Oscillator circuit, oscillator circuit adjusting method, and mass measuring apparatus using oscillator circuit

An oscillator circuit is a closed loop including an amplifying circuit and a feedback circuit. The amplifying circuit includes a pair of amplifiers, which also function as impedance buffers, and a first phase-shift circuit. The feedback circuit includes a second phase-shift circuit and a piezoelectric vibrator. The second phase-shift circuit is capable of adjusting the phase and gain of the feedback circuit. The first phase-shift circuit of the amplifying circuit is arranged between the pair of amplifiers and is separated, in terms of impedance, from the second phase-shift circuit. The first phase-shift circuit is capable of adjusting the phase of the entire closed loop.

RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2003-072363 filed Mar. 17, 2003 and 2004-013195 filed Jan. 21, 2004 which are hereby expressly incorporated by reference herein in their entireties.

BACKGROUND

1. Technical Field

The present invention relates to oscillator circuits for piezoelectric vibrators, and more particularly, to an oscillator circuit suitable for causing a piezoelectric vibrator immersed in liquid to oscillate, an oscillator circuit adjusting method, and a mass measuring apparatus using the oscillator circuit.

2. Background Art

In recent years, quartz crystal microbalances (QCMs) using a quartz vibrator, which is a piezoelectric vibrator, have been receiving attention. QCMs utilize the fact that deposition of a substance on an electrode of a quartz vibrator reduces the oscillation frequency. Since QCMs are capable of detecting masses on the order of nanograms (ng) or less, QCMs are applied, as biosensors, chemical sensors, and the like, to detect micro materials in a wide range of fields, such as medical, biochemical, food, and environmental measurement.

For example, in some cases, quartz vibrators immersed in liquid are used as mass measuring apparatuses. The effective crystal impedance (hereinafter, referred to as a “CI value”) of quartz vibrators in air is very different from that of quartz vibrators in liquid; the CI value in liquid is about ten to thirty times larger than the CI value in air. Since the increase in the CI value makes oscillation of quartz vibrators more difficult, it is difficult to cause quartz vibrators in liquid to oscillate with the same circuit conditions for causing quartz vibrators in air to oscillate. Thus, oscillation of quartz vibrators in liquid has been made possible by increasing the amplification of an oscillator circuit (for example, Japanese Unexamined Patent Application Publication No. 11-163633 (Paragraph No. 0004 and FIG. 1)). Also, an oscillator circuit that causes a plurality of quartz vibrators having different fundamental frequencies to oscillate by the same circuit by using inverters made of high speed CMOS devices has been proposed (Japanese Unexamined Patent Application Publication No. (Paragraph No. 0015)). In Patent Document 2001-289765, however, the operation only in a gas phase is suggested.

Since an oscillator circuit is a feedback circuit that forms an oscillator loop, if oscillation conditions of the oscillator loop are satisfied, oscillation may occur but not by way of a vibrator, or even in the case where the oscillation does occur by way of the vibrator, the oscillation may occur at a frequency that is not the resonance frequency of the vibrator. Thus, if the amplification is increased, as in Patent Document 11-163633, such undesirable oscillation is likely to occur. Also, in a case where the oscillator circuit is incorporated in the actual circuit, the oscillator circuit is electrically connected to various other circuits. Thus, if the amplification of the oscillator circuit is increased, oscillation may occur in the various other circuits that are not provided with vibrators. Therefore, it is difficult to achieve stable oscillation merely by increasing the amplification.

FIG. 20shows the variation in the CI value of a piezoelectric vibrator in air with respect to frequency and the variation in the CI value of a piezoelectric vibrator in liquid with respect to frequency. In air, the CI value of the piezoelectric vibrator that oscillates at a frequency of 148.25 MHz is about 20Ω. In contrast, in liquid, the CI value of the piezoelectric vibrator that oscillates at a frequency of 148.25 MHz is about 300Ω. Thus, it is difficult to cause the piezoelectric vibrator to oscillate in liquid by using the circuit that oscillates in air, as in the technology of Patent Document 2001-289765.

FIG. 21shows the variation in phase of a piezoelectric vibrator in air with respect to frequency and the variation in phase of a piezoelectric vibrator in liquid with respect to frequency. In air, the phase of the piezoelectric vibrator is abruptly changed from −90 degrees to +90 degrees at a frequency near 148.25 MHz. In contrast, in liquid, the phase of the piezoelectric vibrator is only changed between about −90 degrees and about −50 degrees, not only at a frequency near 148.25 MHz, but also in a frequency range between 147.8 MHz and 148.6 MHz. Accordingly, the phase does not abruptly change. Thus, in Patent Document 2001-289765, an amplifier includes an inverter and the phase at the output side of the amplifier is always 180 degrees different from the phase at the input side of the amplifier. In other words, the phase at the output side of the amplifier cannot be equal to the phase at the input side of the amplifier.

In order to solve the problems described above, an object of the present invention is to ensure oscillation both in air and liquid.

Also, another object of the present invention is to ensure prevention of undesirable oscillation, such as parasitic oscillation, spurious oscillation, and feedback oscillation.

