JPH0654262B2 - SAW power sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power sensor used for detecting and measuring the power of light and a wide range of power, and particularly to surface elasticity propagating on the surface of a solid substance. Wave (SAW; Surface A)
The present invention relates to a SAW power sensor that applies the sensitive function of a SAW device using a coustic wave).

[Conventional technology]

A power sensor that detects the power of electric power and light includes a photodiode / phototransistor that uses the photoelectric effect of semiconductors, a thermistor / thermocouple, etc. There is a thermal sensor or the like that converts the electromotive force to detect the change.

These thermal power sensors basically belong to a temperature sensor, and a function of converting electric power or light power into heat quantity is added to the temperature sensor.

A thermistor / thermocouple, which is a thermal power sensor, converts the input power into a heat quantity, and further converts the heat quantity into a resistance value or a thermoelectromotive force, and finally into a voltage. . Therefore, in order to make these conversions highly accurate, an expensive differential amplifier that accurately amplifies the output voltage is required, and when performing measurement or system control using microcomputer technology, expensive A / D conversion is required. Need a vessel.

In terms of manufacturing, in the case of a thermocouple, it is necessary to make a contact accurately with limited two kinds of metals, and complicated processes such as vapor deposition and etching are required. In the case of a thermistor, it is difficult to deposit the thermistor material, which is an oxide, because of its low vapor pressure.Therefore, a method of depositing the metal of the thermistor material and then performing an oxidation treatment has been adopted. A complicated process of using a substance as a component and properly controlling the component ratio is required.

Since the thermal power sensor as described above is complicated to manufacture and the materials used are limited, there have been problems in cost, stability, reliability and mass productivity.

By the way, as one of the methods for detecting the temperature, in the oscillation circuit using the SAW device, the state of the SAW propagation path is thermally distorted and the SAW propagation in the SAW propagation path is distorted.
There is a SAW temperature sensor that changes the oscillation frequency peculiar to the oscillation circuit by changing the propagation velocity or the propagation path length and detects the amount of heat received by the SAW device from the amount of change in the frequency. This SAW temperature sensor is small and lightweight, has high accuracy, and the SAW sensor unit is easy to manufacture and has high reproducibility. Therefore, it is one of the sensors using the SAW, which is being put to practical use.

An oscillator using a SAW device is disclosed in JP-A-54-138480.
As disclosed in Japanese Patent Publication (Temperature Detection Device), a resonance type oscillator and a delay line element type oscillator are known.

Techniques disclosed regarding the SAW temperature sensor include, for example, those described in JP-A-54-138480 (temperature detection device) and JP-A-59-57126 (temperature detector). The temperature detecting device disclosed in Japanese Patent Laid-Open No. 138480/54 constitutes an oscillator whose oscillation frequency depends on temperature by using a SAW device, detects the oscillation frequency at a remote place, and places the oscillator. The temperature of the position is detected. In the temperature detector disclosed in Japanese Patent Laid-Open No. 59-57126, an oscillator is composed of a SAW device serving as a temperature sensing section and an amplifier, and the output signal level of the oscillator is determined by a level determiner, and the measured signal is measured. The temperature of the warm object is detected to be higher or lower than the Curie point. According to the described embodiment, the SAW device is housed in the tip of a cylindrical metal case, and signals are exchanged between the interdigital electrodes and the amplifier or other elements by means of lead wires passing through the metal case. There is.

The SAW temperature sensor is provided with a function of converting the power of light into a calorific value to form an optical power sensor, which solves the problems of the thermal power sensor described above, for example, Japanese Patent Laid-Open No. 59-224527 (optical power meter). ) Is disclosed. The optical power meter is provided with a black body layer on the surface, and a first surface acoustic wave oscillator in which measurement light is incident on the black body layer, and the first surface acoustic wave oscillator is formed of the same material and in the same shape. The black body layer is composed of a second surface acoustic wave oscillator to which measurement light is not incident, and a mixer for sending out a difference in frequency between output signals of the first surface acoustic wave oscillator and the second surface acoustic wave oscillator. The light to be measured is radiated to and the power is measured. In the configuration explanatory view (cross-sectional view) showing the main part described as an example, the interdigitated electrode portion of one surface acoustic waveform oscillator is irradiated with light to be measured to heat the electrode portion. The figure is shown.

[Problems to be solved by the invention]

As described above, the temperature detecting device and the temperature detector are placed in an atmosphere at the temperature to be detected by the entire SAW sensor section, and the optical power meter is a surface acoustic waveform oscillator (resonant type oscillator in the embodiment). The cross finger-shaped electrode part) is heated to detect the optical power.

As shown in these examples, in the conventional technique, not only when using the resonance type oscillator but also when using the delay line element type oscillator, it is attempted to measure the entire SAW sensor unit including the interdigital finger type electrode unit. It was configured to receive the heat (temperature) that it produced.

