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DESCRIPTION JP2008271466

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DESCRIPTION JP2008271466
A condenser microphone of a phase modulation type oscillation detection system capable of
canceling noise generated in a high frequency oscillator while removing a factor that lowers the
amplitude of a high frequency signal oscillated by the high frequency oscillator and enhancing
sensitivity. SOLUTION: A resonant circuit composed of capacitances C1 and C2 of a microphone
unit and resonant coils LA and LB biased by a high frequency generated by an oscillation circuit,
an output signal of the resonant circuit changes electrostatic capacitance of the microphone unit
Demodulation circuits 51 and 52 are provided to demodulate corresponding audio signals. The
fixed electrodes opposed to the diaphragm 34 are divided into first and second fixed electrodes
31 and 32, and the fixed electrodes 31 and 32 have the same area facing the diaphragm 34 and
are equidistant to the diaphragm 34. In the resonance circuit, resonance coils LA and LB are
respectively connected to electrostatic capacitances C1 and C2 formed by the diaphragm 34 and
the fixed electrodes 31 and 32 opposed thereto, and two resonance circuits are configured. .
[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 in
particular, uses a microphone unit that performs push-pull operation with a single diaphragm
and uses two resonance circuits to cancel noise generated by an oscillator. It is characterized by
[0002]
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Since a microphone unit used for a condenser microphone has a high output impedance,
generally, an impedance converter is connected, and the impedance is lowered to be 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 is a phase modulation
oscillation detection method, an example of a conventional phase modulation oscillation
detection condenser microphone will be described in more detail below. The phase modulation
type oscillation detection type condenser microphone is an oscillator that stabilizes the
oscillation frequency with a quartz oscillator, a tank coil (oscillation coil) included in the
oscillator, and a high frequency (8 MHz to 12 MHz) transmitted by the oscillator as a condenser
microphone It has a resonant coil 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 each show an example of a conventional phase modulation type
oscillation detection type condenser microphone.
[0005]
The example shown in FIG. 3 is a circuit example of a phase modulation type oscillation detection
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type condenser microphone provided with a bridge circuit devised so that noise generated in an
oscillator is mutually canceled and does 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. 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 resonant circuit 12 varies corresponding to the
sound wave received by the diaphragm of the microphone unit M, and the resonant frequency in
the resonant circuit 12 varies according to the capacitance change of the microphone unit M
fluctuate. The fluctuation of the resonant frequency in the resonant circuit 12 with respect to a
constant resonant frequency in the resonant circuit 11 is taken out in 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 the direction to cancel the rectified voltage of the high frequency bias.
[0006]
As long as the balance between the resonant circuit 11 and the resonant circuit 12 connected in
series is not broken, no voltage appears on the output side. When a 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 by that amount, and a voltage consisting of a side band
without a carrier wave appears at the output side. As such, since the same high frequency current
flows on both sides of the bridge circuit, all the noise of the oscillator 10 is eliminated.
[0007]
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 serves
as a load of the high frequency oscillator 10 and the amplitude of the high frequency signal
oscillates in the high frequency oscillator 10 Is a factor to lower the When the amplitude of the
high frequency signal oscillated by the high frequency oscillator 10 is lowered, there is a
problem that the sensitivity as the phase modulation type oscillation detection type condenser
microphone is lowered.
[0008]
Next, another example of the conventional phase modulation type oscillation detection type
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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
resonant circuits 21 and 22 coupled to a high frequency oscillator 20. One resonant circuit 21
includes a varicap diode 24, and the other resonant circuit 22 includes the capacitance of the
condenser microphone unit M. The low-frequency amplification transistor 25 amplifies 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 the varicap diode 24 passes
through the filter. It is configured to be fed back. This circuit aims at widening the dynamic range
without impairing the SN ratio by providing a feedback circuit.
[0009]
The prior art example shown in FIG. 4 is basically the same as the prior art example shown in
FIG. 3, and varicap diode 24 serves as a load of high frequency oscillator 20 to reduce the
amplitude of the high frequency signal oscillated by high frequency oscillator 20. It becomes a
factor. As a result, there is a problem that the sensitivity as the phase modulation type oscillation
detection type condenser microphone is lowered.
