JP5861221B2 - Piezoelectric vibration circuit - Google Patents

Description

  It applies to base stations for high-speed data communication, piezoelectric oscillators used as a frequency reference for frequency measuring instruments, and piezoelectric oscillators for sensors using the piezoelectric effect. The present invention relates to an oscillation circuit for piezoelectric vibration for physical measurement and chemical measurement, and more specifically, a viscosity sensor, a flow rate sensor, a pressure sensor, a temperature sensor, a vibration sensor, a suspended particulate sensor, a volatile organic chemical gas, etc. In particular, the present invention relates to a piezoelectric vibration circuit suitable for oscillating a piezoelectric vibrator sensor that is used by being immersed in a fluid as a chemical sensor, an immunosensor, or the like.

  In recent years, a quartz crystal microbalance (QCM) called a microbalance using a crystal resonator, which is a piezoelectric resonator, has attracted attention. This QCM detects the mass of the surface generated by the substance adhering to the electrodes of a crystal resonator as a change in the resonance frequency of a frequency conversion element such as a crystal resonator. Since QCM is capable of detecting mass below nanogram, 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 equivalent circuit constant (crystal impedance, hereinafter referred to as “CI value”) of the vibrator is greatly increased in the air and in the liquid, and the CI value in the liquid is about 10 times that in the air. Therefore, it was difficult to oscillate a crystal resonator in liquid.

A Colpitts type oscillation circuit using a CMOS inverter is presented as a prior art. Furthermore, the following patent documents are presented. In Patent Document 1 and Patent Document 2, a bias setting resistor R ₂ or R ₄ is inserted between the input and output of the inverter. In addition, C ₂ , C ₃ , Cp ₁ , and Cp ₂ pass capacitors are inserted in order to obtain a DC potential separation from the inverter input / output. In contrast, in the present invention, it is not necessary to use a resistor, and there is only one capacitor corresponding to a pass capacitor. The circuit is simplified by reducing the number of capacitors and resistors that need to be fine tuned.

  FIG. 1 shows a circuit diagram of a conventional piezoelectric oscillation circuit called a Colpitts type. FIG. 2 shows the equivalent oscillation circuit-1. Apply Kirchhoff's law to get the formula (Equation 1).

Similarly, the characteristic circuit equation of the formula (Equation 2) is obtained.

Set each impedance as shown in Equation (3), and substitute it into Equation (2) to obtain the equivalent circuit resistance R _C and equivalent circuit capacitance C _{C shown} in FIG. . The result is shown in Formula (Formula 4).

In the Colpitts circuit, the combined equivalent circuit resistance R _{PCCi and the} combined equivalent capacitance C _PCCi on the circuit side from the motion arm are obtained, and Expression (6) is obtained.

When the combined equivalent resistance R _PCCi and the combined equivalent capacitance are calculated using the value of the capacitor C as a parameter, C _PCCi increases as C increases, but R _PCCi decreases. That is, the oscillation possible region is restricted by an increase in capacitance due to a stray capacitance applied from the outside. At 10 MHz, R _PCCi is less than -1 kΩ. Fig. 4 shows the approximate equivalent circuit-3 of the oscillation circuit during steady oscillation, and Fig. 5 shows the approximate equivalent circuit-4. The following shows the derivation process of the equivalent resistance R _CCi , equivalent capacitance C _CCi , equivalent inductance L _CCi , and equivalent resistance R _pCCi , and equivalent capacitance C _pCCi synthesized by including in the circuit side of the Colpitts circuit. FIG. 10 compares the impedance of the oscillation circuit according to the present invention. The Colpitts type oscillation circuit exhibits a large negative resistance in a low frequency region. At 0.6 MHz, the negative resistance maximum value R _pCCimax is approximately equal to −27 kΩ. The dependence on frequency is largely independent of the individual equivalent capacitance C _PCCi from 3 to 15 pF.

JP2008-263272 JP2008-311980

  Provided is a piezoelectric oscillation circuit that obtains a large negative resistance using an inverter and obtains a wider oscillation frequency variable width. And it proves logically that a large negative resistance can be obtained.

  In order to achieve the above object, the piezoelectric vibration circuit includes a circuit in which a capacitor and a piezoelectric vibrator are connected in parallel between the input and output of the inverter, and a circuit in which a first coil and a second coil are connected in series. A pass capacitor is provided between the connection point of the coil and the second coil and the ground point of the circuit, the connection point and the ground point are short-circuited with respect to the AC, and a capacitor is inserted between the power source and the ground to The circuit is short-circuited, and a voltage is applied between a power supply terminal and a ground terminal of the inverter to constitute an oscillation circuit.