As a result of the simulation, the inventors of the present application, in the study of the SAW power sensor, heated the interdigitated electrode part,
Although there is thermal expansion, there is a change in frequency due to temperature changes, but on the other hand, the linearity of sensitivity is adversely affected, and the piezoelectric substrate itself has a large heat capacity, which improves sensitivity. I have found that it is hindering me.

In order to realize a good sensor, it is required that there is a unique relationship between the measured parameter and the measured quantity.
In order to realize a power sensor with a W device), it is required to use only the delay characteristic of the SAW propagation path without involving the temperature-sensitive characteristic of the interdigital electrode.

That is, in order to realize a SAW power sensor with good linearity of sensitivity, the interdigitated electrode portion is less likely to be thermally affected, and the SAW propagation path generates heat corresponding to the measured power, that is, temperature rise. Is desirable.

The present invention has been made in view of such circumstances, and an object of the present invention is to use a SAW device having a feature of outputting a frequency change amount that is easy to manufacture and easy to digitally measure. Using the oscillation circuit described above, it is to provide a SAW power sensor having good linearity of sensitivity.

[Means for solving problems]

In the present invention, a delay line element type oscillator is adopted as an oscillator using surface acoustic waves (SAW), and furthermore, a transmission electrode and a reception electrode (in the embodiment, provided on a piezoelectric substrate having temperature characteristics). Interdigitated electrodes, which is called IDT, and a predetermined distance is set between the IDT (Inter Digital Transducer). Then, on the propagation path serving as the delay line of the SAW formed between both electrodes, at a predetermined distance from each of the electrodes, the power of the signal under measurement is absorbed and heat is generated to heat the propagation path, that is, the delay line. The heating element is provided in contact with or close to it.

The SAW delay line oscillator is provided with counting means for counting the frequency or the amount of change in the frequency caused by the heat generation of the heating element.

[Action]

The heating element absorbs the power of the signal under measurement to generate heat and heat the propagation path of the SAW. The SAW emitted from the transmitting electrode and received by the receiving electrode changes the propagation time between the transmitting and receiving electrodes due to the change in the propagation path length and the change in the propagation velocity due to the heating of the propagation path. As a result, the frequency of the SAW delay line oscillator changes. The amount of heat generation, that is, the power of the signal under measurement is obtained by counting the changed frequency or the amount of change in the frequency.

Since the heating element is provided in contact with or close to each of the electrodes on the SAW propagation path at a predetermined distance from each of the electrodes, the SAW propagation path is mainly heated, and the influence of heat on both electrodes is small.

〔Example〕

 FIG. 1 shows a block diagram of an embodiment of the present invention.

As shown in the drawing, the present invention is provided with SA on the surface of the piezoelectric substrate 1.
Intermittent interdigital electrode (IDT) 2 for firing W
And a receiving cross-shaped electrode (ID for receiving SAW)
T) 3 is provided with a space between both electrodes, and a SAW delay line element that causes a delay corresponding to the propagation time in the SAW propagating in the SAW propagation path defined by this space is formed, and the input power is The heating element 4 that converts heat into the IDT 2 for transmission
It is provided on the SAW propagation path between the receiving IDT 3 and the receiving IDT 3.
In the figure, the heating element 4, the transmitting IDT 2 and the receiving IDT 3 are arranged in the middle so as to be separated by a predetermined distance. The heating element 4 may be arranged in contact (contact) with or in proximity to at least a part of the piezoelectric substrate 1. In short, the heating element 4 needs to be heated by the heating value corresponding to the power to be measured. Just make a change. One electrode of each of the IDTs 2 and 3 is grounded, and an amplifier 5 forming a SAW oscillation circuit is connected between the signal input side electrode of the transmitting IDT 2 and the signal output side electrode of the receiving IDT 3. Has been done. Then, the amplifier 5 and the transmitting IDT 2
When an electrically closed loop circuit is formed by the receiving IDT 3, the amplifier 5 becomes an oscillator, and the SAW delay line oscillator can perform self-excited oscillation with the center frequency of the SAW delay line as the natural frequency. The SAW generated in the transmitting IDT 2 by this self-oscillation advances on the piezoelectric substrate 1 and is received by the receiving IDT 3. When light power is input to the heating element 4 provided on the piezoelectric substrate 1 to generate heat, both I
Since the DTs 2, 3 and the heating element 4 are arranged as described above, the temperature of mainly the SAW propagation path portion of the piezoelectric substrate 1 rises, the SAW propagation speed of the SAW delay line changes, and at the same time both IDTs 2, The distance between 3 changes due to thermal expansion, and the delay time of the SAW delay line changes. Under this influence, the frequency of self-excited oscillation with the natural frequency at the center frequency of the SAW delay line changes. This frequency is the frequency counter 6
If detected by, the amount of strain on the surface of the piezoelectric substrate 1, that is, the magnitude of the power applied to the SAW delay line can be detected.

