JP5409293B2 - Condenser microphone - Google Patents

Description

  The present invention relates to a condenser microphone, and more particularly to an oscillator in a digital microphone that obtains an audio signal by digitally processing a change in capacitance of a microphone unit.

  Capacitor microphones are equipped with a kind of capacitor that consists of a combination of a diaphragm and a fixed pole. When taking out the change in capacitance due to the displacement of the diaphragm due to sound waves as an electrical signal, a direct current is placed between the diaphragm and the fixed pole. It is necessary to apply voltage. This DC voltage is called a polarization voltage, and there are a method of applying a voltage from the outside of the microphone unit and a method of applying a polarization voltage by an electret material.

  In the condenser microphone, since the impedance between the diaphragm and the fixed pole is extremely high, an impedance converter such as an FET or a vacuum tube is used to obtain a voltage signal at a predetermined level.

  However, the conventional condenser microphone is susceptible to the influence of an external electric field or magnetic field when converting a change in capacitance into a voltage. In addition, since the impedance converter is used, for example, when the ambient humidity is high, charge leakage inherent to the impedance converter may occur and noise may be generated.

  Furthermore, the electret condenser microphone can be miniaturized, for example, for a cellular phone. However, since the electret is vulnerable to heat, the reflow solder method cannot be applied when mounted on a substrate. Therefore, it must be mounted separately from chip components such as resistors and capacitors.

  Therefore, Patent Document 1 proposes a condenser microphone in which a change in capacitance of a microphone unit is converted into a digital signal to obtain an audio signal.

  In this digital condenser microphone, a first oscillator that outputs an audio signal having a frequency corresponding to the capacitance of the microphone unit, a second oscillator that outputs a follow-up signal whose frequency changes according to the control voltage, and the audio signal and follow-up A phase comparator that outputs the phase difference signal of the signal, and the addition pulse signal and the subtraction pulse signal obtained from the logical product of the advance / delay phase difference signal and the clock pulse are counted by the addition / subtraction counter and digital audio I try to get data.

  According to this, since the capacitance change is not converted into a voltage, it is not easily influenced by the surrounding electromagnetic field. In addition, noise due to charge leakage of the impedance converter is not generated. Furthermore, since a polarization voltage by an electret material or the like is not required, an advantage that surface mounting by a reflow soldering method together with a chip component such as a resistor or a capacitor is possible is obtained.

JP 2003-23690 A

  By the way, in the condenser microphone according to Patent Document 1, a C (capacitance) -F (frequency) converter is used as the first oscillator that outputs an audio signal having a frequency corresponding to the capacitance of the microphone unit. Yes.

  However, when designing a microphone unit, it is difficult to incorporate a C-F converter into an existing microphone unit due to space restrictions such as component placement, so the design is made from the unit case (housing) itself. Or a C-F converter is externally attached, but in any case, it is not preferable in that the cost is increased and the apparatus becomes large.

  Accordingly, an object of the present invention is to provide an oscillator that outputs an audio signal having a frequency corresponding to the capacitance of a microphone unit using a fixed pole in the microphone unit in a digital condenser microphone described in Patent Document 1. There is in getting.

In order to solve the above-described problems, the present invention includes a microphone unit that includes a diaphragm and a fixed pole that are arranged to face each other via a spacer ring, and whose capacitance changes according to input sound pressure, and the capacitance A first oscillator that outputs an audio signal whose frequency changes according to the frequency, a second oscillator that outputs a tracking signal whose frequency changes according to the control voltage, the audio signal and the tracking signal, and A first control signal is output when the audio signal has a phase advance relative to the signal, and a second control signal is output when the audio signal has a phase lag, so that the follow-up signal and the audio signal are in phase. A phase synchronizing means for controlling the control voltage, a clock generator for generating a clock pulse, and a first AND circuit for outputting an addition pulse signal by the logical product of the first control signal and the clock pulse, A second AND circuit for outputting a subtraction pulse signal by the logical product of the second control signal and the clock pulse; and an addition / subtraction counter for counting the addition pulse signal and the subtraction pulse signal and outputting an audio data signal; In a condenser microphone equipped with
The first oscillator includes an amplifier that amplifies the output signal of the microphone unit, and a three-stage RC network including a resistor R and a capacitor C connected to the output side of the amplifier as a feedback circuit. An oscillator,
The fixed electrode is equally divided into three in the first to third stationary pole piece forms a fan-shaped, three electrostatic capacitance generated between the diaphragm and the respective fixed pole pieces of the RC network capacitor C, and the amplifier is an inverting amplifier, between the output terminal of the inverting amplifier and the first fixed pole piece, between the first fixed pole piece and the second fixed pole piece, and the second fixed pole. The resistor R is connected between each piece and the third fixed pole piece, and the third fixed pole piece is connected to the input terminal of the inverting amplifier to constitute the three-stage RC network. An audio signal whose frequency changes in accordance with the capacitance of the microphone unit is applied to one input terminal of the phase synchronization means from the output terminal of the inverting amplifier .

