JP5023282B2 - Piezoelectric oscillator - Google Patents

Description

  The present invention relates to an oscillation circuit for a piezoelectric vibrator for physical quantity measurement and chemical measurement. More specifically, the present invention relates to a viscosity sensor, a flow rate sensor, a pressure sensor, a temperature sensor, a vibration sensor, a gas sensor, suspended particulates, a chemical sensor, and an immunosensor. In particular, the present invention relates to a piezoelectric oscillator suitable for oscillating a piezoelectric vibrator that is immersed in a liquid and used.

  In recent years, attention has been focused on a quartz crystal microbalance (QCM) called a microbalance using a crystal resonator which is a piezoelectric resonator. This QCM detects the mass of the surface generated by the substance adhering to the electrodes of the crystal resonator as a change in the resonance frequency of a frequency conversion element such as a crystal resonator. Since QCM is capable of mass detection of nanogram (ng) or less, it is applied to detection of trace substances in a wide range of fields such as medicine, biochemistry, food and environmental measurement as a biosensor and chemical sensor.

  For example, there are cases where a crystal resonator is used as a liquid-phase mass measuring device and the crystal resonator is immersed in a liquid. At this time, the effective crystal impedance (hereinafter referred to as “CI value”) of the crystal resonator is greatly increased in the air and in the liquid, and the CI value in the liquid is about 10 to 30 than in the air. Since it becomes twice as large, it is difficult to oscillate the crystal resonator in the liquid.

  As one of the methods for oscillating a conventional crystal resonator even in a liquid, there is a proposal for increasing the amplification degree of an oscillation circuit (for example, Patent Document 1). In addition, an oscillation circuit has been proposed in which a plurality of crystal resonators having different fundamental frequencies are oscillated by the same circuit using an inverter composed of a high-speed CMOS (Patent Document 2). However, Patent Document 2 suggests only an operation in a gas phase.

  In the conventional way of thinking, when a crystal resonator is oscillated even in a liquid, the electrodes provided on both sides of the resonator must be prevented from being short-circuited to each other by a solution. It was considered natural that it was completely insulated.

One method of making one electrode of a conventional vibrator electrode completely insulated is to sandwich a quartz plate with an O-ring. The crystal plate is sandwiched between two O-rings from both sides so that the solution does not enter from the space between the O-ring and the crystal plate. Therefore, a structure has been proposed in which the electrode on one side is exposed to the solution, but the electrode on the other side is not exposed to the solution and is not short-circuited (Patent Document 3). In addition, a structure has been proposed in which a part of the exposed electrode is accommodated on the side covered with the single-sided covering material and is not short-circuited (Patent Document 4).
JP-A-11-163633 JP 2001-289765 A JP 2001-153777 A Japanese Patent Laid-Open No. 11-14525

  However, in Patent Document 1, oscillation data in an actual liquid phase is not listed in the embodiment, and it cannot be confirmed whether oscillation in the liquid phase in the oscillation circuit shown in Patent Document 1 is possible. Also, since the CI value in liquid is about 10 to 30 times larger than in air, it cannot be said that oscillation is difficult. If the negative resistance on the circuit side necessary for oscillation can be created by the oscillation circuit, oscillation will not occur. Is possible.

  In Patent Document 2, only the oscillation data in the gas phase is described in the embodiment, and it cannot be confirmed whether or not oscillation in the liquid phase is possible in the oscillation circuit shown in Patent Document 2.

  In Patent Document 3, the method of sandwiching a quartz plate with an O-ring makes it difficult for the quartz plate to oscillate when tightly sandwiched to prevent the solution from entering. In addition, if the pin is loosely clamped, the solution enters, and the force for pinching the crystal plate tends to become non-uniform, making it difficult to pinch the crystal plate with an appropriate force.

  In Patent Document 4, a conductive adhesive is used when fixing the vibrator to the single-sided covering material, and it is inevitable that the bonded portion affects the oscillation of the vibrator, such as attenuation.