[0010]
The prior art related to the present invention is the invention described in Patent Document 1
according to the application of the present applicant. 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.
[0011]
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
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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.
[0012]
The invention described in Patent Document 2 is similar to the components of the present
invention described later in the point of including the oscillator for converting the vibration of
the diaphragm into the FM wave, but after conversion into the FM wave The circuit configuration
or signal processing is completely different, and the present invention does not have a circuit
configuration for outputting as a digital signal.
[0013]
JP, 2006-101302, A JP, 7-23492, A
[0014]
The present invention solves the problems found in the conventional phase modulation type
oscillation detection type condenser microphone as described with reference to FIGS. 3 and 4,
that is, causes the reduction of the amplitude of the high frequency signal oscillated by the high
frequency oscillator. An object of the present invention is to provide a condenser microphone of
a phase modulation type oscillation detection system capable of canceling out noise generated by
a high frequency oscillator while removing and enhancing sensitivity.
[0015]
According to the present invention, an oscillation circuit including an oscillation coil and a quartz
oscillator, and the oscillation circuit coupled with the oscillation circuit via the coupling means
and biased by a high frequency generated by the oscillation circuit, and the capacitance and
resonance of the condenser microphone unit A condenser microphone comprising: a resonance
circuit constituted by a coil; and a demodulation circuit that demodulates an output signal of the
resonance circuit into an audio signal corresponding to a change in capacitance of the condenser
microphone unit. The fixed electrode disposed opposite to 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
disposed equidistantly to the diaphragm, and the resonance circuit is formed by the diaphragm
and the first fixed electrode and the second fixed electrode opposed thereto. The most important
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feature that the two resonance circuit is constituted by connecting respective resonant coil
capacitance.
[0016]
Since the resonant circuit is constituted by the electrostatic capacitance of the condenser
microphone unit itself and the resonant coil, it is not necessary to connect a capacitor serving as
a load of the oscillator circuit as in the conventional phase modulation type oscillation detection
condenser microphone. This eliminates the cause of the reduction in the amplitude of the high
frequency signal that oscillates in the above, and a condenser microphone of a phase modulation
type oscillation detection system with high sensitivity can be obtained.
In addition, since it is possible to perform the push-pull operation of the condenser microphone
unit and the two resonance circuits and perform the balanced output of the audio signal, it is
possible to cancel the polarities of the noise generated by the oscillator by reversing each other.
[0017]
Hereinafter, an embodiment of a condenser microphone according to the present invention will
be described with reference to FIG. 1 and FIG.
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.
The number of fixed electrodes arranged to face the diaphragm 34 is usually one, but in the
present invention, it is divided into the first fixed electrode 31 and the second fixed electrode 32
so as to be opposed to one diaphragm 34. doing. For example, a synthetic resin film having a
metal vapor deposition film is used as the diaphragm 34, and the 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 with the diaphragm 34 and are
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overlapped by interposing a common ring-shaped spacer (not shown) between the diaphragm 34
and the first fixed electrode 31 and the second fixed electrode 32. It is arranged equidistant to
the diaphragm 34. The diaphragm 34 is connected to the ground via the support ring 35 and the
unit case.
[0018]
FIG. 2 shows an example of the fixed electrode plate 30 provided with the first fixed electrode 31
and the second fixed electrode 32. The first fixed electrode 31 and the second fixed electrode 32
are formed on one surface side of the fixed electrode plate 30 made of an insulating material and
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 radially outward, and a
soldering land 37 electrically connected to the first fixed electrode 31 and a soldering land 38
electrically connected to the second fixed electrode 32 are formed on the protruding portion. ing.
[0019]
In FIG. 1, reference numeral 40 denotes an oscillator circuit that transmits a high frequency
signal of, for example, about 8 MHz to 12 MHz. The oscillation circuit 40 includes a quartz
oscillator 43, two field effect transistors 41 and 42, and an oscillation coil LO. The terminal of the
crystal oscillator 43 is connected to the bases of the transistors 41 and 42, and the sources of the
transistors 41 and 42 are connected in series via two coils LO1 and LO2 of the oscillation coil
LO. The drain of the transistor 41 is connected to ground via the resistor R1, the base of the
transistor 41 via the resistor R2, the base of the transistor 42 via the resistor R3, and the drain of
the transistor 42 via the resistor R4 to ground. .
