JP4823134B2 - Condenser microphone - Google Patents

Description

  The present invention relates to an oscillation detection type condenser microphone, and in particular, uses a microphone unit that performs a push-pull operation with a single diaphragm, and forms two resonance circuits to cancel noise generated by an oscillator. It is characterized by this.

  Since the microphone unit used for the condenser microphone has a high output impedance, an impedance converter is generally connected to output the microphone unit with a low impedance. There is an oscillation detection type condenser microphone as another method capable of outputting a low impedance without using an impedance converter. Oscillation detection type condenser microphones have the problem that the circuit configuration is complicated and difficult to adjust, but are still commercialized because they have the advantage of low inherent noise.

  Oscillation detection type condenser microphones include a phase modulation type and an amplitude modulation type. The phase modulation type can use a crystal resonator as an oscillator and has been used for a long time because of its relatively simple circuit configuration. The amplitude modulation type uses a high-frequency bridge and operates even if the oscillation frequency slightly moves. However, there is a problem that the circuit configuration is complicated as compared with the phase modulation type.

  Since the condenser microphone according to the present invention is a phase modulation type oscillation detection system, an example of a conventional phase modulation type oscillation detection system condenser microphone will be described in more detail below. The phase modulation type oscillation detection system condenser microphone is an oscillator that stabilizes the oscillation frequency with a crystal oscillator, a tank coil (oscillation coil) included in the oscillator, and a high frequency (8 MHz-12 MHz) transmitted by the oscillator. It has a resonant coil that is phase modulated by the unit. Therefore, it is necessary to supply the high frequency oscillated by the oscillator from the tank coil (oscillation coil) to the resonance coil. 3 and 4 show examples of conventional phase modulation type oscillation detection type condenser microphones.

  The example shown in FIG. 3 is a circuit example of a phase modulation type oscillation detection type condenser microphone provided with a bridge circuit designed so that noises generated by the oscillators cancel each other and do not appear on the output side. In this circuit example, two resonant circuits 11 and 12 coupled to a high-frequency oscillator 10 are connected in series with each other. One resonance circuit 11 has a fixed capacitor C, and the other resonance circuit 12 has the capacitance of the condenser microphone unit M itself as a capacitor. Therefore, the capacitance of the capacitor included in the resonance circuit 12 varies in accordance with the sound wave received by the diaphragm of the microphone unit M, and the resonance frequency in the resonance circuit 12 changes according to the capacitance change of the microphone unit M. fluctuate. A fluctuation of the resonance frequency in the resonance circuit 12 with respect to a certain resonance frequency in the resonance circuit 11 is extracted by the ratio detection circuit 13 and output as an audio signal. In the ratio detection circuit 13, two diodes D1 and D2 are connected in a direction to cancel the rectified voltage of the high frequency bias.

  As long as the balance between the resonance circuit 11 and the resonance circuit 12 connected in series is not lost, no voltage appears on the output side. When the sound wave arrives and the capacitance of the microphone unit M changes, the resonance circuit 12 is biased and modulated with respect to the resonance circuit 11, and a voltage having a sideband without a carrier appears on the output side. In this way, since the same high-frequency current flows through both sides of the bridge circuit, all noise of the oscillator 10 is eliminated.

  According to the example of the conventional phase modulation type oscillation detection type condenser microphone shown in FIG. 3, the fixed capacitor C of one resonance circuit 11 is a load of the high frequency oscillator 10, and the amplitude of the high frequency signal oscillated by the high frequency oscillator 10. It is a factor that decreases When the amplitude of the high-frequency signal oscillated by the high-frequency oscillator 10 is lowered, there is a difficulty that sensitivity as a phase modulation type oscillation detection type condenser microphone is lowered.

  Next, another example of the conventional phase modulation type oscillation detection type condenser microphone shown in FIG. 4 will be described. Also in this conventional example, the output of the oscillator 20 is simultaneously supplied to a bridge circuit composed of two resonance circuits 21 and 22 coupled to the high-frequency oscillator 20. One resonance circuit 21 includes a varicap diode 24, and the other resonance circuit 22 includes the capacitance of the condenser microphone unit M. A low frequency amplification transistor 25 is provided for amplifying the audio signal detected and output by the ratio detection circuit 23, and a part of the signal output from the collector of the transistor 25 passes through the filter and then the varicap diode 24. It is configured to be fed back to. This circuit aims at extending the dynamic range without impairing the signal-to-noise ratio by providing a feedback circuit.