SUMMARY

FIG. 22is a block diagram showing the circuit structure of an oscillator circuit. The oscillator circuit includes an amplifying circuit and a feedback circuit, as shown inFIG. 22. The oscillation conditions of the oscillator circuit are represented by the following formulae:
Re(Aβ)≧1(power condition)  Formula 1
Im(Aβ)=0(frequency condition),  Formula 2

where A represents the gain of the amplifying circuit and β represents the feedback ratio of the feedback circuit. Here, Re(Aβ) and Im(Aβ) represent the real part and the imaginary part, respectively, of a complex quantity Aβ. As described above, in order to cause the oscillator circuit to oscillate, the amplification, which is a power condition, must be 1 or more and the phase of an oscillator loop must be 0 degrees. Thus, a phase-shift circuit is provided both in the amplifying circuit and the feedback circuit, so that the phase-shift circuit in the amplifying circuit can adjust the phase of the entire loop. Thus, a phase condition, which is one of the oscillation conditions, is satisfied so that an influence of the feedback circuit (feedback loop) on the gain is minimized.

In other words, an oscillator circuit according to the present invention includes a plurality of amplifiers, which also function as impedance buffers; a first phase-shift circuit connected between the amplifiers, the first phase-shift circuit being capable of adjusting the phase of an oscillator loop; a feedback circuit, an input side of the feedback circuit being connected to an output terminal of the amplifier that is connected to an output side of the first phase-shift circuit, an output side of the feedback circuit being connected to an input terminal of the amplifier that is connected to an input side of the first phase-shift circuit; a second phase-shift circuit provided in the feedback circuit and capable of adjusting the phase and gain of the oscillator loop; and a piezoelectric vibrator provided in the feedback circuit and connected in series with the second phase-shift circuit.

As described above, in order to satisfy the phase condition for oscillation and the gain condition of the oscillator loop both in gas, such as air, and liquid, the amount of phase shift of the entire oscillator loop is adjusted to an appropriate value by the first phase-shift circuit. Also, in order to obtain an oscillator loop gain and to achieve the phase condition for stable oscillation, the reactance of the second phase-shift circuit arranged in the feedback circuit is adjusted. Since the first phase-shift circuit is separated from the second phase-shift circuit in terms of impedance by the amplifiers having a buffer function due to impedance conversion, the phase of the entire closed loop can be adjusted irrespective of the gain characteristics of the feedback circuit. Thus, the phase condition and gain condition of the oscillator circuit are satisfied so that stable and easy oscillation of the piezoelectric vibrator both in air and liquid can be ensured.

The phase-shift circuit may include a tank circuit that resonates at an oscillation frequency of the vibrator. Thus, even if the amplification of the oscillator loop is increased, undesirable oscillation, such as parasitic oscillation, spurious oscillation, and feedback oscillation, can be avoided. In other words, an input frequency equal to the resonance frequency of the tank circuit rapidly increases the impedance when viewed from the closed loop side, and an input frequency deviated from the resonance frequency reduces the impedance. Thus, by setting the resonance frequency of the tank circuit to be substantially equal to the oscillation frequency of the piezoelectric vibrator, the amplification of the closed loop at frequencies other than the oscillation frequency of the piezoelectric vibrator is reduced. Therefore, undesirable oscillation, such as parasitic oscillation, spurious oscillation, and feedback oscillation, can be avoided.

At least one of the first phase-shift circuit and the second phase-shift circuit may include a voltage-controlled phase-shift circuit capable of adjusting the phase of the oscillator loop in accordance with a control voltage from the outside. The impedance characteristics of a piezoelectric vibrator in a liquid change depending on the state of the liquid, the wettability of the liquid to the piezoelectric vibrator, the contact condition of the liquid to the surface of the piezoelectric vibrator, and the like. By the phase-shift circuit formed by the voltage-controlled phase-shift circuit, the oscillator circuit can be precisely adjusted. Thus, a stable operation of the oscillator circuit in liquid can be achieved.

Also, each of the amplifiers may be a differential amplifier including an inverting input terminal, a non-inverting input terminal, an inverting output terminal, and a non-inverting output terminal. The differential amplifier may be an emitter-coupled logic circuit. The piezoelectric vibrator may be any one of an AT-cut quartz vibrator, a reverse-mesa AT-cut quartz vibrator, and a surface acoustic wave (SAW) vibrator.

An adjusting method according to the present invention for adjusting the oscillator circuit includes a gain and phase calculation step of measuring the circuit characteristics of the oscillator circuit and of calculating the gain and phase of the oscillator loop at the oscillation frequency of the oscillator circuit when the piezoelectric vibrator is disposed in gas and when the piezoelectric vibrator is disposed in liquid; and an adjusting step of performing an adjustment so as to satisfy a first phase-adjusting step of changing a circuit constant of the first phase-shift circuit of the oscillator circuit to perform coarse adjustment for adjusting the phase such that the phase of the oscillator loop is about 0 at frequencies near the principal vibration frequency when the piezoelectric vibrator is disposed in gas and when the piezoelectric vibrator is disposed in liquid; a second phase-adjusting step of changing a circuit constant of the second phase-shift circuit of the oscillator circuit to adjust the phase of the oscillator loop to 0 at a frequency near the oscillation frequency and near the resonance frequency of the vibrator when the piezoelectric vibrator is disposed in gas and when the piezoelectric vibrator is disposed in liquid; and a gain adjusting step of adjusting the gain of the oscillator loop to 1 or more.

Preferably, in the gain adjusting step, a negative resistance is set to be three or more times larger than the impedance of the piezoelectric vibrator. Thus, oscillation of the piezoelectric vibrator can be ensured. Also, preferably, each of the steps for adjusting the oscillator circuit is performed when the oscillator circuit is an open loop. Thus, adjustment of the oscillator circuit can be easily performed.

