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DESCRIPTION JP2008306636

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DESCRIPTION JP2008306636
A condenser microphone of a phase modulation type oscillation detection system is provided
which suppresses fluctuation of a resonance frequency even if temperature changes and
prevents deterioration of performance and characteristics of a microphone. The oscillation circuit
1 includes an oscillation coil 4 and a crystal oscillator 3. The oscillation circuit 1 is coupled to the
oscillation circuit and biased by a high frequency signal generated by the oscillation circuit. And
a demodulation circuit 10 for demodulating the output signal of the resonance circuit into an
audio signal corresponding to the change in capacitance of the condenser microphone unit 5.
The resonant coil 6 includes a giant magnetostrictive element 12 whose outer periphery is
wound by a coil, and piezoelectric elements 14 and 15 which apply pressure to the giant
magnetostrictive element, and an error signal of the output level of the demodulation circuit 10
is input to the piezoelectric element The element applies a pressure according to the error signal
to the giant magnetostrictive element 12, and the giant magnetostrictive element changes the
inductance according to the pressure, and corrects the fluctuation of the resonant frequency of
the resonant circuit. [Selected figure] Figure 1
コンデンサーマイクロホン
[0001]
The present invention relates to an oscillation detection type condenser microphone, and is
characterized in particular by the configuration of a temperature characteristic correction circuit
of a resonance circuit.
[0002]
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1
Since a microphone unit used for a condenser microphone has a high output impedance,
generally, an impedance converter is provided to lower the output impedance of the microphone
unit and output.
As another system capable of low impedance output without using an impedance converter,
there is a condenser microphone of an oscillation detection system. Although there is a problem
that the circuit configuration is complicated and the adjustment is difficult, the oscillation
detection type condenser microphone has a merit that the intrinsic noise is small, and it is
commercialized even now.
[0003]
There are a phase modulation type and an amplitude modulation type as the condenser
microphone of the oscillation detection method. The phase modulation type can use a crystal
oscillator as an oscillator, and has been used for a long time because the circuit configuration is
relatively simple. The amplitude modulation type uses a high frequency bridge and operates even
if the oscillation frequency moves slightly, but has a drawback that the circuit configuration
becomes complicated as compared with the phase modulation type.
[0004]
Since the condenser microphone according to the present invention belongs to the phase
modulation oscillation detection method, an example of a conventional phase modulation
oscillation detection condenser microphone will be described in more detail below. In FIG. 3,
reference numeral 1 denotes an oscillation circuit, 4 denotes an oscillation coil, 10 denotes a
ratio detection circuit, and 6 denotes a resonance coil. The oscillating coil 4 comprises three coils
L1, L2 and L3 magnetically coupled to one another.
[0005]
The oscillation circuit 1 includes a transistor 2, a crystal unit 3, and the coil L1 (tank coil). A
capacitor C1 is connected in parallel to the coil L1, a power source PS is connected to one end of
the coil L1, and the other end of the coil L1 is connected to the collector of the transistor 2. A
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resistor R1 is connected between the collector and the base of the transistor 2, a crystal unit 3 is
connected between the base and the ground, and a resistor R2 is connected in parallel with the
crystal unit 3. Capacitors C7 and C8 are also connected in series between the base of transistor 2
and ground, the junction of capacitors C7 and C8 is connected to the emitter of transistor 2, and
the emitter is connected to ground via resistor R3. ing. The coils L2 and L3 are connected in
series and this connection point is connected to ground. The oscillation circuit 1 stabilizes the
oscillation frequency by configuring an oscillation circuit including the crystal oscillator 3.
[0006]
The resonant coil 6 comprises three coils L4, L5 and L6 magnetically coupled to one another.
The ratio detection circuit 10 is one of the demodulation methods for phase-modulated signals,
and is a circuit method for creating a phase difference by combining the coils L5 and L6 and a
capacitor and performing vector-like demodulation. The capacitor is formed between the
condenser microphone unit 5, more specifically, the diaphragm constituting the condenser
microphone unit, and the fixed electrode facing the diaphragm with a gap. It is a capacitor. The
coils L5 and L6 are connected in series, the other end of the coil L5 is connected to the
microphone output terminal OUT via the diode 7, and the other end of the coil L6 is connected to
the microphone output terminal OUT via the diode 8. The diodes 7 and 8 are in the opposite
direction to each other.