Next, a means for improving the sensitivity of the SAW power sensor will be described.

The SAW delay line that constitutes the SAW power sensor has a large heat capacity in the piezoelectric substrate itself. Therefore, the piezoelectric substrate may be thin and thin, but mechanical strength becomes a problem.
To solve this problem, the back surface of the SAW propagation path of the piezoelectric substrate may be etched to thin the piezoelectric substrate so that the transmitting IDT and the receiving IDT are vertically filled.

An example thereof is shown in FIG. This SAW power sensor applies thermal strain between the IDT for transmission and the IDT for reception.
Since the basic principle is to change the W propagation state, the SAW propagation region is thinned and the heat capacity of the piezoelectric substrate 1 is reduced. In addition to this, surface acoustic waves (SAW)
The transmission IDT so that you can take advantage of all of the
2. The IDT 3 for reception is arranged in the vertical direction of the piezoelectric substrate 1. With the two improvements described above, not only the sensitivity but also the response speed can be improved.

Next, the SAW power sensor actually manufactured will be described.

FIG. 3 shows a SAW delay line provided with a heating resistor that constitutes a SAW power sensor actually manufactured. This sensor is for power measurement, and uses the heating resistor 7 as a heating element.
Electrodes 8 for power input are provided at both ends of the heating resistor 7, respectively. Fever house antibody 77 and transmission I
The DT2 and the receiving IDT3 are arranged at a predetermined distance from each other to reduce the influence of heat on the IDTs 2 and 3.

The experimental results are shown in FIG. In the figure, the vertical axis represents the amount of change in the oscillation frequency of the SAW power sensor, and the horizontal axis represents the power input to the heating resistor. As a result of experiments, it was found that the amount of change in the oscillation frequency of this SAW power sensor changes linearly with respect to the input power. The correlation coefficient r was 0.99997. In addition, a sensitivity characteristic of 0.1 mW 10 Hz was obtained, which was easy to measure.

Next, the application aspect will be described. For example, a microwave band power sensor can be realized by forming a matching load on a piezoelectric substrate, absorbing microwaves to generate heat, and detecting the amount of frequency change. In addition, in order to make an infrared sensor, which is a type of optical power sensor, if an absorber such as gold black, which is an infrared absorber, is provided on the piezoelectric substrate, infrared rays are absorbed, and the amount of change in frequency is similarly detected. Good.

〔The invention's effect〕

As described above, in the present invention, the delay line element type oscillator is adopted as the oscillator using the surface acoustic wave, and the predetermined distance is provided between the transmitting electrode and the receiving electrode provided on the piezoelectric substrate. And a heating element for absorbing the power of the signal under measurement and generating heat to heat the propagation path on the propagation path of the SAW formed between the electrodes at a predetermined distance from the electrodes. As a result, a SAW power sensor with little influence of heat on the electrode portion and good linearity of sensitivity was obtained.

[Brief description of drawings]

FIG. 1 is an electric circuit configuration diagram of the power sensor of the present invention, FIG. 2 is a diagram showing an embodiment for the purpose of improving the sensitivity of the SAW power sensor, and FIG. 3 is a configuration of an actually manufactured SAW power sensor. FIG. 4 is a diagram showing a SAW delay line that performs the above, and FIG. 4 is a diagram showing an output result of an actually manufactured SAW power sensor. In the figure, 1 is a piezoelectric substrate, 2 is a transmitting cross-shaped electrode (ID
T), 3 is an interdigital finger electrode (IDT) for reception, 4 is a heating element, 5 is an amplifier (oscillates as a SAW delay line oscillator due to the closed loop configuration), 6 is a frequency counter, 7 is a heating resistor, 8 Indicates an electrode.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-54-138480 (JP, A) JP-A-59-57126 (JP, A) JP-A-59-224527 (JP, A)

Claims (1)

[Claims] 1. A piezoelectric substrate made of a piezoelectric material having temperature characteristics and having a flat surface, a transmitting electrode provided on the surface of the piezoelectric substrate for emitting a surface acoustic wave, and a transmitting electrode. The transmitting electrode and the receiving electrode are provided on the surface of the piezoelectric substrate so as to receive the surface acoustic wave emitted by the trust electrode, and the receiving electrode provided at a predetermined distance from the transmitting electrode. A SAW delay line oscillator in which a surface acoustic wave propagation path that causes a propagation delay for the surface acoustic wave is formed on the surface of the piezoelectric substrate between the transmission electrode and the transmission electrode; A heating element that is provided in contact with or in proximity to the propagation path at a position separated from the reception electrode by a predetermined distance, absorbs power of a signal under measurement to generate heat, and heats the propagation path; The SA generated by the heat generation of the heating element SAW power sensor comprising a counting means for counting the amount of change in the frequency or the frequency of the delay line oscillator.