The capacitor between the input and output of the inverter is a tuning capacitor or a voltage controlled variable capacitance diode.
At this time, the semiconductor constituting the inverter is an FET or a bipolar transistor.

The piezoelectric vibrator is a crystal body of quartz or a sintered body of ceramic.
FIG. 6 shows an oscillation circuit in which the capacitor between the input and output of the inverter of the present invention is a tuning capacitor. Insert a piezoelectric vibrator between the input and output of the inverter, connect an inductance to the input of the inverter, connect an inductance to the output of the inverter, and connect a capacitance between the midpoint connecting the inductances and the circuit ground. Configure the oscillation circuit. In addition, a sufficiently large capacitance for short-circuiting the DC voltage and AC voltage is inserted between the power supply terminal and the ground terminal of the inverter. The circuit according to the invention constitutes a double resonance circuit composed of a combination of the LC resonance state by C _X or C ₄ and L ₂ and L ₃ added to the circuit and the resonance state of the crystal resonator. The maximum value R _CCimax of the negative resistance of the double resonance type oscillation circuit is approximately equal to -7 kΩ at 10 MHz. Furthermore, a rapid change is shown in a narrow frequency region in the vicinity of the resonance frequency. The equivalent resistance indicates an inductance characteristic and a capacitive characteristic in a high frequency region. 7, 8 and 9 show equivalent circuits of the circuit of the present invention. In the circuit of the present invention, functions equivalent to or equivalent to those of the circuits shown in Patent Document 1 and Patent Document 2 are realized with fewer parts. The resistors in the figure are the resistors that give the Q values (= ωL / R) of the inductances L ₂ and L ₃ . Equation (7) is obtained from the current and potential difference from each current source. By combining the conductance of the constant current source, the total conductance of Equation (9) is obtained.

  Applying Kirchhoff's voltage law and current law, we obtain a homogeneous linear equation that forms the basic equation.

  The characteristic equation (Equation 12) is obtained under the condition that the solution of the equation is not always zero.

The impedance shown in the following equation is specified and substituted to obtain the equivalent resistance and equivalent inductance.
Expression (Equation 13) is a definition of the Q value.

  Each impedance is decomposed into an internal resistance and a reactance, and the formula (Equation 14) is obtained.

  The following equation is obtained from the equivalent circuit of the steady oscillation circuit.

In the equivalent circuit-3, Z _xt is an equivalent impedance of the crystal resonator. R _CCi and L _CCi are found by combining C ₀ and combining C _x with the equivalent resistance and inductance of the driver circuit impedance R _c and L _c . Using the angular frequency ω, the inductance L _c can be converted to a capacitance C _c . Similarly, L _CCi can be converted to C _CCi . The definitions of the synthesized equivalent resistance R _CCi and the synthesized equivalent capacitance C _CCi synthesized including the parallel capacitance on the circuit side are given by the following equations (Equation 19) and (Equation 20). The _relationship between the combined equivalent resistance R _CCi , the combined equivalent capacitance C _C , C _CCi, and the combined equivalent inductance L _C , L _CCi as seen from the motion arm is shown in the following equation (Formula 18).

  The composite equivalent resistance and the composite equivalent capacitance are shown.

  FIG. 10 shows an oscillation circuit of the present invention in which the capacitor between the input and output of the inverter is configured with a single variable capacitance diode frequency control system. Hereinafter, this circuit system is referred to as single diode frequency control. The single diode frequency control system has a feature that the number of components is small, but the change in capacitance with respect to the voltage differs in the case of a change on the increasing side and a change on the decreasing side of the control voltage. Therefore, in frequency control, frequency control is always performed using only one side of the rising or falling part of the control voltage gradient.

  In order to solve this problem, FIG. 11 shows an oscillation circuit in which two variable capacitance diodes are connected in reverse polarity and a control bias is applied to the double variable capacitance diode frequency control method. In this circuit diagram, the back-to-back connection is used. However, the control voltage may have a reverse polarity as a head-to-head connection.

