JPH0430699A - Condenser microphone device - Google Patents

Abstract

PURPOSE:To allow the microphone device to be used for detecting a very small static capacitance such as that in the order of 10<-5>pF by employing a ceramic resonator for both a resonator of an oscillating circuit and a resonator for a modulation section and detecting a change in a static capacitance equivalent to a sound pressure detected by a sound pressure detection section through the use of change in a tuning point of the ceramic resonator. CONSTITUTION:An oscillation circuit 1 is oscillated by using a signal of a high frequency as a carrier signal and gives the signal to a ceramic resonator 2 being a modulation section. A sound pressure detection section 3 consists mainly of a diaphragm 7 receiving a sound pressure signal and making vibration and a fixed electrode 9 at a rear pole opposite to the diaphragm 7 at a prescribed interval thereto, the diaphragm 7 connects to ground and the fixed electrode 9 connects to an output electrode of the ceramic resonator 2. An oscillated frequency f1 of the oscillation circuit 1 is set to be slightly deviated with respect to the resonance frequency fo of the ceramic resonator 2. The resonance frequency fo of the ceramic resonator 2 is deviated in response to a change in the static capacitance detected by the sound pressure detection section 3 and when the static capacitance changes by DELTAC, a voltage change of DELTAV is extracted from the ceramic resonator 2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrostatic microphone device that detects a sound pressure signal as a change in capacitance.

[Conventional technology]

In conventional general electrostatic microphone devices, a diaphragm is placed opposite a fixed electrode, and the amplitude displacement of the diaphragm, which vibrates in response to a sound pressure signal, is measured by adjusting the capacitance between the fixed electrode and the diaphragm. This is detected as a change in capacitance, and this change in capacitance is converted into a voltage signal and extracted.

Since the sensitivity is proportional to the tension of the diaphragm, the diaphragm is tensioned to increase the sensitivity of the device.

As methods for tensioning the diaphragm, there are an AC bias method in which an AC voltage is applied between a fixed electrode and the diaphragm, and a DC bias method in which a DC voltage is applied between the diaphragm and the fixed electrode.

[Problem to be solved by the invention]

The type of device that tensions the diaphragm using the AC bias method connects the diaphragm directly to the tuner of the oscillation circuit, and changes the oscillation frequency based on the change in capacitance detected as the diaphragm vibrates. Although the sound pressure signal is detected, the capacitance resolution of this type of device is lXl0-' ~ IXl0
-zPF is low, and there is a problem that a minute sound pressure signal cannot be detected even with high sensitivity, and there is a drawback that it cannot be developed into a zoom stereo microphone.

Furthermore, in a device in which the diaphragm is stretched using a DC bias method, it is necessary to increase the electric field between the diaphragm and the fixed electrode in order to increase detection sensitivity.

To increase this electric field, increase the applied voltage or
Alternatively, it is necessary to reduce the distance between the diaphragm and the fixed electrode, but if the distance is reduced, the potential gradient increases, causing an inconvenience that electrical discharge occurs between the diaphragm and the fixed electrode. - On the other hand, if the applied voltage is increased, the withstand voltage of elements such as transistors used in the circuit must be increased, and the cost of the device also increases.

In this way, the DC bias method also has a certain limit on the gap between the diaphragm and the fixed electrode, and moreover, the voltage applied to the fixed electrode and the diaphragm has a limit of approximately 200V. There is also a certain limit to the detection sensitivity, making it impossible to detect minute sound pressures at ultra-high sensitivity.

The present inventor constructed the resonator of the oscillation circuit and the resonator of the tuning circuit using separate ceramic resonators, and
We have developed a ceramic resonant talking capacitive sensor that can detect minute changes in capacitance with ultra-high sensitivity. If this ceramic resonant talking type electrostatic sensor is applied to an electrostatic microphone device, it becomes possible to solve the problems of conventional electrostatic microphone devices at once. The present invention has been made with attention to this point, and an object thereof is to provide an electrostatic microphone device that can detect minute sound pressure signals with ultrahigh sensitivity.

