JPH0692998B2 - Quartz crystal Q factor measuring device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal resonator Q value measuring device capable of measuring the Q value of a crystal resonator in real time.

[0002]

2. Description of the Related Art Conventionally, a crystal oscillator has been used for various purposes, for example, as a gas sensor. In a gas sensor using this crystal oscillator, electrodes are formed on both surfaces of a crystal plate, and an oscillation circuit is connected to these electrodes. Then, gas sensitive films are formed on the electrodes,
The mass change of the adsorbed substance is measured based on the change of the oscillation frequency accompanying the adsorption and desorption of gas molecules on the gas sensitive film.

[0003]

The above-described quartz oscillator gas sensor can be easily manufactured by applying an adsorption film to a quartz plate, and its operation is stable, so that it has been widely put into practical use. However, this known gas sensor can accurately detect the mass change of adsorbed molecules, but cannot detect odor information. On the other hand, the lipid membrane, which is a component of the biological cell membrane, changes its viscoelasticity with the adsorption of odor molecules, so if the change in the viscoelasticity of the lipid membrane can be detected, the odor information can be detected and the usefulness as an odor sensor is achieved. To be done.

When odor molecules are adsorbed on the adsorption film formed on the crystal unit, the viscosity of the adsorption film changes, which affects energy loss against vibration, and the Q value of the crystal unit also changes. Therefore, if the Q value change of the crystal unit can be measured, the odor information can be identified, and the information obtained from the sensor can be diversified.

On the other hand, an impedance analyzer is known as a device for measuring the Q value of a crystal unit. However, the impedance analyzer is not only expensive but also has the drawback of not being suitable for real-time measurement.
Therefore, an object of the present invention is to provide a device that can measure the Q value or Q change of a crystal unit with a simple configuration and can measure in real time.

[0006]

A Q-value measuring device for a crystal unit according to the present invention includes a crystal unit whose Q-value is to be measured,
An oscillation circuit that supplies a fundamental wave signal whose amplitude is constant and whose frequency changes with time, a current-voltage converter that converts the current flowing through the crystal unit into a voltage signal, and the output from this voltage-current converter The crystal oscillator is equipped with a frequency conversion circuit that detects the voltage component in phase with the fundamental wave signal supplied to the crystal unit from the signal, and a low-pass filter that extracts the low-frequency signal component from the output signal of this frequency conversion circuit. It is characterized in that the maximum conductance at the resonance frequency is detected by sweeping.

[0007]

FIG. 1 shows an equivalent circuit of a crystal unit in which an adsorption film is formed on a crystal plate. The admittance Y of this crystal unit is given by the following equation.

## EQU1 ## Y = Y _d + Y _mot Y _d = jωC _d Y _mot = 1 / {R + j (ωL-1 / ωC _p ), where Y _d represents braking admittance and Y _mot represents dynamic admittance. Further, C _d represents a binding capacitance (parasitic capacitance), and C _p represents a piezoelectric capacitance. As is clear from FIG. 1, the resonance angular frequency ω ₀ of the crystal unit is represented by the following equation.

[Equation 2] Therefore, the Q value of this crystal oscillator is given by the following equation.

## EQU3 ## Q = 1 / ω ₀ C _p R = ω ₀ L / R
Increases and the series resonance frequency decreases. On the other hand, when the adsorption film has viscoelasticity, R is greatly affected in addition to L,
R changes more greatly than the change of ω ₀ L. Therefore, the Q value can be considered to be proportional to 1 / R. Based on these studies, in the present invention, the maximum conductance G at the resonance angular frequency ω ₀ is obtained, and the Q value is obtained from the obtained maximum conductance.

The admittance locus of the crystal unit is shown in FIG. Since the Q value of the crystal unit is sufficiently large, the admittance locus becomes a circle. Therefore, the maximum conductance is obtained by sweeping the frequency, and the value becomes 1 / R. Therefore, in the present invention, the maximum conductance value proportional to the Q value is obtained by sweeping the frequency.