The test was performed using the voltage controlled crystal oscillation circuit of FIG. The circuit constants of the circuit are as follows. Circuit constants: Pass capacitor C ₂ = 10 μF, C ₃ , C ₄ , C ₈ , C ₉ , C ₁₀ = 1000 pF, C ₆ = 10 pF, C ₇ = 0.1 μF; variable capacitance diodes C _x1 , C _x2 : 1SV149B, CMOS inverter IC ₁ : TC7SHU04; L ₂ , L ₃ = 2.7 μH; R _f4 , R _f5 = 100 Ω; R ₆ , R ₇ , R ₈ = 1 MΩ.

FIG. 12 shows how the frequency is variable by single variable capacitance diode frequency control. Absolute values of observed frequency jump and frequency deviation in variable-capacitance diode voltage-controlled oscillator ■: Frequency deviation f _vd1 (↑) when the bias voltage is increased, □: Frequency deviation f _vd1 (↓) when the bias voltage is decreased, ● : Normalization of frequency deviation f _vd1 (↑) | Δf _vd1 (↑) / f ₁ |, ○: Normalization of frequency deviation f _vd1 (↓) | Δf _vd1 (↓) / f ₁ |, circuit constant: L _{2 , L 3 = 2.7 μH, Q} 2, Q 3 = 100, C o = 10 pF, G M = 10 mA / V. Equivalent circuit constant of crystal resonator: C ₀ = 5.122 pF, L ₁ = 12.71 mH, C ₁ = 24.61 pF, R
₁ = 24.15Ω and f ₁ = 8.9989 MHz.

FIG. 13 shows the negative resistance and the variable frequency displacement rate by theoretical calculation. C oscillation frequency when the bias voltage falling as a function of _x and the normalized frequency deviation is equivalent negative resistance. Regarding the dependence of the negative resistance on the capacitance C _x , the oscillation frequency is locked before the negative resistance reaches the maximum value of 36Ω when the bias voltage increases. Crystal resonance occurs at the maximum value 10 kΩ of the negative resistance R _dci when the bias voltage drops. The same value was observed when the same value R _dci was _equal to the crystal resonance frequency f _xt . The mode transition from LC resonance to crystal resonance was observed at the same frequency during the increase and decrease of the bias voltage.

FIG. 14 shows frequency control characteristics of the double variable capacitance diode frequency control method. The crystal resonance occurs at a lower capacitance value and oscillates continuously until the frequency deviation shows a steep change of 10 ¹ ppm.

FIG. 15 shows the theoretical analysis. Quartz resonance was observed at C _x of about 56 pF. Here, the double resonance appears in a wider capacitance range. At this point, the absolute value of R _dci is about 10 kΩ. At this time, the motion arm of crystal resonance leaves the resonance state, and the oscillation mode transitions to LC oscillation.

  16 and 17 show the examination results of the linearity of the dependence of the frequency difference on the control voltage in the variable capacitance diode control circuit. The single variable capacitance diode control circuit shows a mismatch between the crystal resonance and the LC resonance, but the oscillation frequency at which the LC resonance is locked and the crystal resonance frequency match in the double variable capacitance diode control circuit. Furthermore, the double-resonance crystal oscillation circuit of the present invention exhibits linearity with a proportionality factor of about 1 when the control voltage is in the range of 3 to 7V.

In the single variable capacitance diode control, the frequency shift shown in the increase / decrease of the bias is different, whereas in the double variable capacitance diode control, the same frequency displacement is shown.
Frequency stability is important in voltage controlled oscillator circuits. The sensor drive circuit is related to the resolution and reproducibility of the measurement, and the communication application is related to separation between adjacent frequency bands. Allan dispersion is shown as an index of short-term stability in the LC oscillation at a frequency away from the crystal oscillation frequency and in the frequency locked state near the crystal oscillation frequency. Here, the Allan variance is defined as shown in Equation 21. The Allan dispersion of the double resonance showed a magnitude of ^10-10 . In this measurement, environmental drift was reduced by the shield case. This result shows a reasonable value compared with a standard crystal oscillation circuit.

18 and 19 show the Allan variance σ when the bias voltage drops. The Allan variance for a finite sample is defined by equation (21). The oscillation frequency f ^τ _n of the circuit is the frequency indicated by the counter. τ is a gate time, and n is a number representing an order. The deviation of the oscillation frequency is the resonance frequency f ₀ and the deviation Δf ^τ _n . The normalized frequency shift is defined by a dimensionless quantity such as y ^τ _n .

The normalized and dimensionless frequency displacement is y ^τ _n .