A mass measuring apparatus according to the present invention includes the oscillator circuit described above. Thus, a mass measuring apparatus achieving stable oscillation both in gas and liquid can be provided, and a QCM with high reliability can be provided.

DETAILED DESCRIPTION

The best mode for an oscillator circuit, an oscillator circuit adjusting method, and a mass measuring apparatus using the oscillator circuit according to the present invention will be described in detail with reference to the drawings.

FIG. 1is a block diagram showing an oscillator circuit10for a piezoelectric vibrator according to a first embodiment. InFIG. 1, the oscillator circuit10includes an amplifying circuit20and a feedback circuit30. The amplifying circuit20includes a plurality of amplifiers22and24, which also function as impedance buffers, and a first phase-shift circuit26. The first phase-shift circuit26is arranged between the amplifiers22and24. The input side of the first phase-shift circuit26is connected to the output terminal of the amplifier22and the output side of the first phase-shift circuit26is connected to the input terminal of the amplifier24.

Also, the feedback circuit30includes a second phase-shift circuit32and a piezoelectric vibrator34connected to the input side of the second phase-shift circuit32. The input side of the second phase-shift circuit32is connected to the output terminal of the amplifier24with the piezoelectric vibrator34therebetween. The output side of the second phase-shift circuit32is connected to the input terminal of the amplifier22. Accordingly, the oscillator circuit10is a closed loop including the amplifying circuit20and the feedback circuit30. An output buffer12is connected to the output side of the first phase-shift circuit26of the amplifying circuit20so as to be in parallel with the amplifier24.

The piezoelectric vibrator34may be connected to the output side of the second phase-shift circuit32, in other words, to the input side of the amplifier22. Also, the second phase-shift circuit32may include a plurality of phase-shift circuit elements, and the piezoelectric vibrator34may be arranged among the phase-shift circuit elements. The output buffer12may be connected to the output side of the amplifying circuit20, in other words, to the output terminal of the amplifier24.

Each of the amplifiers22and24, which also function as impedance buffers, may be an amplifying circuit using a transistor, an operational amplifier (Op-Amp), a differential amplifier, emitter coupled logic (ECL), positive ECL (PECL), or the like. The piezoelectric vibrator34may be a quartz vibrator or a lithium tetraborate (Li2B4O7(LBO)) vibrator. Also, the piezoelectric vibrator34may be an AT-cut vibrator, a BT-cut vibrator, a GT-cut vibrator, an SC-cut vibrator, a quartz-crystal filter, a SAW vibrator, or a SAW filter.

Examples of equivalent circuits and structures of the first phase-shift circuit26and the second phase-shift circuit32will now be described. Each equivalent circuit of the first phase-shift circuit26and the equivalent circuit of the second phase-shift circuit32can be formed by a phase-advance circuit for advancing the phase, a phase-delay circuit for delaying the phase, or a resonance circuit. For example,FIGS. 2(A)to (C) show an example of the phase-advance circuit.FIGS. 3(A)to (C) show an example of the phase-delay circuit. Also,FIG. 4(A)shows an example of a series resonance circuit, andFIG. 4(B)shows an example of a parallel resonance circuit. Furthermore,FIG. 5shows a voltage-controlled reactance control circuit using a variable capacitance diode40. As described above, the equivalent circuit of the first phase-shift circuit26and the equivalent circuit of the second phase-shift circuit32can be formed by any circuit shown inFIGS. 2 to 5.

FIGS.6(A)–(C) include specific examples of the structure of the first phase-shift circuit26or the structure of the second phase-shift circuit32.FIG. 6(A)shows an example in which amplifiers200are connected in series with each other. Here, the number of the amplifiers200is not particularly limited as long as one or more amplifiers200are provided.FIG. 6(B)shows an example in which a delay element is arranged. The delay element may be a distributed-constant element, such as a variable capacitance diode or a thermistor.FIG. 6(C)shows an example in which the first phase-shift circuit26or the second phase-shift circuit32is formed by a line. The line may be a stripline, a microstrip line, or the like.

In the oscillator circuit10according to the first embodiment arranged as described above, the loop phase and the loop gain can be individually adjusted such that the conditions for oscillation of the piezoelectric vibrator34both in air and liquid are satisfied. Thus, the oscillator circuit10according to the first embodiment is able to ensure oscillation of the piezoelectric vibrator34both in air (gas) and liquid. Also, stable oscillation can be achieved with respect to the characteristics of various liquids. Accordingly, the use of the oscillator circuit10easily provides a mass measuring apparatus with high reliability, such as a chemical sensor, biosensor, and a QCM.

FIG. 7is a block diagram showing a piezoelectric vibrator oscillator circuit50according to a second embodiment. InFIG. 7, the oscillator circuit50is a closed loop including the amplifying circuit20and a second phase-shift circuit52functioning as a feedback circuit. The second phase-shift circuit52includes a phase-shift circuit part54, the piezoelectric vibrator34, and a tank circuit56. The piezoelectric vibrator34may be arranged on the input side of the second phase-shift circuit52.

