JPH07112307B2 - Electrostatic microphone device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrostatic microphone device that detects a sound pressure signal as a change in capacitance.

[Prior Art] In a conventional general electrostatic microphone device, a diaphragm is arranged so as to face a fixed electrode, and an amplitude displacement of the diaphragm vibrating in response to a sound pressure signal is generated between the fixed electrode and the diaphragm. Is detected as a change in the electrostatic capacity, and the change in the electrostatic capacity is converted into a voltage signal and taken out.

In this type of electrostatic microphone device, since the detection sensitivity of sound pressure is proportional to the tension of the diaphragm, the diaphragm is stretched to increase the sensitivity of the apparatus.

As a method of stretching the diaphragm, there are an AC bias method of applying an exchange voltage between the fixed electrode and the diaphragm, and a DC bias method of applying a DC voltage to the diaphragm and the fixed electrode.

[Problems to be Solved by the Invention]

The type of device that stretches the diaphragm by 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 with the vibration of the diaphragm. Although it detects sound pressure signals, the resolution of the capacitance of this type of device is as low as 1 x 10 ^-1 to 1 x 10 ^-2 PF, and it is said that minute sound pressure signals cannot be detected with high sensitivity. However, there is a problem that it cannot be applied to zoom stereo microphones.

Further, in a device in which the diaphragm is stretched by the DC bias method, it is necessary to increase the electric field between the diaphragm and the fixed electrode in order to increase the detection sensitivity. In order to increase this electric field, it is necessary to increase the applied voltage or decrease the distance between the diaphragm and the fixed electrode. However, if the distance is decreased, the potential gradient increases and the diaphragm and the fixed electrode are fixed. The inconvenience of causing an electric discharge between the electrodes occurs.
On the other hand, when the applied voltage is increased, it is necessary to increase the breakdown voltage of elements such as transistors used in the circuit, which causes a problem that the price of the device also becomes expensive. As described above, the DC bias method also has a certain limit on the gap between the diaphragm and the fixed electrode, and the voltage applied to the fixed electrode and the diaphragm is about 200 V. ,
There is a circumstance in which the detection sensitivity also has a certain limit, and it is not possible to detect a minute sound pressure under ultrahigh sensitivity.

The present inventor configured the resonator of the oscillation circuit and the resonator of the tuning circuit by separate and independent ceramic resonators, and a ceramic resonator capable of detecting a change in micro capacitance with an ultrahigh sensitivity of 10 ⁻⁵ PF. Type electrostatic sensor was developed. If this ceramic resonator type electrostatic sensor is applied to an electrostatic microphone device, the problems of the conventional electrostatic microphone device can be solved at once. The present invention has been made paying attention to such a point, and an object thereof is to provide an electrostatic microphone device capable of detecting a minute sound pressure signal with ultrahigh sensitivity.

[Means for Solving the Problems]

The present invention is configured as follows to achieve the above object. That is, the electrostatic microphone device of the present invention includes an oscillation circuit that uses a ceramic resonator that oscillates a carrier signal and a sound pressure that detects a vibration displacement of a diaphragm that receives a sound pressure signal and vibrates as a change in capacitance. The resonance frequency is biased in response to a change in the capacitance detected by the detection unit and the sound pressure detection unit, and the voltage change corresponding to the change in the tuning point with the oscillation circuit is AM-modulated using the carrier signal. It is characterized in that it has a ceramic resonator of the oscillation circuit that outputs as a signal and a modulation section that is independent of the ceramic resonator, and that the AM output from the modulation section.
An antenna circuit for wirelessly transmitting a modulation signal may be provided, a detection circuit for detecting an AM modulation signal output from the modulation unit to extract a signal corresponding to a sound pressure signal, and an amplification circuit for amplifying the detection output may be provided. Furthermore, it is a characteristic configuration of the present invention that an AFC circuit in which variable capacitance diodes are connected in a reverse bias state is added to the modulator.

[Action]

In the present invention, when the sound pressure signal is input, the diaphragm receives the sound pressure signal and vibrates. Then, the amplitude displacement of the diaphragm is detected as a change in capacitance, and the detected change in capacitance is applied to the ceramic resonator of the modulator.
The ceramic resonator shifts the resonance frequency in response to the change in capacitance and changes the tuning point with the oscillation circuit. Then, a voltage change corresponding to the change of the tuning point is added to the carrier signal added from the oscillation circuit to create an AM modulation signal,
This is output as a sound pressure detection signal.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. First
A circuit diagram of a first embodiment of an electrostatic microphone device according to the present invention is shown in the drawing. The device of this embodiment includes an oscillation circuit 1, a ceramic resonator 2 that functions as a modulator, and
Sound pressure detector 3, amplification circuit 5, antenna circuit 6, AF
And a C (Automatic Frequency Control) circuit 8.