The short-term stability in the case of single variable capacitance diode control is shown. When locked to the crystal oscillation frequency, the Allan dispersion is generally on the order of 10 ⁻⁹ to 10 ^{−8 and} 10 ⁻¹⁰ depending on the gate time range. Next, short-term stability of double variable capacitance diode control is shown. Similarly, Alain dispersion generally exhibits a size of 10 ⁻⁹ to 10 ⁻⁸ units.

Modeling when the vibrator is immersed in water will be described. When the crystal unit is immersed in a liquid, an increase in leakage current, an increase in parallel capacitance, energy dissipation due to viscosity, and the like occur, and an increase in the dielectric constant increases the parallel capacitance C ₀ between the electrodes. This increase in capacitance is described as C _0W . Leakage current is described by the resistor R _W. An increase in viscosity in the liquid results in an increase in equivalent resistance. The increased equivalent resistance is described by R ¹ _W. The double resonance type oscillation circuit is connected to a crystal resonator in liquid. If C _0W and C ₀ are introduced into R _C and L _C , the equivalent resistance is R ¹ _CCi and the equivalent capacitance is C ¹ _CCi , then L ¹ _CCi is expressed by the terms C ¹ _CCi and resonance angular frequency ω . R _WCCi and C _WCCi are obtained by combining R _W with R ¹ _CCi and C ¹ _CCi .

  With the piezoelectric vibration circuit of the present invention, a change in dielectric constant or electrical conductivity in a liquid medium can be detected as a change in oscillation frequency. In water, metal ions, OH ions, and the like can be quantified as minute changes in electrical conductivity, and therefore can be used for monitoring ion reactions in water. Since the oscillation frequency of the crystal unit is not affected by the strong magnetic field, the same measurement is possible in a liquid immersion environment in a strong magnetic field.

  In addition, a crystal resonator having a large area electrode in gas may not oscillate due to an increase in interelectrode capacitance. However, when the large interelectrode capacitance is regarded as a load capacitance, it can be excited by the circuit according to the present invention. is there.

  Furthermore, by changing the tuning capacitor to a voltage-controlled variable capacitance diode, when used as a sensor drive circuit, continuous oscillation in a liquid medium is possible due to the large negative resistance realized by the conventional double resonance circuit. In addition to tracking the oscillation frequency from the crystal resonance frequency to the low frequency side by changing the constant, automatic tracking can be performed by voltage adjustment.

  When the tuning capacitance is replaced, the power source needs to be turned off once. However, since it can be continuously changed, the frequency can be changed while the sensor driving circuit is in an operating state. Since it automatically shifts to LC oscillation outside the tuning region of the crystal oscillation circuit, there is an effect that the continuous frequency variable range becomes remarkably wide.

Circuit diagram of conventional oscillator circuit Equivalent circuit of conventional circuit-1 Equivalent circuit of conventional circuit-2 Equivalent circuit of conventional circuit-3 Equivalent circuit of conventional circuit-4 Circuit diagram of oscillation circuit according to the present invention Equivalent circuit of the oscillation circuit according to the present invention-1 Equivalent circuit of the oscillation circuit according to the present invention-2 Equivalent circuit of oscillation circuit according to the present invention-3 Oscillation circuit configured with single variable capacitance diode frequency control system Oscillation circuit with double variable capacitance diode frequency control system Frequency control characteristics of single variable capacitance diode frequency control system Theoretical analysis results of frequency control characteristics Frequency control characteristics of double variable capacitance diode frequency control system Theoretical analysis results of frequency control characteristics Observation of linearity in single variable capacitance diode control. Observation of linearity in double variable capacitance diode control Allan variance σ at bias voltage drop in single variable capacitance diode control Allan variance σ when bias voltage drops under double variable capacitance diode control Comparison of negative resistance and equivalent capacitance of conventional circuit (Colpitts circuit) and invented oscillation circuit (Double resonance) Frequency dependence of negative resistance of conventional circuit (Colpitts circuit) with parallel capacitance C ₀ as parameter Frequency dependence of the negative resistance of the circuit of the present invention (Double resonance) with the parallel capacitance C ₀ as a parameter Conventional circuit negative resistance of the parallel capacitance Gm parameters and the frequency dependence of (Colpitts circuit) (circuit constants:. Varied as G _m parameters; C ₀ is _{0 pF C 2, C 3 =} 20 pF, R 2 = 1 MΩ) Frequency dependence with the parallel capacitance Gm of the negative resistance of the circuit of the present invention (Double resonance) as a parameter (circuit constant: change as parameter; C ₀ is 0 pF. L ₂ , L ₃ = 30 μH; Q value Q ₂ , Q ₃ = 100) Crystal resonator model when immersed in water Equivalent circuit of oscillation circuit using quartz crystal model when immersed in water-1 Equivalent circuit of oscillation circuit using quartz crystal model when immersed in water-2 Equivalent circuit of an oscillation circuit using a crystal unit model immersed in water-3 Presented circuit constant as a function of frequency of negative resistance for oscillation in water Solid line C _X = 0 pF; Dashed line 15 pF; Dotted line 30 pF; R _W = 10 kΩ constant Other constants: C _0W = 30 pF G _m = 10 mA / V; L ₂ , L ₃ = 10 μH; Q value Q ₂ , Q ₃ = 100. Display circuit constant by frequency function of negative resistance in case of underwater oscillation Solid line R _W = 10kΩ; Broken line 3 kΩ; Dash-dot line 1 kΩ; C _x = 30 pF constant. Other circuit constants C _0W = 30 pF; G _m = 10 mA / V; L ₂ , L ₃ = 10 μH; Q values Q ₂ and Q ₃ = 100. Outline of implementation of piezoelectric vibration excitation in water Presenting experimental results in water Presentation of experimental results in air