The phase-shift circuit part54can be arranged in a similar manner to the first phase-shift circuit26and the second phase-shift circuit32(seeFIG. 1). The input side of the phase-shift circuit part54is connected to the output terminal of the amplifier24constituting the amplifying circuit20. Also, the output side of the phase-shift circuit part54is connected to one electrode of the piezoelectric vibrator34. The other electrode of the piezoelectric vibrator34is connected to the input terminal of the amplifier22constituting the amplifying circuit20. The tank circuit56is a parallel resonance circuit including a capacitance element and an inductance element. One end of the tank circuit56is connected between the piezoelectric vibrator34and the input terminal of the amplifier22. The other end of the tank circuit56is grounded via a capacitor58. The tank circuit56is adjusted such that resonance occurs at a predetermined frequency, that is, the oscillation frequency of the piezoelectric vibrator34. The capacitor58is grounded such that the tank circuit56DC-floats.

FIG. 23shows a modification of the second embodiment. In this modification, each of the first phase-shift circuit26and the phase-shift circuit part54of the second phase-shift circuit52is a voltage-controlled phase-shift circuit and includes a voltage-controlled reactance control circuit. Accordingly, when the piezoelectric vibrator34is immersed in liquid, stable oscillation can be achieved irrespective of the type of liquid. In other words, the impedance characteristics of piezoelectric vibrators in a liquid change depending on the state of the liquid, the wettability of the liquid to the piezoelectric vibrator, the contact condition of the liquid to the surface of the piezoelectric vibrator, and the like. By providing the voltage-controlled reactance control circuits in the phase-shift circuits, as shown inFIG. 23, a circuit constant of the oscillator circuit50can be precisely adjusted. Thus, stable operation of the oscillator circuit50in liquid can be achieved, and stable oscillation of the piezoelectric vibrator34can thus be ensured. Of course, each of the first phase-shift circuit26and the second phase-shift circuit32in the first embodiment may be arranged so as to include a voltage-controlled reactance control circuit.

FIG. 8shows a specific example of the oscillator circuit50according to a modification of the second embodiment. InFIG. 8, each of the first phase-shift circuit26and the second phase-shift circuit52includes a voltage-controlled reactance control circuit using a variable capacitance diode, a coil functioning as a phase-shift circuit element, and DC-cutting capacitors. In this specific example, the output buffer12is connected to the output terminal of the amplifier24, which is the output terminal of the amplifying circuit20.

The first phase-shift circuit26of the amplifying circuit20includes a variable capacitance diode60, two coupling capacitors62and64for cutting DC components, a coil66functioning as a phase-shift element, and two resistors68and70. In other words, in the first phase-shift circuit26, the anode of the variable capacitance diode60is connected to the output terminal of the amplifier22with the coupling capacitor62therebetween and is grounded via the resistor68. Also, the cathode of the variable capacitance diode60is connected to the input terminal of a control voltage VC1input from the outside via the resistor70and is connected to one end of the coil66. The other end of the coil66is connected to the input terminal of the amplifier24via the coupling capacitor64. Accordingly, the variable capacitance diode60and the two resistors68and70constitute the voltage-controlled reactance control circuit.

In contrast, the second phase-shift circuit52includes a variable capacitance diode74, two coupling capacitors76and78for cutting DC components, a coil80functioning as a phase-shift element, and two resistors82and84. The anode of the variable capacitance diode74is connected to one end of the coupling capacitor76and is grounded via the resistor82. The other end of the coupling capacitor76is connected to one end of the coil80. The other end of the coil80is connected to one electrode of the piezoelectric vibrator34. Also, the cathode of the variable capacitance diode74is connected to one end of the coupling capacitor78and is connected to the input terminal of a control voltage VC2input from the outside via the resistor84. The other end of the coupling capacitor78is connected to the output terminal of the amplifier24. Accordingly, the variable capacitance diode74and the two resistors82and84constitute the voltage-controlled reactance control circuit.

The other electrode of the piezoelectric vibrator34is connected to the input terminal of the amplifier22via a coupling capacitor86and the tank circuit56. The tank circuit56is a parallel resonance circuit in which a resistor88for adjusting the Q factor of the tank circuit56, a capacitor90, and a coil92are connected in parallel with each other. In the second embodiment, in order to prevent undesirable oscillation, the inductance of the coil92and the capacitance of the capacitor90are determined in the tank circuit56such that the range of the resonance frequency is reduced.

The influence on the phase change with respect to frequency and the influence on the gain with respect to frequency of a circuit that changes the phase will now be described.

FIG. 9shows a state in which the amount of variation in the phase is changed by using only the second phase-shift circuit52. As described above, when the piezoelectric vibrator34is immersed in liquid after the oscillator circuit50is adjusted such that the piezoelectric vibrator34oscillates in air, the piezoelectric vibrator34cannot oscillate because the impedance increases and the phase of the piezoelectric vibrator34does not change very much. A state in which the amount of phase shift in the feedback circuit is changed only by using the second phase-shift circuit52such that the piezoelectric vibrator34oscillates even in liquid is shown inFIG. 9. For example, when the inductance of the coil80constituting the second phase-shift circuit52is adjusted and the phase of the feedback circuit is delayed, the amount of change in the phase is increased from the curved lines A to E in that order. Delaying the phase of the feedback circuit increases the width of frequencies at which the phase changes. Thus, the piezoelectric vibrator34oscillates at a frequency that is away (far) from the oscillation frequency of the piezoelectric vibrator34. Therefore, a current flowing in the piezoelectric vibrator34is reduced, thus reducing the stability of a frequency at which the piezoelectric vibrator34oscillates. In other words, in a case where such the piezoelectric vibrator34is used for an apparatus for measuring a very small amount of mass, the S/N ratio is deteriorated, thus reducing the measurement accuracy.