[0007]
The oscillation circuit 1 oscillates a high frequency signal of, for example, 8 MHz to 12 MHz, and
one end of the coil L2 and one end of the coil L4 are connected in order to supply the oscillated
high frequency signal from the oscillation coil 4 to the resonant coil 6. One end of is connected to
the connection point of the coils L5 and L6. The high frequency signal oscillated by the
oscillation circuit 1 is supplied to the resonant coil 6 from the coils L2 and L3. The electrostatic
capacitance of the microphone unit 5 is connected in series to the coil L4 constituting the
resonant coil 6. The capacitance of the condenser microphone unit 5 is changed by vibrating
according to the sound wave received by the diaphragm. The capacitance of the microphone unit
5 and the inductance of the resonance coil 6 constitute a resonance circuit, and a high frequency
signal generated by the oscillation circuit 1 is added as a bias to the resonance circuit. The high
frequency signal is phase-modulated by the microphone unit 5 with an audio signal subjected to
electroacoustic conversion. The modulated signal is demodulated by the ratio detection circuit 10
which is a demodulation circuit and output from the microphone output terminal OUT. The coil
L4 constituting the resonance coil 6 resonates in series with the capacitance of the microphone
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unit 5, so the impedance at the resonance frequency becomes extremely low. If the impedance at
the resonance frequency is not lowered as described above, the sensitivity is lowered.
[0008]
In the above-mentioned example of the conventional phase modulation type oscillation detection
type condenser microphone, the resonance circuit is constituted by the resonance coil 6
composed of a plurality of coils L1, L2 and L3 magnetically coupled at the core, and the
condenser microphone unit 5. The core is a movable core so that the resonance frequency can be
adjusted by adjusting the degree of magnetic coupling of the plurality of coils.
[0009]
In addition, there exists invention of patent document 1 as a prior art relevant to this invention.
The invention described in Patent Document 1 is a condenser microphone in which the first fixed
electrode and the second fixed electrode, which have the same facing area to the diaphragm and
are equidistant to the diaphragm, are used for each fixed electrode. An impedance converter is
connected, and a polarized voltage of reverse polarity is applied to the first fixed electrode and
the second fixed electrode. The invention described in Patent Document 1 aims to reduce the
output distortion of the impedance converter at the time of an excessive signal input without
impairing the directional frequency response in the high frequency band.
[0010]
Another prior art related to the present invention is the invention described in Patent Document
2. The invention described in Patent Document 2 relates to a digital microphone, and an
oscillator for converting the vibration of a vibrating plate that receives and vibrates sound waves
to a change in oscillation frequency, ie, an FM wave, and an FM signal output from this oscillator
as a digital audio signal. A difference between a reference value of the same frequency as that at
the time of no modulation of FM and the above-mentioned counted value is calculated, a gate
circuit which gates at a clock cycle of sampling frequency, a pulse count section which counts the
number of gated FM signals. And a digital signal is output according to the period of the
sampling frequency.
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[0011]
The invention described in Patent Document 2 is similar to the configuration of the invention of
the present invention described later, insofar as it has an oscillator for converting the vibration of
the diaphragm to an FM wave, but the circuit configuration after conversion to an FM wave Also,
the signal processing is completely different, and the present invention does not have a circuit
configuration for outputting as a digital signal. JP, 2006-101302, A JP, 7-23492, A
[0012]
An object of the present invention is to solve the problems found in the conventional phase
modulation type oscillation detection type condenser microphone as described with reference to
FIG. That is, since the above-mentioned resonant coil 6 and condenser microphone unit 5 of the
resonant circuit constituted of the plurality of coils L 1, L 2 and L 3 described above and the
condenser microphone unit 5 have a temperature coefficient, After adjusting the resonance
frequency to the optimum value, there is a problem that the resonance frequency deviates from
the optimum value as the temperature changes, and the performance or characteristics of the
microphone deteriorate. Therefore, the present invention, by devising the configuration of the
resonant circuit, suppresses the fluctuation of the resonant frequency even if the temperature
changes, and prevents the deterioration of the performance or characteristics of the microphone.
It aims at providing a microphone.
[0013]
The present invention provides an oscillation circuit including an oscillation coil and a quartz
oscillator, and a resonance that is biased by a high frequency signal coupled to the oscillation
circuit and generated by the oscillation circuit and configured by the capacitance of the
condenser microphone unit and the resonance coil. A condenser microphone comprising: a
circuit; and a demodulation circuit that demodulates an output signal of the resonance circuit to
an audio signal corresponding to a change in capacitance of the condenser microphone unit,
wherein the resonance coil has a coil wound around its periphery And a piezoelectric element for
applying pressure to the giant magnetostrictive element, the error signal of the output level of
the demodulation circuit is connected to be input to the piezoelectric element, and the
piezoelectric element is responsive to the error signal. Pressure is applied to the giant
magnetostrictive element, the inductance of the giant magnetostrictive element changes
according to the pressure, and the resonant circumference of the resonant circuit The most
important features to modify the variations of the number.