As shown in FIG. 21, the circuit shown in FIG. 6 was prepared, and the experiment was performed by immersing the piezoelectric vibration element 3 in the water 5 in the container 4. The oscillation circuit 1 was disposed immediately above the liquid level.
The experimental results in water are shown in FIG. In the figure, ○ is the bias current of the circuit, △ is the L / C resonance frequency at non-resonance, □ is the double resonance frequency of the crystal resonator, and the beginning of double resonance due to resonance frequency jump at 20 pF Is shown. Frequency locking was observed between 20 pF and 36 pF. At 20 pF, the bias current suddenly increases and then the bias current gradually decreases. The resonance frequency of the L / C resonance was adjusted by the capacitor _CX .

  Table 1 shows the equivalent circuit constants of the crystal units used in the experiment.

The circuit shown in FIG. 6 was produced, and the experimental results in air are shown in FIG. In the figure, ○ is the bias current of the circuit, △ is the L / C resonance frequency at non-resonance, □ is the frequency of the double resonance of the crystal resonator, and an example of the start of double resonance by jumping the resonance frequency at 4 pF Show. Frequency locking was observed between 4 pF and 40 pF. The resonance frequency of the L / C resonance was adjusted by the capacitor _CX . At 4 pF, the bias current suddenly increases, and then the bias current gradually decreases. The same quartz crystal was used in underwater and air oscillation experiments. In the oscillation circuit according to the present invention, both electrode portions of the crystal resonator are immersed in water. In contrast to QCM (One Face Open QCM) where only one side is exposed for liquid use, both sides are in contact with the liquid medium in the experiment of the circuit of the present invention. The conventional Colpitts oscillation circuit stops oscillating when both electrodes are immersed in water. In the oscillation circuit according to the present invention, oscillation characteristics can be obtained even if both crystal electrodes are immersed in water. Is obtained.

DESCRIPTION OF SYMBOLS 1 Oscillation circuit 2 Output 3 Piezoelectric vibration element 4 Liquid container 5 Liquid (water)

Claims (4)

A circuit in which a capacitor and a piezoelectric vibrator are connected in parallel between the input and output of the inverter;
A circuit comprising a first coil and a second coil connected in series;
Said first coil, said second comprising a path condenser between the ground point of the connection point and the circuit of the coil, the are short connection point and to the AC between the ground point, between more power and ground capacitor A piezoelectric vibration circuit characterized in that an oscillating circuit is configured by inserting a short circuit with respect to alternating current and applying a voltage between a power supply terminal and a ground terminal of the inverter. 2. The piezoelectric vibration circuit according to claim 1, wherein the capacitor between the input and output of the inverter is a tuning capacitor or a voltage-controlled variable capacitance diode . The piezoelectric vibration circuit according to claim 1, wherein the semiconductor constituting the inverter is an FET or a bipolar transistor . The piezoelectric vibration circuit according to claim 1, wherein the piezoelectric vibrator is a crystal of crystal or a sintered body of ceramic .