trimmed by a laser, and once trimmed excessively, the resistance value cannot be lowered. However, as is clear from the equation (8), this negative capacitance is Since the value is expressed by [-Cf · Rf / Rs], increasing Rf of the numerator increases the absolute value of the negative capacitance value, and increasing Rs of the denominator decreases the absolute value of the negative capacitance value. The capacitance value between the crystal unit and ground can be increased or decreased as desired.

The conventional voltage controlled crystal oscillator is
The variable capacitance diode was connected in parallel with the capacitor connected to the shunt between the crystal unit and the ground. The variable-capacitance diode is an element that changes the capacitance value seen from both ends according to the voltage applied to both ends.In the above-mentioned voltage-controlled crystal oscillator, a voltage is externally supplied to both ends of the variable-capacitance diode. The oscillation frequency was controlled by changing the voltage. However, since this variable capacitance diode cannot be manufactured by a CMOS manufacturing process, it cannot be formed on the same chip together with other elements such as an amplifier. On the other hand, for example, by adopting a voltage-controlled semiconductor resistance variable element that can be easily manufactured by a CMOS manufacturing process for at least one of Rf and Rs in FIG. Circuit 1
It is possible to configure an IC for a voltage controlled crystal oscillator that is made into a chip. By using this IC, it becomes possible to configure a more compact voltage controlled crystal oscillator. Further, since it is possible to reduce the total capacitance value of the capacitors connected to the shunt between the crystal unit and the ground, it is possible to increase the frequency variable width.

Although the present invention has been described by taking the oscillator using the crystal oscillator as an example, the present invention can be applied to not only the oscillator but also the filter using the piezoelectric oscillator. FIG. 6 (a),
(B) is a quartz substrate with one surface separated by a predetermined gap 2
One electrode is arranged, a larger electrode is arranged on the other surface so as to face the two electrodes, and a multi-mode filter (monolithic crystal) utilizing acoustic coupling of a plurality of vibration modes generated in the vicinity of the electrodes is arranged. Filter: MC
It is drawing which shows sectional drawing of F), and its equivalent circuit. As is clear from the equivalent circuit of FIG. 6B, the interelectrode capacitances Cs1 and Cs2 are inevitably connected to the shunt between the dotted line in the figure functioning as a series resonance circuit and the ground.

For example, when the terminals 1 and 3 are an input terminal pair and the terminals 2 and 4 are an output terminal pair, a part of the input signal leaks to the ground side through the interelectrode capacitance Cs1 on the input terminal pair side. The signal that has passed through the series resonant circuit portion is further leaked to the ground due to the interelectrode capacitance Cs2 on the output terminal pair side, which is one of the causes of the reduction of the pass frequency bandwidth of the filter. Therefore, in order to reduce the interelectrode capacitances Cs1 and Cs2 connected to the shunt between the series resonance circuit portion of this filter and the ground and to make the capacitance almost zero, one embodiment in which the present invention is applied to the filter is described. It is FIG. In the figure, the interelectrode capacitances Cs1 and C
A negative value capacitor means based on the circuit of FIG. 2B is added to each of s2, and each element value is set so that the interelectrode capacitances Cs1 and Cs2 become substantially zero. In this way, the capacitor connected to the shunt between the resonant circuit and ground,
By applying the negative value condenser means for adjusting in the decreasing direction to the filter, it is possible to improve the reduction of the pass frequency bandwidth.

Further, in the above example, an analysis was attempted on the assumption of an ideal operational amplifier, but in the actual operational amplifier, the effects substantially shown by the equations (8) and (14) are obtained. Needless to say, can be expected. Furthermore,
It is obvious that the present invention can obtain the same effect by forming a well-known differentially operating amplifier circuit by combining a transistor, a resistor, and a capacitor and using it instead of an ideal operational amplifier. Similarly, although the impedance, the amplifier gain, and the output impedance have finite values, and there is a slight difference from the ideal operational amplifier in that input / output phase rotation occurs, the expressions (8) and (14) are practically used. The effect shown in can be expected. Therefore, in this case, it is well known that a slight correction coefficient should be added when performing the analysis.

Although the present invention has been described above by taking an oscillator or filter using a crystal oscillator as an example, the present invention is not limited to these embodiments, and piezoelectric materials other than quartz, such as LiTaO3 and LiNbO3, are used. , Li2
A piezoelectric vibrator or piezoelectric resonator made of a piezoelectric single crystal such as B4O7 or a piezoelectric ceramic is also applicable. It is also obvious that not only a multimode filter using acoustic coupling such as a monolithic crystal filter but also a general resonator type filter or a surface acoustic wave device may be used. . Further, it is obvious that the present invention can be applied not only to an oscillator or a filter using a piezoelectric vibrator but also to an oscillator and a filter using a resonance circuit in which a coil or a capacitor such as an LC resonance circuit is combined. Other applied piezoelectric devices, such as piezoelectric transducers, ultrasonic delay lines, or electromechanical functional devices such as vibrating gyros may be applied.

[0023]

As described above, according to the present invention, in the oscillator and the filter using the resonance circuit, the capacitance value connected to the shunt between the resonance circuit and the ground can be adjusted in the direction of decreasing, It is possible to perform fine frequency adjustment without a capacitor element, to be able to configure a voltage-controlled piezoelectric oscillator without a variable-capacitance diode, and to significantly reduce the pass frequency band of the filter, which is extremely effective.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram for explaining a negative value capacitor means according to the present invention and its principle.

3A and 3B are diagrams showing a configuration of an embodiment of a crystal oscillator according to the present invention.

4A and 4B are views showing the configuration of another embodiment of the crystal oscillator according to the present invention.

FIG. 5 is a diagram showing the configuration of still another embodiment of the crystal oscillator according to the present invention.

6A and 6B are diagrams showing a configuration diagram of a monolithic filter and an equivalent circuit, respectively.

FIG. 7 is a diagram showing an embodiment of a monolithic filter according to the present invention.

FIG. 8 is a diagram showing a configuration of a conventional crystal oscillator.

FIG. 9 is a diagram illustrating a defect of a conventional crystal oscillator.

[Explanation of symbols]

A, A1, A2 ... Operational amplifiers C1, C2, Cf, Cs, Cf1, Cs1, Cf2, Cs2.
..Capacitors Rf, Rs, Rf1, Rs1, Rf2, Rs2 ... Resistance-Cv ... Negative capacitance value

Claims (2)

(57) [Claims] 1. An oscillator and a filter using a resonance circuit, wherein a capacitor Cs connected to or developed in a shunt between the resonance circuit and ground, and the capacitor Cs.
s and a negative value capacitor means connected in parallel for giving a negative capacitance value, wherein the negative value capacitor means is an operational amplifier and a capacitor Cf connected between the negative input terminal of the operational amplifier and ground. A resistor Rf connected between the negative input terminal and the output terminal of the operational amplifier, and a resistor Rs connected between the positive input terminal and the output terminal of the operational amplifier.
An oscillator and a filter, wherein the positive input terminal of the operational amplifier is connected to the common circuit side terminal of the capacitor Cs. 2. The oscillator and the filter according to claim 1, wherein a piezoelectric vibrator is used as the common circuit.