  According to the present invention, the first oscillator that outputs an audio signal whose frequency changes in accordance with the capacitance of the microphone unit is a phase-shifting oscillator, and the microphone is connected to the capacitor C included in each RC network. By using three capacitances formed between the diaphragm and the fixed pole of the unit, a phase-shifting oscillator can be easily configured in the microphone unit. That is, the phase shift oscillator can be configured by equally dividing the fixed pole into three in a fan shape and connecting a resistance element to each fixed pole piece.

1 is a circuit diagram of a condenser microphone according to an embodiment of the present invention. The schematic diagram which shows the structure of the 1st oscillator used for the said condenser microphone. The schematic diagram which shows the electrical constitution of the said 1st oscillator. FIG. 3 is an equivalent circuit diagram of the first oscillator. The wave form diagram of the sound pressure input into a microphone unit, and the wave form diagram of the audio | voice signal output from a 1st oscillator. FIG. 4 is a comparison waveform diagram of an audio signal and a tracking signal, and a waveform diagram of a control signal output from a phase comparator. The output waveform figure of a charge pump. The wave form diagram which shows a clock pulse, an addition pulse, and a subtraction pulse.

  Next, an embodiment of the condenser microphone of the present invention will be described with reference to FIGS. 1 to 8, but the present invention is not limited to this.

  As shown in FIG. 1, the condenser microphone M is output from a first oscillator 2 having a microphone unit 1, a second oscillator 3 composed of a VCO (voltage controlled oscillator), and first and second oscillators 2 and 3. A phase comparator 4 for comparing phases of respective frequencies, a charge pump 5 for supplying electric charge including a pair of FETs 6 and 7, a loop filter 8 comprising a resistor and a capacitor, and a clock generator such as a crystal oscillation circuit 11, a first logical product circuit 12, a second logical product circuit 13, and an addition / subtraction counter 14 that outputs digital audio data.

  As shown in FIG. 3, the microphone unit 1 is stretched over a diaphragm ring 111, and is arranged to face the diaphragm 110 via a diaphragm 110 that vibrates with an input sound pressure and a spacer ring (not shown). The electrostatic capacity changes due to the displacement of the diaphragm 110. In the present invention, the microphone unit 1 may have the simplest configuration, and no polarization voltage by an electret material or the like is necessary.

  The first oscillator 2 is an oscillator whose oscillation frequency changes in accordance with the change in capacitance of the microphone unit 1. In the present invention, the first oscillator 2 is connected to an amplifier that amplifies the output signal of the signal source and a feedback circuit on the output side of the amplifier. And a three-stage RC network including a resistor R and a capacitor C.

  2 and 3, in the present invention, a capacitance between diaphragm 110 and fixed pole 120 is used as capacitor C included in each RC network of the phase-shifting oscillator.

  If the amplifier 201 is an inverting amplifier whose input voltage and output voltage are 180 ° out of phase, the feedback circuit must also be 180 ° out of phase, so a three-stage RC network is required for the feedback circuit. Is done.

  Therefore, as shown in FIG. 2, the fixed pole 120 is equally divided into three first to third fixed pole pieces 121, 122, 123 having a fan shape. Thereby, three capacitors C are formed between the diaphragm 110 and the diaphragm 110.

  For example, assuming that the first fixed pole piece 121 is connected to the output side of the amplifier 201, the first fixed pole piece 121 and the second fixed pole piece 122 are provided between the first fixed pole piece 121 and the output terminal of the amplifier 201. , A resistor R is connected between each of the second fixed pole piece 122 and the third fixed pole piece 123, whereby a three-stage RC network is obtained. The resistor R is preferably a chip component that can be easily mounted.