FIG. 10shows a variation in the gain when the phase is changed by using only the second phase-shift circuit52. When the amount of change in the phase is increased by using the second phase-shift circuit52, the gain of the feedback circuit is increased due to a change in the impedance of the feedback circuit. Thus, undesirable oscillation is, unfortunately, likely to occur. In other words, due to the use of a phase-adjusting coil in order to increase the amount of change in phase, a frequency at which a series resonance caused by an inter-electrode capacitance C0of the piezoelectric vibrator34and the phase-adjusting coil occurs is generated. The gain at this frequency may be greater than 1 due to an increase in gain of the entire oscillator circuit. In this case, since the oscillation starting time of the series resonance, which is not a mechanical vibration, caused by the phase-adjusting coil and the inter-electrode capacitance C0is short, the series resonance may start oscillation earlier than the oscillation of the piezoelectric vibrator34. Also, when a phase-adjusting coil with a large inductance is used in order to increase the amount of change in the phase, the series resonance frequency of the coil and the inter-electrode capacitance C0is reduced to near the oscillation frequency of the piezoelectric vibrator34. Thus, the gain of a series resonance circuits composed of the phase-adjusting coil and the inter-electrode capacitance C0is likely to be increased, and the series resonance caused by the phase-adjusting coil and the inter-electrode capacitance C0occurs before the oscillation of the piezoelectric vibrator34. InFIG. 9, in order to simplify the comparison of the amount of change in the phase, a phase at a frequency that is away to some extent from the resonance frequency is set to 0 degrees.

In order to overcome the inconveniences described above, in the oscillator circuit10(seeFIG. 1) or50(seeFIG. 7) according to the present invention, the phase of the entire loop of the oscillator circuit10or50is adjusted by the first phase-shift circuit26that is separated, in terms of impedance, from the second phase-shift circuit32(seeFIG. 1) or52(seeFIG. 7) due to the amplifiers22and24, which also function as impedance buffers. In other words, adjusting the phase so that the piezoelectric vibrator34can oscillate both in air and liquid by the second phase-shift circuit32or52provided in the feedback circuit is limited. Thus, the entire phase-shift condition is moved to an appropriate frequency by means of the first phase-shift circuit26provided in the amplifying circuit20. Then, the gain at a predetermined oscillation frequency is obtained by the second phase-shift circuit32or52, and the phase is adjusted to 0 degrees. Accordingly, a measuring apparatus with an excellent S/N ratio can be achieved. Also, undesirable oscillation can be avoided.

FIG. 11shows the variation in phase of the entire loop of the oscillator circuit10or50by means of the first phase-shift circuit26. As shown in the drawing, even if the reactance of the first phase-shift circuit26is adjusted in order to increase the amount of change in the phase, the width of frequencies at which the phase changes is not increased. Thus, even if the phase is changed by the first phase-shift circuit26, oscillation at a frequency that is away (displaced) from the oscillation frequency of the piezoelectric vibrator34does not occur. Therefore, the phase condition for oscillation of the oscillator circuit10or50can be satisfied, while the frequency at which the oscillation occurs is stabilized. When the phase is changed by the first phase-shift circuit26, there is no change in the gain of the feedback loop. Thus, the phase condition of the entire oscillator loop can be moved to an appropriate frequency by using the first phase-shift circuit26. Therefore, the phase condition for oscillation both in air and liquid can be easily adjusted by using the second phase-shift circuit32or52. Consequently, stable oscillation of the piezoelectric vibrator34both in air and liquid can be ensured.

Also, in the oscillator circuit50according to the second embodiment, due to the tank circuit56provided in the second phase-shift circuit52, not only can the series resonance caused by the inter-electrode capacitance C0of the piezoelectric vibrator34and the coil be prevented, but undesirable oscillation, such as parasitic oscillation, spurious oscillation of the piezoelectric vibrator34, and feedback oscillation on the circuit, can also be prevented. In other words, the gain of the closed loop composed of the amplifying circuit20and the second phase-shift circuit52is changed by the tank circuit56, as shown inFIG. 12. Here, when the tank circuit56is viewed from the loop side of the oscillator circuit50, resonance of the tank circuit56at the resonance frequency significantly increases the impedance of the tank circuit56. Thus, the loss in the gain of the closed loop due to the tank circuit56is 0. However, if an AC signal input to the tank circuit56is deviated from the resonance state, the impedance of the tank circuit56when viewed from the closed loop side is abruptly reduced, and the AC signal (current) deviated from the resonance frequency flows to the ground side via the capacitor58and reduces the gain of the closed loop. Thus, by setting the resonance frequency of the tank circuit56equal to the oscillation frequency of the piezoelectric vibrator34, the gain of the closed loop at a frequency other than the oscillation frequency is reduced. Therefore, undesirable oscillation can be suppressed.

FIG. 13is a flowchart showing an adjusting method for causing the piezoelectric vibrator34to oscillate both in air and liquid by using the oscillator circuit50. A process for adjusting the piezoelectric vibrator34to oscillate both in air and liquid first by adjusting the piezoelectric vibrator34when in air and then by adjusting the piezoelectric vibrator34when immersed in liquid is shown inFIG. 13.