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[0014]
If the frequency of the signal output from the resonance circuit is a predetermined frequency and
the DC output of the demodulation circuit is a predetermined voltage, no error signal of the
output level of the demodulation circuit is output, and the frequency of the output of the
resonance circuit is maintained as it is Be done.
When the frequency of the output of the resonant circuit fluctuates due to a temperature change
or the like, an error signal of the output level of the demodulation circuit is output, this error
signal is input to the piezoelectric element, and the pressure due to the piezoelectric action of the
piezoelectric element is applied to the giant magnetostrictive element. The giant magnetostrictive
element changes its inductance according to the pressure. This change in inductance is the
change in resonant frequency of the resonant circuit including the resonant coil. Therefore, the
change in the resonance frequency is connected so as to correct the DC output of the
demodulation circuit to a predetermined voltage. By this, even if the temperature changes, the
fluctuation of the resonance frequency can be suppressed, and the deterioration of the
performance or characteristics of the microphone can be prevented.
[0015]
Hereinafter, embodiments of a condenser microphone according to the present invention will be
described with reference to the drawings. The same components as those of the conventional
example shown in FIG. 3 are denoted by the same reference numerals. In FIG. 1, reference
numeral 1 denotes an oscillation circuit, 4 denotes an oscillation coil, 10 denotes a ratio
detection circuit which is a demodulation circuit, and 6 denotes a resonance coil. The oscillating
coil 4 comprises three coils L1, L2 and L3 magnetically coupled to each other with the
interposition of a core.
[0016]
The oscillation circuit 1 includes a transistor 2, a crystal unit 3, and the coil L1 (tank coil). A
capacitor C1 is connected in parallel to the coil L1, a power source PS is connected to one end of
the coil L1, and the other end of the coil L1 is connected to the collector of the transistor 2. A
resistor R1 is connected between the collector and the base of the transistor 2, a crystal unit 3 is
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connected between the base and the ground, and a resistor R2 is connected in parallel with the
crystal unit 3. Capacitors C7 and C8 are also connected in series between the base of transistor 2
and ground, the junction of capacitors C7 and C8 is connected to the emitter of transistor 2, and
the emitter is connected to ground via resistor R3. ing. The coils L2 and L3 are connected in
series and this connection point is connected to ground. The oscillation circuit 1 stabilizes the
oscillation frequency by including the crystal oscillator 3.
[0017]
The resonant coil 6 comprises three coils L4, L5 and L6 magnetically coupled to each other with
the interposition of a core. The ratio detection circuit 10 is one of the demodulation methods for
phase-modulated signals, and is a circuit method for creating a phase difference by combining
the coils L5 and L6 and a capacitor and performing vector-like demodulation. The condenser is
formed between the condenser microphone unit 5, more specifically, the diaphragm constituting
the condenser microphone unit 5 and the fixed electrode facing the diaphragm with a gap.
Capacitor. A resonant circuit is constituted by the electrostatic capacitance composed of the
condenser microphone unit 5 and the resonant coil 6. The coils L5 and L6 are connected in
series, the other end of the coil L5 is connected to the microphone output terminal OUT via the
diode 7, and the other end of the coil L6 is connected to the microphone output terminal OUT via
the diode 8. The diodes 7 and 8 are in the opposite direction to each other.
[0018]
The resonant coil 6 is characterized by the structure of the core. FIG. 2 shows in detail the
configuration of the resonant coil 6 including the configuration of the core. In FIG. 2, the core of
the resonant coil 6 includes a support member 11, a giant magnetostrictive element 12, a first
piezoelectric element 14 and a second piezoelectric element 15.