  The conventional example shown in FIG. 4 is basically the same as the conventional example shown in FIG. 3, and the varicap diode 24 serves as a load of the high-frequency oscillator 20 and reduces the amplitude of the high-frequency signal oscillated by the high-frequency oscillator 20. It becomes a factor. As a result, there is a problem that the sensitivity as a phase modulation type oscillation detection type condenser microphone is lowered.

In addition, there exists invention of patent document 1 concerning the application of this applicant as a prior art relevant to this invention. The invention described in Patent Document 1 uses a first fixed electrode and a second fixed electrode, which have the same facing area with respect to the diaphragm and are arranged at an equal distance from the diaphragm, in the condenser microphone. An impedance converter is connected, and an opposite polarity polarization voltage is applied to the first fixed electrode and the second fixed electrode.
The invention described in Patent Document 1 aims to reduce output distortion of an impedance converter when an excessive signal is input without impairing a directivity frequency response in a high frequency range.

  As another prior art related to the present invention, there is an invention described in Patent Document 2. The invention described in Patent Document 2 relates to a digital microphone. An oscillator that converts vibration of a diaphragm that receives and vibrates a sound wave into an oscillation frequency change, that is, an FM wave, and an FM signal that is output from the oscillator are converted into digital audio signals A gate circuit that gates at a sampling frequency clock period, a pulse count unit that counts the number of gated FM signals, and a difference between the reference value and the counted value at the same frequency as when FM is not modulated. And an arithmetic unit, which outputs digital data according to a sampling frequency period.

  The invention described in Patent Document 2 is similar to the constituent part of the present invention described later as long as it is provided with an oscillator that converts vibrations of the diaphragm into FM waves. The circuit configuration or signal processing is completely different, and the present invention does not include a circuit configuration for outputting as a digital signal.

JP 2006-101302 A Japanese Patent Laid-Open No. 7-23492

  The present invention eliminates the problems seen in the conventional phase modulation type oscillation detection type condenser microphone as described with reference to FIGS. 3 and 4, that is, a factor for reducing the amplitude of the high frequency signal oscillated by the high frequency oscillator. An object of the present invention is to provide a phase modulation type oscillation detection type condenser microphone capable of canceling noise generated by a high frequency oscillator while eliminating the noise and enhancing the sensitivity.

  The present invention relates to an oscillation circuit including an oscillation coil and a crystal resonator, and is coupled to the oscillation circuit via a coupling means and biased at a high frequency generated by the oscillation circuit, and also to electrostatic capacitance and resonance of the condenser microphone unit. A condenser microphone comprising: a resonance circuit configured by a coil; and a demodulation circuit that demodulates an audio signal corresponding to a change in capacitance of the capacitor microphone unit from an output signal of the resonance circuit, and receiving a sound wave The fixed electrode arranged facing the vibrating diaphragm is divided into a first fixed electrode and a second fixed electrode, and the first fixed electrode and the second fixed electrode have the same facing area with the diaphragm, and The resonance circuit is arranged at an equal distance from the diaphragm, and is formed by the diaphragm, the first fixed electrode and the second fixed electrode facing the diaphragm. The most important feature that the two resonance circuit is constituted by connecting respective resonant coil capacitance.

Since the resonance circuit is composed of the capacitance of the condenser microphone unit itself and the resonance coil, there is no need to connect a capacitor that becomes the load of the oscillation circuit unlike the conventional phase modulation type oscillation detection type condenser microphone. This eliminates the cause of the amplitude reduction of the high-frequency signal that oscillates at, so that a highly sensitive phase modulation type oscillation detection type condenser microphone can be obtained.
In addition, since it is possible to push-pull the condenser microphone unit and the two resonance circuits and to output the audio signal in a balanced manner, it is possible to cancel the polarities of the noises generated by the oscillators opposite to each other.

Hereinafter, embodiments of a condenser microphone according to the present invention will be described with reference to FIGS.
In FIG. 1, reference numeral 31 denotes a first fixed electrode, 32 denotes a second fixed electrode, and 34 denotes a diaphragm. The first fixed electrode 31, the second fixed electrode 32, and the diaphragm 34 constitute the main part of the condenser microphone unit. In general, the number of the fixed electrodes arranged facing the diaphragm 34 is one. However, in the present invention, the fixed electrode 31 is divided into the first fixed electrode 31 and the second fixed electrode 32 so as to face the single diaphragm 34. is doing. As the diaphragm 34, for example, a synthetic resin film having a metal vapor deposition film is used, and an outer peripheral edge portion is stretched on the support ring 35 with a predetermined tension. The first fixed electrode 31 and the second fixed electrode 32 have the same facing area to the diaphragm 34 and are overlapped with the diaphragm 34 with a common ring spacer (not shown) interposed therebetween. The diaphragm 34 is disposed at an equal distance. The diaphragm 34 is connected to the ground via the support ring 35 and the unit case.