First, part of the closed loop of the oscillator circuit50is released to open the loop. For example, the oscillator circuit50becomes an open loop by separating the tank circuit56from the amplifying circuit20, and a measuring device, such as a network analyzer, is connected at the separated portion. Then, the circuit characteristics of the open loop are measured by the measuring device in order to calculate the gain and phase of the open loop (step S100). In other words, the piezoelectric vibrator34is excited both in air and liquid in the open loop state, and the excitation frequencies both in air and liquid are measured. Then, gain and phase calculation processing for obtaining the changes in the gain and phase near a desired frequency at which oscillation is desired to occur is performed. In order to adjust the phase when the oscillator circuit50is a closed loop, the calculation of the gain and phase of the open loop is compensated in consideration of the difference of the impedances between the open loop and the closed loop.

FIG. 14shows an example of the measurement of a change in the phase when the oscillator circuit is an open loop. In the drawing, there is a possibility for the oscillator circuit to oscillate at frequencies at which the curved lines indicating the phase change cross 0 degrees. Then, in step S102, it is determined whether or not phase condition1is satisfied. Phase condition1is to adjust the phase such that, when the piezoelectric vibrator34is caused to oscillate both in air and liquid, the phase is 0 and the gain is 1 or less in a frequency range higher than the frequency of a principal vibration except for frequencies near the frequency of the principal vibration. If phase condition1is not satisfied, the process proceeds to step S104to perform first phase-shift adjusting processing. In the first phase-shift adjusting process, the amount of phase shift due to the first phase-shift circuit26is calculated, and, in a state in which the oscillator circuit is an open loop, a circuit constant of the first phase-shift circuit26is changed such that phase condition1is satisfied in a state in which the oscillator circuit is a closed loop. For example, the impedance of the coil66or the coupling capacitor64shown inFIG. 8is changed. Accordingly, as shown by an arrow115inFIG. 14, the curved lines indicating the change of the phase are moved in the vertical direction, and the phase at a desired frequency can be adjusted. Then, the process returns to step S100to measure the circuit characteristics of the open loop again. Then, it is determined whether or not phase condition1is satisfied (step S102).

If phase condition1is satisfied, the process proceeds from step S102to step S106to determine whether or not phase condition2is satisfied. Phase condition2is to adjust the phase such that, when the piezoelectric vibrator34is caused to oscillate both in air and liquid, the phase is approximately 0 degrees at a desired frequency near the resonance frequency of the piezoelectric vibrator34in a frequency range in which the phase significantly changes. If phase condition2is not satisfied, second phase-shift adjusting process is performed. In other words, the amount of phase shift due to the second phase-shift circuit52is calculated, and, in a state in which the oscillator circuit is an open loop, a circuit constant of the second phase-shift circuit52is changed such that phase condition2is satisfied in a state in which the oscillator circuit is a closed loop (step S108). In other words, for example, the inductance of the coil80of the phase-shift circuit part54shown inFIG. 8is changed. Then, the process returns to step S100to perform the processing of steps100to106.

If phase condition2is satisfied in step S106, it is determined whether or not gain condition2is satisfied (step S110). Gain condition2is to adjust the gain such that the gain of the closed loop is 1 or more when phase conditions1and2are satisfied. If gain condition2is not satisfied in Step S110, after performing the processing of step S108, which is a gain adjusting process, the process proceeds to step S100to perform the processing of steps S100to S110again. In this gain adjustment, in order to ensure the oscillation of the piezoelectric vibrator34, a negative resistance is set to be about three times larger than the impedance of the piezoelectric vibrator34. If gain condition2is satisfied in step S110, the process proceeds from step S110to step S112to check the operation. In other words, the oscillator circuit50is changed to a closed loop, and the oscillation operation is actually performed both in air and liquid. Accordingly, oscillation of the piezoelectric vibrator34both in air and liquid can be ensured. Thus, by using the oscillator circuit according to the embodiments described above, a mass measuring apparatus with high reliability can be achieved.FIG. 24shows a state of oscillation both in air and liquid when the oscillator circuit is adjusted as described above.

When the piezoelectric vibrator34is immersed in liquid, the characteristics of the piezoelectric vibrator34are changed in various ways depending on the type and state of the liquid. The use of a voltage-controlled reactance control circuit in a phase-shift circuit causes the piezoelectric vibrator34to stably oscillate easily; thus a stable oscillation frequency can be achieved. For example, as shown inFIG. 25, the resistor70provided between the cathode of the variable capacitance diode60and a voltage input terminal and the resistor84provided between the cathode of the variable capacitance diode74and a voltage input terminal are changed to variable resistors. Here, resistors202and204connected to the cathodes of the variable capacitance diodes60and74, respectively, are provided for dividing the control voltages VC1and VC2, respectively. For example, the values of the control voltages VC1and VC2when the oscillator circuit is adjusted and the resistances of the variable resistors70and84are set as initial voltages. Then, when the piezoelectric vibrator34is immersed in any liquid, the resistances of the variable resistors70and84are adjusted on the basis of the set values so as to correspond to the change in the characteristics in the liquid. Thus, an oscillation operation in the liquid is stabilized. Therefore, the use of the oscillator circuit provides a measuring apparatus with an excellent S/N ratio. Here, the first phase-shift circuit26may include a plurality of amplifiers connected in series with each other. Also, the first phase-shift circuit26may include a temporal delay circuit including a delay element, a delay line, or the like. Also, three or more amplifiers that also function as impedance buffers may be provided.