[Means to solve the problem]

In order to achieve the above object, the present invention is configured as follows. That is, the electrostatic microphone device of the present invention includes an oscillation circuit using a ceramic resonator that oscillates a carrier signal, and a sound pressure circuit that detects the vibration displacement of a diaphragm that vibrates in response to a sound pressure signal as a change in capacitance. AM modulation using the carrier signal, which biases the resonant frequency in response to a change in capacitance detected by the detection section and the sound pressure detection section, and generates a voltage change corresponding to a change in the tuning point with the oscillation circuit. The structure is characterized in that it has a modulation section consisting of a ceramic resonator separate from the ceramic resonator of the oscillation circuit that outputs as a signal, and the A output from the modulation section is
Installing an antenna circuit that wirelessly transmits the M modulation signal,
A detection circuit that detects the AM modulation signal output from the modulation section and extracts a signal corresponding to the sound pressure signal, and an amplification circuit that amplifies the detection output are provided, and a variable capacitance diode is reverse biased in the modulation section. A characteristic configuration of the present invention is that an AFC circuit connected in a shape is added.

[Function] In the present invention, when a sound pressure signal comes in, the diaphragm receives this sound pressure signal and vibrates, and the amplitude displacement of the diaphragm is detected as a change in capacitance. This change in capacitance is applied to the ceramic resonator of the modulation section.

In response to changes in capacitance, the ceramic resonator shifts its resonant frequency and changes the tuning point with the oscillation circuit. Then, a voltage change corresponding to a change in the tuning point is added to the carrier signal applied from the oscillation circuit to create an AM modulation signal, which is output as a sound pressure detection signal.

〔Example〕

Embodiments of the present invention will be described below based on the drawings. 1st
The figure shows a circuit diagram of a first embodiment of an electrostatic microphone device according to the present invention. The device of this embodiment includes an oscillation circuit 1, a ceramic resonator 2 functioning as a modulation section,
Sound pressure detection section 3, amplifier circuit 5, antenna circuit 6, and A
FC (Automation Frequency)
control) circuit 8.

The oscillation circuit 1 has an extremely high frequency, in this embodiment IGHz~
It is equipped with a ceramic resonator lO which oscillates at a fixed and constant oscillation frequency within the range of 10 GHz.

This oscillation circuit 1 oscillates the high frequency signal as a carrier signal, and applies this to a ceramic resonator 2 as a modulation section via a high impedance conversion circuit within the oscillation circuit 1.

The sound pressure detection unit 3 has as main elements a diaphragm 7 that vibrates in response to a sound pressure signal, and a fixed electrode 9 on the back electrode side that faces the diaphragm 7 at a fixed interval. The diaphragm 7 is connected to ground, and the fixed electrode 9 is connected to the output electrode of the ceramic resonator 2.

The ceramic resonator 2 has frequency characteristics as shown in FIG. 2, and the resonant frequency f0 of the ceramic resonator 2 is
oscillation frequency f of the oscillation circuit 1.

is set at a slightly shifted position. The oscillation frequency f can be set using the so-called up porch method, in which the oscillation frequency is set at a frequency position to the right of the resonance frequency f0 of the ceramic resonator 2, and the back porch method, in which the oscillation frequency is set at a frequency position to the left of the resonance frequency f0. Either method may be used, but in this embodiment, the oscillation frequency f,
is set using the back porch method. The resonant frequency f0 of the ceramic resonator 2 shifts according to a change in capacitance detected by the sound pressure detection section 3. For example, when the capacitance changes by ΔC, the resonant frequency f0 shifts by Δf. At this time, the oscillation frequency of the oscillation circuit 1 is fixed at fl, so when there is no change in capacitance, an output voltage of vo is sent out from the ceramic resonator 2, but when the capacitance changes by ΔC, the ceramic resonance It is taken out from the device 2 as a voltage change of Δ■. In this way, the ceramic resonator 2
The resonance frequency of deviates, and accordingly, the tuning point changes from ^ to A2, and an output voltage corresponding to the change in capacitance is sent out from the ceramic resonator 2. Specifically, The change in resonance frequency Δf corresponding to the change in capacitance is multiplied by the carrier signal output from the oscillation circuit l, and the voltage change corresponding to the change in capacitance becomes a carrier wave (carrier signal).
It is then taken out as an AM modulated signal. The output electrode of this ceramic resonator 2 is a capacitor 11
and an inductance element 12 through a high impedance circuit to an amplifier circuit 5 constituted by an operational amplifier.

This amplifier circuit 5 amplifies the AM modulation signal supplied from the ceramic resonator 2 and applies it to the antenna circuit 6. The antenna circuit 6 wirelessly sends the AM modulated signal amplified by the amplifier circuit 5 to a receiving circuit (not shown).