  In general, a piezoelectric vibrator is easily affected by parasitic capacitance that is inevitably generated in a soldering or printed circuit board or the like that directly contacts the piezoelectric vibrator. Therefore, when the piezoelectric vibrator is oscillated in water, it is expected that when the vibrator is immersed in water, the dielectric constant of water is large, so that a very large parasitic capacitance Cα is generated relative to the interelectrode capacitance C0 of the piezoelectric vibrator. Is done. FIG. 1 is an equivalent circuit of a piezoelectric oscillation circuit. The electrical equivalent circuit 20 of the piezoelectric vibrator is configured by parallel connection of the series arm 10 of the piezoelectric vibrator and the interelectrode capacitance C0. The series arm 10 of the piezoelectric vibrator is constituted by a series connection of an equivalent series resistance R1, an equivalent series capacitance C1, and an equivalent series inductance L1 of the piezoelectric vibrator. The oscillation circuit 30 excluding the piezoelectric vibrator is configured by a series connection of an equivalent input inductance Lci and a negative resistance -Rci. Conventionally, C0 and Cα are synthesized on the series arm side of the vibrator for circuit analysis. However, if Cα increases, the negative resistance -Rci necessary for oscillation cannot be obtained with this concept.

It is said that when a piezoelectric vibrator is immersed in water, the equivalent series resistance R1 of the vibrator becomes extremely large (20 to 30 times) due to the viscosity of water. FIG. 2 shows an equivalent circuit of the piezoelectric oscillation circuit when C0 and Cα are combined on the series arm side of the piezoelectric vibrator. The combined equivalent circuit is configured by an effective resistance RL connected in series with the effective inductance Le and a parallel connection of a negative resistance -Rci connected in series with the load capacitance Cci. The condition for the piezoelectric vibrator to oscillate is expressed by the equation (1). Despite the fact that -Rci required for oscillation cannot be obtained sufficiently due to the parasitic capacitance as described above, RL will inevitably increase if R1 becomes extremely large, and the oscillation condition of equation (1) is satisfied. Absent.

  The present invention has been made to eliminate the above-mentioned drawbacks of the prior art, and it is possible to immerse the vibrator electrode directly in a liquid using a coating material without insulating both surfaces. A piezoelectric oscillator that can oscillate stably is provided. Furthermore, the present invention generates a sufficient negative resistance and enables stable oscillation even when the parasitic capacitance Cα and the equivalent series resistance R1 of the vibrator accompanying the immersion in water are increased. This oscillation circuit has completely new circuit contents and configuration, and cannot be predicted or predicted from conventional electronic circuits.

  In order to achieve the above object, a piezoelectric oscillator according to the present invention includes a piezoelectric vibrator excited at a predetermined frequency, a logic element for exciting the piezoelectric vibrator, and a capacitor connected in parallel to the piezoelectric vibrator. A piezoelectric oscillator comprising: a first inductor and a first pass capacitor connected to the output of the logic element; and a second inductor and a second pass capacitor connected to the input of the logic element. The capacitor, the first inductor, and the second inductor are grounded from the output of the logic element through the first inductor and grounded from the input of the logic element through the second inductor. However, a tank circuit that resonates at the oscillation frequency of the piezoelectric vibrator is configured.

  By performing the configuration of the oscillation circuit as described above, it is possible to easily perform oscillation particularly in liquid. These features have a great effect on sensor oscillation such as QCM (micromass measurement using quartz crystal), and are expected to develop technology in these fields.

FIG. 3 shows an equivalent circuit after C0 and Cα are synthesized on the circuit side. -Rcci is the combined equivalent resistance. The oscillation condition in that case is shown in the equation (2). When C0 + Cα increases, −Rcci decreases extremely. However, in the circuit of the present invention, when the resonance frequency of C0 + Cα and Lci in FIG. 1 is matched with the resonance frequency of L1 and C1, the combined equivalent resistance −Rcci increases, and the circuit reactance at this time is the circuit inductive reactance. Using the feature of Lcci or circuit capacitive reactance Ccci, a piezoelectric vibrator immersed in a liquid can be oscillated.