FIG. 15is a block diagram showing an oscillator circuit120according to a third embodiment. An example in which the first phase-shift circuit26includes a plurality of amplifiers is shown inFIG. 15. In the oscillator circuit120according to the third embodiment, the first phase-shift circuit26provided in the amplifying circuit20includes two amplifiers122and124. Each of the amplifiers122and124may be an amplifying circuit using a transistor, an operational amplifier, a differential amplifier, ECL, PECL, or the like, as in the amplifiers22and24. Also, in the oscillator circuit120, the second phase-shift circuit32and the piezoelectric vibrator34constitute a feedback circuit. The input side of the second phase-shift circuit32is connected to the output terminal of the amplifier24, and the output side of the second phase-shift circuit32is connected to the input terminal of the amplifier22via the piezoelectric vibrator34. Also, the tank circuit56is provided between the piezoelectric vibrator34and the amplifier22. The tank circuit56is grounded via the capacitor58.

In the oscillator circuit120arranged as described above, the two amplifiers122and124constituting the first phase-shift circuit26control the phase of the entire closed loop. Advantages similar to the oscillator circuit according to the embodiments described above can also be achieved in the oscillator circuit120according to the third embodiment. Here, three or more amplifiers constituting the first phase-shift circuit26may be provided.

FIG. 16is a block diagram showing an oscillator circuit130according to a fourth embodiment. In the oscillator circuit130, amplifiers132and134constituting the amplifying circuit20and functioning as impedance buffers each include an inverting input terminal, a non-inverting input terminal, an inverting output terminal, and a non-inverting output terminal. Also, an output buffer140is an amplifier including an inverting input terminal, a non-inverting input terminal, an inverting output terminal, and a non-inverting output terminal.

The non-inverting input terminal of the amplifier132constituting the amplifying circuit20is connected to the other electrode of the piezoelectric vibrator34and to one end of the tank circuit56. Also, the inverting input terminal of the amplifier132is grounded via a capacitor136and is connected to the other end of the tank circuit56. The input side of the first phase-shift circuit26is connected to the non-inverting output terminal and the inverting output terminal of the amplifier132. Also, the output side of the first phase-shift circuit26is connected to the non-inverting input terminal and the inverting input terminal of the amplifier134and to the non-inverting input terminal and the inverting input terminal of the output buffer140. The non-inverting output terminal of the amplifier134is connected to the input side of the second phase-shift circuit32. The output of the oscillator circuit130is captured from the non-inverting output terminal of the output buffer140.

FIG. 17shows a specific example of the oscillator circuit130according to the fourth embodiment. In this specific example, the second phase-shift circuit32constituting the feedback circuit is arranged in a similar manner to the phase-shift circuit part54of the second phase-shift circuit52in the second embodiment shown inFIG. 7. Also, the tank circuit56includes a parallel circuit composed of the resistor88, the capacitor90, and the coil92. The first phase-shift circuit26constituting the amplifying circuit20may be a capacitance element, an inductance element, a variable capacitance diode, a delay element, an operational amplifier, a differential amplifier, or the like, as described above. Also, the differential amplifier may be ECL or PECL.

FIG. 18is a sectional view of a mass measuring apparatus150provided with the oscillator circuit according to any one of the embodiments described above. The mass measuring apparatus150includes a case152accommodating the oscillator circuit. The case152includes a box-type case main unit154and a cover156. The cover156is formed by a flat plate and is fixed (watertight) to an opening of the case main unit154with an adhesive158in order to prevent intrusion of sample liquid into the case152. Also, a window162for exposing a piezoelectric vibration reed160functioning as a piezoelectric vibrator is arranged in the cover156.

In the embodiments, the piezoelectric vibration reed160is set to achieve high-frequency oscillation at a frequency of, for example, approximately 150 MHz by processing an AT-cut piezoelectric vibration plate to have a so-called reverse-mesa structure. The piezoelectric vibration reed160includes an excitation electrodes164(164aand164b) on both sides of the reverse-mesa part. The excitation electrode164ais provided with a sensitive film (not shown) for attracting a substance to be detected in the sample liquid. The window162of the cover156causes the sensitive film provided on the excitation electrode164ato be exposed to contact the sample liquid. Then, the piezoelectric vibration reed160is firmly fixed around the window162inside the cover156with a conductive adhesive166.

Also, the piezoelectric vibration reed160includes connection electrodes168(168aand168b) which are integral with the excitation electrodes164. In contrast, a circuit pattern part170made of a conductive material is provided on the inner surface of the cover156. Also, an IC chip174is firmly fixed to the inner surface of the cover156with an adhesive172. The IC chip174is an integrated circuit including the amplifiers22and24, the output buffer12, and the like of the oscillator circuit (for example, the oscillator circuit50shown inFIG. 7). The connection electrode168aof the piezoelectric vibration reed160is electrically connected to the circuit pattern part170with the conductive adhesive166therebetween. The circuit pattern part170includes a wiring pattern. Also, the coil66of the first phase-shift circuit26, the coil80of the phase-shift circuit part54constituting the second phase-shift circuit52, and the like, which are not illustrated inFIG. 18, are arranged in the circuit pattern part170. The circuit pattern part170is electrically connected to the IC chip174with a plurality of wires176made of gold or the like. Also, the connection electrode168bof the piezoelectric vibration reed160is electrically connected to the IC chip174with the wires176. Furthermore, the case main unit154has a through hole178in one side, and a cable180is connected to the through hole178. The cable180includes a power line, a signal output line, and the like. The ends of such lines are connected to the IC chip174.