  FIG. 2 shows an example of a fixed electrode plate 30 having a first fixed electrode 31 and a second fixed electrode 32. On the one surface side of the fixed electrode plate 30 made of an insulating material, the first fixed electrode 31 and the second fixed electrode 32 are formed by being divided into two along a diameter line passing through the center of the fixed electrode plate 30. A part of the outer periphery of the fixed electrode plate 30 protrudes outward in the radial direction, and a soldering land 37 that is conductive to the first fixed electrode 31 and a soldering land 38 that is conductive to the second fixed electrode 32 are formed at the protruding portion. ing.

  In FIG. 1, reference numeral 40 denotes an oscillation circuit that transmits a high-frequency signal of, for example, about 8 MHz to 12 MHz. The oscillation circuit 40 includes a crystal resonator 43, two field effect transistors 41 and 42, and an oscillation coil LO. The terminal of the crystal unit 43 is connected to the bases of the transistors 41 and 42, and the sources of the transistors 41 and 42 are connected via two coils LO1 and LO2 of the oscillation coil LO in series. The drain of the transistor 41 is connected to the ground via the resistor R1, the base of the transistor 41 is connected to the ground via the resistor R2, the base of the transistor 42 is connected to the resistor R3, and the drain of the transistor 42 is connected to the ground via the resistor R4. .

  The oscillation coil 40 includes other coils LO3 and LO4 that are magnetically coupled to the coils LO1 and LO2. These coils LO3 and LO4 function as coupling means for coupling the oscillation circuit 40 to the resonance circuit in order to bias the resonance circuit at a high frequency generated by the oscillation circuit 40. The coils LO3 and LO4 have center taps, and these center taps are connected to the ground. One end of the coil LO3 is connected to the first fixed electrode 31 through the coil LA1 constituting the resonance coil LA, more specifically, to the soldering land 37 of the first fixed electrode 31 shown in FIG. The other end of the coil LO3 is connected to the second fixed electrode 32 through the coil LB1 constituting the resonance coil LB, more specifically, to the soldering land 38 of the second fixed electrode 32 shown in FIG. One end of the coil LO4 is connected to the center tap of the coil LA2 magnetically coupled to the coil LA1, and the other end of the coil LO4 is connected to the center tap of the coil LB2 magnetically coupled to the coil LB1.

  Both ends of the coil LA2 constituting the resonance coil LA are connected to the output terminal 2 via the diodes D11 and D12 constituting the ratio detection circuit 51, respectively, and are connected to the ground via the capacitor C3. Both ends of the coil LB2 constituting the other resonance coil LB are connected to the output terminal 3 via diodes D21 and D22 constituting the ratio detection circuit 52, respectively, and are connected to the ground via a capacitor C4. The output terminals 2 and 3 together with the output terminal 1 connected to the connection point of the oscillation coils LO1 and LO2 constitute a three-terminal balanced output terminal. The terminal 1 is a ground terminal, the terminal 2 is a hot-side signal terminal, and the terminal 3 Is a signal terminal on the cold side. In the example shown in FIG. 1, the terminals 2 and 3 are connected to both ends of the primary winding of the output transformer 45 so that an audio signal is output from the secondary winding of the transformer 45.

  The microphone unit has two fixed electrodes 31 and 32 that face one diaphragm 34, and includes a capacitor C <b> 1 composed of the diaphragm 34 and the first fixed electrode 31, and the diaphragm 34 and the second fixed electrode 32. A resonance circuit that performs a push-pull operation is configured by connecting inductances including the coils LA1 and LB1 to the configured capacitor C2. The resonance circuit having the capacitor C1 and the coil LA1, and the resonance circuit having the capacitor C2 and the coil LB1 are biased by high-frequency oscillation signals output from the coil LO3 included in the oscillation circuit 40, respectively. The high-frequency signals supplied from the coil LO3 to the two resonance circuits are supplied from one end and the other end of the coil LO3 having a center tap, and thus become high-frequency signals having opposite phases.