  FIG. 4 shows a block diagram of the oscillation circuit (piezoelectric oscillator 1). INV indicates inverter logic (logic element). The inverter logic is preferably a high speed CMOS. Cp1 (first pass capacitor) and Cp2 (second pass capacitor) are pass capacitors and do not affect the oscillation. Zxt is a piezoelectric vibrator. When there is no Zxt, the circuit operates as an LC oscillation circuit, and when Zxt is a piezoelectric vibrator, the circuit operates as a piezoelectric oscillation circuit. Ce is an additional capacitor (capacitance element), R2 is a resistor (first resistor), z2 (first inductor), z3 (second inductor) are both inductors, and r2 and r3 are resistance components of the respective inductors. , L2 and L3 represent inductor components of the respective inductors. That is, the circuit of the present invention (piezoelectric oscillator 1) is connected in parallel to the piezoelectric vibrator Zxt, the inverter logic INV for exciting the piezoelectric vibrator Zxt, the additional capacitor Ce connected in parallel to the inverter logic INV, and the additional capacitor Ce. A piezoelectric oscillator composed of a resistor R2 and grounded from an INV output via an inductor z3 and a pass capacitor Cp2, and grounded from an input of the INV via an inductor z2 and a pass capacitor Cp1. Composed. The above contents are realized by configuring a resonance circuit of the additional capacitor Ce, the inductor Z2 and the inductor Z3 and making the resonance frequency coincide with the resonance frequencies of L1 and C1 of the vibrator.

  A circuit analysis is performed based on this block diagram, and the circuit characteristics of the inventive circuit are described in detail below.

FIG. 5 shows a block diagram obtained by the circuit analysis of FIG. Rc is a combined series resistance and Lc is a combined series inductance, which are represented by equations (3) and (4), respectively. Gm in the equations (3) and (4) is a mutual conductance. The two-terminal impedance circuit Za is composed of Lc connected in series with Rc.

In the case of an LC oscillation circuit without Zxt, the two-terminal impedance circuit Zb shown in FIG. 6 obtained as a result of synthesizing the resistance component of the combined impedance of Ce and R2 into Za is obtained from a negative resistance Rec connected in series with Lc. Composed. Rec is expressed by equation (5), and the oscillation frequency fec is expressed by equation (6).

  FIG. 7 shows the simulation results of the negative resistance Rec and the oscillation frequency fec using the additional capacitor Ce as a parameter when the inventive circuit operates as an LC oscillation circuit. The simulation was performed based on the equations (5) and (6), and the simulation conditions were f = 3.3 MHz, L2 = L3 = 10 μF, r2 = r3 = 2.9Ω, R2 = 1 MΩ, and gm = 15 mA / V. When operating as an LC oscillation circuit, the resonance frequency significantly decreases as Ce increases. Negative resistance increases rapidly to the increasing Ce = 10pF, and then keeps a constant value.

FIG. 8 is a block diagram showing a result of combining the combined impedance of the additional capacitor Ce and the resistor R2 with Za when FIG. 4 operates as a piezoelectric oscillation circuit. The circuit shown in FIG. 8 includes a startup circuit inductive reactance Leci connected in series with a negative resistance Reci and connected in parallel with Zxt. Reci is expressed by equation (7) and Leci is expressed by equation (8).

Ld and Rd in the expressions (7) and (8) are expressed by the expressions (9) and (10), respectively.

  FIG. 9 shows the simulation results of the negative resistance Reci and the start-up circuit inductive reactance Leci using the additional capacitor Ce as a parameter when the inventive circuit operates as a piezoelectric oscillation circuit. The simulation was performed based on the equations (9) and (10), and the simulation conditions were f = 3.3 MHz, L2 = L3 = 10 μF, r2 = r3 = 2.9Ω, R2 = 1 MΩ, and gm = 15 mA / V. When operating as a piezoelectric oscillation circuit, in the inventive circuit, Leci sharply decreases from a positive value to a negative value in the vicinity of Ce = 21 pF, and the oscillation circuit changes from inductive to capacitive. At this time, Reci showed the maximum value, and it was confirmed that it coincided with the above contents. The piezoelectric oscillator 1 is preferably used for chemical measurement, physical measurement, or mass measurement.

  Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, as long as there is no specific description, the component, kind, combination, shape, relative arrangement, and the like described in this embodiment are merely examples and not intended to limit the scope of the present invention. In addition, the piezoelectric vibrator is an AT-cut quartz crystal vibrator, an inverted mesa-type AT-cut quartz crystal vibrator, an SC-cut quartz crystal vibrator, a SAW (Surface Acoustic Wave) vibrator, a Lame mode (contour vibration mode) crystal vibrator or a Lamb wave It may be any one of crystal resonators.

  FIG. 10 shows a circuit diagram used in the embodiment. INV is an inverter logic (logic element), OUT is an output terminal, and C3 is a capacitor. Cp1 (first pass capacitor) and Cp2 (second pass capacitor) are pass capacitors and do not affect the oscillation. Zxt is a piezoelectric vibrator, C4 is an additional capacitor, Cx is a variable capacitor, Ce (capacitance element) is the combined capacitance of C4 and Cx, R2 is a resistor (first resistor), L2 (first inductor) and L3 Both (second inductors) are inductors. Setting constants are R2 = 1MΩ, L2 = L3 = 10μH, Cp1 = Cp2 = 1μF, C3 = 47μF, INV: TC7SU04F (Toshiba), Zxt: At-Cut crystal resonator (3.25MHz), 3.25MHz without Zxt C4 = 47pF was selected as the value of C4 for LC oscillation.

  In the following, circuit characteristics when a piezoelectric vibrator is immersed in water will be described based on an example performed using the circuit of the set constant described in the previous paragraph. FIG. 11 shows the characteristics of the power supply voltage and the oscillation frequency when the vibrator is immersed in water. Crystal oscillation starts from 3.4V around the reference frequency due to the voltage rise, and then shifts to LC oscillation from around 4V.

  FIG. 12 shows a continuous frequency change when the crystal unit is immersed in water while the oscillation of the crystal unit is continued in the atmosphere at the power supply voltage Vcc = 3.4V. The elapsed time was oscillated in the atmosphere for up to 7 minutes, immersed in water at 7 minutes, and then the vibrator was in water, but it was confirmed that the oscillation continued in water. As described above, the piezoelectric vibrator Zxt can stably oscillate even when both surfaces thereof are immersed in the liquid.

  FIG. 13 shows the oscillation frequency characteristics with respect to variable Cx when Vcc = 3.4V. When the crystal unit is in the liquid, the frequency change of Cx is almost equivalent to LC oscillation. However, the frequency change is small around Cx = 40pF, which indicates that it has shifted to crystal oscillation.

  FIG. 14 shows negative resistance characteristics with respect to variable Cx. Although almost the same negative resistance is generated in both LC oscillation and crystal oscillation, the negative resistance is generally lowered in the oscillation in water. This difference is an increase in resistance of the crystal resonator due to water.

  In the following, the sensor characteristics of the circuit of the present invention will be described with reference to an embodiment of a sensor in which a piezoelectric vibrator is immersed in water using a circuit having set constants shown in FIG. FIG. 15 shows a temporal change in oscillation frequency when 1 ml of ethanol is added to 58 ml of distilled water of a vibrator oscillating in distilled water using the circuit of the present invention. The set constant of the oscillation circuit is the same as that used in the first embodiment. It is detected as a change in frequency of 100 ppm, indicating that it can be used as a physical quantity sensor in liquid. As in the case of the conventional QCM, it is possible to increase the sensitivity by forming a detection film on the vibrator.