In the mass measuring apparatus150arranged as described above, the signal output line of the cable180is connected to a frequency counter190, as shown inFIG. 19. The output side of the frequency counter190is connected to a computer192, and the oscillation frequency of the mass measuring apparatus150is input to the computer192. Then, the mass measuring apparatus150is immersed in a sample liquid196stored in a sample container194. A substance to be detected in the sample liquid196becomes attached to the sensitive film provided on the excitation electrode164aof the piezoelectric vibration reed160. The computer192compares the oscillation frequency of the piezoelectric vibration reed160when the substance is deposited, the oscillation frequency being output from the frequency counter190, with a reference oscillation frequency (oscillation frequency before deposition of the substance), and calculates the mass and concentration of the substance deposited on the sensitive film (the excitation electrode164a) in accordance with an algorithm that is given in advance. In the oscillator circuit, only the piezoelectric vibration reed160may be accommodated inside the case152. In this case, since the size of the case152is reduced, the mass measuring apparatus150can be arranged or inserted in a smaller part.

A specific method for measurement in liquid by the mass measuring apparatus150is performed as described below. First, liquid (solvent or solution) not including a substance to be detected is stored in the sample container194, and the mass measuring apparatus150is immersed in the liquid. Then, after waiting until the resonance of the piezoelectric vibration reed160in the liquid is stabilized, the resonance frequency is stored as a reference frequency in the computer192. Then, a predetermined amount of a sample (liquid) including the substance to be detected is added to the liquid in the sample container194to be diffused, and the added substance in the sample is deposited on the sensitive film on the piezoelectric vibration reed160.

Also, measurement can be performed as described below. First, two sample containers194are provided. Only a liquid (for example, water or alcohol) not including a substance to be detected is poured in one of the sample containers194. A sample including a substance dissolved or diffused in water or alcohol is poured in the other one of the sample containers194. Then, a reference frequency (oscillation frequency) of the piezoelectric vibration reed160in liquid in the one of the sample containers194is obtained. After that, the mass measuring apparatus150is immersed in the other one of the sample containers194to measure the substance. Accordingly, the concentration and the like of the substance can be easily and precisely obtained. Alternatively, the mass measuring apparatus150in which the substance to be detected is deposited or responds to the sensitive film is immersed in liquid, such as water or alcohol. Then, a chemical agent that desorbs or resolves the substance is added to the liquid, so that the substance is eliminated from the sensitive film. Accordingly, the amount and the like of the substance deposited on the sensitive film can be measured.

In the oscillator circuit according to each of the embodiments described above, for example, an inter-electrode capacitance C0of the piezoelectric vibrator increases in accordance with a reduction in the size of the piezoelectric vibrator. Thus, the oscillator circuit can also be applied to a case where the amount of change in the phase of a piezoelectric vibrator is small.

Also, although the case in which the oscillator circuit according to each of the embodiments described above is applied to the mass measuring apparatus150for detecting a particular substance in liquid has been described above, the oscillator circuit according to the present invention can also be used for a measuring apparatus for measuring various types of minute physical quantities. For example, the oscillator circuit may be used as an odor sensor, a moisture sensor, a plating thickness monitor, an ion sensor, a viscosity/density sensor, and the like. When the oscillator circuit is used as an odor sensor, a sensitive film that selectively attracts an odor substance is applied to the surface of an excitation electrode. Also, when the oscillator circuit is used as a moisture sensor, a water-absorbing film is applied (refer to Japanese Unexamined Patent Application Publication No. 7-209165).

When the oscillator circuit is used as a plating thickness monitor, a piezoelectric vibrator for mass measurement is immersed in a plating liquid, together with a substance to be plated. In this case, the resonance frequency of a piezoelectric vibration reed is reduced in accordance with an increase in the plating thickness deposited on the surface of an excitation electrode. Thus, the plating thickness of the substance can be detected. Also, when the oscillator circuit is used as an ion sensor, an ion absorption substance is applied as a sensitive film. By measuring the amount of change in frequency of the piezoelectric vibrator reed by depositing the ion on the sensitive film, quantitative analysis of the ion in the sample liquid can be performed. It is desirable that sensitive films be provided on the excitation electrodes164that are arranged on both sides of the piezoelectric vibration reed160when the oscillator circuit is used in gas, for example, as an odor sensor. In this case, both sides of the piezoelectric vibration reed160are exposed so as to contact an odor substance. Accordingly, the amount of odor substance deposited on the piezoelectric vibration reed160is increased. Thus, the detection sensitivity is increased, and the measurement accuracy can thus be increased.

The mass measuring apparatus according to the present invention is capable of measuring minute physical quantities other than mass. The measurement principle for the mass measuring apparatus according to the present invention used as a viscosity/density sensor will be described below. An AT-cut piezoelectric vibrator performs thickness-shear vibration along the surface thereof. When the AT-cut piezoelectric vibrator is caused to oscillate in liquid, shear stress is generated between the AT-cut piezoelectric vibrator and the liquid. From Newton's Viscosity Law and the quartz vibrator vibration formula, the following equation representing the amount of change in the frequency due to the liquid viscosity can be obtained:

where df represents the amount of change in the resonance frequency of the piezoelectric vibrator reed, f0represents an initial value of the resonance frequency of the piezoelectric vibrator reed, η represents the liquid viscosity, ρLrepresents the liquid density, and μ represents the modulus of elasticity of the piezoelectric material. In the above equation, if one of the liquid viscosity η and the liquid density ρLis constant, the other one corresponds to the amount of change in the resonance frequency in a one-to-one relationship. Thus, by measuring the amount of change in the resonance frequency, the change in the liquid viscosity or the change in the liquid density can be obtained.