  Ratio detection circuits 51 and 52 are connected to the two resonance circuits via coils LA2 and LB2 that are magnetically coupled to the resonance coils LA1 and LB1, respectively. In the ratio detection circuit 51, the difference between the oscillation signal of a constant frequency from the oscillation circuit 40 and the resonance signal of the first resonance circuit in which the capacitance of the capacitor C1 changes and the frequency fluctuates in response to the vibration of the diaphragm 34. Is detected and output from the terminal 2 as an audio signal. In the other ratio detection circuit 52, an oscillation signal having a constant frequency by the oscillation circuit 40, and a resonance signal of the second resonance circuit in which the capacitance of the capacitor C2 changes corresponding to the vibration of the diaphragm 34 and the frequency varies. Is detected and output from the terminal 3 as an audio signal. These audio signals are balanced and output from terminals 2 and 3. This balanced output can be extracted to the outside by a two-core microphone cable whose outer periphery is shielded. Alternatively, as shown in FIG. 1, the terminals 2 and 3 may be connected to both ends of the primary winding of the output transformer 45 and output from both ends of the secondary winding to the outside.

  According to the embodiment described above, the noise generated by the high-frequency oscillator 40 is input to the two resonance circuits as a balance signal, and detected by the ratio detection circuits 51 and 52 corresponding to each resonance circuit. In FIG. 1, the waveforms shown beside the terminals 2 and 3 are waveforms of a sine wave and noise signals n1 and n2 as a model of the audio signal. The phases of the audio signals output from the terminals 2 and 3 are opposite to each other. Since the noise signals n1 and n2 appearing at the terminals 2 and 3 are opposite in phase, the noise signals n1 and n2 are canceled.

  Further, the condenser microphone unit is composed of one diaphragm 34 and the two fixed electrodes 31 and 32 facing the diaphragm 34, so that a push-pull operation can be performed, and the two fixed electrodes 31 and 32 and one vibration can be obtained. Two resonant circuits including two capacitors C1 and C2 constituted by the plate 34 are formed. Therefore, the two resonance circuits are constituted by the capacitance of the condenser microphone unit itself and the resonance coil, and a capacitor serving as a load of the oscillation circuit is connected like the conventional phase modulation type oscillation detection type condenser microphone. This eliminates the need to reduce the amplitude of the high-frequency signal oscillated by the oscillator, and a highly sensitive phase modulation type oscillation detection type condenser microphone can be obtained.

It is a circuit diagram which shows the Example of the condenser microphone which concerns on this invention. It is a front view which shows the example of the fixed electrode of the condenser microphone unit used for the said Example. It is a circuit diagram which shows the example of the conventional phase modulation type | mold oscillation detection system condenser microphone. It is a circuit diagram which shows another example of the conventional phase modulation type | mold oscillation detection system condenser microphone.

Explanation of symbols

31 First Fixed Electrode 32 Second Fixed Electrode 34 Diaphragm 40 Oscillation Circuit 43 Crystal Vibrator 51 Ratio Detection Circuit 52 Ratio Detection Circuit LO Oscillation Coil LA Resonance Coil LB Resonance Coil

Claims (4)

An oscillation circuit including an oscillation coil and a crystal unit;
A resonance circuit coupled with the oscillation circuit via a coupling means and biased at a high frequency generated by the oscillation circuit and configured by a capacitance of a condenser microphone unit and a resonance coil;
A demodulator circuit that demodulates an audio signal corresponding to a change in capacitance of the condenser microphone unit from the output signal of the resonance circuit, and a condenser microphone comprising:
The fixed electrode disposed opposite to the diaphragm that vibrates in response to sound waves is divided into a first fixed electrode and a second fixed electrode,
The first fixed electrode and the second fixed electrode have the same opposing area to the diaphragm and are arranged at an equal distance from the diaphragm,
The resonance circuit includes two resonance circuits by connecting a resonance coil to the capacitance formed by the diaphragm and the first fixed electrode and the second fixed electrode facing the diaphragm. Condenser microphone.   The condenser microphone according to claim 1, wherein the first fixed electrode and the second fixed electrode are connected to a resonance circuit so as to perform a push-pull operation with respect to one diaphragm.   The condenser microphone according to claim 1, wherein a ratio detection circuit is connected to each resonance circuit as a demodulation circuit, and an audio signal is output from each ratio detection circuit.   4. The condenser microphone according to claim 3, wherein the audio signal output from each ratio detection circuit is balanced.

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