[0019]
The support member 11 is a member for stacking the first piezoelectric element 14, the giant
magnetostrictive element 12, and the second piezoelectric element 15 in series and sandwiching
these members, and is made of a rigid body such as metal. In the example shown in FIG. 1, the
support member 11 is a bolt (hereinafter, the support member 11 is referred to as a “bolt 11”)
passing through the central holes of the giant magnetostrictive element 12 and the piezoelectric
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elements 14 and 15. A nut 19 is screwed in. The first piezoelectric element 14, the giant
magnetostrictive element 12, and the second piezoelectric element 15 are fastened in a state of
being mechanically joined by fastening means including the bolt 11 and the nut 19. Therefore, in
the giant magnetostrictive element 12, the first piezoelectric element 14, and the second
piezoelectric element 15, the mechanical vibration system is closely coupled, and the conversion
efficiency is high. In addition, since it is fastened by the bolt 11 and the nut 19, it is equivalent to
positioning the giant magnetostrictive element 12 between so-called Langevin type piezoelectric
electrons, and the giant magnetostrictive element that was necessary when used alone as a giant
magnetostrictive element There is no need for a coil spring to support the Both ends of the first
piezoelectric element 14, the giant magnetostrictive element 12, and the second piezoelectric
element 15 stacked in series, that is, between the head of the bolt 11 and the first piezoelectric
element 14, the nut 19 and the second piezoelectric element Washers 17 and 18 intervene
between the elements 15, respectively. The first piezoelectric element 14, the giant
magnetostrictive element 12, and the second piezoelectric element 15 are fastened with a stable
fastening force by the washers 17 and 18.
[0020]
The material of the giant magnetostrictive element 12 is, for example, a single crystal alloy
mainly composed of terbium, dysprosium, iron, and the like. When an external magnetic field is
applied, the Joule effect causes a change in dimension extending along the direction of the
external magnetic field, or when an external stress is received, the billy effect causes a change in
the amount of magnetization to cause compressive deformation. It has the property that the
inductance changes. In addition, the giant magnetostrictive element 12 is made of powder
metallurgy, and has extremely high mechanical strength such as compressive strength of 600 ×
10 <6> (Pa) and tensile strength of 20 × 10 <6> (Pa). In addition, Young's modulus is also
extremely high at 2.0 × 10 <6> (N / m <2>). As described above, since the giant magnetostrictive
element 12 is formed of an element having high mechanical strength, it is not broken even if
stress is applied. In addition, the high Young's modulus provides good response to high
frequencies.
[0021]
The first piezoelectric element 14 and the second piezoelectric element 15 generate mechanical
distortion due to the piezoelectric effect when an electric signal is input from the electrodes at
both ends, and both ends of the giant magnetostrictive element 12 in the displacement direction
It is arranged. The first piezoelectric element 14 and the second piezoelectric element 15 are
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arranged such that their polarities face each other as shown by the arrows in FIG. Further, the
first piezoelectric element 14 and the second piezoelectric element 15 have a cylindrical shape
having the same diameter as the giant magnetostrictive element 12 formed in an annular shape,
having a central hole, and the bolt 11 is inserted inside (center) It is done. Therefore, the super
magnetostrictive element 12, the first piezoelectric element 14, and the second piezoelectric
element 15 stacked in series are in a continuous cylindrical shape.
[0022]
It is preferable that the first piezoelectric element 14 and the second piezoelectric element 15 be
configured in a stack shape in which a plurality of layers are stacked. By laminating a plurality of
piezoelectric elements, they are electrically connected in series, and a high voltage can be
generated from both ends of each of the laminated piezoelectric elements. Further, output
terminals are provided at both ends of each of the stacked piezoelectric elements. 1 and 2, the
first terminal portion 24A is disposed on one end side of the first piezoelectric element 14, and
the second terminal portion 24B is disposed on the other end side. On the other hand, the third
terminal portion 25A is disposed on one end side of the second piezoelectric element 15, and the
fourth terminal portion 25B is disposed on the other end side. That is, the first terminal portion
24A is disposed between the washer 17 and one end side of the first piezoelectric element 14,
and the second terminal portion 24B is located between the other end side of the first
piezoelectric element 14 and the super magnetostrictive element 12. It is arranged. On the other
hand, the fourth terminal 25 B is disposed between the giant magnetostrictive element 12 and
the other end of the second piezoelectric element 15, and the third terminal 25 A is between the
one end of the second piezoelectric element 15 and the washer 18. It is arranged. Therefore,
from each terminal part of the both ends in the 1st piezoelectric element 14 and the 2nd
piezoelectric element 15, the output voltage of each laminated piezoelectric element is added and
it outputs.