  In this embodiment, the output terminal of the first oscillator 2 (terminal connected to one input terminal 4a of the phase comparator 4) is drawn from between the output terminal of the amplifier 201 and the first-stage resistor R.

With reference to the equivalent circuit diagram of FIG. 4, the oscillation conditions of the phase-shifting oscillator will be described. In FIG. 4, A _v V ₁ is a voltage source of the amplifier 201, and R ₀ is an output resistance of the amplifier 201. R is a resistance value of the resistor R, and C is a capacitance value of the capacitor C. V ₁ is an output voltage of the third stage RC network, and V ₂ and V ₃ are voltages generated in the first and second stage RC networks, respectively.

The amplitude condition (voltage gain) and phase condition (oscillation frequency) are obtained using Kirchhoff's law. According to Kirchhoff's law, the relationship between the currents I _{1 to} I ₆ is expressed by the following equations (1), (2), and (3).

Therefore, the values of the currents I _{1 to} I ₆ are as follows.

Substituting the values of the currents I _{1 to} I _{6 into} the equations (1), (2), and (3), and formulating the voltages V ₁ , V ₂ , and V ₃ , the following equations (4), (5), (6).

These are arranged to obtain the following equations (7), (8), (9).

Since the simultaneous equations constituted by the equations (7), (8), and (9) are all zero on the right side, in order to have a meaningful solution, the following determinant may be set to zero.

This is developed and organized into a real part and an imaginary part to obtain the following expression (10).

This becomes the amplitude condition and phase condition of the oscillation condition. As shown in the following equation (11), when the imaginary part of the equation (10) is 0, the phase condition (oscillation frequency) of the following equation (12) is obtained.

Here, if R ₀ << R, the expression (12) becomes the following expression (13).

As shown in the following equation (14), when the real part of the equation (10) is set to 0 and the equation (13) is substituted, the amplitude condition (voltage amplification factor) shown in the following equation (15) is obtained.

Here, if R ₀ << R, Expression (15) becomes Expression (16) below.

  In this way, the oscillation frequency that is the phase condition of the phase-shifting oscillator can be obtained from the equation (13), and the voltage gain that is the amplitude condition can be obtained from the equation (16).

  According to the present invention, the fixed pole 120 of the microphone unit 1 is divided into three parts, a resistor R is connected to each of the fixed pole pieces, and the amplifier 201 is connected to the RC network thus formed. A phase-shifting oscillator as the first oscillator 2 can be incorporated in the unit 1.

  The first oscillator 2 supplies an audio signal having a frequency proportional to the sound pressure input to the microphone unit 1 to one input terminal 4 a of the phase comparator 4. The other input terminal 4 b of the phase comparator 4 is given a follow-up signal for the audio signal from the second oscillator 3.

  The phase comparator 4 compares the phase of the audio signal from the first oscillator 2 and the follow-up signal from the second oscillator 3, and when the audio signal has a phase advance with respect to the follow-up signal, the phase comparator 4 outputs the first signal to the first output terminal 4u. 1 control signal S1 is output, and the second control signal S2 is output to the second output terminal 4d when the audio signal has a phase delay with respect to the follow-up signal.

  In this embodiment, the phase comparator 4 is a phase frequency comparator that operates at the rising edge of an input waveform, and for example, an integrated circuit MC4044 manufactured by Motorola or the like can be used.

  The phase comparator 4 forms a phase locked loop of the second oscillator 3 together with the charge pump 5 and the loop filter 8 connected to the output side thereof. The charge pump 5 includes a pair of FETs 6 and 7. The gate terminal of one FET 6 is connected to the first output terminal 4u of the phase comparator 4, and the gate terminal of the other FET 7 is connected to the second output terminal 4d. Has been.

  The drain terminal of the FET 6 is connected to the in-device power supply Vcc, and the drain terminal of the FET 7 is grounded. The source terminals of the FETs 6 and 7 are connected to each other, the connection point is the output terminal OUT of the charge pump 5, and this output terminal OUT is connected to the control terminal of the second oscillator 3 via the loop filter 8. .