The AFC circuit 8 includes an operational amplifier 13, a capacitor 14, a diode 15, a resistor 16, a variable capacitance diode 17, and a coupling capacitor 18 as main circuit elements. A negative terminal (inverting input terminal) of the operational amplifier 13 is connected to an inverting input terminal of the amplifier circuit 5 via a resistor.

A parallel circuit of a capacitor 14 and a resistor 16 is connected between the negative terminal and the output terminal of the operational amplifier 13, and a variable capacitor 17 is connected to the output terminal of the operational amplifier 13 in a reverse bias manner. The cathode side is connected, and the anode side of the variable capacitor 17 is connected to ground. Further, the cathode side of the variable capacitance diode 17 is connected to the output electrode of the ceramic resonator 2 via a coupling capacitor 18.

This AFC circuit 8 converts the AM modulation signal applied from the ceramic resonator 2 into a low frequency, almost direct current signal by the rectifying action of the diode 15 and the smoothing action of the capacitor 14, and then amplifies it to the required level. The very low signal component is applied to the variable capacitance diode 17.

The variable capacitance diode 17 changes its capacitance according to the voltage applied from the operational amplifier 13, and transmits this capacitance change to the ceramic resonator 2 via the coupling capacitor 18.
The resonant frequency of the resonator 2 is changed. In other words, if the resonant frequency of the ceramic resonator 2 is greatly deviated for some reason and the tuning point is out of the set range, the resonant frequency can be automatically adjusted by adding an AFC signal to the ceramic resonator 2. By shifting in the opposite direction to the biasing direction, the tuning point is prevented from going out of the set range, and the tuning point of the ceramic resonator 2 is adjusted to an appropriate ratio.

Note that in this embodiment, the ceramic resonator 1 of the oscillation circuit 1
The characteristics of the ceramic resonator 2 in the modulation section, that is, the overall characteristics such as frequency characteristics, Q characteristics, temperature characteristics, and shape dimensions, are almost the same. This facilitates impedance matching between the ceramic resonator 1 and the ceramic resonator 2 as a modulator, thereby stabilizing the circuit operation.

Furthermore, in this embodiment, since the diaphragm 7 is connected to the ground, it is possible to remove unnecessary radio waves emitted from the ceramic resonator 10.2, etc., thereby preventing mutual interference between the radio waves and the circuit. Further stability and reliability of operation is ensured.

The first embodiment is configured as described above, and its operation will be described below.

As shown in FIG. 2, the oscillation frequency f of the oscillation circuit l with respect to the resonance frequency (tuning frequency) fo of the ceramic resonator 2.
When a sound pressure signal enters the sound pressure detection section 3 in a state where the sound pressure signal is set at a position slightly apart from 1, the diaphragm 7 vibrates in response to the sound pressure of this sound pressure signal. This amplitude displacement of the diaphragm 7 changes the capacitance between the diaphragm 7 and the fixed electrode 9, and this change in capacitance is applied to the ceramic resonator 2. The ceramic resonator 2 includes the sound pressure detection section 3
Until a sound pressure signal enters, the resonant frequency is fo, but when a sound pressure signal enters and the capacitance changes as described above, the resonant frequency is shifted by Δf. Due to this deviation of the resonant frequency, the tuning point changes from 8 to A2, and therefore, the output voltage from the ceramic resonator 2 is taken out as a voltage of v0+Δ■ increased from V by Δ■. Specifically, this ceramic resonator 2 multiplies the carrier signal applied from the oscillation circuit 1 by the change component Δf of the resonant frequency, adds a voltage change component corresponding to the change in capacitance to the carrier signal, and generates an AM signal. create a modulated signal,
The amplifier circuit 5 and the AFC circuit 8 use this as a sound pressure detection signal.
Add to.

The amplifier circuit 5 amplifies the AM modulated signal and applies it to the antenna circuit 6. The antenna circuit 6 transmits wirelessly to a desired receiving circuit.

On the other hand, the AFC circuit 8 converts the AM modulation signal into a nearly direct current signal by the rectification effect of the diode 15 and the smoothing effect of the capacitor 14, and further amplifies the signal to a required level and transfers it to the variable capacitance diode 17. Add. The variable capacitance diode 17 changes its capacitance according to this applied signal, and the resonance frequency fo of the ceramic resonator 2 is optimally adjusted by this capacitance change. That is, when the capacitance of the ceramic resonator 2 changes due to environmental changes such as temperature and the resonant frequency shifts, the capacitance change component due to the variable capacitance diode 17 increases the capacitance of the ceramic resonator 2 due to the environmental change. This cancels out the variation, prevents fluctuations in the tuning point of the ceramic resonator 2 due to environmental changes, stabilizes the tuning point, that is, provides an appropriate ratio of resonance frequencies, and stabilizes the operation of the device.