When actually determining the Q value, the reference material is measured using the apparatus according to the present invention and the maximum conductance is measured using the impedance analyzer, and the correspondence relationship between these results is determined in advance. Then, the Q value is obtained based on the following equation.

[Equation 4] Here, P _q is the density of the crystal, d is the thickness of the crystal, ε ₃₅ is the piezoelectric constant of the crystal, and A is the area of the crystal.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a diagrammatic sectional view showing the structure of an example of a crystal resonator according to the present invention. Electrode films 2 on both sides of crystal plate 1
a and 2b are formed, and the gas sensitive film 3a is formed on these electrode films.
And 3b, respectively. In this example, an AT cut oscillator with a display frequency of 10 MHz is used as the crystal plate. Further, a lecithin film, which is abundant in human cell membranes, is used as the gas reaction film. The lecithin films 3a and 3b
The resonance frequency shift after coating is 13.659 kHz.

FIG. 4 is a circuit diagram showing a configuration of an example of a Q value measuring device for a crystal unit according to the present invention. Voltage divider consisting of two resistors R and r on the crystal unit 10 whose Q value is to be measured
A voltage controlled crystal oscillator (VCXO) 12 is connected via 11, and a function generator 13 is connected to this VCXO. As shown in FIG. 5, the function generator 13 is a rectangle that generates a sawtooth voltage signal (0 to 20 V) for sweep driving the VCXO, a reset pulse of a peak hold circuit and a sample pulse of a sample hold circuit, which will be described later. Generates a wave (5V). The frequency of the sawtooth wave output from the function generator 13 is 2H
Let z. The reason for this is that the resonance output of the crystal oscillator gas sensor decreases when set to a higher frequency. The oscillation frequency of the VCXO12 is about 10 MHz and its frequency sweep range is 30 kHz. VCXO12
A signal whose amplitude is constant and whose frequency changes with the wave interval is output from the crystal oscillator 10 via the voltage divider 11.
Supply to. The current flowing through the crystal unit 10 is converted into a voltage signal by the current-voltage converter 14, and this output signal is supplied to the frequency converter (DBM) 15. Phase delay occurs when the impedance at the time of resonance of the crystal unit 10 changes,
The magnitude of the phase delay also changes depending on the magnitude of the impedance. As a result, the output signal from the current-voltage converter 14 is out of phase with the fundamental wave output from the VCXO 12. Therefore, in the present invention, the frequency conversion
Using the reference numeral 15, only the signal component in phase with the fundamental wave (LO signal) output from the VCXO is extracted from the fundamental wave signal output from the current-voltage converter. Current - The output signal from the voltage converter, since produce a phase difference φ with respect to the output signal from the VCXO by the braking admittance Y _d of the crystal oscillator, the representative of the output signal from the VCXO at V ₁ cos .omega.t current - voltage The output signal from the converter is V ₂ cos (ωt + φ)
Can be expressed as On the other hand, since the frequency converter 15 has a function as a multiplier, its output signal S is given by the following equation.

[Equation 5] As is clear from the above equation, the output signal from the frequency converter 15 is converted into the sum component and the difference component of the LO signal and the RF signal in the high frequency relationship, and the first term of the above equation represents the harmonic component. The second term represents the difference component. The second term is a component of the RF signal that is in phase with the LO signal and is the conductance component of the crystal unit.

The output signal from the frequency converter 15 is supplied to the low-pass filter 16 to cut harmonic components. The output signal of the low-pass filter 16 is supplied to the peak hold circuit 17, the maximum output voltage is peak-held within the sweep frequency, and then the output voltage corresponding to the maximum conductance G is formed through the sample hold circuit 18 and recorded in the recorder 19. To do. The peak hold circuit 17 and the sample hold circuit 18 are controlled based on the signal supplied from the function generator 13 shown in FIG. It is also possible to connect a signal processing circuit that determines the Q value by a calculation process in the subsequent stage of the sample and hold circuit and display the Q value based on the output voltage from the sample and hold circuit.