[0023]
Further, the first terminal portion 24A of the first piezoelectric element 14 and the third terminal
portion 25A of the second piezoelectric element 15 are connected to each other to form a first
terminal 21. On the other hand, the second terminal portion 24B of the first piezoelectric
element 14 and the fourth terminal portion 25B of the second piezoelectric element 14 are
connected to each other to form a second terminal 22. Signals are input from the terminals 21
and 22 to the piezoelectric elements 14 and 15 as described later. Although the first piezoelectric
element 14 and the second piezoelectric element 15 are connected in parallel in the illustrated
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example, they may be connected in series.
[0024]
Returning to FIG. 1, the first piezoelectric element 14 and the second piezoelectric element 15
are electrically connected so that an error signal of the output level of the ratio detection circuit
10 which is a demodulation circuit is input. More specifically, between the microphone output
terminal OUT and the ground, a type of filter in which a resistor R6 and a capacitor C10 are
connected in series is connected. This filter is a filter that passes frequency signals in the voice
band, and is connected so that the terminal voltage of the capacitor C10 is input to the negative
terminal of the differential amplifier 20. The positive terminal of the differential amplifier 20 is
connected so as to input a reference voltage generated by dividing the voltage of the power
supply PS by the resistors R4 and R5. The differential amplifier 20 detects an error in the output
level of the microphone with respect to the reference voltage, amplifies and outputs this error,
and is connected to be input to the first piezoelectric element 14 and the second piezoelectric
element 15. ing. That is, the output terminal of the differential amplifier 20 is connected to the
first terminal 21 and the second terminal 22 is connected to the ground.
[0025]
The operation of the embodiment configured as described above will now be described. The basic
operation as a phase modulation type oscillation detection condenser microphone is as described
above. That is, the capacitance of the condenser microphone unit 5 is changed by vibrating
according to the sound wave received by the diaphragm. The electrostatic capacitance of the
microphone unit 5 and the inductance of the resonant coil 6 constitute a resonant circuit, and a
high frequency signal generated by the oscillator circuit 1 is added as a bias to the resonant
circuit. This high frequency signal is phase-modulated by the microphone unit 5 with the audio
signal subjected to electroacoustic conversion. This modulated signal is demodulated by the ratio
detection circuit 10 which is a demodulation circuit and output from the microphone output
terminal OUT. The resonance frequency is set so as to achieve the optimum operation in the
optimum operation state when the DC component of the output terminal OUT is 0V. When the
DC component of the output signal is 0 V, the differential amplifier 20 is adjusted so as not to
output a signal, and the resonant circuit including the resonant coil 6 operates without change.
[0026]
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10
Now, assuming that the frequency of the output of the resonant circuit fluctuates due to a
temperature change or the like, the output level of the ratio detection circuit 10 fluctuates and
the DC component of the output terminal OUT becomes a positive or negative voltage other than
0V. A signal corresponding to this voltage is output from the differential amplifier 20 and input
to the piezoelectric elements 14 and 15, and the pressure by the piezoelectric action of the
piezoelectric elements 14 and 15 increases or decreases to increase or decrease the pressure
applied to the giant magnetostrictive element 12 The inductance of the magnetostrictive element
12 changes according to the pressure. By changing the resonance frequency of the resonance
circuit including the resonance coil 6 due to the change of the inductance, the DC component of
the signal output from the ratio detection circuit 10 is corrected to 0V. As described above, even
if the temperature changes, the resonance frequency of the resonance circuit can be
automatically adjusted to be an optimum value, and deterioration of the performance or
characteristics of the microphone due to the temperature change can be prevented.
[0027]
In the illustrated embodiment, the piezoelectric elements are disposed on both sides of the giant
magnetostrictive element, and the piezoelectric element is configured to sandwich the giant
magnetostrictive element from both sides, but the pressure of the piezoelectric element is applied
from one side of the giant magnetostrictive element. Even if configured as described above, the
desired effect can be obtained.
[0028]
It is a circuit diagram showing an example of a condenser microphone concerning the present
invention.
It is an expansion longitudinal cross-sectional view which shows the structure of the resonance
coil in the said Example. It is a circuit diagram which shows the example of the conventional
phase modulation type | mold oscillation detection system condenser microphone.
Explanation of sign
[0029]
DESCRIPTION OF SYMBOLS 1 oscillation circuit 2 transistor 3 crystal oscillator 4 oscillation coil
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5 condenser microphone unit 6 resonance coil 10 ratio detection circuit as a demodulation
circuit 11 bolt as fastening means 19 nut as fastening means 12 super magnetostrictive element
14 piezoelectric element 15 piezoelectric element 20 amplifier
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