[0020]
The oscillation coil 40 has other coils LO3 and LO4 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, which are connected to
ground. One end of the coil LO3 is connected to the first fixed electrode 31 through the coil LA1
constituting the resonant coil LA, more specifically, to the soldering land 37 of the first fixed
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electrode 31 shown in FIG. The other end of the coil LO3 is connected to the second fixed
electrode 32 via 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.
[0021]
Both ends of the coil LA2 constituting the resonant coil LA are connected to the output terminal
2 through the diodes D11 and D12 constituting the ratio detection circuit 51, and are connected
to the ground through the capacitor C3. Both ends of the coil LB2 constituting the other
resonance coil LB are connected to the output terminal 3 through the diodes D21 and D22
constituting the ratio detection circuit 52, and are connected to the earth through the 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 balanced output terminal of
three terminals, the terminal 1 is a ground terminal, the terminal 2 is a hot signal terminal, the
terminal 3 Is a signal terminal on the cold side. The terminals 2 and 3 are connected to both ends
of the primary winding of the output transformer 45 in the example shown in FIG. 1 so that an
audio signal is output from the secondary winding of the transformer 45.
[0022]
The microphone unit has two fixed electrodes 31 and 32 opposed to one diaphragm 34, and
includes a capacitor C 1 configured of the diaphragm 34 and the first fixed electrode 31, the
diaphragm 34 and the second fixed electrode 32. The inductances formed of the coils LA1 and
LB1 are respectively connected to the configured capacitor C2 to configure a resonant circuit
that performs push-pull operation. The resonance circuit having the capacitor C1, the coil LA1,
the resonance circuit having the capacitor C2, and the resonance circuit having the coil LB1 are
respectively biased by the high frequency oscillation signal output from the coil LO3 included in
the oscillation circuit 40. The high frequency signals supplied from the coil LO3 to the two
resonant circuits are respectively supplied from one end and the other end of the coil LO3 having
a center tap, and thus become high frequency signals in opposite phase to each other.
[0023]
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Ratio detection circuits 51 and 52 are connected to the two resonance circuits via coils LA2 and
LB2 magnetically coupled to the resonance coils LA1 and LB1, respectively. In the ratio detection
circuit 51, the difference between the oscillation signal of the constant frequency by 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 according 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 of 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 and the frequency fluctuates according to the vibration of the diaphragm
34 Is detected and output from the terminal 3 as an audio signal. These audio signals are
balancedly output from the terminal 2 and the terminal 3. This balanced output can be pulled out
by means of 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 the both ends of the secondary winding to the
outside.
[0024]
According to the embodiment described above, the noise generated by the high frequency
oscillator 40 is input to the two resonant circuits as balanced signals and detected by the ratio
detection circuits 51 and 52 corresponding to the respective resonant circuits. In FIG. 1, the
waveforms described near the terminals 2 and 3 are a sinusoidal signal as a model of an audio
signal and waveforms of noise signals n1 and n2. 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 to each other, the noise signals n1 and n2 are
canceled out.
[0025]
In addition, the condenser microphone unit is configured of a single diaphragm 34 and two fixed
electrodes 31 and 32 opposed thereto to make a push-pull operable unit, and vibration of the
two fixed electrodes 31 and 32 is one The two resonant circuits including two capacitors C1 and
C2 configured by the plate 34 are configured. Therefore, the two resonance circuits are
constituted by the electrostatic capacitance of the condenser microphone unit itself and the
resonance coil, and the condenser serving as the load of the oscillation circuit is connected like a
conventional phase modulation type oscillation detection condenser microphone There is no
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need for this, and the cause of the reduction in the amplitude of the high frequency signal
oscillated by the oscillator is eliminated, and it is possible to obtain the condenser microphone of
the phase modulation type oscillation detection system with high sensitivity.
[0026]
It is a circuit diagram showing an example of a condenser microphone concerning the present
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 sign
[0027]
31 1st fixed electrode 32 2nd fixed electrode 34 diaphragm 40 oscillation circuit 43 crystal
oscillator 51 ratio detection circuit 52 ratio detection circuit LO oscillation coil LA resonance coil
LB resonance coil
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