Next, description will be given of experimental results obtained by measuring the quartz resonators on which various gas sensitive films are formed by using the above-mentioned Q value measuring device. Lecithin, cholesterol, and polyethylene glycol were used as the gas-sensitive film, and the maximum contactance at the resonance frequency was measured using an impedance analyzer, and the maximum conductance was measured using the above-described Q value measuring device. The result of this comparison is shown in FIG. In FIG. 6, the vertical axis represents the measurement result (circuit output voltage) of the Q value measuring device according to the present invention, and the horizontal axis represents the maximum conductance G (mS) measured by the impedance analyzer. As is clear from FIG. 6, a substantially linear correspondence is recognized between the measurement result of the device according to the present invention and the measurement result of the impedance analyzer.

Next, the measurement results of the Q change measurement due to the adsorption of odorous molecules will be described. Ethanol, apple flavor and grape flavor are used as odor molecules,
A lecithin film was used as the gas sensitive film. In the measurement, using a flow system, dry air is flown for 30 seconds, then a sample is flowed for 30 seconds, and this is repeated alternately. The results of these measurements are shown in FIG. In FIG. 7, the horizontal axis represents time and the vertical axis represents the measured circuit output (maximum conductance). Since the circuit output when dry air is flown is 1.0V,
Maximum conductance is about 30mS, Q value is about 20000
Met. On the other hand, when ethanol was flown, the maximum conductance changed to 6 mS, and the Q value was about 4500. The apple flavor and the grape flavor both changed to 12 mS, and the Q value was about 9000. From these results, it was confirmed that the Q change can be measured by the Q value measuring device according to the present invention.

[0015]

As described above, according to the present invention, the Q value of the crystal unit can be measured in real time. Moreover, since the device can be constructed with a simple circuit, the manufacturing cost of the measuring device can be reduced. Furthermore, since the maximum conductance value is measured by the dynamic admittance method, the effect of being less affected by the parasitic capacitance of the crystal unit is achieved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an equivalent circuit of a crystal unit.

FIG. 2 is a diagram showing an admittance locus of a crystal unit.

FIG. 3 is a schematic cross-sectional view showing the structure of an example of a crystal resonator.

FIG. 4 is a circuit diagram showing a circuit configuration of a crystal resonator Q value measuring device according to the present invention.

FIG. 5 is a diagram showing an output signal from a function generator.

FIG. 6 is a graph showing a correspondence relationship between a circuit output and a measured value by an impedance analyzer.

FIG. 7 is a graph showing the measurement results of various samples.

[Explanation of symbols]

 10 Crystal oscillator 11 Voltage divider 12 Voltage controlled crystal oscillator 13 Function generator 14 Current-voltage converter 15 Frequency converter 16 Low-pass filter 17 Peak hold circuit 18 Sample hold circuit 19 Display device

Claims (4)

[Claims] 1. A transmission circuit for supplying a crystal oscillator whose Q value is to be measured with a fundamental wave signal whose amplitude is constant and whose frequency changes temporally, and a current which converts a current flowing through the crystal oscillator into a voltage signal. -A voltage converter, a frequency conversion circuit for detecting a voltage component in phase with the fundamental wave signal supplied to the crystal unit from an output signal from the voltage-current converter, and a low frequency signal output from the frequency conversion circuit. A Q-value measuring device for a crystal resonator, comprising: a low-pass filter for extracting a frequency signal component, wherein the crystal resonator is frequency-swept to detect the maximum conductance at a resonance frequency. 2. A Q-value measuring device for a crystal unit, wherein a peak hold circuit and a sample hold circuit are connected to the latter stage of the low pass filter. 3. The Q value measurement of the crystal oscillator according to claim 1, wherein a voltage divider is connected between the oscillator circuit and the crystal oscillator to divide the output voltage of the oscillator circuit. apparatus. 4. The oscillator according to claim 1, further comprising a function generator for generating a low frequency sawtooth voltage signal and a voltage controlled oscillator.
Item 3. A quartz resonator Q value measuring device according to item.