It is an electrical equivalent circuit diagram of the piezoelectric oscillation circuit of a prior art example. FIG. 6 is an equivalent circuit diagram of a piezoelectric oscillation circuit in which C0 is synthesized on the series arm side of a piezoelectric vibrator in a conventional example. FIG. 10 is an equivalent circuit diagram of a piezoelectric oscillation circuit in which C0 is synthesized on the oscillation circuit side other than the series arm in the conventional example. It is a block diagram of the oscillation circuit showing the piezoelectric oscillator which concerns on this invention. It is the block diagram obtained by analyzing the block diagram of the oscillation circuit showing the piezoelectric oscillator which concerns on this invention. FIG. 3 is a circuit diagram of a two-terminal impedance circuit Zb when a circuit representing a piezoelectric oscillator according to the present invention operates as an LC oscillation circuit. It is a figure which shows the result of having simulated the change of the oscillation frequency of a negative resistance and a piezoelectric vibrator when changing additional resistance as LC oscillation circuit. It is a block diagram when the circuit showing the piezoelectric oscillator which concerns on this invention operate | moves as a piezoelectric oscillation circuit. It is a figure which shows the result of having simulated the change of the negative resistance when changing an additional resistance as a piezoelectric oscillation circuit, and the circuit inductive reactance at the time of starting. It is a circuit diagram which shows the specific example of embodiment of the piezoelectric oscillator which concerns on this invention. It is a figure which shows the change of the oscillation frequency when changing a power supply voltage. It is a figure which shows the oscillation state in the air and the liquid. It is a figure which shows the change of the oscillation frequency deviation when changing a variable capacity | capacitance. It is a figure which shows the change of a negative resistance when changing a variable capacity | capacitance. It is a figure which shows the oscillation state when ethanol is added in the liquid.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Piezoelectric oscillator 10 Series arm of piezoelectric vibrator 20 Electrical equivalent circuit of piezoelectric vibrator 30 Oscillation circuit excluding piezoelectric vibrator
C0 Electrode capacitance
Cα parasitic capacitance
Ce addition capacitor (capacitance element)
R2 first resistor
R1 Equivalent series resistance
C1 Equivalent series capacitance
L1 Equivalent series inductance
Lci equivalent input inductance
Le effective inductance
RL effective resistance
Cci load capacity
-Rcci Composite equivalent resistance
Ccci circuit capacitive reactance
Lcci circuit inductive reactance
Fec oscillation frequency
Leci circuit inductive reactance at start-up
-Rci, Rec, Reci Negative resistance
INV Inverter logic (logic element)
gm mutual conductance
Cp1 pass capacitor (first pass capacitor)
Cp2 pass capacitor (second pass capacitor)
C3 capacitor
C4 LC resonance capacitor
Cx variable capacitor
z2 inductor (first inductor)
z3 inductor (second inductor)
r2, r3 Inductor resistance component
Inductor component of L2 and L3 inductors
Zxt At-Cut crystal resonator (piezoelectric resonator)
Vcc supply voltage
Rc Composite series resistance
Lc Composite series inductance
Za, Zb 2-terminal impedance circuit

Claims (6)

A piezoelectric vibrator excited at a predetermined frequency;
A logic element for exciting the piezoelectric vibrator;
A capacitive element connected in parallel to the logic element; a resistor connected in parallel to the capacitive element;
A first inductor and a first pass capacitor connected to the output of the logic element;
A piezoelectric oscillator comprising a second inductor and a second pass capacitor connected to the input of the logic element;
The capacitance element is grounded from the output of the logic element via a first inductor and a first pass capacitor, and grounded from the input of the logic element via a second inductor and a second pass capacitor. And a first inductor and a second inductor constitute a tank circuit that resonates at an oscillation frequency of the piezoelectric vibrator.   2. The piezoelectric oscillator according to claim 1, wherein the logic element is a high-speed CMOS.   3. The capacitor element, the piezoelectric vibrator, and the first resistor are incorporated by being connected in parallel in a closed circuit that connects the input and output of the logic element. Piezoelectric oscillator.   The piezoelectric oscillator according to claim 1, wherein the piezoelectric vibrator is a quartz crystal vibrator.   The piezoelectric oscillator according to claim 1, wherein the piezoelectric vibrator is a SAW vibrator.   The piezoelectric oscillator according to any one of claims 1 to 3, wherein the piezoelectric vibrator is a Lamb wave vibrator.

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