  When the audio signal has a phase advance with respect to the tracking signal, one FET 6 is turned on by the first control signal S1 appearing at the first output terminal 4u, and a predetermined voltage is supplied from the in-device power supply Vcc through the loop filter 8. 2 is added to the control terminal of the oscillator 3.

  On the other hand, when the audio signal has a phase lag with respect to the follow-up signal, the other FET 7 is turned on by the first control signal S2 appearing at the second output terminal 4d, and a predetermined value is applied from the control terminal side of the second oscillator 3. Voltage is pulled to ground. In this way, the follow-up signal is controlled to be in phase with the audio signal. In this embodiment, MOS type FETs 6 and 7 are used, but junction type FETs may be used.

  The clock output terminal of the clock generator 11 is connected to one input terminal of the two-input type first AND circuit 12 and also connected to one input terminal of the two-input type second AND circuit 13. Has been.

  The other input terminal of the first AND circuit 12 is connected to the first output terminal 4 u of the phase comparator 4, and the other input terminal of the second AND circuit 13 is connected to the second output of the phase comparator 4. Terminal 4d is connected.

  The output terminal of the first AND circuit 12 is connected to the addition pulse input terminal 14u of the addition / subtraction counter 14, and the output terminal of the second AND circuit 13 is connected to the subtraction pulse input terminal 14d of the addition / subtraction counter 14. Yes.

  Next, the operation of the condenser microphone M will be described with reference to FIGS. For example, when a sound pressure W as shown in FIG. 5A is input to the microphone unit 1, the capacitance of the microphone unit 1 changes, and the oscillation frequency of the first oscillator 2 changes according to the change in capacitance. .

  That is, the capacitance of the microphone unit 1 is converted into a frequency by the first oscillator 2 and is output to one input terminal 4a of the phase comparator 4 as the audio signal W1 shown in FIG. In FIG. 5B, the vertical axis is the frequency axis, and the horizontal axis is the time axis.

  A tracking signal is input from the second oscillator 3 to the other input terminal 4b of the phase comparator 4, and FIG. 6A shows the relationship between the audio signal W1 and the tracking signal W2. The phase comparator 4 compares the phases of the audio signal W1 and the tracking signal W2. However, since the phase-locked loop has a propagation time required for the control of the second oscillator 3, the tracking signal W2 is the same as the audio signal W1. It takes time corresponding to the time difference ΔT to reach the oscillation frequency.

  In the example of FIG. 6A, in the section T1 and the section T3, since the frequency of the audio signal W1 is higher than the frequency of the tracking signal W2, the audio signal W1 has a phase advance with respect to the tracking signal W2. become.

  Therefore, as shown in FIG. 6B, in the section T1 and the section T3, the first control signal S1 is output from only one output terminal 4u of the phase comparator 4. The pulse width of the first control signal S1 changes in proportion to the phase difference.

  On the other hand, in the section T2, since the frequency of the audio signal W1 is smaller than the frequency of the tracking signal W2, the audio signal W1 has a phase lag with respect to the tracking signal W2.

  Therefore, as shown in FIG. 6C, in the section T2, the second control signal S2 is output only from the other output terminal 4d of the phase comparator 4. The second control signal S2 also changes the pulse width according to the phase delay amount.

  In this manner, in the phase comparator 4, the input sound pressure W in FIG. 5 is PWM (Pulse Width Modulation) by the phase difference between the audio signal W1 and the follow-up signal W2, and the first and first pulse widths corresponding to the phase difference are obtained. Second control signals S1 and S2 are output.

  The FETs 6 and 7 of the charge pump 5 are turned on and off by the first and second control signals S1 and S2, and a voltage V1 as shown in FIG. 7 appears at the output terminal OUT of the charge pump 5.

  That is, in the sections T1 and T3 in which the first control signal S1 is output, the voltage V1 rises stepwise according to the pulse width, and in the section T2 in which the second control signal S2 is output, The voltage V1 decreases stepwise according to the pulse width.

  The harmonic component and noise included in the voltage V1 are removed by the loop filter 8 and input as the control voltage CV of the second oscillator 3. As a result, the second oscillator 3 operates to oscillate at the same frequency as the first oscillator 2. The second oscillator 3 is exemplified by an integrated circuit MC4024 manufactured by Motorola.