The oscillating circuit 1 has an extremely high frequency, in this embodiment, 1 GHz to 10 GH.
It comprises a ceramic resonator 10 which oscillates at a fixed and constant oscillation frequency within the range of z. This oscillating circuit 1 oscillates the high frequency signal as a carrier signal, and adds this to a ceramic resonator 2 as a modulator via a high impedance conversion circuit in the oscillating circuit 1.

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

The ceramic resonator 2 has a frequency characteristic as shown in FIG. 2, and the oscillation frequency f ₁ of the oscillation circuit ₁ is set at a position slightly displaced from the resonance frequency f ₀ of the ceramic resonator 2. It This oscillation frequency f ₁ is set by a so-called up-porch method in which the oscillation frequency is set at a frequency position on the right side of the resonance frequency f ₀ of the ceramic resonator 2, and a back position by which it is set at a frequency position on the left side of the resonance frequency f _0. There is a pouch method, and either method may be used, but in this embodiment, the oscillation frequency f ₁ is set by the back porch method. The resonance frequency f ₀ of the ceramic resonator 2 is displaced according to the change of the electrostatic capacitance detected by the sound pressure detection unit 3. For example, when the electrostatic capacitance is changed by ΔC, the resonance frequency f ₀ is deviated by Δf. At this time, since the oscillation frequency of the oscillation circuit 1 is fixed to f ₁ , the output voltage of V ₀ is sent from the ceramic resonator 2 when there is no change in capacitance, but when the capacitance changes by ΔC, It is taken out from the ceramic resonator 2 as a voltage change of ΔV. In this way, the resonance frequency of the ceramic resonator 2 is deviated in accordance with the change in the electrostatic capacitance detected by the sound pressure detection unit 3, and the tuning point is changed from A ₁ to A ₂ accordingly, and the ceramic resonance is changed. The output voltage V corresponding to the capacitance change is sent from the device 2.
Specifically, a change Δf in resonance frequency corresponding to a change in capacitance is multiplied by a carrier signal output from the oscillation circuit 1, and a voltage change corresponding to the change in capacitance causes a carrier wave (carrier Signal) and extracted as an AM modulated signal. The output electrode of the ceramic resonator 2 is connected to an amplifier circuit 5 composed of an operational amplifier via a high impedance circuit including a capacitor 11 and an inductance element 12.

The amplifier circuit 5 is supplied from the ceramic resonator.
The AM modulation signal is amplified and added 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. The negative terminal (inverting input terminal) of the operational amplifier 13 is connected to the inverting input terminal of the amplifier circuit 5 via a resistor. In addition, 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 the output terminal of the operational amplifier 13 has a variable capacitor 17 connected in reverse bias. The cathode side is connected, and the anode side of the variable capacitor 17 is connected to ground. The cathode side of the variable capacitance diode 17 is connected to the output electrode of the ceramic resonator 2 via the coupling capacitor 18.

This AFC circuit 8 is an AM added from the ceramic resonator 2.
By the rectifying action of the diode 15 and the smoothing action of the capacitor 14, the modulation signal is a low frequency signal close to DC,
In addition, a very low signal component amplified to a required level 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, transmits this capacitance change to the ceramic resonator 2 via the coupling capacitor 18, and changes the resonance frequency of the resonator 2. That is, the resonance frequency of the ceramic resonator 2 is largely deviated for some reason,
When the inconvenience that the tuning point deviates from the setting region occurs, the resonance frequency is automatically shifted in the opposite direction to the bias direction by adding the AFC signal to the ceramic resonator 2 so that the tuning point deviates from the setting region. Prevent
The tuning point of the ceramic resonator 2 is optimized.

In this embodiment, the ceramic resonator 10 of the oscillation circuit 1
And the characteristics of the ceramic resonator 2 of the modulator, that is, the overall characteristics such as the frequency characteristics, the Q characteristics, the temperature characteristics, and the shape dimensions are substantially the same, and thereby the impedance of the circuit is increased and the oscillation circuit 1 Impedance matching between the ceramic resonator 2 as the modulator and the ceramic resonator 2 is facilitated to stabilize the circuit operation.

Further, in this embodiment, since the diaphragm 7 is connected to the ground, unnecessary radio waves emitted from the ceramic resonators 10 and 2, etc. can be removed, whereby mutual interference of radio waves can be prevented and the circuit can be prevented. The operation ensures more stability and reliability.

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

As shown in FIG. 2, the sound pressure detection unit is set in a position slightly deviated from the oscillation frequency f ₁ of the oscillation circuit 1 with respect to the resonance frequency (tuning frequency) f ₀ of the ceramic resonator 2. When the sound pressure signal enters into 3, the diaphragm 7 vibrates under the sound pressure of this sound pressure signal. Due to the amplitude displacement of the vibration plate 7, the capacitance between the vibration plate 7 and the fixed electrode 9 changes, and this change in capacitance is applied to the ceramic resonator 2. The ceramic resonator 2 has a resonance frequency of f ₀ before a sound pressure signal enters the sound pressure detector 3, but when the sound pressure signal enters and the capacitance changes as described above, The resonance frequency is biased by Δf. The biasing of the resonance frequency, the tuning point is changed from A ₁ to A _2, therefore, the output voltage from the ceramic resonator 2 is taken as the voltage V ₀ + [Delta] V, which increased by Δ from V _0. Specifically, the ceramic resonator 2 multiplies the carrier signal applied from the oscillation circuit 1 by the resonance frequency change component Δf, and adds the voltage change component corresponding to the capacitance change to the carrier signal to AM. A modulated signal is created and added to the amplifier circuit 5 and the AFC circuit 8 as a sound pressure detection signal.