FIG. 3 shows a second embodiment of the electrostatic microphone device according to the present invention. This second embodiment differs from the first embodiment in that a detection circuit 4 is interposed between the output side of the ceramic resonator 2 and the amplifier circuit 5, and the AM modulation applied from the ceramic resonator 2 is A signal is detected by a detection circuit 4 and extracted as a signal corresponding to a sound pressure signal, and this detection output is amplified by an amplifier circuit 5 and applied to a signal processing unit such as a recorder or a loudspeaker (not shown). The other configurations are the same as those of the first embodiment.

In this second embodiment, the detection output is applied to the AFC circuit 8, and therefore the AF of this second embodiment
The configuration of the C circuit 8 is simpler than that of the AFC circuit 8 of the first embodiment.

Note that the present invention is not limited to the above embodiments,
Various implementation modes are possible.

〔Effect of the invention〕

In the present invention, both the resonator of the oscillation circuit and the resonator of the modulation section are configured by ceramic resonators, and the change in capacitance corresponding to the sound pressure detected by the sound pressure detection section is transmitted to the ceramic resonator of the modulation section. Because it detects using changes in the tuning point of This makes it possible to develop zoom stereo microphones, which was difficult to do with conventional devices, and has tremendous technical value.

Furthermore, the device can be made smaller and lighter, and the device is also very easy to handle.

Furthermore, in a configuration in which an AFC circuit is added, the resonant frequency of the modulation section can be optimized in response to environmental changes, etc., and the reliability of the device operation can be greatly improved.

Furthermore, by configuring the characteristics of the ceramic resonator of the oscillation circuit and the ceramic resonator of the modulation section to be almost the same, stabilization of Q and high impedance matching of the circuit are facilitated, which improves the performance of the device and the circuit. It is possible to further stabilize the operation.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an electrostatic microphone device according to the present invention, FIG. 2 is an explanatory diagram of the operation of a modulation section in the same embodiment, and FIG. 3 is a circuit diagram showing a first embodiment of an electrostatic microphone device according to the present invention. FIG. 2 is a circuit diagram showing a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Oscillation circuit, 2... Ceramic resonator (modulation part), 3... Sound pressure detection part, 4... Detection circuit, 5...
Amplification circuit, 6... Antenna circuit, 7... Diaphragm, 8
...AFC circuit, 9.-Fixed electrode, 10.. Ceramic resonator, 11.. Capacitor, 12.. Inductance element, 13.. Operational amplifier, 14.. Capacitor, 15.. Diode , 16...Resistor, 1
7... Variable capacitance diode, 18... Coupling capacitor. Applicant Murata Manufacturing Co., Ltd. Representative Patent Attorney Igarashi Kiyoshi Diatu Diagram T Zhou Heshu

Claims (5)

[Claims] (1) An oscillation circuit using a ceramic resonator that oscillates a carrier signal, a sound pressure detection section that detects the vibration displacement of a diaphragm that vibrates in response to a sound pressure signal as a change in capacitance, and this sound pressure detection The oscillation circuit biases the resonance frequency in response to a change in capacitance detected by the oscillation circuit, and outputs a voltage change corresponding to a change in a tuning point with the oscillation circuit as an AM modulation signal using the carrier signal. An electrostatic microphone device comprising a ceramic resonator and a modulation section consisting of a separate ceramic resonator. (2) Claim 1, which is provided with an antenna circuit that wirelessly transmits the AM modulated signal output from the modulation section.
Electrostatic microphone device. (3) Claim 1, which is provided with a detection circuit that detects the AM modulation signal output from the modulation section and extracts a signal corresponding to the sound pressure signal, and an amplification circuit that amplifies the detection output. Electrostatic microphone device. (4) The electrostatic microphone device according to any one of claims 1 to 3, wherein the modulation section includes an AFC circuit in which a variable capacitance diode is connected in a reverse biased manner. (5) According to any one of claims 1 to 4, the ceramic resonator of the oscillation circuit and the ceramic resonator of the modulation section are constituted by ceramic resonators having substantially the same characteristics. Microphone device as described.