  As shown in FIG. 8A, the clock generator 11 generates a clock pulse CK having a sufficiently short pulse width compared to the time widths of the first and second control signals S1 and S2, and the first logical product. The signal is input to the circuit 12 and the second AND circuit 13.

  The first AND circuit 12 obtains a logical product of the clock pulse CK and the first control signal S1 at the leading phase, and adds the addition pulse signal P1 as shown in FIG. This is given to the input terminal 14u.

  Further, the second AND circuit 13 obtains a logical product of the clock pulse CK and the second control signal S2 at the delayed phase, and generates a subtraction pulse signal P2 as shown in FIG. This is given to the subtraction pulse input terminal 14d.

  That is, PNM (Pulse Number Modulation) is performed by the first and second AND circuits 12 and 13, and the pulse widths of the first and second control signals S1 and S2 are converted into the number of clock pulses CK.

  The addition / subtraction counter (up / down counter) 14 performs reversible addition / subtraction based on the addition pulse signal P1 and the subtraction pulse signal P2, and outputs the audio data from the bit terminals MSB to LSB as parallel data.

  The adder / subtractor counter 14 keeps the state before the phase changes, and counts up or down only when the phase changes. Also, the audio data output from the adder / subtractor 14 is D / A converted as necessary, and then supplied to an audio output unit (not shown).

  In the above embodiment, a crystal oscillation circuit is used as the clock generator, but a CR oscillation circuit, an LC oscillation circuit, a multivibrator, or the like may be used instead. The addition / subtraction counter may be a microprocessor or the like. Furthermore, the phase comparator 4 may be an analog type phase comparator. In this case, the output signal is A / D converted, for example, and input to the AND circuit.

  In the above embodiment, the fixed pole is divided into three. However, in the case of a single phase-shifting oscillator, a capacitor including a diaphragm and a fixed pole may be assigned to each RC network.

DESCRIPTION OF SYMBOLS 1 Microphone unit 2 1st oscillator 3 2nd oscillator 4 Phase comparator 4a, 4b Input terminal 4u 1st output terminal 4d 2nd output terminal 5 Charge pump 6, 7 FET
8 Loop filter 11 Clock generator 12 First AND circuit 13 Second AND circuit 14 Addition / subtraction counter 14u Addition pulse input terminal 14d Subtraction pulse input terminal 110 Diaphragm 120 Fixed pole 121, 122, 123 Fixed pole piece 201 Amplifier C Capacitor R Resistance

Claims (1)

A microphone unit that includes a diaphragm and a fixed pole that are arranged to face each other via a spacer ring and whose capacitance changes according to the input sound pressure, and outputs an audio signal whose frequency changes according to the capacitance. A first oscillator, a second oscillator that outputs a follow-up signal whose frequency changes according to a control voltage, the sound signal and the follow-up signal are obtained, and the sound signal has a phase advance with respect to the follow-up signal. Phase synchronization means for outputting a first control signal at a time, outputting a second control signal when having a phase delay, and controlling the control voltage so that the follow-up signal and the audio signal have the same phase; A clock generator for generating a clock pulse, a first AND circuit for outputting an addition pulse signal by a logical product of the first control signal and the clock pulse, the second control signal and the clock pulse A second AND circuit for outputting a logical product by the subtraction pulse signal, the condenser microphone and a subtracting counter for outputting an audio data signal by counting the addition pulse signal and the subtraction pulse signal,
The first oscillator includes an amplifier that amplifies the output signal of the microphone unit, and a three-stage RC network including a resistor R and a capacitor C connected to the output side of the amplifier as a feedback circuit. An oscillator,
The fixed electrode is equally divided into three in the first to third stationary pole piece forms a fan-shaped, three electrostatic capacitance generated between the diaphragm and the respective fixed pole pieces of the RC network capacitor Used as C ,
The amplifier is an inverting amplifier, between the output terminal of the inverting amplifier and the first fixed pole piece, between the first fixed pole piece and the second fixed pole piece, the second fixed pole piece and the third fixed pole piece. The resistor R is connected to each of the fixed pole pieces, the third fixed pole piece is connected to the input terminal of the inverting amplifier, and the three-stage RC network is configured.
A condenser microphone, wherein an audio signal whose frequency changes in accordance with the capacitance of the microphone unit is applied to one input terminal of the phase synchronization means from an output terminal of the inverting amplifier .

Images