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

On the other hand, the AFC circuit 8 converts the AM modulated signal into a signal close to direct current by the rectifying action of the diode 15 and the smoothing action of the capacitor 14, and further amplifies the signal to a required level and outputs it to the variable capacitance diode 17. Add. The variable capacitance diode 17 changes the capacitance according to the applied signal,
The resonance frequency f ₀ 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 resonance frequency is biased, the capacitance change component of the varactor diode 17 changes the capacitance of the ceramic resonator 2 due to the environmental changes. The change is canceled, the tuning point of the ceramic resonator 2 is prevented from fluctuating due to environmental changes, the tuning point is stabilized, that is, the resonance frequency is optimized, and the operation of the device is stabilized.

FIG. 3 shows a second embodiment of the electrostatic microphone device according to the present invention. This second embodiment is different from the first embodiment in that a detection circuit 4 is provided 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 performed. A signal which is detected by a detection circuit 4 and extracted as a signal corresponding to a sound pressure signal, and the detected output is amplified by an amplification circuit 5 and added to a signal processing unit (not shown) such as a recorder or a loudspeaker. The rest of the configuration is the same as that of the first embodiment.
In the second embodiment, the detection output is applied to the AFC circuit 8. Therefore, the configuration of the AFC circuit 8 of the second embodiment is simpler than that of the AFC circuit 8 of the first embodiment. Has been converted.

The present invention is not limited to the above embodiments,
Various embodiments can be adopted.

〔The invention's effect〕

According to the present invention, the resonator of the oscillation circuit and the resonator of the modulation unit are both configured by a ceramic resonator, and a ceramic resonator of the modulation unit is used to detect a change in capacitance corresponding to the sound pressure detected by the sound pressure detection unit. Since it is detected by using the change of the tuning point of, it becomes possible to detect a small electrostatic capacitance of about 10 ^-5 PF,
As a result, a weak sound pressure signal can be reliably detected under ultra-high sensitivity, and it can be applied to a zoom stereo microphone, which was difficult with a conventional device, and its technical value is enormous.

Further, the size and weight of the device are reduced, and the device is very easy to handle.

Further, in the configuration in which the AFC circuit is added, the resonance frequency of the modulation unit can be optimized with respect to environmental changes and the like, and the reliability of the device operation can be significantly improved.

Further, by making the characteristics of the ceramic resonator of the oscillation circuit and the ceramic resonator of the modulation section substantially the same, it becomes easy to stabilize Q and to match the high impedance of the circuit, thereby improving the performance of the device and operating the circuit. Can be further stabilized.

[Brief description 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 operation explanatory diagram of a modulator in the same embodiment, and FIG. 3 is an electrostatic microphone device according to the present invention. 2 is a circuit diagram showing a second embodiment of FIG. 1 ... Oscillation circuit, 2 ... Ceramic resonator (modulator),
3 ... Sound pressure detector, 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 ... Opamp, 14 ... Capacitor, 15 ... Diode, 16 ... Resistor, 17 ... Variable capacitance Diode, 18 ... Coupling capacitor.

Claims (5)

[Claims] 1. An oscillation circuit using a ceramic resonator that oscillates a carrier signal, and a sound pressure detection unit that detects a vibration displacement of a diaphragm that receives a sound pressure signal and vibrates as a change in capacitance.
The resonance frequency is biased in response to a change in capacitance detected by the sound pressure detection unit, and a voltage change corresponding to a change in the tuning point with the oscillation circuit is output as an AM modulation signal using the carrier signal. An electrostatic microphone device having a ceramic resonator of the oscillation circuit and a modulator formed of a ceramic resonator independent of the ceramic resonator. 2. The electrostatic microphone device according to claim 1, further comprising an antenna circuit for wirelessly transmitting the AM modulated signal output from the modulator. 3. A detection circuit for detecting an AM-modulated signal output from a modulation unit to extract a signal corresponding to a sound pressure signal, and an amplification circuit for amplifying the detection output. The electrostatic microphone device of paragraph. 4. The electrostatic microphone device according to claim 1, wherein an AFC circuit in which variable capacitance diodes are connected in reverse bias is added to the modulation section. 5. The ceramic resonator of the oscillating circuit and the ceramic resonator of the modulator are constituted by ceramic resonators having substantially the same characteristics. Microphone device.